Etienne Save
email: etienne.save@univ-provence.fr

Bruno Poucet

Role of the parietal cortex in long-term representation of spatial information in the rat

The processing of spatial information in the brain requires a network of structures within which the hip-pocampus plays a prominent role by elaborating an allocentric representation of space. The parietal cor-tex has long been suggested to have a complementary function. An overview of lesion and unit recording data in the rat indicates that the parietal cortex is involved in different aspects of spatial information pro-cessing including allocentric and egocentric processing. More specifically, the data suggest that the pari-etal cortex plays a fundamental role in combining visual and motion information, a process that would be important for an egocentric-to-allocentric transformation process. Furthermore, the parietal cortex may also have a role in the longterm storage of representation although this possibility needs further evi-dence. The data overall show that the parietal cortex occupies a unique position in the brain at the inter-face of perception and representation.

Introduction

Spatial behaviors are essential to survival of most animal species. Evolution yielded the emergence of spatial strategies that allow animals to maintain their navigational capability and their spatial memory in spite of environmental modifications. Understanding how the brain processes spatial information has motivated a huge amount of work. It is now well established that the processing of spatial information in the brain requires a network of cortical and subcortical structures within which the hippocampus plays a central role by implementing an allocentric representation of space. One of the most striking evidence in favor of such a role comes from the existence in CA1 and CA3 of pyramidal neurons characterized by location-specific firing, the so-called place cells [START_REF] O'keefe | The hippocampus as a spatial map. Preliminary evidence from unit activity in the freely moving rat[END_REF][START_REF] O'keefe | The hippocampus as a cognitive map[END_REF]. The discovery of place cells in the 1970s has had a great conceptual influence and contributed to promote a ''hippocampus-centered" view of the processing of spatial information in the brain. However, that a phylogenetically preserved, paleocortical structure such as the hippocampus could be the neural substrate of high level cognitive processes implicitly raised the question of the role of the neocortex in rodents. In this respect, the influence of Lashley's theories was still perceptible in the 1970s [START_REF] Mcdaniel | Posterior association cortex and visual pattern discrimination in the rat[END_REF][START_REF] Thomas | Mass function and equipotentiality: A reanalysis of Lashley's retention data[END_REF][START_REF] Thomas | The effects of lesions in the frontal or posterior association cortex of rats on maze III[END_REF]. As the main proponent of a holistic view of cortical functions in learning twenty years before, Lashley had postulated that cortical areas do not have spe-cific functions as far as learning is concerned and can substitute for each other when a lesion is made (equipotentiality principle). Several decades later, this theory motivated studies that examined the effects of lesioning various parts of the cortex on learning performance. Lesions of the posterior association cortex, frontal cortex and temporal cortex produced different effects on various learning tasks thus questioning Lashley's equipotentiality principle and perhaps more importantly, suggesting a specific contribution of the posterior association cortex (McDaniel & Thomas, 1978;[START_REF] Thomas | Mass function and equipotentiality: A reanalysis of Lashley's retention data[END_REF][START_REF] Thomas | The effects of lesions in the frontal or posterior association cortex of rats on maze III[END_REF]. In the context of a strong disagreement in the literature regarding the existence of a posterior association cortex in the rat, these seminal studies are among the first to propose that this region has a distinct role in spatial learning and memory. This renewal of interest for the parietal cortex in the rat produced a large number of studies that sought to characterize this cortical area both neuroanatomically and functionally. The results provide a great deal of evidence in favor of a role in the formation of long-term spatial representations. This aim of this review is to summarize this evidence and to suggest possible directions for further work.

The parietal cortex is involved in multimodal processing: Anatomical evidence

The hypothesis of the existence of a posterior association cortex (hereafter referred to as parietal cortex) in the rat has been initially founded on neuroanatomical bases. Using cytoarchitectonic characteristics, Krieg described a parietal region subdivided into six areas [START_REF] Krieg | Connexions of the cerebral cortex. 1. The albino rat. A. Topography of the cortical areas[END_REF], three primary somatosensory areas (labeled 1 1, 2, 3 according to Brodmann's nomenclature) and three areas putatively involved in multisensory integration (labeled 7, 39, 40). Subsequently, Krieg's area 7 was considered as corresponding to the parietal cortex by [START_REF] Kolb | Behavioral and anatomical studies of the posterior parietal cortex in the rat[END_REF]. This area, lying between the rostral primary somatosensory areas and the caudal secondary visual areas, would differ from the neighboring regions by a reduction in layer thickness and fewer fibers [START_REF] Kolb | Posterior parietal and temporal association cortex[END_REF]. The parietal cortex was also described on the basis of its thalamic inputs. Authors agreed that the thalamic projections to the parietal cortex originated from the lateroposterior and laterodorsal nuclei [START_REF] Chandler | Thalamocortical connexions of rat posterior parietal cortex[END_REF][START_REF] Kolb | Behavioral and anatomical studies of the posterior parietal cortex in the rat[END_REF][START_REF] Lashley | Thalamo-cortical connections of the rat's brain[END_REF]McDaniel, McDaniel, & Thomas, 1978;[START_REF] Reep | Rat posterior parietal cortex: Topography of corticocortical and thalamic connexions[END_REF]. However, such connection are not specific since the lateroposterior nucleus also have extensive projections to various cortical areas including primary and secondary visual cortex, medial prefrontal and anterior cingulate cortex (Musil & Olson, 1988a, 1988b) and subcortical regions such as the striatum [START_REF] Kamishina | Striatal projections from the rat lateral posterior thalamic nucleus[END_REF]. Whether there is topographic organization of the neurons within the lateroposterior thalamus with respect to their cortical site of projection is not clearly established. Most importantly, strong support for the hypothesis of an associative function in the parietal cortex is provided by the pattern of corticocortical connections. As shown in Fig. 1, the parietal cortex receives inputs from various sensory regions including the somatosensory cortex (Par 1 according to Zilles' nomenclature, Zilles, 1985), primary and secondary visual cortex (Oc1, Oc2L, Oc2M), and the auditory cortex (Te1) [START_REF] Kimura | Efferent connections of ''posterodorsal'' auditory area in the rat cortex: Implications for auditory spatial processing[END_REF][START_REF] Kolb | Posterior parietal and temporal association cortex[END_REF][START_REF] Kolb | Behavioral and anatomical studies of the posterior parietal cortex in the rat[END_REF][START_REF] Miller | Direct connexions of rat visual cortex with sensory, motor, and association cortices[END_REF][START_REF] Reep | Rat posterior parietal cortex: Topography of corticocortical and thalamic connexions[END_REF][START_REF] Torrealba | Cortical connexions of the anteromedial extrastriate visual cortex in the rat[END_REF]. It is also connected to cortical regions involved in goal-directed behavior such as the orbitofrontal, and medial prefrontal cortices (LO, VLO, Fr2) [START_REF] Kolb | Behavioral and anatomical studies of the posterior parietal cortex in the rat[END_REF][START_REF] Nelson | Prefrontal cortical modulation of acetylcholine release in posterior parietal cortex[END_REF][START_REF] Reep | Rat posterior parietal cortex: Topography of corticocortical and thalamic connexions[END_REF]. Interestingly, the parietal cortex receives inputs from the cerebellum suggesting a direct link with motor systems [START_REF] Giannetti | Cerebellar input to the posterior parietal cortex in the rat[END_REF]. It may also have some connection with the vestibular system either monosynaptically [START_REF] Guldin | Monosynaptic input from the cerebral cortex to the vestibular brainstem nuclei in the rat[END_REF] or polysynaptically via the lateroposterior thalamic nucleus [START_REF] Smith | The effects of vestibular lesions on hippocampal function in rats[END_REF] but this remains to be clarified. Consistent with the hypothesis of a role in spatial memory, the parietal cortex is connected to the limbic system and in particular to the hippocampal formation via the retrosplenial and the postrhinal cortex [START_REF] Burwell | Cortical afferents of the perirhinal, postrhinal, and entorhinal cortices of the rat[END_REF]. Note how-ever that nothing is known about the topographical organization of the projections within the parietal cortex. One can assume that the projections are not intermingled over the whole parietal surface but on the contrary segregated but this hypothesis remains to be confirmed. Overall, this complex pattern of connection strongly suggests that the parietal cortex is part of various networks involved in the processing of sensory, motor information and in memory. It therefore may play a unique role in multimodal processing and, as a result, would be an important actor in many cognitive processes in the rat.

