Keywords: Settlements, peasants-soldiers, Eurasia, Indian Ocean, Russia, steppe, filles du roi, Quebec

This chapter seeks to provide a comprehensive view of forms of indentured migrations which goes beyond conventional approaches limiting this phenomenon to post-slavery era, essentially in the Atlantic and debating whether it was free or unfree labor. Instead, it is argued that this form of extreme dependence related to migration developed on both sides of Eurasia (in Central Asia, Russia and China and in Western Empires) since the seventeenth century onwards. This global phenomenon expressed the persistent overlapping between labor and debt across the centuries and the continents. The global diffusion of indentured migrations will be also related to Empire building. Eurasian land empires will be contrasted to European maritime empires, in terms of forms of labor dependence and migrants' inclusion/exclusion.

Historiography

Indentured migration is usually understood as a temporary solution between slavery and free wage labor market, a kind of adjustment estate owners, producers and merchants needed to resettle labor and product markets. As such, indentured migration is usually encapsulated into a period running from the 1830s through the early twentieth century and WWI. 1 From this standpoint, indentured contracts and indentured immigrants are relevant focuses to discuss the tensions between free wage labor and slavery and the role of both in global capitalism. 2

Debates mostly focused on whether indentured migrants were disguised slaves or not.

According to a first approach, the indentured contract resembled forced labour and slavery and contracts were said to express a "legal fiction". This interpretation was advanced by

in inner Asia as well as the contemporary consolidation of Western European Empires across the Atlantic. On both sides of the northern hemisphere, bonded settlers contributed to occupy the so called nomadic lands.

Indentured migrants and settlers in Eurasia, sixteenth-eighteenth centuries

Western maritime empires are said to bring military and commercial superiority over Asian (and central European) land Empires, together with innovative institutions (the Atlantic and the glorious revolution in Britain). It is argued that Asian empires like Russia and China, lack of capital was compensated by maximum coercion. Instead, the European nation-states are said to have achieved the right mix of capital and coercion. [START_REF] Tilly | Coercion, Capital and European States. AD[END_REF] The case of indentured migrants and colonists is a good test to evaluate this argument.

In the space of three centuries (from the fifteenth to the seventeenth), Moscow built the largest empire in the world along with China; 7 together, they shared the whole of southern and central Asia. Muscovite expansion was achieved through the systematic displacement and submission of more or less "Nomadic" people of the steppe. Nogays, Kalmyks and other "nomads" were not only military aggressed, they were also deprived of their main monetary and in kind resources. The problem in the newly conquered regions was the lack of peasants.

The first waves of settlement took place in the sixteenth century after seizing Kazan in 1552, and again during the 1580s and 1590s following the withdrawal of the Tatars of Crimea and the Nogays, and the agricultural crisis in central Russia. The Muscovite authorities allowed the peasants and peasant that performed their military service in these regions to pass their small estates on to their descendants. These policies contrasted those enhanced during this same period in the muscovite mainland where peasants were increasingly bound to their masters and lands. Indeed, there was a link between these two apparently contradictory processes: the progressive institutionalization of serfdom, between the late sixteenth century and the statute of 1649 actually sought to compensate estate owners from the loss of peasants due to military conscription and colonial settlement.

Moreover, the tsarist authorities did not always enforced these rules and let fugitive peasants stay in the new occupied areas. 8 For a few fugitives who were returned to their "legitimate owners", millions of other peasants were left in their new places. In the eyes of some tsarist elites, geopolitical considerations overwhelmed the political and social defense of estate owners in the central areas of Russia. Colonization could not have worked without a more or less accepted emigration and, thus, a limitation of the power of estate owners of central Russia on their peasants.9

Migrants entered some main legal and social categories. To encourage military enlistment and strengthen the army, the Muscovite princes increased pomest'e (land grants in return for military service) in frontier regions, particularly in the south. This trend took another turn with the legal creation of the odnodvortsy (individuals unmarried owners) who were identified as landowners by their tax and military system in decrees 7 M. Khodarkovsky, Russia's Steppe Frontier. The Making of a Colonial Empire, 1500-1800 (Bloomington and Indianapolis: Indiana University Press, 2002). W. Sunderland, Taming the Wild Field. Colonization and Empire on the Russian Steppe (Ithaca: Cornell University Press, 2004). 8 RGIA (Russian Imperial Archives), fond 379 opis'1; PSZ (Full Compilation of Russian Laws), sery 1, vol. 40 n 21779; vol. 32 n. 25150. adopted between 1710 and 1724. At the time, the odnovortsy included 600,000 Muscovites and their dependents. The odnodvortsy were similar to state peasants: like the latter, they paid taxes (in money and in kind), but they also had military obligations and enjoyed property rights over the land,10 which meant land ownership was no longer a distinguishing feature of the "nobility" as opposed to the peasantry.

Official rules also regulated forms of indenturship: This particular form of contract entered a broader category, called kholopstvo, a wide range of forms of human bondage including chattel slavery, debt slavery, indenturship, war captives, and domesticity. These relationships were extended to some migrants to whom the state, or their master, paid the transfer and the purchase of tools.11 In exchange, migrants worked for free several years. Like other forms of kholopstvo, the contract and dependence could last one (renewable) year or, most often, until the death of the master or of the migrant. Instead, unlike western slavery, this status was not transferable to the children.

Unlike the Steppe, Siberia did not benefit from a positive image among the peasants, despite the fact that serfdom was banned in Siberia. As a result, in the 1770s, Siberia had roughly a half-million inhabitants, but only two-thirds of them were Slavs. Overall, in much the same way as in Canada and other northern areas of the Americas, the relative lack of women drove the authorities to encourage family migration, household resettlements, or simply the transfer of "women of ill repute" from Russian prisons and hospices. Kidnapping was also practiced.

Women were often raped during their journey to Siberia and once they reached their destination.

In sum, Russian expansion drew its strength not from serfdom and absolutism, but from its empire with relatively flexible institutions and social hierarchies in which the real center of gravity lay in the periphery, in the steppe, rather than in Moscow and still less in Saint Petersburg. The Tsars and the Moscow elites enjoyed considerable power, but they had to negotiate constantly with the other strata of society, especially in newly conquered territories. The latter, in return, had an ongoing influence on the policies and balance of power in the "center".

The outcome was that between 1678 and 1897, peasants' settlements in the central forest heartland fell from 69.9 to 41.22 percent of the total cultivated land, while those in the steppe areas increased from 28.78 to 41.22 percent. During this same period, settlement in Siberia rose from 1.32 to 7.54 percent. In the southern and eastern settlement areas, one-third of the population increase was due to natural growth and two-thirds to immigration. By the 1680s the peasant population in the Ukrainian territories was about half a million people; it doubled by 1720. By 1678, 3.7 million peasants had emigrated and settled in Siberia, the northwestern areas, the Urals, the south-eastern steppe, and the Volga. Yet, the distribution of migrants was very unequal: most settlements took place in left-bank Ukraine (909 651 male souls in 1719) and Middle Volga (651405), while all together the lower Volga, southern Urals and don Cossacks lands counted only 200 000 male souls. 12 In the early 1770s, squeezed by the growing number of Russian settlers as well as by Russian government interference on pastures, 150 000 Kalmyks left the lower Volga for Chinese Jungaria. Unlike the territories beyond the Urals, the steppe was much often considered as "Russia", not as its colony. As a consequence, this "empty space" had to be secured with Russian colonists, military garrisons and eventually new annexations beyond the border line in order to secure the new areas and settlements. 13 Between 1796 and 1835, some 1.7 million new migrants moved to the region, more than tripling the total migration of the 1762-1795 period.

Quite similarly, in China, the Qing encouraged colonization in the north-east and north-west.

In the former case (and in the Liaoning region in particular), the Qing set up a system of colonization which gave peasants the specific status of "State peasants", this also being widespread in the area around Beijing. At the same time, in Liaoning, these peasants were placed under the authority of the banners, a military-administrative system. It was a hereditary status which linked peasants to the land by significantly restricting mobility. 14 In the north-west, the situation was both similar to and different from that in Liaoning; between 1660 and 1750 there were major migrations from the south to the north-west of China. Using the same strategy as that employed by the Ming and the Yuan, but on a far larger scale, the Qing decided to use soldier-colonists. The soldiers cultivated their own land, thus providing a large part of their own supplies. This solution was designed to reduce transport costs and simultaneously occupy the frontier regions. The colonists came from the north-west and were Han, or else Turks from southern Xinjiang. The criminals became the garrisons' bonded servants. A differentiated tax system was introduced. On Muslim lands the Qing took 40% of the harvests as payment in kind. The farmers were considered to be State peasants 15 . On the other hand, in the Han military colonies, all surpluses belonged to the State and the colonists were attached to the land. The colonists also had duties to the State in the form of public works and road-building. Finally, banner troops owned the land collectively, in the name of the garrison to which they belonged. Soldiers were used not just to defend borders but also to clear land. Once their military service came to an end, they were allowed to stay on as colonists.

The Chinese as well introduced a system very much like that of England's indenture, China used them to supply its army at home and along the frontiers. At first the Chinese State, and later on an increasing number of intermediaries, paid in advance for migrants' transport and clothing. In exchange, the latter were expected to spend five years working on land clearance, giving almost all of their harvests to the State. Once their debts had been repaid, they became self-employed farmers. However, compared to Europeans in America, the Qing attached greater importance to land clearance and to soldier-colonists than to civilian colonists 16 .

To sum up, Eurasian Empires like China and Russia adopted forms of indentured migration to colonize central Asia; migrants (mostly peasant-soldiers) were under coercion and strong military control (most were peasants-soldiers) but still their conditions were far better compared to those of most laborers in the mainland. In the steppe, serfdom was either mild or non existent, while it was forbidden in Siberia. This outcome was completely different from that in the Western maritime Empires, in particular in the Atlantic, where the conditions of indentured migrants were much worse than those of people in the mainland.

Indentured migrations in maritime Empires. The British case

As early as 1560, in a book on the Elizabethan polity, Sir Thomas Smith argued that "the necessity and want of bondmen have made men to use free men as bondmen to all servile services: but yet more liberally and freely, and with more equality and moderation than in time of slaves." 17 The English experience in Ireland marked a turning point. The "Plantation of Ulster" provided a middle ground for Scots and Englishmen alike to pursue a common scheme of plantation and civilization in a potentially pan-British enterprise. 18 In 1609, English and Scottish settlers were sent to the Ulster plantation to develop Irish land, along with a thousand Irishmen enrolled as soldiers in the service of the King. Both the settlements of Ireland and of the American colonies entailed dispossessing native peoples of their lands; both created polities that were in a dependent relationship with England or, after 1707, Britain.

Tudor attempts to establish plantations in Ireland reflected the hope that England could repeat in Irish territory the exploits of Spain's conquistadores, but the results were very different.

There was no swift conquest, no gold or silver, and Irish armed resistance proved effective at least until the end of the seventeenth century. Profits from the Ulster plantation were scarce and land leased out to Irish Catholic tenants reaped a low rate of return. Many of the English fortune-hunting gentry who sought to subjugate Ireland later embarked on privateering expeditions to the Caribbean.

The regulation of labor and the meaning of freedom were situated at the intersection of institutional dynamics: the forms and institutions of labor in Britain itself, the relationship between the Crown and the Parliament, and the fiscal and military state.

Between 170 000 and 225 000 emigrants left the British isles for America between 1610 and 1660. 110 000 went to Caribbean, 50 000 to Virginia and 20-25000 to New England. 19 In the mainland, the male female ratio was 6:1 in the 1630s, dropping to 3:1 then 2:1 during the second half of the century. In the Caribbean, indentured migrants were essentially men, then with the raise of slavery, mostly men. Outside of the Caribbean, between the 1630s and 1780, a range of 472,000-510,000 migrants are estimated, of which 50,000 were convicts and the rest were half indentured and half "voluntary" migrants. However, indentured in percentage were about 60-65 in the seventeenth century and 40-42 percent in the following century. 20

They mostly came from southeast England and later, from southwest as well. This was linked to the transformations of agriculture in these areas, the seventeenth century enclosures in particular. Indentured labor was not presented (by authorities, masters, and legal scholars) as a form of coerced labor until after the abolition of slavery in the 1830s (in the British Empire); until that moment, the indentured contract was an extension, not the negation, of existing contracts in the mainland. The migrant was committed to labor for a negotiated period on terms agreed with a shipper prior to embarkation in exchange for transportation. The shipper recovered transportation costs and margins by selling the servant's indenture on arrival. The planter bought the service of the immigrants according to "the custom of the country". 21 These relationships found their origin in rural contracts, and in particular, servants in husbandry and rural apprenticeship. Like apprentices, indentured immigrants were under seven years contract, during which they could be transferred, but could not move without the authorization of the master. Instead, apprentices could not be properly sold without their consent or those of their father. This meant that English migration was built upon institutions which specifically aimed at controlling labor service and time of all working people and with a specific attention devoted to vagrants, poor and young: rules on runaways were adopted not only in Colonial Americas for slaves and for indenture workers, but also in Great-Britain, where runaway workers, journeymen, etc. were submitted to quite similar rules, under the master and servant laws and the statute of artificers (1562). Apprenticeship, advances on wages and raw materials, and also simple master-servant relations justified for these provisions. Free labour, despite the existence of a contract, was considered as being a "property" of the employer and a resource for the whole community to which she/he belonged 22 .

However, transatlantic migration also raised two novelties: first, credit played a crucial role.

The advance of the trip costs was much more important than advances on wages in England and gave rise to longer dependency (five to seven years). Second, credits as property meant that the migrant was an article of commerce and, as such, he could be sold, transferred or leased. Once in America, two supplementary problems rose for masters: the immigrant's motivation; her/his temptation to escape. Coercion and violence were the most common solutions to the first problem, but sometimes masters also sought to offer better conditions of working and leaving. Contracts show that masters offered better conditions to more skilled and productive immigrants. The size of the farm and the activity was important as well: large estates offered better conditions than small ones. The enforcement of the rules was harsher in the colonies than in England (Britain since 1701); English law forbade beatings, while several American statutes forbade masters from "excessive beating and abusing" their servants. A further innovation took place: masters had to provide specifically enumerated goods, called freedom dues, to their servants at contract's end. In the late eighteenth century, freedom dues could amount to 66% of the contract's price or 28% of yearly wages.

We may distinguish two main periods and regions of immigration: the Chesapeake Bay colonies (Virginia and Maryland) between 1630 and the early 1700s, the Delaware Valley (Pennsylvania, New Jersey) and Maryland between the 1670s and the 1770s. From the mid-1620s through the end of the century, between 100,000 and 110,000 English migrants arrived in the Chesapeake region. 23 They mostly came from southeast England and later from the southwest due to the agricultural transformations in those areas, particularly seventeenthcentury enclosures.

In Virginia, In the 1630s, when the number of migrants rose in connection with the expanding tobacco plantations, the Assembly decided that laborers and artificers should be prohibited from leaving unfinished work. Servitude became a rigorous condition of subordination to a master. The legal form of indentured servitude in Virginia also owed a great deal to the explicit binding of parish orphans and pauper children into husbandry apprenticeship in the mainland and the law of vagrancy with its obsession with the control of deviants.

Unlike the Virginia colony statutes, the measures adopted in Pennsylvania put greater emphasis on court oversight of the master-servant relationship and left much less to the discretion of the master. Similar rules were adopted in Virginia and Chesapeake for all persons living under household government, i.e. children, women and servants. In those provisions, the political, the economic, and the domestic were not separate realms; they all participated in the social order. Master-servant relations, marriage, and paternal authority entered into a single idea of community and its discipline at work. Thus, by the eighteenth century, American colonists spoke of indentured labor in terms of property: runaway servants had to be returned to their legitimate master and owner. As a consequence, creditors could attach servants to satisfy their master's debt and indentured immigrants were sold at public fairs. 24 Prices depended on the age, sex, and skill of the indentured immigrant; young, healthy men were better quoted than women, children, and the unskilled.

Despite coercion, the profitability of white indentured immigrants was limited by the fact that they did not transmit their status: they were not chattel slaves and could not be bound for life.

Moreover, the rate of emigration Britain produced before the diffusion of the plantation system seemingly was a considerable achievement and could not be pushed further anymore.