Effects of parietal cortex lesions in the processing of allocentric information

Parietal cortex lesion studies were performed not only to uncover the role of this structure in spatial learning but also to discriminate it from that of the hippocampus. The possibility that the cognitive map or at least an elementary form was elaborated in the parietal cortex before being fully realized in the hippocampus was raised. To investigate the contribution of the parietal cortex in long-term representation of spatial information, a number of studies examined the effects of parietal lesions in place navigation tasks that involve the formation and use of an allocentric spatial representation. Most of these studies used the Morris water maze but a few used alternative situations such as the cheese board task, a dry version of the water maze [START_REF] Kesner | Place and taste aversion learning: Role of basal forebrain, parietal cortex, and amygdala[END_REF]. In the Morris water maze, the animals are required to locate a submerged platform by using a configuration of environmental cues. Lesions yielded variable effects. Rats with parietal cortex lesions were at best non affected [START_REF] Compton | The flexible use of multiple cue relationships in spatial navigation: A comparison of water maze performance following hippocampal, medial septal, prefrontal cortex, or posterior parietal cortex lesions[END_REF][START_REF] Kolb | A comparison of the contributions of the frontal and parietal association cortex to spatial localization in rats[END_REF]Save & Poucet, 2000a) and at worst mildly impaired in the acquisition of this task [START_REF] Kolb | Dissociation of the medial prefrontal, posterior parietal, and posterior temporal cortex for spatial navigation and recognition memory in the rat[END_REF][START_REF] Kolb | Recovery from early cortical lesions in rats. III. Neonatal removal of posterior parietal cortex has greater behavioral and anatomical effects than similar removals in adulthood[END_REF][START_REF] Kolb | Behavioral and anatomical studies of the posterior parietal cortex in the rat[END_REF][START_REF] Save | Effects of lesions of the associative parietal cortex in the acquisition and use of spatial memory in egocentric and allocentric navigation tasks in the rat[END_REF]. In contrast, a marked deficit was found by DiMattia and [START_REF] Dimattia | Spatial cognitive maps: Differential role of posterior parietal cortex and hippocampal formation[END_REF] and Hoh and co-workers (2003). However, in the DiMattia and Kesner study, it is possible that the deficits would result from larger lesion size and more anterior lesion location than the other studies. We also showed that the parietal cortex is not recruited when the hippocampus is inactivated during place learning (Parron, Poucet, & Save, 2001). Using a distributed learning procedure, we found that short-lasting reversible inactivation of the dorsal hippocampus during the navigation trials in the Morris water maze did not prevent learning of a platform location, therefore suggesting the involvement of another structure for acquisition, storage and off line processing. The possibility that this structure could be the parietal cortex was ruled out since rats with parietal cortex lesions that had their hippocampus inactivated were able to perform the task as well as rats with just an inactivated hippocampus. Thus, the parietal cortex does not appear to be the brain area that compensates for a dysfunctioning hippocampus. As a whole, the results suggest that the parietal cortex plays a role in the formation of spatial representations but this role appears to be not critical for place learning and navigation.

That hippocampal lesions had much more deleterious effects on place navigation than parietal cortex lesions (e.g. [START_REF] Compton | The flexible use of multiple cue relationships in spatial navigation: A comparison of water maze performance following hippocampal, medial septal, prefrontal cortex, or posterior parietal cortex lesions[END_REF][START_REF] Morris | Place navigation impaired in rats with hippocampal lesions[END_REF] suggested that these two regions contribute to different aspects of the processing of spatial information. Interestingly, Kolb and colleagues observed that rats with parietal cortex lesions were able to learn the general location of the platform by using room cues but had difficulties in adjusting their movement toward the goal [START_REF] Kolb | Dissociation of the medial prefrontal, posterior parietal, and posterior temporal cortex for spatial navigation and recognition memory in the rat[END_REF][START_REF] Kolb | Behavioral and anatomical studies of the posterior parietal cortex in the rat[END_REF]. Because this deficit did not appear when the animals had to navigate toward a visible goal, the possibility of a pure motor impairment is unlikely. A more appealing hypothesis would consider the possibility that the parietal lesion is involved in the conjoint integration of spatial features and motion information, a process that may be important for the formation of the hippocampal cognitive map [START_REF] Mcnaughton | Cortical-hippocampal interactions and cognitive mapping: A hypothesis based on reintegration of the parietal and inferotemporal pathways for visual processing[END_REF][START_REF] Mcnaughton | Cortical representation of motion during unrestrained spatial navigation in the rat[END_REF]. A study in which rats were trained to navigate to a submerged platform with cues/objects directly placed in the pool provided results consistent with this hypothesis. This study was based on the assumption that building an allocentric representation requires extraction of spatial invariants in the environment. It has been proposed that this process involves conjoint integration of different views of the environment with movements connecting these views [START_REF] Poucet | Spatial cognitive maps in animals: New hypotheses on their structure and neural mechanisms[END_REF][START_REF] Poucet | The neuropsychology of spatial cognition in the rat[END_REF]. Because of parallax effects, using intramaze cues may require enhanced association between views and motion to form a spatial representation. Rats with parietal cortex lesions were impaired when they had to use intramaze cues but not when they had to use distant cues whereas rats with hippocampal lesions were impaired regardless the kind of cues they used (Save & Poucet, 2000a). The notion that the parietal cortex cooperates with the hippocampus and is involved in the formation of spatial representations based on proximal objects is further supported by a study that examined the effects of parietal lesions on hippocampal place activity. Place cells were recorded in parietal-lesioned rats as the animals performed a pellet chasing task in a circular arena containing three objects [START_REF] Save | Functional interaction between the parietal cortex and hippocampal place cell firing in the rat[END_REF]. Room cues were made irrelevant by placing a curtain around the arena. We found that place field stability was perfectly controlled by the intramaze objects in control rats. Ninety degree rotation of the set of objects in the absence of the rat resulted in equivalent rotation of the place fields. In contrast, in parietal-lesioned rats, the control of place fields by the objects was much weaker since a majority of fields did not rotate but remained stable relative to the room reference frame. We also examined whether the rats could use olfactory and idiothetic cues to maintain stable fields by removing the objects. This was the case for control rats in which place fields remained stable. In contrast, place fields in lesioned rats shifted to their initial position, suggesting that place cells did not use combined olfactory and idiothetic cues but background cues to maintain stable fields (see Fig. 2). We assumed that because parietal-lesioned rats were unable to properly use proximal objects, place cells eventually used non controlled cues. If this hypothesis is correct, then place cell activity in parietal-lesioned rats should be normally controlled by distal cues. Beyond this assumption, this study is a demonstration that the parietal and the hippocampus are functionally related in spite of indirect neuroanatomical relationships.

As a whole, the data are compatible with the idea that the parietal cortex plays a role in the combination of visuo-spatial and motion information but also provide some new hints on the organization of cue encoding processes. Proximal cues, i.e. objects placed in the animal's locomotor space, and distal cues, i.e. room cues, may be differentially encoded by different structures in the brain. These results and others suggest that the parietal cortex is preferentially involved in the processing of proximal cues whereas the entorhinal cortex is preferentially involved in the processing of distal cues. The hippocampal system would have a major role in combining the two kinds of information to form an integrated spatial representation [START_REF] Parron | Entorhinal cortex lesions impairs the use of distal but not proximal landmarks during navigation in the rat[END_REF]Save & Poucet, 2000a;[START_REF] Van Cauter | Unstable CA1 place cell representation in rats with entorhinal cortex lesions[END_REF].

The effects of parietal cortex lesions were also examined in nonassociative tasks. Save and colleagues used a habituation/dishabituation situation in which the rats explored an arena containing several objects [START_REF] Save | Objects exploration and reaction to a spatial and a non-spatial change in the rat following damage to the posterior parietal cortex or the dorsal hippocampus[END_REF]. The objects formed a particular configuration that remained constant during the habituation sessions but was modified during the spatial change and non-spatial change sessions. It is important to note that each animal was run only one time and that the whole sequence of session was completed in approximately 1 h. When repeatedly exposed to the initial configuration, both sham-operated and parietal-lesioned rats exhibited habituation, i.e. a decrease of object exploration and locomotor activity. It was assumed that rats would form a spatial representation during habituation. To test this hypothesis, the effects of changing the spatial configuration were examined by displacing some objects. Such a manipulation induced a renewal of exploration specifically directed toward the displaced objects in sham-operated rats. In contrast, rats with parietal lesions did not display such a renewal, suggesting that they were impaired in elaborating a spatial representation on the basis of the object configuration. In addition it was demonstrated that this deficit is specific of manipulation of spatial relationships since a non-spatial change (replacing a familiar object with a novel object) induced a renewal of exploration in the two groups. In this situation, rats with hippocampal lesions displayed similar pattern of performance as rats with parietal lesions: they were impaired in the detection of spatial but not non-spatial change. At this point, the comparison of the impact of parietal Fig. 2. Examples of firing rate maps of two hippocampal place cells recorded in four successive sessions in control and parietal-lesioned-rats. Rats were foraging in a circular arena containing three objects (represented as a black square, white circle, and white polygon). Dark pixels indicate the place field of the cell. In control rats, place field were controlled by the objects, i.e. rotated an equivalent amount after object rotation, and remained stable after object removal suggesting a control by non visual (e.g. idiothetic, olfactory) cues. In contrast, in parietal-lesioned rats, a number of place fields were not controlled by the objects (not illustrated). In addition, after object removal, place fields shifted back to their initial location (Standard 1/Standard 2).

lesions in associative and non-associative tasks is instructive. First, the parietal cortex seems to play a greater role when the animal has to use a configuration of intramaze objects than a configuration of extramaze cues to form a spatial representation. This supports the hypothesis that the parietal cortex is involved in the association between visuo-spatial and motion-related information. Second, the contribution of the parietal cortex would be fully revealed when encoding of spatial information is performed within a limited amount of time that does not allow for compensatory processes and/or neural activation. Such processes may account for the reduced deficits when training is distributed over days.

Both the parietal cortex and the hippocampus are involved in the processing of complex spatial information but their specific contributions remain unclear. In the perspective of disentangling their respective roles, studies compared the effects of parietal cortex and hippocampal lesions in various spatial tasks. In a scene discrimination task, both rats with parietal lesions and hippocampal lesions exhibited impaired detection of spatial and spatial/object changes but not object changes [START_REF] Decoteau | Effects of hippocampal and parietal cortex lesions on the processing of multiple-object scenes[END_REF], an effect in line with Save et al.'s findings (1992). Later it was suggested that this effect could result from the inability of parietal-lesioned rats to process topological information and of hippocampal-lesioned rats to process metric information [START_REF] Goodrich-Hunsaker | Human topological task adapted for rats: Spatial information processes of the parietal cortex[END_REF][START_REF] Goodrich-Hunsaker | Dissociating the role of the parietal cortex and dorsal hippocampus for spatial information processing[END_REF]. In contrast, neither the parietal cortex nor the hippocampus seem to be involved in the discrimination of spatial location or allocentric distance as measured in a delayed-matching-to-sample go/no go task [START_REF] Long | The effects of dorsal versus ventral hippocampal, total hippocampal, and parietal cortex lesions on memory for allocentric distance in rats[END_REF][START_REF] Taube | Head direction cells and the neurophysiological basis for a sense of direction[END_REF]. However, the parietal cortex would play a weaker role in the memory for allocentric distance than the hippocampus in a similar paradigm [START_REF] Long | The effects of dorsal versus ventral hippocampal, total hippocampal, and parietal cortex lesions on memory for allocentric distance in rats[END_REF]. Rats with parietal lesions were not impaired in learning an allocentric version of a Hebb-Williams maze but were impaired in an egocentric version. Rats with hippocampal lesions were impaired in the allocentric but not egocentric version [START_REF] Rogers | Lesions of the dorsal hippocampus or parietal cortex differentially affect spatial information processing[END_REF]. These results do not provide a clear picture of the respective roles of the parietal cortex, hippocampus, and parietal-hippocampal relationships. They nevertheless suggest that the parietal cortex has a more subtle role in spatial representation than the hippocampus. In particular, parietal lesions appeared to affect or spare the ability to process and memorize spatial information depending on the experimental set up and task used. Thus, a possible explanation for these inconsistent effects is that the involvement of the parietal cortex and therefore of the parietal-hippocampal interaction would be highly dependent on the cognitive demand, task contingencies, and behavioral constraints. This hypothesis is supported by the results of a recent study in which Rogers and Kesner used a disconnection procedure to more directly examine the importance of cortical-hippocampal interaction in the processing of spatial information [START_REF] Rogers | Hippocampal-parietal cortex interactions: Evidence from a disconnection study in the rat[END_REF]. In this procedure, the rats received unilateral lesions in both the hippocampus and the parietal cortex either on ipsilateral or contralateral hemisphere. If the two structures interact, contralateral lesions are expected to completely disrupt the parietal-hippocampal relationships therefore producing profound deficits whereas ipsilateral lesions that partially damage the relationships would produce only mild deficits. Rats were trained in different spatial tasks including an object-place paired associate learning task, a dry-land place navigation task and a reaction-to-change task. Rats with contralateral lesions were more impaired than rats with ipsilateral lesions in the object-place learning task and the dry-land task. In contrast, both groups were similarly impaired in the reaction-to change task. Although all three tasks have been shown to be sensitive to both parietal cortex lesions and hippocampal lesions, the results suggest that the parietal cortex and the hippocampus cooperate in the object-place and dry-land navigation task but not in the reaction-to change task. As pointed out by the authors one possible explanation is that parietal-hippocampal interaction would be important in spatial tasks that induce a gradual (over days) formation of a spatial representation. On the contrary, rapid acquisition of environmental information during exploration involves both the parietal cortex and the hippocampus but do not require an interaction between these two structures. Interestingly, in a recent study using similar disconnection procedure and equivalent tasks, we looked at the interaction between the entorhinal cortex and the hippocampus [START_REF] Parron | Cooperation between the hippocampus and the entorhinal cortex in spatial memory: A disconnection study[END_REF] and obtained opposite effects suggesting entorhinal-hippocampal interactions in the reaction-to-change task but not in the place navigation task. This suggests that formation of a representation and detection of a spatial change during object exploration requires cooperation between the entorhinal cortex and the hippocampus but not between the parietal cortex and the hippocampus. This outcome is consistent with the notion that the cortical-hippocampal interactions are modulated by the task requirements.