Thus, at the turn of the seventeenth to the eighteenth century, the increasing arrival of slaves was not a response to lack of labor per se, but to a lack of coerced labor, particularly indentured immigrants. Indentured servants were one third of the population in the 1640s, one-fifth in 1670 and 12 percent in 1700. In the meantime, the ratio slave/servant population moved to 30 percent to 45.9 percent. 25 In the mid-seventeenth century, the conditions and treatment of white indentured servants and African immigrants were quite similar. Gradually, the distinction between them became clear-cut, starting with the English Civil War and its impact on the American colonies. Then, precisely in response to the legal and racial identification of slaves, indentured servitude evolved. Although the terms servant was still in use in legal proceedings, wage workers often refused to be called so; progressively, indentured labor was confined to minors alone and the previous representation of labor relationship as a continuum, from free workers and servants to indentured immigrants, left the place to a discontinuous representation of it, made of two poles, free (wage earners) and unfree (slaves) labor. Bipolarity was enhanced by the extension of slavery and its racial identification. These changes won the support of all those who argued that white people were the real slaves, particularly when Britain failed to guarantee all the liberties enjoyed in the mainland. If British subjects in the colonies did not enjoy the traditional rights of English people, then they were not freemen, but slaves. 26 After the American independance, indentured servitude evolved into a thoroughly commercialized institution. Decreasing transportation costs encouraged shipowners to enter the emigrant market, where labor became a tradable commodity. White indentured declined during the 1820 and stopped in the following decade. A number of states limited indentured labor to immigrants and minors and excluded native-born locals. Thus, the previous representation of labor relationship in the North as a continuum, extending from free workers and servants to indentured immigrants, gave place to a discontinuous representation made up of two poles: free and unfree labor. As a result, at the turn of the nineteenth century, indentured servitude as a whole was gradually shifting from a free contract to a form of slavery. In abolitionist states, slave masters began to use the still legal status of indentured service as a way to keep former slaves in bondage. American courts enhanced the rights of indentured immigrants against those of their masters. The latter lost their direct power to coerce and engage in physical punishment, but retained the power to indirectly coerce by withholding from workers their means of subsistence. As a consequence, involuntary servitude (slavery) and voluntary service (indentured labor) gradually converged in slaveholder practices as well as in anti-slavery discourse and action. From the 1820s onwards, through the efforts of the abolitionist movement, indentured immigration came to be viewed as a form of forced labor tantamount to slavery, and as such unacceptable, for whites in particular.

By the late 1830s, there were no more penal sanctions for breach of contracts concerning white adult workers. Several factors intervened to bring about this change: the identification of white servitude with slavery in abolitionist discourse; second, increasing political action on the part of white wage workers; third, the early dissemination of suffrage to many states. 27 In short, employers did not abandon servitude because they found wage labor to be superior; the opposite was true: As immigrants abandoned servitude, employers found it easy to shift to wage labor because they regarded the two types of labor to be interchangeable.

French indentured migrants (engagés)

As in the British empire, in the French colonies as well the contract of engagement was developed in the seventeenth century. Transport expenses were advanced by employers or their middlemen in exchange for a commitment to work for several years. The engagés were subject to criminal penalties and could be transferred along with their contract to other masters. Owing to the close resemblance between wage earners and domestic servants (especially under the ancien régime) and the survival of forms of domestic service into the nineteenth century, the contract of engagement should not be understood in opposition to these other labor relationships, but rather as an extension and of them in the colonial situation.

In other words, whereas our own accustomed categories contrast free wage work in France with slavery and indentured service in its colonies, the actors at the time regarded the contract of engagement as a free contract, and the penalties for breach of contract were quite similar to those applied to laborers. The oldest contracts of engagement were thus the ones that still had some connection to rent paid by tenants to feudal lords and leases for tenant farming.

Later, at the mid-seventeenth century, contracts of engagement explicitly relied on two types of contracts that already existed: the agricultural journeyman's contract and the sailor's contract. [START_REF] Debien | Les engagés pour les Antilles 1634-1715[END_REF] The engagé rented his services, i.e., the totality of his time, to his master, and terminating a contract was difficult, especially for the engagé. [START_REF]Minutier Teuleron[END_REF] The engagés were not allowed to marry without authorization from the master, but an engagé had the right to redeem his indenture and could oblige his master to agree to do so. Contracts of engagement explicitly invoked apprenticeship contracts: the master had the same requirement to provide for the care of the engagé as he did for the apprentice, the same expenses in case of illness, and the same word in the margins: bondage. Sometimes the close relationship between engagement and apprenticeship was explicit, and the expression engagement-apprentissage appeared. In this case, the engagé departed and returned with his master to work on all "his affairs, trade, and commerce." These engagés were not apprentice-settlers but apprenticemerchants, without wages. Indeed, the father or mother of the engagé paid a lump sum to the merchant or the settler.

In the seventeenth and eighteenth centuries, the contract of engagement concerned mainly whites who went to the French West Indies and Canada, but also to the Indian Ocean. [START_REF] Dechêne | Habitants et marchands de Montréal au XVIIe siècle[END_REF] Between 1660 and 1715, 5,200 engagés left for the French West Indies from La Rochelle alone. This figure is much smaller than the 210,000 indentured Britons who left for North America between 1630 and 1700.

Why so few French? Some attributed French (and British) attitudes to "cultural" rather than economic forces (pull and push). [START_REF] Moogk | Reluctant Exiles : Emigrants from France in Canada Before 1760[END_REF] In particular, communal traditions, the weakness of recruiting system and of commercial carriers (French trade brokers and maritime capital did not find particularly appealing the trade in migrants) surely intervened as well as the comparatively low attractive power of Quebec compared to mainland North America. Also, much more than in Britain, where some pamphleteers sustained colonization, in France the opposite attitude expressed.

The royal administration, and Colbert in particular, also feared that France would be weakened militarily and economically by the loss of any part of its population. Colbert was against. 32

There was also a correlation between migration from the Western French Littoral to the Americas in the seventeenth century and periods of famine, hunger and riot in western France.

Yet despite the frequency of these events, even more than in Britain at this time, it did not suffice to push people to cross the Ocean, other opportunities being available, such as temporary migration to town or even sealing. Therefore, coercion intervened: private as well as state companies kidnapped people in ports and littoral villages and towns. Companies and captains as well did not seem to care about traveling conditions: the rate of mortality during the middle passage was high: to New France, 17% in 1662, 20% the following year. Using the French Royal Navy for transport did not improve matters. The mortality rate during the crossing increased astronomically: 33 passengers out of 200 died during the 1662 recruitment.

The following year, of 300 passenger recruits, 60 died during the voyage and 12 more soon after arrival. The authorities decided to turn to Dutch transporters first, and then the West Indian Company. The mortality rate fell considerably, although there were discrepancies between the passenger manifest upon departure and those accounted for in New France. The reason for this, in part, is that the Company did not want to be held responsible for losses.

Once a recruit had finished his service and if he was free of debt, he could decide to go home to France or to settle down as a colonist. He had the right to receive wages at the end of his five years' service. However, the master would deduct the cost of transport, drinks, lost or damaged tools, any advances paid, laundry and medicine. Consequently, there was little left for the recruit at the end of his service.

Initially only men were recruited; The reason for this is that land clearers were the most in demand, while domestics were recruited either among former recruits or among the indigenous people. In addition to the relatively high costs of transport and modest profits to be made, the recruiters had very little interest in enrolling women.

in the mid-1660s, because of frequent rapes of native women and therefore raising conflicts 46,3 (1989): 463-505. 32 Talon to Colbert, Oct. 4, i665, Rapport de l'Archiviste, I930-I931, 36; Colbert to Talon, Jan. 5, I 666, ibid., 4I .

with local population, but also because of Colbert's ambition to colonize the new World, the quest for indentured women began. French authorities decided to tap into women retained in poorhouses, workhouses and monasteries. As in England, penitentiaries for women in France in the seventeenth, eighteenth and nineteenth centuries were rather widespread, holding women serving ordinary sentences, prostitutes, beggars, and single mothers, but also women who were deemed to be "indocile", "possessed" or "mad". The French seized the youngest among prisoners, offered them a small dowry and sent them across the Atlantic. 31 women and teens arrived in 1653 and 1659; 770 between 1663 and 1673, mostly from Paris La Salpetrière and from Rouen. After being raped during the travel and again to destination (while waiting of their allocation) they were eventually given to colonists who, again, often made use of violence and after abandoned them. A few of them survived, raised a family and are now the well celebrated filles du Roi in Quebec, mothers' of the country. 76 out of 770 filles du Roi arriving between 1663 and 1673 were aged 12-15 years ; 247 were between 16 and 20 years old and 204 between 21 and 25. 4% never married, 80% did it within 6 months but about 1/7 were repudiated in a few months and the marriage invalidated under the argument that the women were not "pure" or, worse, already pregnant (despite the fact that this had took place during the travel or even in New France). Women made 4% of Quebec population in 1650 and 1760, soldiers and indentured migrants were 60%, the rest being war captives and slaves (15%), merchants officers and priests (15%).