Effects of parietal cortex lesions in the processing of egocentric information

As in allocentric tasks, parietal cortex lesions produced variable effects in egocentric tasks. Once again, the diversity of the tasks used may account for such inconsistency. Actually, egocentric tasks include a heterogenous amount of behavioral situations ranging from visually guided navigation in the water maze to path integration in an arena. Thus, these tasks involve different sensory inputs and processes that may be mediated by different brain structures. It not surprising therefore that parietal cortex lesions do not disrupt all egocentric learning tasks. Parietal cortex lesions did not impair navigation to a visible platform in the water maze [START_REF] Kolb | Behavioral and anatomical studies of the posterior parietal cortex in the rat[END_REF]Save & Poucet, 2000a), learning of an egocentric version of the radial maze [START_REF] Kesner | Double dissociation of egocentric and allocentric space following medial prefrontal and parietal cortex lesions in the rat[END_REF][START_REF] Kolb | Dissociation of the medial prefrontal, posterior parietal, and posterior temporal cortex for spatial navigation and recognition memory in the rat[END_REF]. Memory for egocentric distance in a delayed-matching-to-sample go/no go task was not affected [START_REF] Long | Effects of hippocampal and parietal cortex lesions on memory for egocentric distance and spatial location information in rats[END_REF]. In contrast, deficits were found in acquisition of a response-learning task in a greek cross-shaped water maze [START_REF] Mcdaniel | Unilateral injury of posterior parietal cortex and spatial learning in hooded rats[END_REF] and route learning task in a Hebb-Williams maze [START_REF] Rogers | Lesions of the dorsal hippocampus or parietal cortex differentially affect spatial information processing[END_REF].

Few studies were conducted to investigate the possibility that the parietal cortex is involved in the processing of motion information. Generally, in these studies, allothetic cues are removed or made inconsistent in order to encourage the use of idiothetic cues by the animals. For example, [START_REF] Save | Effects of lesions of the associative parietal cortex in the acquisition and use of spatial memory in egocentric and allocentric navigation tasks in the rat[END_REF] trained rats to reach a platform in the water maze from a fixed start position in total darkness. Lesioning the parietal cortex resulted in inaccurate trajectories so that lesioned rats could not learn the task. Consistent with these results, parietal lesions had a deleterious effect in rats that were trained by using a disorientation procedure to neglect allocentric cues and to rely on egocentric cues to navigate to the platform [START_REF] Commins | Disorientation combined with bilateral parietal cortex lesions causes path integration deficits in the water maze[END_REF]. The hypothesis that the parietal cortex is involved in the processing of idiothetic information was further investigated in studies that examined the rats' capability to navigate by path integration. Rats were trained to perform a homing task in a large circular arena. They had to climb on the arena, explore it to find a piece of food hidden in one of 17 food wells and carry the food back, straight to the home cage. Because distant visual cues, directional auditory cues and local olfactory cues were made not relevant, it was assumed that the animals would rely on path integration to return to their home cage. Rats with parietal lesions made more errors, i.e. did not exhibit a correct return, suggesting a path integration deficit. Note that both hippocampal lesions and entorhinal cortex lesions also produced an impairment in this task (Parron & Save, 2004;[START_REF] Save | Dissociation of the effects of lesions of the dorsal hippocampus and parietal cortex on path integration in the rat[END_REF]. However, unlike parietal and entorhinal cortex-lesioned rats, hippocampallesioned rats exhibited slower acquisition of the basic requirements for the task that may reflect a more general learning impairment. Together, these results suggest that the parietal cortex plays an important role in path integration and that path integration is dependent on the recruitment of a large functional network including at least the parietal cortex, the entorhinal cortex and the hippocampus.

Acquisition vs. retention: A role in memory storage?

There is a large consensus that a dialog between the neocortex and the hippocampus is essential for the formation of long-term memory [START_REF] Mcclelland | Why there are complementary learning systems in the hippocampus and neocortex: Insights from the successes and failures of connectionist models of learning and memory[END_REF]. The hippocampus would be necessary for rapid acquisition of new information and short term storage whereas the neocortex would be involved in the storage of remote memories [START_REF] Frankland | The organization of recent and remote memories[END_REF]. The parietal cortex has been hypothesized to be activated during consolidation [START_REF] Maviel | Sites of neocortical reorganization critical for remote spatial memory[END_REF]. Accordingly, post acquisition parietal cortex lesions should affect retention in spatial tasks. This aspect of parietal functioning has been poorly investigated, most studies reporting the effects of lesions made before learning. Among the studies that addressed the acquisition vs. retention issue, Cho and Kesner showed that rats with parietal cortex lesions did not interfere with the retention of two previously learned spatial discriminations in the radial maze. In addition, lesioned rats were able to relearn new discriminations [START_REF] Cho | Retrograde and anterograde amnesia for spatial discrimination in rats: Role of hippocampus, entorhinal cortex and parietal cortex[END_REF]. In a subsequent study using a similar task, the same authors found a non-temporally graded retention deficit in rats with parietal lesions [START_REF] Cho | Involvement of entorhinal cortex or parietal cortex in long-term spatial discrimination memory in rats: Retrograde amnesia[END_REF]. Post acquisition lesions affected the retention of an egocentric navigation task in the water maze and produced a transient deficit in the place navigation (allocentric) version of the task [START_REF] Commins | Disorientation combined with bilateral parietal cortex lesions causes path integration deficits in the water maze[END_REF][START_REF] Save | Effects of lesions of the associative parietal cortex in the acquisition and use of spatial memory in egocentric and allocentric navigation tasks in the rat[END_REF]. [START_REF] Hoh | Role of the neocortex in the water maze task in the rat: A detailed behavioral and Golgi-Cox analysis[END_REF] did not find any retention deficit in the place navigation task. Parietal-lesioned rats were found to be impaired in both acquisition and retention of allocentric and egocentric maze learning using the Hebb-Williams maze [START_REF] Rogers | Lesions of the dorsal hippocampus or parietal cortex differentially affect spatial information processing[END_REF]. Thus, in tasks that yielded no or mild acquisition deficits such as the place navigation task in the water maze, parietal lesions were not found to significantly impair retention. In contrast, in tasks that yield acquisition deficits such as egocentric navigation in darkness or route learning in the Hebb-Williams maze, parietal lesions produced retention deficits. Clearly, more data are needed but those available suggest that when the parietal cortex is necessary, it is involved in both initial acquisition and long-term storage of spatial information.

Neural activity in the parietal cortex: Unit recordings

Only a very few studies have managed to record unit activity in the parietal cortex in the rat. McNaughton and collaborators recorded parietal neurons as the rats performed a radial maze task. They found that a substantial number of cells exhibited movement correlates, discriminating between right turns, left turns and forward motion. Interestingly, there were cells that appeared to be modulated by a combination of motion and spatial correlates. For examples, some cells were preferentially activated for outwardly directed forward motion [START_REF] Mcnaughton | Cortical representation of motion during unrestrained spatial navigation in the rat[END_REF]. Other cells fired selectively during specific turns at the end but not at the center of the maze. Still more selective spatial correlates were found since there were cells that fired during specific turns in circumscribed parts of the maze, for example in the western arms [START_REF] Mcnaughton | Cortical-hippocampal interactions and cognitive mapping: A hypothesis based on reintegration of the parietal and inferotemporal pathways for visual processing[END_REF]. Thus, the parietal cortex contains cells that have correlates ranging from pure motion to conjunctions of motion and spatial correlates.

Further investigations allowed to identify a small number of cells in the parietal cortex that had head direction firing properties (Chen, Lin, Barnes, & McNaughton, 1994b;Chen, Lin, Green, Barnes, & McNaughton, 1994a). Similar cells were also found in the retrosplenial cortex (see also [START_REF] Cho | Head direction, place, and movement correlates for cells in the rat retrosplenial cortex[END_REF]. Some cells exhibited behavioral modulations in addition to their direction-specific firing. For example, there were head direction neurons that were more active for specific movements, e. g. right turns. Head direction-specific firing of parietal neurons was shown to be controlled by environmental and idiothetic cues (Chen et al., 1994b). Some of these neurons showed activity modulation in response to vestibular stimulation [START_REF] Chen | Head-centered representation and spatial memory in rat posterior parietal cortex[END_REF]. This is consistent with the properties of head direction cells recorded in other regions of the brain [START_REF] Taube | Head direction cells and the neurophysiological basis for a sense of direction[END_REF], for a review) and suggests that parietal neurons incorporate information from both environmental cues and movement-related cues including motor and vestibular cues.