From early modern to modern times. Continuities and changes of indenturship At the turn of the Eighteenth and nineteenth century, the great divergence between the West and Asia took its full strength and the opposition between maritime and land empires lost its meaning not only because of the overall power of the steam (on land and on sea) but also because migration within and across Eurasian empires entered global dynamics in which western powers led the game. Thus, the rapid growth of Chinese emigration in the nineteenth century was closely related to deteriorating economic and social conditions. Economic dependence, periodic natural disasters, along with growing political instability drove many to leave home. Massive migration took place from overpopulated southern provinces to the northern frontier. Yet, fewer than three-quarters of a million Chinese migrants signed indentured contracts with European employers. The Chinese financed their voyage with money advanced by families or future employers through a "credit ticket" system. Recruits were delivered to the European firms by Chinese brokers who by-passed Chinese legal prohibitions of indentured emigration. Women still were a minority but their proportion varied according to the destination: about 5% to the US, 17% to Dutch and British Guyana.

Sometimes they arranged marriages, sometimes labor contract for domestic service, in many cases this turned out into prostitution. 33 Indians made up the largest immigrant population in the Indian Ocean during the nineteenth century. The size of the exodus from India reflected rising distress in the territory and its integration into the British Empire. Of the two million Indians who immigrated to other regions of the British empire between 1834 and 1912, 95 percent came from Bengal. [START_REF] Mna | Mauritius National Archives) B1B, First Immigration, Arrivals of Indian Immigrants[END_REF] There was considerable migration from rural areas to the cities, short-term migration within British India for seasonal work, indentured migration to Assam tea plantations, and recruitment for public works. The overlapping between labor and credit markets provided an overall connection between different forms of mobility. Creditors exchanged small advances for large quantities of work. Landless workers lacked collateral and they were forced to offer months or years of future labor to repay the loan and to secure the debt. These arrangements were deeply embedded in networks of reputation, deference and community-regulated-norms. [START_REF] Prakash | Bonded histories. Genealogies of Labor Servitude in Colonial India[END_REF] The indentured immigrants from Calcutta often belonged to the caste of untouchables or came from tribal areas in the interior and very often from the northeast. Many came from the lower castes and classes, but this was not necessarily the case and middle and upper castes migrated too, as detailed studies on Mauritius and Natal confirm. [START_REF] Bhana | Indentured Indian Emigrants to Natal, 1860-1902. A Study Based on Ships' List[END_REF] Female immigration remained marginal, at least initially, and had to be overseen by the state. [START_REF] Carter | [END_REF] It did not develop rapidly until the 1860s, due to the arrival of new indentured servants who came with their families and the considerable demand for domestic and urban labor as well as more traditional labor on the sugar plantations.

The living and working conditions of immigrants remained extremely harsh, particularly during the 1850s and much of the 1860s, despite the efforts of the abolitionist movement. In Mauritius, Immigrants mostly complained of ill treatment, wage retention and poor food.

Overall, of one million pounds in wages due in 1874, the masters withheld 229,225 pounds for absenteeism and 91,000 pounds for illness. 38 In Assam as well, tea was produced in huge plantations making use of indentured migrants. The Company's labor agents conducted operations in Bengal, tapping into the labor market of agricultural workers, seasonal migrants and the developing indentured labor market for overseas colonies. Three-year contracts were the rule but they were seldom enforced in the courts. Production and labor immigration increased in the 1850s, but the Company complained about the coolies' continuous desertion, absenteeism and lack of motivation. The mortality rate of indentured immigrantsabout 1.5 to 5 percent during voyages to Mauritius and Reunion Islandsjumped to 20 percent and as high as 50 percent for migrants to Assam. 39 Unlike the early "white" indentured migration, one-third of the indentured immigrants to Reunion island, Mauritius, the Caribbean, Surinam, and Jamaica returned home. It was still far from the 70 percent repatriation figure recorded in Thailand, Malaya, and Melanesia.

Returns were first strongly opposed by small "white" estate owners, during the first couple of decades after the abolition of slavery. Later on, returns were encouraged with the increasing flow of immigrants and the increasing concentration and mechanization of the estates which reduced the demand for labor. migrants opposed "colored" coolies, peones and indentured migrants. These last were still important among Indians moving across the Indian Ocean and the Pacific. Increasing living conditions of white migrants therefore contrasted with persistent abuses on Indian and Chinese migrants, while women still were under physical and psychological violence.

Even worse, the so called scramble for Africa and imperialism in the name of freedom (the argument European powers rose to occupy Africa was to free local people from slavery) actually enhanced coercion. These policies took place at the moment when in France, Belgium, Germany, Italy labor rules changed: the Masters and Servants Acts were repealed in 1875, new labor rules and the first welfare state were introduced in Britain, but also in continental. Thus, colonial powers in Africa were immediately concerned with the question of deciding whether emancipated slaves should benefit of similar rules. The unanimous answer was negative: Africans were not yet civilized enough to benefit of modern forms of welfare insofar they were not keen to work with increasing wages and they relied upon kinship aids.

Instead, old rules were resumed: the Masters and Servants Acts, hirings for labor in French and Belgium colonies and the indentured (engagisme) contract for inter-african migration (and for Indians moving to Africa as well). 40

Conclusion

Long term relationships between migration, debt and bonded labor -what was lately known in western Europe as indentured migration (or its French equivalent, engagisme)-actually reflected several related dimensions, to begin with the fact that those colonists-migrants were supposed to occupy the lands of more or less presumed Nomadic populations. These populations were under attack on both sides of the northern hemisphere in the Americas and in Inner Asia.

Yet beyond similarities, differences were also important. The Russians and the Chinese land empires reinforced the legal constraints on peasant mobility in the mainland while partially relaxing them in the new colonies. In the Atlantic, quite the opposite, first indentured migrants were under the authority of their master and their contracts were strongly inspired by military discipline.French and British migrants were under harsher legal and social conditions than laboring people in the mainland.

This was so first of all because these more or less forced migrants were at the very margin of the transforming European societies: women with "bad reputation" or simply in conflict for inheritance; indebted people pushed out by the transformation of the countryside, they were all supposed to colonize the new world. Western European empires were "exclusionary"

Empires: conquered people and even their own laboring force were deprived of any right and voice. Quite the contrary, Asian and Eurasian Empires played the card of a relative inclusiveness, of both local elites and their own migrants.

This situation changed at the turn of the eighteenth and nineteenth century when land empires met increasing difficulties to face the expansion of western maritime empires. The abolition of slavery produced a massive revival of indenturship, but this time it was reserved to colored migrants, while white European migrants travelled as "free" workers. During the nineteenth century, this process produced a great divergence between the rights and conditions non only of white and colored migrants, but also between mainland and colonial subjects. The former benefited of increasing welfare security while the latter still were under the burden of coercive rules.

During all these centuries, women were exposed to violent forms of indenturship because of their legal status (inferior to men), lack of credit and resources; they also were seized, locked and then sent overseas for reasons related to "morality" or inheritance. Still at the turn of the nineteenth and twentieth century, as André Brink's great novel "Chain of voices", inspired of true facts, informs us, most western powers sent women with "a bad reputation" to the colonies to marry soldiers and colonists along a path reminiscent of the "filles du roi".

Further readings

Carter, Marina, Servants, Sirdars, Settlers (New Delhi : Oxford University Press, 1995).