Pursuing the idea that parietal neurons encode motion information, Nitz recorded parietal neurons as rats ran along a familiar path in a complex maze including one right and one left turns [START_REF] Nitz | Tracking route progression in the posterior parietal cortex[END_REF]. He found that parietal neuron activity was modulated by a variety of motor behaviors (straight run, right, left turn, straight + right turn, etc.). Such activity was correlated to the sequence of movements irrespective of the places where these movements occurred. A direction-dependent activity was found only in restricted portions of the paths suggesting a limited influence of the spatial context to parietal neuron activity. Thus, the finding suggests that parietal neurons encode representations of routes mainly in terms of movement sequences. Consistent with previous data, Nitz's results indicate that the parietal cortex not only is involved in the processing of motion information but also encodes representation of complex sensory motor behavior.

The role of the parietal cortex in the formation of spatial representation

Overall, lesion and electrophysiological studies provide a complex pattern of results that may reflect the multiple facets of parietal cortex functioning. It may also reflect neuroanatomical heterogeneity of this area. Note that little is known about the organization of inputs and outputs within the cortex parietal. It is likely that there is a subregional specificity. For example, a subregion may be preferentially involved in the processing of sensory information (e.g. visual) and another subregion in the association between tactile and visual information. Uncovering this organization may be very helpful to enhance our understanding of the multiple functional aspects of the parietal cortex.

The picture that emerges from the data ascribes to the parietal cortex a role in the processing of both allocentric and egocentric information. It is clear however that the parietal cortex does not play a role in all allocentric and egocentric processes but has a more specific function. The data first suggest that the parietal cortex is important when the animal has to form an allocentric spatial representation by using nearby cues. We have previously suggested that, in the nearby object situation, extraction of spatial invariants is strongly dependent on the association between different views of the environment and the movements connecting these views [START_REF] Poucet | Spatial cognitive maps in animals: New hypotheses on their structure and neural mechanisms[END_REF][START_REF] Poucet | The neuropsychology of spatial cognition in the rat[END_REF]. Thus, one basic function of the parietal cortex would be to perform associations between visual information and motion information, a process that is an early and important step for the formation of a spatial representation [START_REF] Mcnaughton | Neural association of movement and space: Preliminary steps toward a non-cartographic theory of spatial representation and learning[END_REF]. This hypothesis is consistent with neuroanatomical and functional data, accounting for the diversity of behavioral lesion effects and firing correlates of parietal neurons reviewed above.

In a previous theoretical work [START_REF] Save | Hippocampal-parietal cortical interactions in spatial cognition[END_REF], we have proposed that the parietal cortex primarily processes spatial information in an egocentric frame of reference. Indeed, visual input from an animal's point of view as well as movement-related information are basically egocentric. Associations between views and motions are assumed to initiate the transformation of egocentric into allocentric information. Evidence for such a gradual process in the parietal cortex is provided, in particular, by unit recordings that have identified firing correlates ranging from pure motor to complex visuo-spatial-motor combinations (Chen et al., 1994a;[START_REF] Mcnaughton | Cortical-hippocampal interactions and cognitive mapping: A hypothesis based on reintegration of the parietal and inferotemporal pathways for visual processing[END_REF][START_REF] Mcnaughton | Cortical representation of motion during unrestrained spatial navigation in the rat[END_REF][START_REF] Nitz | Tracking route progression in the posterior parietal cortex[END_REF]. Accordingly, a number of deficits in allocentric tasks may be a consequence of upstream alteration in the transformation process. The egocentric-to-allocentric hypothesis still holds at the light of the most recent results even if it raises several questions. First, to what extent does the parietal cortex mediate the egocentric-to-allocentric conversion? Second, provided that the conversion is important for elaborating an allocentric representation, why should a parietal lesion not impair all behaviors requiring such a representation? Third, how is it possible to relate this particular function to other potential functions of the parietal cortex for example in attentional processes (e.g. [START_REF] Cabeza | The parietal cortex and episodic memory: An attentional circuit[END_REF]? Fourth, is this role compatible with a possible implication in the long-term storage of allocentric spatial representations? Clearly, any global model of parietal functioning must answer these questions and integrate all these aspects.

The parietal cortex as an element of the cortical-hippocampal interaction

Understanding the role of the parietal cortex in spatial information processing requires taking into account its interactions with other cortical and subcortical regions. The data demonstrate that the parietal is functionally related to the hippocampus, thus supporting the idea that the parietal cortex is part of a functional network that allows continuous dialog between the neocortex and the hippocampus. The data also indicate that the parietal cortex and the hippocampus have distinct roles in spatial tasks. As proposed by [START_REF] Burgess | Integrating hippocampal and spatial functions: A spatial point of view[END_REF], there are two theoretical extreme views of the cooperation between the parietal cortex and the hippocampus. On the one hand, these two structures can be hypothesized to work in series. The parietal cortex would process sensory information in a format that could be used and further processed by the hippocampus. On the other hand, these two structures can be hypothesized to work in parallel. The parietal cortex and the hippocampus would mediate the formation of complementary spatial representations and parietal-hippocampal cooperation would then take place at all levels of processing. According to the serial hypothesis, a parietal cortex lesion may produce equivalent deficits as those resulting from hippocampal lesions, which is clearly not the case as shown by the short review of the literature presented above. According to the parallel hypothesis, a parietal lesion would be less disruptive than in the serial model, due to the spared capacity of the hippocampus to generate a spatial representation. This hypothesis accounts only partially for the behavioral effects of parietal cortex lesions. Thus, it could be useful to consider a model that comprises both serial and parallel properties. One possibility for integrating these two aspects would be to ascribe to the parietal cortex a role in both the processing of sensory information (serial processing in the egocentric-to-allocentric hypothesis) and in the long-term storage of spatial representations (parallel processing). This latter aspect is supported by metabolic imaging studies [START_REF] Bontempi | Time-dependent reorganization of brain circuitry underlying long-term memory storage[END_REF][START_REF] Maviel | Sites of neocortical reorganization critical for remote spatial memory[END_REF]) but needs to be further investigated.

The interest for the parietal cortex function in the spatial processing of information may have a new impetus as a consequence of the discovery of grid cells in the entorhinal cortex. These cells exhibit location-specific activity and generate multiple fields with regular spacing, therefore forming a grid-like firing pattern [START_REF] Hafting | Microstructure of a spatial map in the entorhinal cortex[END_REF]. It has been hypothesized that grid cells are involved in path integration, a basic navigation strategy requiring the use of movement-related information [START_REF] Mcnaughton | Path integration and the neural basis of the ''cognitive map[END_REF] for a review). How grid cell activity is generated remains unknown so far. That the parietal cortex is involved in the processing of movement-related information and in path integration [START_REF] Save | Dissociation of the effects of lesions of the dorsal hippocampus and parietal cortex on path integration in the rat[END_REF] suggests that it could contribute to the generation of the grid cell signal. Other cortical areas projecting to the entorhinal cortex may also contribute to grid cell activity, in particular, the retrosplenial cortex. This region has been shown to be connected to both the parietal and entorhinal cortices and contains spatial, head direction and movement-related signals (Chen et al., 1994a;[START_REF] Cho | Head direction, place, and movement correlates for cells in the rat retrosplenial cortex[END_REF].

The parietal cortex occupies a unique position in the brain linking perception to spatial representations. Data have accumulated across years but the role of this region remains unclear. The difficulty is to integrate the different facets of it function into a coherent model. This model would necessarily have to also integrate the notion of a functional cooperation with the hippocampus and other cortical areas. Further anatomical, lesion and recordings studies are needed to enter into the details of the parietal contribution to spatial processing. In particular, the possibility of functional subdivisions may be investigated. Because there are similarities between rodents and primates, a functional model of the parietal cortex in the rat may be useful to understand the normal and pathological functioning of the human parietal cortex.

Fig. 1 .