  After the great divergence, what Polanyi called the Great transformation, that is the second industrial revolution and the transformation of capitalism and liberalism, added a further major shift in indenturship and migration more generally. Migratory flux increased in the last quarter of the nineteenth century when decreasing costs in transportation and structural economic and social transformation intervened worldwide. In Europe, mechanization and concentration compelled people to migrate, while massive population flow helped create a single global economy for both labor and capital. Thus between 1840 and 1940, 55-58 million Europeans and 2.5 million Africans and Asians reached the Americas; during this same period, 29 million Indians, 19 million Chinese, and 4 million Africans and Europeans moved to Southeast Asia, the Pacific islands, and the Indian Ocean rim. Finally, 46-51 million people from northeastern Asia and Russia moved (or were compelled to move) to Siberia, Manchuria, and Central Asia. Migration became increasingly racialized; white "free" 38 ANOM, FM/SG REU c380 d3228, minute 6. 39 British Parliamentary Papers, 1863(41) 271, Statement Exhibiting the Moral and Material Progress and Condition of India.

40

  Paul Lovejoy, Transformations of Slavery (Cambridge: Cambridge University Press, 2000).

  Northrup, David. Indentured Labor in the Age of Imperialism, 1834-1922 (Cambridge: Cambridge University Press, 1995). Perdue, Peter. China Marches West. The Qing Conquest of Central Eurasia (Harvard, Mass: Belknap Press, 2005). Labor in the Fringes of Empire. Voice, Exit and the Law (New York: Palgrave, 2018). Steinfeld, Robert. The Invention of Free Labor. The Employment relation in English and American Law and Culture, 1350-1870 (Chapel Hill and London: The University of North Carolina Press, 1991). Tomlins, Christopher. Freedom Bound. Law, Labor, and Civic Identity in Colonizing British America, 1580-1865 (Cambridge: Cambridge University Press, 2010).

Stanziani, Alessandro. Bondage. Labor and Rights in Eurasia, 17 th -20 th centuries (New York: Berghahn, 2014) Stanziani, Alessandro.

D. Moon, The Russian Peasantry, 1600-1930 (London and New York: Longman, 1996).

Thomas Esper, "The odnodvortsy and the Russian Nobility", Slavonic and East European Review, 45 (1967):124-135.

Alessandro Stanziani, After Oriental Despotism (London: Bloomsbury, 2014).

R. Menard, Migrants, Servants and Slaves: Unfree Labor in Colonial British America (Aldershot: Ashgate, 2001).

Steinfeld, The Invention: 89.

C. Tomlins, Freedom Bound. Law, Labor, and Civic Identity in Colonizing British America, 1580-1865 (Cambridge: Cambridge University Press, 2010): 41.

J. Greene, Negotiated Authorities: Essays n Colonial Political and Constitutional History (Charlottsville: University of Viriginia Press, 1994).

R. Steinfeld, Coercion, Contract and Free Labor in the 19 th century (Cambridge: Cambridge University Press, 2001): 254.


Andrea Alamia

Rufin Vanrullen

A travelling waves perspective on temporal binding

Brain oscillations are involved in many cognitive processes, and several studies have investigated their role in cognition. In particular, the phase of certain oscillations has been related to temporal binding and integration processes, with some authors arguing that perception could be an inherently rhythmic process. However, previous research on oscillations mostly overlooked their spatial component: how oscillations propagate through the brain as travelling waves, with systematic phase delays between brain regions. Here, we argue that interpreting oscillations as travelling waves is a useful paradigm shift to understand their role in temporal binding and address controversial results. After a brief definition of traveling waves, we propose an original view on temporal integration that considers this new perspective. We first focus on cortical dynamics, then speculate about the role of thalamic nuclei in modulating the waves, and on the possible consequences for rhythmic temporal binding. In conclusion, we highlight the importance of considering oscillations as travelling waves when investigating their role in cognitive functions.

Introduction

Neuronal oscillations have been reliably associated with many cognitive processes, such as perception [START_REF] Dugué | The phase of ongoing oscillations mediates the causal relation between brain excitation and visual perception[END_REF][START_REF] Giraud | Cortical oscillations and sensory predictions[END_REF][START_REF] Vanrullen | Perceptual echoes at 10 Hz in the human brain[END_REF], memory [START_REF] Benchenane | Oscillations in the prefrontal cortex: A gateway to memory and attention[END_REF][START_REF] Lundqvist | Gamma and beta bursts during working memory readout suggest roles in its volitional control[END_REF], attention [START_REF] Bollimunta | Neuronal Mechanisms and Attentional Modulation of Corticothalamic Alpha Oscillations[END_REF][START_REF] Dugué | Attention Reorients Periodically[END_REF][START_REF] Klimesch | Alpha-band oscillations, attention, and controlled access to stored information[END_REF][START_REF] Sauseng | A shift of visual spatial attention is selectively associated with human EEG alpha activity[END_REF][START_REF] Thut | α-Band electroencephalographic activity over occipital cortex indexes visuospatial attention bias and predicts visual target detection[END_REF][START_REF] Vanrullen | Perceptual Cycles[END_REF], time perception [START_REF] Van Wassenhove | Temporal cognition and neural oscillations[END_REF], and even consciousness (Di [START_REF] Gregorio | Tuning alpha rhythms to shape conscious visual perception[END_REF][START_REF] Engel | Temporal binding and the neural correlates of sensory awareness[END_REF][START_REF] Palva | New vistas for α-frequency band oscillations[END_REF]. Several studies have demonstrated that oscillatory dynamics play an active, causal role in modulating effective neuronal connectivity and shaping cortical plasticity [START_REF] Galuske | Relation between gamma oscillations and neuronal plasticity in the visual cortex[END_REF][START_REF] Steriade | Coherent oscillations and short-term plasticity in corticothalamic networks[END_REF]. Yet, despite a myriad of studies investigating oscillatory processes, most research focused on their temporal features, such as phase, frequency and amplitude, overlooking their spatial component, that is how oscillations propagate through brain regions. Even when considering different cortical areas, most studies focused on zero-lag synchrony, i.e. oscillations sharing precisely the same phase across cortical regions (figure 1B). However, recent studies are challenging this approach, supporting a novel interpretation that characterizes oscillations as travelling waves, which propagate across cortical regions with different phase offsets (figure 1C) [START_REF] Alexander | Large-scale cortical travelling waves predict localized future cortical signals[END_REF][START_REF] Muller | Cortical travelling waves: Mechanisms and computational principles[END_REF][START_REF] Zhang | Theta and Alpha Oscillations Are Traveling Waves in the Human Neocortex[END_REF]. This new view provides novel insights to open questions about the role of oscillations in various cognitive functions. Here, after a brief introduction on travelling waves, we discuss how oscillatory travelling waves can provide an original view on the question of temporal binding.

Oscillations as travelling waves

Different phenomena have been characterized as "travelling waves" in the literature.

Previous work described as travelling waves the spatial propagation of non-periodic evoked responses to stimulus onset, for instance rising from occipital to frontal areas [START_REF] Klimesch | P1 and Traveling Alpha Waves: Evidence for Evoked Oscillations[END_REF][START_REF] Zauner | Lexical access and evoked traveling alpha waves[END_REF], or sweeping across a single retinotopic region [START_REF] Bringuier | Horizontal propagation of visual activity in the synaptic integration field of area 17 neurons[END_REF][START_REF] Reynaud | Dynamics of local input normalization result from balanced short-and long-range intracortical interactions in area V1[END_REF] Similarly, periodic stimulation, such as a flickering visual stimulus, gives rise to oscillatory waves that share the same temporal features as the entraining stimulus [START_REF] Sato | Traveling Waves in Visual Cortex[END_REF]. Here, we focus on intrinsic oscillatory travelling waves, i.e., truly periodic waves with an intrinsic frequency that does not necessarily match that of the sensory stimulus, and generated by endogenous processes (for an overview of wave generation, see [START_REF] Ermentrout | Traveling electrical waves in cortex: Insights from phase dynamics and speculation on a computational role[END_REF] but also [START_REF] Alamia | Alpha oscillations and traveling waves: Signatures of predictive coding?[END_REF]. Despite a growing number of studies investigating this type of wave, little is known about their functional role and relation to cognitive functions. Interestingly, oscillatory travelling waves can propagate at different speeds, both at the mesoscopic scale (within brain regions, Fakche & Dugué, 2022;[START_REF] Han | Reverberation of Recent Visual Experience in Spontaneous Cortical Waves[END_REF][START_REF] Sreekumar | Low frequency traveling waves in the human cortex coordinate neural activity across spatial scales[END_REF][START_REF] Zanos | A Sensorimotor Role for Traveling Waves in Primate Visual Cortex[END_REF] as well as at the macroscopic one (between cortical areas, [START_REF] Halgren | The generation and propagation of the human alpha rhythm[END_REF][START_REF] Pang | Turning the stimulus on and off changes the direction of α traveling waves[END_REF][START_REF] Zhang | Theta and Alpha Oscillations Are Traveling Waves in the Human Neocortex[END_REF], and they have been reported in several frequency bands, e.g., theta [START_REF] Bhattacharya | Traveling waves in the prefrontal cortex during working memory[END_REF][START_REF] Zhang | Traveling Theta Waves in the Human Hippocampus[END_REF], alpha [START_REF] Alamia | DMT alters cortical travelling waves[END_REF][START_REF] Lozano-Soldevilla | The Hidden Spatial Dimension of Alpha: 10-Hz Perceptual Echoes Propagate as Periodic Traveling Waves in the Human Brain[END_REF][START_REF] Luo | Conscious perception and perceptual echoes: a binocular rivalry study[END_REF] and beta [START_REF] Das | Spontaneous Neuronal Oscillations in the Human Insula are Hierarchically Organized Traveling Waves[END_REF]. In our previous work investigating macroscopic alpha-band travelling waves, we proposed a key distinction between two main categories determined by the waves' direction. Specifically, forward waves propagate from lower to higher sensory regions and reflect sensory processes [START_REF] Alamia | Alpha oscillations and traveling waves: Signatures of predictive coding?[END_REF][START_REF] Lozano-Soldevilla | The Hidden Spatial Dimension of Alpha: 10-Hz Perceptual Echoes Propagate as Periodic Traveling Waves in the Human Brain[END_REF][START_REF] Pang | Turning the stimulus on and off changes the direction of α traveling waves[END_REF]. On the contrary, backward waves propagate in the opposite direction and primarily relate to top-down functions, such as attention and visual selection [START_REF] Alamia | Distinct roles of forward and backward alpha-band waves in spatial visual attention[END_REF][START_REF] Maris | Rhythmic neuronal synchronization in visual cortex entails spatial phase relation diversity that is modulated by stimulation and attention[END_REF]. In addition, forward and backward waves may rely on different anatomical substrates. In line with previous studies [START_REF] Bastos | Canonical microcircuits for predictive coding[END_REF][START_REF] Bastos | A DCM study of spectral asymmetries in feedforward and feedback connections between visual areas V1 and V4 in the monkey[END_REF][START_REF] Buffalo | Laminar differences in gamma and alpha coherence in the ventral stream[END_REF], it's possible to separate forward and backward streams in superficial and deep layers, respectively. However, it's important to keep in mind the cortical laminae's spectral profile, as superficial layers relate primarily to faster gamma-band oscillations, whereas lower beta-band oscillations originate in deeper layers. In this paper, we mainly focused on alpha-band oscillations, assuming that the interaction between brain regions generates the oscillatory dynamics, focusing on the global rather than local level (i.e., not at a specific layer). In other words, the interaction between cortical areas generates alpha oscillatory dynamics, which propagate forward and backward, coordinating the activity of faster oscillations in each region [START_REF] Fries | Rhythms for Cognition: Communication through Coherence[END_REF][START_REF] Lisman | The Theta-Gamma Neural Code[END_REF][START_REF] Mclelland | Theta-Gamma Coding Meets Communication-through-Coherence: Neuronal Oscillatory Multiplexing Theories Reconciled[END_REF].

Yet, what makes an oscillation a traveling wave? Considering non-invasive EEG recordings (for example, along the electrode midline, as in figure 1B), we can assume that each electrode mainly records activity from a limited and distinct cortical area (it is important, though, to keep in mind several limitations when analyzing travelling waves in EEG recordings: for example, in EEG recordings, only waves longer than twice the electrode separation distance can be resolved (Nyquist criterion in space), and lower spatial resolution and volume conduction may mask shorter waves providing different results than invasive cortical recordings, as detailed in [START_REF] Alexander | Large-scale cortical travelling waves predict localized future cortical signals[END_REF][START_REF] Freeman | Analysis of spatial patterns of phase in neocortical gamma EEGs in rabbit[END_REF][START_REF] Nunez | The brain wave equation: a model for the EEG[END_REF]. Given this assumption, an oscillatory traveling wave is defined by a phase shift between signals from different electrodes, as shown in figure 1C: if the phase shift grows from occipital to frontal electrodes, the wave propagates in the forward direction; otherwise, it flows backward. In other words, the sign of the phase shift determines whether the waves travel from occipital to frontal regions or vice-versa. In some of our recent work combining EEG experiments and computational modeling [START_REF] Alamia | Alpha oscillations and traveling waves: Signatures of predictive coding?[END_REF], we showed that a predictive coding system with biologically plausible neural time constants and hierarchical communication delays would naturally produce alpha-band travelling waves (figure 1D). These waves could flow in either the forward or backward direction, depending on the behavioral task and the state of the system; however, in either case, the phase gradient of the wave was driven by inter-areal communication delays, and thus directly followed the bottom-up or top-down activation latencies across the hierarchy of brain areas.

What can this traveling wave approach to brain oscillations tell us about temporal binding? Figure 1. Difference between standing and travelling waves. A) Consider an experiment in which two visual stimuli, S1 and S2, are displayed sequentially at times t1 and t2. B) Oscillatory waves can be measured for instance along the mid-line of scalp EEG electrodes. Depending on the timings t1 and t2 and on the phase of the ongoing oscillation, the two stimuli may be perceived as one "merged" item or two distinct ones [START_REF] Baumgarten | Beta oscillations define discrete perceptual cycles in the somatosensory domain[END_REF][START_REF] Valera | Perceptual framing and cortical alpha rhythm[END_REF][START_REF] Vanrullen | Is perception discrete or continuous?[END_REF]. In the case of a standing wave, the oscillations share the same phase over all electrodes, and there is only one phase relative to the two stimuli across electrodes. C) If the oscillation is actually a forward (or backward) traveling wave, it presents a phase offset increasing (or decreasing) across electrodes. In this case, the onset of the two stimuli, t1 and t2, falls at different relative phases along the line of electrodes; this could challenge the idea of a unique optimal (or poor) phase for temporal integration across the visual system. D) A cortical model of predictive coding with mutually connected hierarchical regions exchanging predictions (via feedback connections, y C ) and prediction errors (x C ). In a predictive coding implementation, higher regions predict the activity of the lower regions, and the difference between the predictions and the actual activity (i.e., the prediction error) is used to update the next prediction. When taking into account neural time constants and interareal communication delays (ΔT C ), this model naturally produces alpha-band travelling waves as in panel C), flowing in the forward direction during sensory input processing, and in the backward direction when top-down priors are present in the absence of stimuli [START_REF] Alamia | Alpha oscillations and traveling waves: Signatures of predictive coding?[END_REF]. The key intuition is that communication delays introduce a temporal offset in the predictions that generate oscillatory dynamics due to overshooting and compensations. Interestingly, as we showed in our previous work, the temporal delay relates to the frequency of the oscillations: shorter delays (more local interactions) generate faster dynamics, in line with the biological plausible interpretation of such dynamics.

Is a hierarchical system more rhythmic during globally synchronized oscillations, or during traveling waves?

For many years, some authors have debated whether relying on oscillatory neural mechanisms makes the brain a periodic system, with "optimal" and "poor" phases for temporally integrating information [START_REF] Busch | The phase of ongoing EEG oscillations predicts visual perception[END_REF]Fakche et al., 2022;[START_REF] Herbst | Rhythms for cognition: The case of temporal processing[END_REF]Ronconi et al., 2017[START_REF] Ronconi | Alpha-band sensory entrainment alters the duration of temporal windows in visual perception[END_REF]Ronconi & Melcher, 2017;Samaha et al., 2015a;[START_REF] Venskus | Individual differences in alpha frequency are associated with the time window of multisensory integration, but not time perception[END_REF][START_REF] Wiener | Frequency tuning for temporal perception and prediction[END_REF][START_REF] Wutz | Frequency modulation of neural oscillations according to visual task demands[END_REF]. The notion of rhythmic or periodic processing implies that information is preferentially processed by each brain region at the "optimal" oscillation phase. In this case, two sequential objects with the proper temporal delay could be "merged" in one percept when they fall within one phase or "segregated" in two distinct percepts when landing on different phases (as schematically shown in figure 1). This experimental prediction, which stems from considering sensory systems as rhythmic, has been tested in several experimental works, providing arguably controversial results. On the one hand, some authors found experimental evidence that corroborates this hypothesis, demonstrating that individual participants' alpha-band features predict temporal aspects of visual perception [START_REF] Ronconi | Alpha-band sensory entrainment alters the duration of temporal windows in visual perception[END_REF][START_REF] Samaha | Top-down control of the phase of alphaband oscillations as a mechanism for temporal prediction[END_REF]Samaha & Postle, 2015;[START_REF] Wutz | Frequency modulation of neural oscillations according to visual task demands[END_REF]; on the other hand, other authors failed in reproducing these results using similar paradigms, leaving controversial the question about the role of phase in temporal binding processes [START_REF] Buergers | The role of alpha oscillations in temporal binding within and across the senses[END_REF][START_REF] Ruzzoli | The relevance of alpha phase in human perception[END_REF]. However, reinterpreting oscillations as travelling waves challenges the whole narrative and provides a more subtle view that allows re-evaluating -if not reconciling-previous findings.

As described above, when interpreting oscillations as waves propagating through brain regions, it is natural to obtain a gradient of phases throughout the sensory system (figure 1C). Hence, at a given moment, some brain regions within the visual stream may be at the "optimal" phase for temporal integration while other regions would simultaneously reach their "poor" phase. For example, suppose feedforward latencies and communication delays drive the phase gradient across areas, and we can assume a 50ms difference between optimal and poor phase in the alpha band, corresponding to activity in V1 and IT. In this case, these two regions may be simultaneously at opposite alpha phases, one in the optimal and one in the poor phase. That is, the travelling wave perspective puts into question the assumption that there is one unique phase across the whole visual system at any given time; instead, instantaneous oscillatory phase would be specific to each brain region. Considering the example shown in figure 1A with two temporally sequential stimuli, it now becomes less obvious to answer the question of whether we perceive 1 or 2 objects, if these can be simultaneously "merged" in one region (because they are processed at a given phase) and "segregated" in the next brain area (because they are processed at the opposite phase).

More generally, one could argue that oscillatory phase has more functional influence on sensory perception when each area happens to be at its "optimal" (or. poor) phase at the precise moment when the relevant stimulus information reaches it. The "optimal" phase with respect to stimulus onset would thus differ for each region, being mainly driven by the activation latency delay between successive hierarchical regions. As mentioned above, in our previous work [START_REF] Alamia | Alpha oscillations and traveling waves: Signatures of predictive coding?[END_REF] we proposed a model of cortical information processing based on Predictive Coding principles, which could naturally produce oscillatory feedforward traveling waves with a phase gradient that followed the activation delays across the cortical hierarchy (Figure 1D). In such a system, one could argue that temporal binding would be most rhythmic during forward oscillatory waves, when the successive activation delays imply that each area receives the stimulus information at its own optimal (or poor) phase; in turn, the system would behave less rhythmically during standing waves (i.e., synchronized oscillations, having the same phase across regions, see fig. 1B), and even less so during backward waves. Note, however, that this reasoning mainly characterizes the rhythmicity of temporal binding processes associated with bottom-up stimulus processing (e.g. judging the temporal order of two successive flashes); in a task whose outcome depends on top-down information transfer (e.g. related to attention or expectations), the same reasoning could imply that backward-travelling waves [START_REF] Alamia | Distinct roles of forward and backward alpha-band waves in spatial visual attention[END_REF][START_REF] Alamia | Alpha oscillations and traveling waves: Signatures of predictive coding?[END_REF][START_REF] Pang | Turning the stimulus on and off changes the direction of α traveling waves[END_REF] are the most conducive to rhythmic perception… In short, while a "naïve" view of brain oscillations would suggest that rhythmic temporal binding corresponds with episodes of large-scale phase synchronization across the brain, the more nuanced view detailed here implies that temporal binding could be less rhythmic in such a globally synchronized state, and most affected by oscillatory activity when the taskrelevant sequence of cortical activations follows a feedforward or a feed-back travelling wave pattern (depending on exact task specifications). Importantly, this new view supports the counterintuitive conclusion that contrary to common intuition, global synchronization and rhythmic perception are not necessarily identical: a globally synchronized state, in which all the regions have the same phase, acts less rhythmically than a system having a gradient of phases. This complexity could go some way to explaining the observed discrepancies in the literature.

The pulvinar as the conductor orchestrating the alpha-band symphony?

However complicated it might be to establish the role of oscillations in a (simplified) cortical system with a strict hierarchical structure (as in Figure 1D), things get even more difficult when considering the potential implication of sub-cortical regions that could be reciprocally connected with multiple cortical areas. We have long known that non-cortical nuclei play an important role in the generation and synchronization of oscillatory rhythms in the brain.

Here, we consider the role of a specific thalamic nucleus, the pulvinar, in synchronizing the whole cortical symphony, and the associated consequences for the temporal integration of sensory information.

Thalamic nuclei, such as the pulvinar and the reticular nuclei, are known to generate oscillatory dynamics, especially in the alpha rhythms [START_REF] Fiebelkorn | The Puzzling Pulvinar[END_REF][START_REF] Halgren | The generation and propagation of the human alpha rhythm[END_REF][START_REF] Lopes Da Silva | Relative contributions of intracortical and thalamo-cortical processes in the generation of alpha rhythms, revealed by partial coherence analysis[END_REF][START_REF] Saalmann | The pulvinar regulates information transmission between cortical areas based on attention demands[END_REF]. In addition, the pulvinar reciprocally and topographically connects with several cortical regions, including visual areas at different hierarchical levels. Electrophysiological recordings demonstrated that alpha oscillations modulate the interaction between cortical regions and the pulvinar [START_REF] Halgren | The generation and propagation of the human alpha rhythm[END_REF][START_REF] Lopes Da Silva | Relative contributions of intracortical and thalamo-cortical processes in the generation of alpha rhythms, revealed by partial coherence analysis[END_REF]. Interestingly, previous studies demonstrated the critical role of the pulvinar in several cognitive and perceptual tasks, coordinating the activity along both feedforward and feedback pathways [START_REF] Jaramillo | Engagement of Pulvino-cortical Feedforward and Feedback Pathways in Cognitive Computations[END_REF]. Specifically, the pulvinar is crucially involved in spatial attention [START_REF] Saalmann | The pulvinar regulates information transmission between cortical areas based on attention demands[END_REF][START_REF] Zhou | Pulvinar-Cortex Interactions in Vision and Attention[END_REF] and visual working memory [START_REF] Rotshtein | The role of the pulvinar in resolving competition between memory and visual selection: A functional connectivity study[END_REF]. Given its oscillatory dynamics, its widespread connectivity pattern, and its implication in cognitive and perceptual tasks, it is logical to consider the pulvinar as a potential candidate for generating and coordinating oscillatory traveling waves through the cortex.

The "hubness" property of the pulvinar, reciprocally connected with distinct cortical regions (figure 2), implies that its activity would have an overall synchronizing effect on the oscillatory dynamics in the cortex. That is, common inputs from the pulvinar would tend to increase global synchronization of oscillatory phases across multiple cortical regions. This influence should be directly related to the strength of thalamo-cortical connections: the stronger the connections, the more synchronous the activity across cortical regions, i.e., the smaller the phase difference between regions; in the extreme case, when cortical activity would be dominated by the common thalamic signals, forward or backward waves could become nearly indistinguishable from standing waves, as schematically shown in figure 2.

Interestingly, the influence of thalamo-cortical connections may play an important role in processes such as phase resets [START_REF] Lakatos | The Leading Sense: Supramodal Control of Neurophysiological Context by Attention[END_REF], as strong sensory activations may reset the phases in each cortical regions, thus increasing system's global synchronization (and reducing the overall amount of travelling waves). Following our reasoning described in the previous section, the effect of such an external synchronizing drive over the cortex could be to reduce the overall periodicity of the system (e.g., because the phase gradients across cortical regions would not strictly follow the activation latencies anymore). However, this previous reasoning was based on the phase value in each cortical region at the moment when it receives task-relevant sensory inputs from the preceding cortical region (or, when it receives task-relevant top-down "priors" from the next cortical region). If we now consider the functional role of the pulvinar to be more than a mere sensory relay of information, but to also receive and convey specific task-relevant signals, then additional consequences for rhythmic temporal binding could ensue, as detailed below.

A framework to characterize travelling waves and cortico-thalamic dynamics

In the early 90s, David Mumford proposed a compelling hypothesis about the role of the pulvinar in perception, namely the 'active blackboard' theory [START_REF] Mumford | On the computational architecture of the neocortex -I. The role of the thalamocortical loop[END_REF], which falls in line with current Predictive Coding theories (and, more broadly, generative Bayesian models). The 'active blackboard' theory recently gained renewed interest in several computational and experimental studies [START_REF] Ketz | Thalamic pathways underlying prefrontal cortexmedial temporal lobe oscillatory interactions[END_REF][START_REF] O'reilly | Deep predictive learning in neocortex and pulvinar[END_REF]. The core idea is that each cortical region behaves as a generative model whose activity represents the sensory input. Each cortical area projects its specific representation of the sensory data into the pulvinar, which serves as a 'blackboard' to integrate all cortical projections. In other words, the pulvinar acts as a central integration space, which combines all cortical representations and feeds back in the cortex the most likely interpretation of the data. This functional interpretation of the cortico-thalamic connections suggests that the pulvinar represents the sensory 'prior' interpretation of the data (as provided by the cortical input). It forwards such prior information (or predictions) to the cortical regions, which update their representation accordingly. It's important to keep in mind, however, that irrespective of the functional role of the pulvinar, the pulvinar may still synchronize phase across cortical regions, making rhythmic perception easier to detect with EEG recordings. That is, contrary to the previous scenario where cortico-cortical bottom-up or top-down hierarchical communication was assumed to dominate processing, in the current scenario it is the synchronized pulvinar input that would mainly govern cortical processing. As we described above, such a common influence of the pulvinar across multiple cortical areas would be accompanied by increased overall synchronization and reduced phase shifts between cortical regions (see above and figure 2). However, this increased synchronization would now enhance the rhythmicity of the system, because each cortical region would tend to receive the relevant pulvinar information around the same oscillatory phase. In other words, when information from the pulvinar to the cortex is instrumental for the task at hand, the global synchronization of cortical activity could increase its rhythmicity, and favor a more 'periodic' temporal binding.

Traveling waves unveil new possibilities for understanding oscillations.

At the beginning of this paper, we challenged the naïve notion that "periodic temporal binding" would mostly happen in a system with globally phase-synchronized oscillations. By considering a hierarchical system performing perceptual or cognitive tasks that mainly rely on bottom-up or top-down cortical information transfer, we have established that periodic temporal binding could instead correspond to episodes of forward or backward travelling waves activity, respectively. Then, we considered the additional influence of a thalamic hub, the pulvinar, on cortical oscillatory dynamics: by synchronizing cortical oscillations, pulvinar activity could reduce travelling waves, and thus go against the idea of periodic temporal binding. However, in the last section, we showed that the pulvinar hub operating as a functional 'blackboard' could actually have the opposite influence-by synchronizing the phase of cortical oscillations at the exact moment they receive the central 'blackboard' signals, it would favor rhythmic temporal binding. All in all, depending on whether relevant information for the task at hand is relayed by the thalamic nucleus 'blackboard' or mainly runs through the bottom-up or top-down cortical pathways, rhythmic temporal binding in the brain could be associated with globally synchronized oscillatory activity, or with forwardor backward-travelling waves, thus generating three possible scenarios that can be tested -a priori-via M/EEG recordings. In the first case, assuming near-threshold visual stimuli that do not elicit a robust thalamic response, we would predict that the phases of the alpha band traveling waves propagating forward through the cortical hierarchy determine the temporal segregation/integration: the sensory information will have the same delayed phase across each cortical regions (i.e., non-simultaneously, but based on the temporal delays), and such phase -relative to each stimulus-would determine the consequent segregation or integration. In the second case, we consider a stimulus that elicited a more robust response from the pulvinar, supposedly involving spatial attention or the binding of visual features. As described above, the cortico-thalamic connections would produce a more powerful synchronization across the cortex, whose phase would determine whether the stimuli are integrated or segregated in the final perception. Lastly, it's possible to consider the third case of contextual processing. When top-down expectations are involved in the task, we may expect a more prominent role of the phase of alpha-band backward waves (rather than the forward one, as in the previous case) in determining whether the stimuli are integrated or perceived as distinct. We note that there is no currently accepted way to determine, in advance, which of the above situations would be engaged by any particular cognitive task. In addition, EEG recordings pose additional challenges in addressing these three scenarios. Specifically, considering EEG recordings, we could unambiguously predict a rhythmic relation between segregation/integration and the EEG alpha-band phase only in the case of global synchronization driven by the pulvinar. On the contrary, when FW or BW waves dominate the sensory, the relevant phase is region-dependent. Then, identifying the specific region involved in the segregation/integration process becomes crucial. Moreover, the EEG phase at the scalp level may be composed of different sources [START_REF] Alamia | Distinct roles of forward and backward alpha-band waves in spatial visual attention[END_REF], including an ongoing alpha rhythm that is not involved in the traveling wave. This uncertainty may explain, in part, discrepant results in the literature on periodic perception: negative results do not necessarily imply that brain oscillatory phase has no influence on temporal binding; it could just be that the relevant oscillation (or traveling wave) is not measured by the experimenter.

In conclusion, the framework of travelling waves can provide new and essential insights to re-interpret oscillations and their role in different cognitive functions. Here, we took the case of temporal binding and showed how considering the spatio-temporal propagation of oscillations challenges the well-established narrative, providing an original angle with novel solutions (and novel problems). In particular, considering the spatial component of oscillations allows disentangling of different sources. For instance, analysis of EEG recordings is more likely to be blind to various sources of alpha-band oscillations when not considering their spatial component: the "largest" alpha-drive, possibly driven by top-down dynamics, may overwhelm the other "less prominent" alpha-band sources related to sensory processes [START_REF] Sokoliuk | Two spatially distinct posterior alpha sources fulfill different functional roles in attention[END_REF][START_REF] Zhigalov | Travelling waves observed in MEG data can be explained by two discrete sources[END_REF]. On the other hand, an approach based on travelling waves allows disentangling top-down and bottom-up sources [START_REF] Alamia | Distinct roles of forward and backward alpha-band waves in spatial visual attention[END_REF], providing a more comprehensive view of oscillatory dynamics and thus potentially shedding light on some of the controversial results in previous studies. Nevertheless, it's critical to keep in mind task features and cognitive functions involved in each experimental design: forward and backward waves may play an important role in understanding oscillatory dynamics in tasks based on bottom-up processes and top-down expectations, respectively; on the other hand, tasks involving sensory integration or other pulvinar functions may reveal yet a different oscillatory signature with different rhythmic properties for temporal binding.

All in all, travelling waves offer a new perspective and a critical paradigm shift in thinking and interpreting results about neural oscillations, providing a richer and more nuanced scenario.

Figure 2 .

 2 Figure 2. A cortico-thalamic model of oscillatory traveling waves. The schematic represents a model of the interplay between the pulvinar and hierarchical cortical areas (C1 to C4). Following the active blackboard theory, the connections between all areas are based on Predictive Coding principles with temporal delays (ΔTc for cortical inter-areal connections, and ΔTp for pulvinar-cortex connections, see Alamia & VanRullen, 2019 for details), where y Ci , y Pi represent the predictions, and x Ci , x Pi represent the prediction errors. The strength of the cortico-thalamic connections determines the influence of the pulvinar on the synchronization state of the cortex: a stronger effect increases cortical synchronization, progressively reducing the phase-lag between regions from travelling to standing waves.