 1 Fig. 1. Main cortical and subcortical connections of the parietal cortex in the rat.


Mouna Abdelhamid
email: abdelhamid.mouna@gmail.com

Nadia Farjallah #
email: nadiafarjallah3@gmail.com

Imène Guetat
email: iguetat@yahoo.fr

Economic growth, government size and political instability

Keywords: Economic growth, Political instability, government size, GMM system estimator and MENA 1. INTRODUCTION

To empirically determine the effects of political instability and size of government on economic growth, we use the GMM system estimator for linear dynamic panel data models on a sample covering up to 19 countries from 1980 to 2012. The major empirical is that higher degrees of political instability are associated with lower growth rates of GDP per capita, unlike the size of government which has a positive effect on economic growth. Also, we figure out that political instability adversely affects growth by lowering the rates of productivity growth and physical and human capital accumulation. Finally, democracy and inflation have a negative effect, while economic freedom is beneficial to growth.

of research which attracted the attention of several researchers in economic and social materials, particularly in the 80s with the proliferation of coups in Africa. The economic dimension of the political instability generated many papers in the literature, in particular, its relation and its interaction with the economic performances. It is in this context that [START_REF] Rodrik | Policy uncertainty and private investment in developing countries[END_REF] affirms that political instability has a negative impact on macroeconomic indicators, such as investment, unemployment, and inflation. In fact, a low economic growth rate may be the result of political unrest during the change of government [START_REF] Kuznets | Modern Economic Growth: Rate, Structure, and Spread[END_REF].

As a result, a politically unstable economy is likely to cause corruption and other distorting activities.

Therefore, political instability is likely to have a negative impact on economic growth. Empirically, [START_REF] Aisen | Political Instability and Inflation Volatility[END_REF] have shown that higher inflation volatility is associated with higher levels of political instability, fragmentation of the political system and lower economic freedom. Furthermore, they argued that the policies in politically unstable countries tend to be more frequently interrupted by comparison with countries that are politically stable. [START_REF] Alesina | Why are stabilizations delayed?[END_REF] have shown that the delay in the implementation of inflation stabilization programs is associated with greater political instability in countries.

In fact, several empirical studies have shown that political instability has a negative impact on the main macroeconomic variable as GDP, private investment, and inflation. Jong-A-Pin (2009) examined the causal effect of political instability on economic growth (using GMM method). He showed that the unstable political regime has a significant negative effect on economic growth. Political instability considerably reduced economic growth, both statistically and economically [START_REF] Aisen | How does political instability affect Economic Growth?[END_REF]. Similar studies have reported a negative and significant correlation between political instability and economic growth (e.g [START_REF] Gupta | The Economics of Political Violence[END_REF]; Barro, 1991;Alesina et al, 1996;[START_REF] Ades | Thy Neighbor's curse: regional instability and economic growth[END_REF].

Various economists (Alesina et al., 1996;[START_REF] Mauro | Corruption and growth[END_REF][START_REF] Özler | External shocks, politics and private investment: Some theory and empirical evidence[END_REF][START_REF] Alesina | Income distribution, political instability, and investment[END_REF] showed that GDP growth is much less weaker in countries where there is a significant tendency for their government to collapse than other countries. Tang According to the empirical study of Barro (1996), democracy is a slight negative effect on economic growth, with evidence of nonlinearity where democracy increases the growth to low levels of democracy, or reduced to higher levels [START_REF] Helliwell | Empirical Linkages between Democracy and Economic Growth[END_REF]. According to [START_REF] Azam | Risque politique et croissance en Afrique[END_REF], the emergence of political disturbances is determined by economic variables such as health spending, defense spending, the enrollment rate in primary and secondary …etc.

y i,t -y i,t-1 = α y i,t-1 + β ' x i,t + μ i + ε i,t (1)
Where y is the logarithm of GDP per capita, x represents the explanatory variables other than the 2 System-GMM is a useful method to estimate the effects of political instability on growth because it provides a clear solution to the endogeneity problem involving these two variables. To differentiate equation ( 1), Arellano and Bond (1991) propose:

(y i,t -y i,t-1 )-(y i,t-1 -y i,t-2 ) = α (y i,t-1 -y i,t-2 ) + β ' (x i,t -x i,t-1 ) + (ε i,t -ε i,t-1 ) (2)
Although differentiation eliminates countries specific effect but it offers a new way for construction of the new error term, (ε i,t -ε i,t-1 ), which is correlated with the lagged dependent variable (y i,t-1 -y i,t-2 ). As a result, the explanatory variables are strongly exogenous. Arellano and Bond (1991) propose the following moment conditions:

E [y i,t-s (ε i,t -ε i,t -1 )
]= 0 to s ≥ 2 and t = 3, ..., T.

(

E [x i,t-s (ε i,t -ε i,t-1 )]= 0 to s ≥ 2 and t = 3, ..., T. 3)
Arellano and Bond (1991) propose a two-step GMM estimator, using the moment conditions. For the first step (3), the error term is assumed to be independent and homoscedastic across countries and over time. In the second step, residues reached from the preceding step, are used to construct a regular estimate of the variance-covariance matrix.

As a result, we obtain the assumptions of homoscedasticity and independence. We obtain an asymptotically efficient estimator in the first step. Instruments for the regression system delay differences of the corresponding variables.

Conditions moment's estimator for the system is:

E [(y i,t-s -y i,t-s-1 )(μ i -ε i,t )]=0 to S=1 E [ (x i,t-s -x i,t-s-1 )(μ i -ε i,t-1 )]=0 to S=1
The

(ε i,t ) is not correlated in series.
The failure to reject the primary hypotheses of two tests gives support to our model. Both difference estimator and system estimator found some problems with small samples. For two-step estimators, asymptotic standard errors are biased (Arellano and Bond, 1991;Bundell and Bond, 1998).

Table 1 provides details on all variables along with their definitions and sources used in this paper. From strongly autocratic (-10) to strongly democratic [START_REF] Tang | The impacts of tourism, energy consumption and political instability on Economic Growth in MENA Countries[END_REF]. This variable is our proxy for democracy.

Database ( The final consumption expenditure of government (formerly general government consumption) includes all government current expenditures for purchases of goods and services (including compensation of employees). These expenditures also include most of the expenses for defense and national security, but does not include military expenditures that are part of government capital formation of government.

World Bank's World Development Indicators (WDI)

Index of Economic Freedom

The Index of Economic Freedom Takes a broad and comprehensive view of Economic Freedom, measuring country performance in 10 separate Areas. The 10 measured aspects of Economic Freedom may be grouped Into four broad categories: 1. Rule of Law (property rights, freedom from corruption); 2. Government size (fiscal freedom, government Spending) 3. Regulatory efficiency (business freedom, labor freedom, monetary freedom); 4. Market and openness (trade freedom, investment freedom, and financial freedom).

Freedom House political risk

The measure of political constraints employed in this paper estimates the feasibility of policy change (the extent to which a change in the preferences of any one actor may lead to a change in government policy) Henisz, W. J. (2002).

The assessment and measurement of the role of government changes or adjustments is not an easy target to achieve. In this paper, we used the concept of measuring the political instability as the propensity for government change, which has attracted a considerable attention in previous research (Alesina et al., 1996). However, our measures of political instability are somewhat different from those of previous works. In fact we used two variables. The first called political risk which estimates the feasibility of policy change (the extent to which a change in the preferences of any one actor may lead to a change in government policy), and the second is the Democracy (Polity IV) which is our proxy for democracy. However, we believe that application of those variables may also provide some interesting conclusions as it allows comprehensive comparisons between the effects of major and regular government transfers.

Table 2 contains some basic information about our data, which shed some light on the possibility of the existence of simultaneous relations between economic growth and both indicators of political instability. The average growth rate of the studied countries was around 3.677 current US $. On the other hand the average of political risk was at a level of 0.126, which indicates that the change in government policy frequency was prominent. In addition, the democracy level was too low since it did not exceed the rate of -5.

Moreover, to determine the strength of the statistical relationships between all the variables, table 3 represents the correlation matrix. 4). Inflation, GDP deflator (annual %). [START_REF] Alesina | Distributive Politics and Economic Growth[END_REF]. Democracy (polity IV). ( 6). Political risk. [START_REF] Alesina | Political instability and economic growth[END_REF]. Index of Economic Freedom. [START_REF] Bergh | Government Size and Growth: A Survey and Interpretation of the Evidence[END_REF]. size of government.

At the correlation matrix, it can be seen that the factors moderately correlate, which implies that they indeed reflect different dimensions of political instability, although some correlation coefficients do significantly differ from 0.

THE EMPIRICAL RESULTS

We choose a growth model on a panel of 19 MENA developing countries (Annex 1) selected according to data availability for the period from 1980 to 2012. The empirical analysis is divided into two parts. First, we test the hypothesis that the instability of political institutions has a negative effect on economic growth. In the second part of the empirical analysis, we study the channels through which political instability affects economic growth. Our primary interest is to determine the tests of the validity of instruments (Sargan) and the lack of residuals serial autocorrelation (Arellano and Bond (1991).

Political instability and economic growth

Empirically, we accept the presence of an AR [START_REF] Ades | Thy Neighbor's curse: regional instability and economic growth[END_REF] for residues and the absence of an AR (2) effect. This is in accordance with the formulated hypotheses. Besides, the tests of Sargan validate the choices of instruments. For this study, variables: size of government and political instability are represented by proxy variables which are respectively the final consumption expenditure of general government as a percentage of GDP and political risk [START_REF] Henisz | The Institutional Environment for Infrastructure Investment[END_REF]. Specifically, we use the dynamic panel GMM system estimator of [START_REF] Blundell | Initial Conditions and Moment Restrictions in Dynamic Panel Data Models[END_REF]. This choice is motivated by the fact that this estimator allows us to model both the lagged dependent variable and the fixed country effects. In our opinion, including country fixed effects in the model is particularly important because most of the significant variables identified by the empirical growth literature (such as the ethnolinguistic or geographical splitting variables) are time-invariant (an overview of determinants of economic growth can be found in [START_REF] Durlauf | Growth Econometrics, Handbook of Economic Growth[END_REF]. In addition, the GMM approach can be used to take into account the potential endogeneity of political instability by using political instability lagged variables as instrumental variables. Results of the data regressions are represented in Table 1.

In fact, considering macroeconomic variables, the results of the different models are similar to those provided. The hypothesis that political instability negatively affects economic growth gets a clear empirical support. The estimated coefficient implies when there is an additional change in political risk, the annual growth rate decreases. Consequently, the low economic growth may increase the volatility of government (Alesina et al., 1996).The initial GDP per capita has a negative coefficient, which is compatible with the conditional convergence income across countries. Investment [START_REF] Mankiw | A contribution to the empirics of Economic Growth[END_REF]) and tertiary enrollment rates have positive and statistically significant coefficients, indicating that investment and education promote growth.

Inflation has a negative and statistically significant effect on economic growth due that high inflation negatively affects growth [START_REF] Edison | International financial integration and economic growth[END_REF][START_REF] Elder | Another perspective on the effects of inflation uncertainty[END_REF].

The Index of Economic Freedom is included in the model in column 2 to explain the favorable economic institutions. This index is statistically significant and has a positive sign as expected.

Similarly, the size of government has a positive and significant effect on economic growth. In contrast, democracy has a significant and negative effect on economic growth. The one-unit increase in the index of democracy decreases the economic growth rate of 1.9 per cent. Similarly, the empirical analysis of Barro (1996) shows a negative relationship between democracy and economic growth. This implies that democracy promotes economic growth to low levels of political freedom, although tends to decrease it when a certain level of freedom is achieved.

Transmission channels

We study mechanisms by which political instability affects economic growth since political instability is associated with greater uncertainty about future economic policy. Hence, it is likely to affect negatively on investment and thus on physical capital. Various studies have verified a negative relationship between political instability and investment [START_REF] Alesina | Income distribution, political instability, and investment[END_REF][START_REF] Mauro | Corruption and growth[END_REF][START_REF] Özler | External shocks, politics and private investment: Some theory and empirical evidence[END_REF][START_REF] Perotti | Growth, income distribution, and democracy. What the data say?[END_REF]. The accumulation of human capital could be disturbed by political instability because uncertainty about the future can encourage less investing in education. In developing countries, human capital formation may be adversely affected by political instability in two ways (Gyimah-Brempong and Camacho (1998)). First, a greater political instability can bring those who have high levels of human capital to emigrate. The second source is due to the allocation of resources by the government. Devereux and Wen (1998) argue that greater political instability leads to a higher share of public spending in GDP, which may require a misallocation of resources and slow productivity growth. Political instability affects growth through the accumulation of physical and human capital, note that the first having a slightly greater effect than the second. Government stability is an important feature of political systems. Political instability leads to uncertainty about future political which encourages leaders to adopt a predatory behavior towards the resources of private economic resources. One of the main characteristics of democracy is providing transparent rules to facilitate the transaction between political forces. Consequently, democracies can have a peaceful and predictable transfer of political power; nevertheless, autocracies may experience violent and irregular changes. Empirically, Alesina et al. (1996) found that political instability has a negative effect on growth. The estimation results of the regressions are shown in the table 5. So far, we have examined the overall effect of political instability on economic growth without trying to exactly distinguish the influence of the accumulation of production factors. By adding the interaction term between the production factors and political instability, we find that the interaction term between political instability and human capital is positive and significant at 5% level. Then the interaction term between political instability and physical capital is significant and negative at 5% level. In addition, there is a term negative and significant interaction between democracy and political instability. In contrast, there is not an interaction term between political instability and government size. Finally, we can conclude that political instability can influence economic growth, through indirect effects via human capital. In other words, greater political stability will affect economic growth, if and only if these countries are characterized by a productive human capital.

CONCLUSION

In this study, we have tried to contribute to the resolution of fundamental questions concerning impact of political instability on economic growth and the transmission channels through which it affects economic growth. To do this, we used the GMM system estimator for linear dynamic panel data models on a sample covering up to 19 countries in the MENA region during the period 1980-2012. As part of this empirical study, we tested the effects of political instability, democracy and government size on economic growth. The key findings emerged from this empirical analysis show a negative impact of political instability and democracy on economic growth, which is opposite to the effect of government size on economic growth. In addition, increased political instability is aggregated to a decline in economic growth through the channels of human capital, physical capital, government size and democracy. More generally, since 2011, several countries in the MENA region recorded a significant slowdown in tourism, lower remittances from migrants, worsening budget deficits and a raise of their debt level. These factors explain the decline in the growth rate in these countries which record negative rates in the case of transitions countries (Libya and Syria), and others weak and volatile ones for most countries in the region. We conclude, that this analysis allowed us, even in part, to show the existence of a relationship between political instability and economic performance and some key channels through which the effects of political instability could affect the performance of countries in the MENA region. Although, it is important to note that, despite the importance of empirical results that led this work, shortcomings might arise namely existence of other possible mechanisms to examine this relationship that have not been considered and the causality problem that also has not been treated.

  and Abosedra (2014) found that political instability prevents the process of growth and economic development in the MENA region (use of 24 countries in the MENA region). Economic theory suggests several mechanisms by which government activities can affect growth. Literature concerning the relationship between government size and economic growth is full of contradictory results. This conflict is explained by variations in definitions and studied countries. There are many reasons to expect a relationship that is inversely U-shaped, a hypothesis that is sometimes indicated under the name of Armey curve (Armey 1995). For less developed countries, there is a positive link between tax revenue and growth because a state managed typically to collect taxes if it succeeded in providing the stability necessary for economic activity begins to grow (Besley and Persson 2009). The most basic government functions such as protection of property rights and Law enforcement can be performed at low levels of taxation. If productive public spending is characterized by diminishing returns, the negative impact of taxes financing public spending can dominate the positive impact of government activities promoting growth. Generally, in poor countries, the public sectors are insignificant, and the relationship between government size and growth is positive. Unlike in rich countries, public sectors are great, and the relationship between government size and growth is less positive than in poor countries, and possibly negative1. Concerning the interaction between 1 Andreas Bergh and Magnus Henrekson (2011). Government Size and Growth: A Survey and Interpretation of the Evidence. IFN Working Paper No. 858, 2011 democracy and economic growth, Acemoglu et al. (2014) found a positive effect of democracy on growth. Economists, such as Alesina and Rodrik (1994) and Persson and Tabellini (1994), said that the democratic redistribution is a distortion and it will discourage economic growth. As a matter of fact, Acemoglu et al. (2008) argues that democratic institutions can create distortions because of their redistributive tendencies.

Figure 1 :Figure 1 ,

 11 Figure 1: Growth and political stability in MENA 2014

  Concerning, the case where the explanatory variables persistence,[START_REF] Blundell | Initial Conditions and Moment Restrictions in Dynamic Panel Data Models[END_REF] and Alonso-Borrego andArellano (1996), verified that the delayed levels of these variables are weak instruments for the regression of the difference equation. Asymptotically, it will have an increase in the variance of the coefficients. Monti Carlo simulations for small sample sizes verified that the weaknesses of the instruments can establish biased coefficients.[START_REF] Arellano | Another Look at the Instrumental-Variable Estimation of Error Components Models[END_REF],[START_REF] Blundell | Initial Conditions and Moment Restrictions in Dynamic Panel Data Models[END_REF] have proposed an estimator system to reduce the potential bias and imprecision associated with the difference estimator.

  consistency of the GMM estimator depends on the validity of hypotheses of autocorrelation absence of error terms and instruments. For the validity of these assumptions, we use two tests proposed by Arellano and Bond (1991), Blundell and Bond (1998) together with Arellano and Bover (1995). The first Sargan test of over-identification tests the complete validity of the instruments and the second test verifies the assumption that the error

  Annual data on economic and political variables from 1980 to 2012 were collected for 19 countries, covering the MENA region (the Middle East and North Africa). Economic data sources were found on the World Development Indicators of the World Bank (WDI, 2007). The political data was obtained by Henisz (2002) and the Polity IV database (Marshall and Jaggers, 2009). The following work will focus on determining the impact of political instability and the size of government on economic growth.

  Inflation as Measured by the annual growth rate of the GDP implicit deflator shows the rate of price change in the economy as a Whole. The GDP implicit deflator is the ratio of GDP in current local currency to GDP in constant local currency.The total Enrollment in tertiary education (ISCED 5 and 6) Regardless of age, Expressed as a percentage of the total population of the five-year age group Following on from secondary school leaving.

			Table1: List of variables, definitions and sources
	Indicator		Definition	Source
	Inflation,	GDP		World Bank's World
	deflator (annual %)		Development Indicators
				(WDI)
	Investment Share of	The share of investment as a percentage of GDP.	World Bank's World
	GDP (%)			Development Indicators
				(WDI)
	School Enrollment,		World Bank's World
	tertiary (% gross)		Development Indicators
				(WDI)
	Democracy	(Polity
	IV)

  GDP per capita is gross domestic product divided by midyear population. GDP is the sum of gross value added by all resident producers in the economy plus any product taxes and minus any subsidies not included in the value of the products. It is calculated without making deductions for depreciation of fabricated assets or for depletion and degradation of natural resources. Data are in current U.S. dollars.

				Marshall and
				Jaggers, 2009)
	GDP	per	capita	World Bank's World
	(current US $)		Development Indicators
				(WDI)
	General government
	final	consumption
	expenditure (% of
	GDP)

Table 2 :

 2 Descriptive statistics of examined data

	Variables	Obs	Mean	Standard	Min	Max
				deviation
	growth of GDP per capita	500	3.677	0.563	2.452	4.967
	Investment Share of GDP (%)	400	29.698	11.442	7.976	80.120
	School Enrollment, tertiary (% gross). 411	19.536	13.196	-12.732	62.375
	Inflation, GDP deflator	476	10.893	26.818	-25699	390.678
	(Annual %)
	Democracy (polity IV)	509	-5.332	5.208	-10	10
	political risk	523	0.126	0.201	0	0.647
	Index of Economic Freedom	213	6.446	1.141	3.1	8.1
	size of government	458	19.338	7.116	5.745	76.222

Table 3 :

 3 Correlation Matrix

	Variable	1	2	3	4	5	6	7	8
	1	1
	2	0.004	1
	3	0.099	-0.028	1
	4	-0.069	-0.045	0.121	1
	5	-0.078	-0.036	0.633	0.302	1
	6	-0.047	-0.104	0.375	0.118	0.645	1
	7	0.633	0.150	0.184	-0.304	-0.059	-0.148	1
	8	0.184	0.018	0.075	0.165	0.089	0.146	-0.109	1

[START_REF] Ades | Thy Neighbor's curse: regional instability and economic growth[END_REF]

. growth of GDP per capita.

[START_REF] Aisen | Political Instability and Inflation Volatility[END_REF]

.Investment Share of GDP (%).

[START_REF] Aisen | How does political instability affect Economic Growth?[END_REF]

. School enrollment, tertiary (% gross). (

Table 4 :

 4 Economic growth and instability of political institutions

	Dependent Variable : real GDP growth per capita
	1	2	3	4	5
	lagged real GDP growth per capita	-0.101 (0.000)**	-0.069 (0.032)**	-0.059 (0.000)**	-0.028 (0.006) * *	-0.159 (0.037)**
	Investment Share of GDP (%)	0.001 (0.000)**	-0.001 (0.440)	0.012 (0.000)**	0.002 ( 0.002) * *	0.027 ( 0.005) * *
	School enrollment, tertiary (% gross)	0.001 (0.000)**	0.017 (0.000)**	0.008 (0.000)**	0.004 (0.000)**	0.021 (0.003) * *
	political risk	-1.279	-0.463	-0.882	-0.980	-0.987
		(0.000)**	(0. 009) * *	(0.000)**	(0.020) * *	(0.000)**
	Index of Economic Freedom		1.448 (0.000)**
	Inflation, GDP deflator (annual %)			-0.003	-0.005
				(0.010) * *	(0.001) * *
	size of government				0.004
					(0.000)**

Table 5 :

 5 Effect of political instability

			Dependent Variable : real GDP growth per capita
			1	2	3	4
	lagged real GDP growth per capita	-0.086	-0.097	-0.098	-0.091
			(0.000)**	(0.000)**	(0.000)**	(0.000)**
	Investment Share of GDP (%)	0.008	0.008	0.008	0.008
			(0.000)**	(0.000)**	(0.000)**	(0.000)**
	political risk* School enrollment, tertiary	0.007
	(% gross)		(0.000)**
				-0.006
	Political risk* Investment Share of GDP		(0.085) *
	(%)
	political risk* size of government			-0.007
					(0.002) * *
	political risk* democracy				-0.001
						(0.031) * *
	constant		3.781	3.834	3.828	3.813
			(0.000)**	(0.000)**	(0.000)**	(0.000)**
	Number of observations	276	276	276	276
	Number of countries	31	31	31	31
	Hansen test (p-value)	0.14	0.14	0.13	0.13
	Arellano-Bond test for AR(1) (p-value)	0.000	0.000	0.000	0.000
	Arellano-Bond test for AR(2) (p-value)	0.17	0.147	0.164	0.141
	Notes:
		System-GMM estimates for dynamics panel-data models. Sample period: 1980-2012

Annex: List of countries used in the sample

We used the MENA countries as a sample of our study except it has no standardized definition; different organizations define the region as consisting of different territories, so we choose to limit this analyze to the following list of countries: