Léo Varnet

Julien Meyer

Michel Hoen
email: michel.hoen@inserm.fr

Fanny Meunier
email: fanny.meunier@ish-lyon.cnrs.fr

Phoneme resistance during speech-in-speech comprehension

Keywords: Speech-in-speech, Energetic masking, Informational masking, Phoneme resistance

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Introduction

Most of the time in real-life listening situations, we have to deal with environmental noise or concurrent speech partly masking target speech signals, yet we are still able to decipher the information they contain. However different types of backgrounds have been shown to differently affect speech comprehension [START_REF] Bronkhorst | The cocktail party phenomenon: a review of research on speech intelligibility in multiple-talker conditions[END_REF]. In the present paper we tested the effect of different backgrounds on a word identification task. For speech target, two types of masking effects must be considered: energetic masking and informational masking [START_REF] Brungart | Informational and energetic masking effects in the perception of two simultaneous talkers[END_REF]; [START_REF] Brungart | Informational and energetic masking effects in the perception of multiple simultaneous talkers[END_REF]. Energetic masking occurs when speech and masker noises overlap, even partially, in time and frequency. Informational masking concerns the type of information carried by the two signals. Although there is not necessarily any physical overlap in the signals from target-and masker-sounds, competition between information carried by the two signals will compete during highlevel processes [START_REF] Dirks | Masking effects of speech competing messages[END_REF], [START_REF] Festen | Effects of fluctuating noise and interfering speech on the speech-reception threshold for impaired and normal hearing[END_REF]. In the context of speech-in-speech comprehension, some energetic masking certainly does occur, although it has been shown to be responsible for only a relatively small part of the overall masking phenomenon which occurs in this listening situation [START_REF] Brungart | Isolating the energetic component of speech-on-speech masking with ideal time-frequency segregation[END_REF]. Indeed during speech-in-speech comprehension informational masking plays a predominant role on the intelligibility of target speech signals. While informational masking has until now been considered as monolithic, it seems clear that in the particular case of speech, such a view is limited given the numerous types of linguistic information involved during comprehension (for example phonological information and lexical one). In a previous paper [START_REF] Hoen | Phonetic and lexical interferences in informational masking during speech-in-speech comprehension[END_REF] we examined the different effects of acoustic-phonetic and lexical content of 4-to 8-talker babble on word identification. Our results showed that the nature and amount of interfering linguistic information available from background babble varied with the decrease in spectro-temporal saturation caused by reducing the number of talkers in the babble. This was associated with different types of linguistic competition for target-word identification, reaching the lexical masking effect when only 4 talkers constituted the background noise (see also [START_REF] Van Engen | Sentence recognition in native and foreign-language multi-talker background noise[END_REF]). While in our previous work we focused on word identification performances, i.e. the proportion of reported words that corresponded to target words, in the present paper we analyzed performances of masked word identification at a phonemic level in order to test resistance of different French phonemes to different types of masking.

The present study

Our experiment studied the impact of different types of babble backgrounds during word identification on phonological information, with an increasing number of simultaneous talkers. To avoid unmasking effects mostly due to the processing of pitch information observed with babble sounds made of up to 3-talkers [START_REF] Brungart | Informational and energetic masking effects in the perception of multiple simultaneous talkers[END_REF], we focused on situations with 4, 6 and 8 talkers where individual voice characteristics are less predominant. We contrasted situations where the babble was made of natural speech and therefore contained real words (natural speech) vs. situations in which only partial phonetic information was available (reversed speech) vs. situations in which no phonetic information was available (speech derived noise). As babble sounds, we used signals composed of 4, 6 and 8 simultaneous talkers (S4, S6 and S8). In order to dissociate the spectrotemporal saturation effect from potential linguistic masking effects, the same speech sounds were also presented reversed along their temporal axis (reversed babble sounds, later referred to as R4, R6, R8). Time reversal of speech signals has been claimed to be the most drastic degradation one can apply to speech [START_REF] Saberi | Cognitive restoration of reversed speech[END_REF]. However, not only does reversed speech 'sound' like speech, but partial phonetic information present in natural speech remain intelligible (like vowels or fricatives for example). Moreover, when different reversed speech streams are mixed together, the resulting babble sounds like normal speech babble and phonemes can be perceived, although it does not contain words. Reversed babble stimuli were thus considered in the experiment as an intermediate situation where speech sounds contained phonetic but no lexical information. To further obtain a reference measure of a pure energetic masking effect, we added one condition where speech was presented against a broadband noise background (later referred to as N). This noise was designed to have similar spectro-temporal characteristics as our most spectro-temporally saturated natural and reversed babble signals (i.e. S8 and R8). These 7 background noise types (S4, S6, S8, R4, R6, R8 and N) were all tested at 4 different SNRs of 3, 0, +3 and +6 dB, yielding a total of 28 main experimental conditions.

Experiment

Materials and Methods

Concurrent sounds: Multitalker babble sounds, reversed babble sounds and associated broadband noise

The babble signals were created with groups of 4, 6 and 8 talker voices. Each voice was first recorded separately in a sound-proof room, reading extracts from the French press. Individual recordings were modified according to the following protocol: (i) removal of silences and pauses of more than 1 s, (ii) suppression of sentences containing pronunciation errors, exaggerated prosody or proper nouns, (iii) noise reduction optimized for speech signals, (iv) intensity calibration in dB-A and normalization of each source at 80 dB-A and (v) final mixing of individual sources into cocktail party sound tracks. Reversed babble sounds were obtained by reversing the previously generated speech babble stimuli along their temporal dimension. We created a broadband noise with spectro-temporal characteristics comparable to those of our most saturated natural and reversed babble, i.e. the 8-talker babble (see [START_REF] Hoen | Phonetic and lexical interferences in informational masking during speech-in-speech comprehension[END_REF] for details).

Target words

Two hundred and eighty-eight French mono-syllabic, triphonemic words were recorded in a sound-proof booth by a male native French speaker. Words were selected in a middle range of frequency of occurrence (ranging from 0.19 to 146.71 per million; mean = 20.96, SD = 21.37), according to the French database Lexique2 [START_REF] New | Lexique 2: a new French lexical database[END_REF].

Stimuli and word lists

Stimuli consisted of the 288 single target-words mixed together with 4 s samples of background noise. Target words were always inserted 2.5 s from the start of the stimulus. Stimuli were composed by mixing one chunk of background noise, randomly selected from 40 chunks extracted from the original noise files, with one target word. Individual intensity levels for background noise and target-words were adjusted according to the global rms power of the original sounds to be mixed. As this resulted in some intensity modulation of the final stimuli and in order to avoid global intensity of stimuli to become predictive of the SNR, a final randomized intensity roving over a ±3 dB range in 1 dB steps was applied. Thirty-six different listsone for each participantwere generated, each list containing every target word only once (288 stimuli). Across lists, all target words were presented against the 28 background conditions.

Participants and procedure

Thirty-six volunteers participated in the experiment, they were native French speakers, aged 18-32 years, with no known hearing or language disorders. Participants sat in a quiet room, facing a computer monitor. Stimuli were delivered diotically via headphones (Beyerdynamic DT 48, 200 X) at an individually adjusted comfortable sound level. The task for participants consisted in a single-word transcription, participants being asked to type the sounds they heard on a computer keyboard. Before testing phase, participants were given 12 practice items to accommodate to stimulus presentation mode and target's voice. The experiment lasted on average 45 min.

Results

We analyzed the phonemic decomposition of the responded words. To assess the influence of each factor, the phoneme error rate was computed separately for each condition, each SNR, each position in the syllabic structure, and each type of phoneme (12 vowels: [a, ã, ə, e, ɛ, i, ɛ , ɔ, o, ɔ , y, u] and 18 consonants: [p, t, k, f, s, ʃ, b, d, g, v, z, ʒ, j, ʁ, l, m, n, ɲ]). Altogether, a total of 858 phonemes were heard by each of the 36 participants, yielding a total of 30 888 observations. Overall, we obtained a mean phoneme error rate of 20% across all conditions. As expected, the percentage of errors in phonemes decreased with SNR (7% at +6dB, 12% at +3dB, 21% at 0dB and 39% at -3dB, p<.001). Figure 1 shows performance scores for each type of background noise separately. The lowest phoneme error rate was observed in condition N (14%, all ps<.001). It appears that Speech and Reversed speech conditions lead to a similar decrease in performances between 6 talkers and 8 talkers (19% for S6 and R6 and 21% for S8 and R8). Conversely, for 4 talkers, a difference of 5.1% is observed in the percentage of incorrectly reported phonemes between these conditions (R4: 18.1%, S4: 23.2%, p<.001), suggesting that the information carried by the 4 talkers' speech interfered more with target identification.

Figure 1. Phoneme error rates in all conditions

To further investigate this difference, we dissociate errors in phoneme recognition by studying separately confusions (phonemes mistaken for one another), deletions (suppression of a phoneme in the syllabic structure of the responded word), and insertions (addition of a phoneme in the syllabic structure of the responded word). Confusions and insertions turned out to have similar evolutions with SNR whatever the type of background noise (see Figure 2): in all condition we observed a decrease in confusion rate from 20% at -3 dB to approximately 5% at 6 dB, while insertion rate stands below 5%. Conversely, our analysis revealed an important variation in deletion rate (p<.001) depending on the kind of noise: low (<7%) and quasi-constant in condition N, but on the contrary strongly correlated with SNR in condition S4, from 2% to 26%. Additional evidences of this difference in S4 were found in distinguishing between errors occurring in vowels or in consonants. Overall the mean error rate was 9% in vowels, and 25.4% in consonants. We observed a lower error rate in vowels than in consonants for each kind of background noise (N: 4.3%, S4: 12.3%, S6: 8.7%, S8: 9.7%, R4: 10.4%, R6: 8.0%, R8: 9.8% for vowels, and N: 19.5%, S4: 29.2%, S6: 24.9%, S8: 27.5%, R4: 24%, R6: 25.4%, R8: 27.2% for consonants). The evolution of degradation in recognition performances with SNR confirmed that deletion dramatically increases with noise level in condition S4 but not in condition N. Confusion, deletion and insertion rates depend on the type of sound (p<.001), deletion rate reaching over 25% at -3 dB for consonants but less than 10% for vowels, in all seven conditions, whereas deletion rate is similar for vowels and consonants. The main source of errors on consonants at any SNR was confusion, whereas on vowels, confusion and deletion rates are equivalent, except at -3dB where the major part of errors was due to an absence of answer (not in condition N). A more detailed observation of errors on consonants revealed that, in all conditions, deletion rates are equivalent on initial and final consonants, but confusion rates are considerably higher in the first case. More precisely, when comparing recognition performances for the 17 most played consonants independently (Figure 3), specific distribution patterns are seen in condition N, S and R, with clear differences in scores for [f] (ranging from 66% (S6) to 48% (S8)), [b] (from 76% (N) to 53% (S4)), [g] (from 83% (N) to 64% (S8)) or [j] (from 96% (N) to 69% (S4)). The increasing difficulty in identification of phonemes indicated above is mostly due to variations in confusion rates with the type of background noise, deletion rates being constant over conditions. The sibilant consonants [ʃ], [Ʒ] and [s] were the best recognized in every condition, but the score of the consonant [z] depends on the type of background noise (81% for (R4) but 56% for (S4)), demonstrating that the sibilant frication has to be completed by phonetic power to be a critical factor for good identification. The consonants with the poorest recognition performances are not identical for all types of background noise (e.g. [f] and [v] for N, [b, d, v, z] for S4, and [d, v] for R4).Vowels with a sufficient number of occurrences have similar patterns of recognition for conditions S4, S6, S8, R4, R6 and R8, with a general percentage of not reported phonemes around 9%, whereas in the case of a broadband noise (N), percentage of not reported phonemes falls below 3% (see Figure 3).

Discussion & Conclusion

In this experiment we tested phoneme identification performances in situations where the babble was made of natural speech vs. situations in which only partial phonetic information was available vs. situations in which no phonetic information was available. The babble sounds were composed of 4, 6 and 8 simultaneous talkers.

Results are coherent with those of a previous analysis performed at the word level: we observed more errors in babble sounds (natural or reversed) compared to broadband noise and also a monotonic decrease in speech comprehension rates with the increasing number of simultaneous talkers between 6 talkers and 8 talkers (for natural and reversed babbles). When only 4 talkers are present the pattern is different despite the fact that a signal analysis showed a monotonic increase in spectro-temporal saturation with an increasing number of simultaneous talkers, for both natural and reversed speech. This suggest that the rise of informal masking observed in R4 (that gave similar performances than R6) and even more in S4 corresponded to acoustic-phonetic masking (which is fairly similar in the reversed and natural conditions) and higher level linguistic masking (only present in natural babble). Both effects depend on the number of talkers in the background babble.

Our results also showed that vowels are much more resistant than consonants and that errors on consonants were mostly confusions whereas deletion is also source of errors on vowels. Overall there were more confusions than deletions and quite rare insertions. Performed analyses clearly showed the different impact of background sounds on phoneme intelligibility. With few talkers, deletions appear as rare as insertions for higher SNRs, but increased at less favorable SNRs to reach confusion rate in R4 and even overtake it in S4. Also while some phonemes presented stable patterns over conditions as [v]

Figure 2 .

 2 Figure 2. Percentage of deletion, insertion and confusion in function of RSB, in condition N, R4 and S4.

Figure 3 .

 3 Figure 3. Confusion rate and no answer rate per phoneme, in condition N (A) and in condition S4 (B)In order to clarify the origin of confusions between phonemes, we displayed confusion matrix for consonants and vowels individually for each type of background noise (the two most contrasting conditions being reported on Figure4). It appears that the most frequent inter-vowels confusions corresponded to the situations when [o] was mistaken for a [u], [u] was mistaken for a [y], and [y] was mistaken for a [o]. The most frequent interconsonants confusion were obtained when [f] was mistaken for a [p], [p] was mistaken for a [b], [t] was mistaken for a [p], [v] was mistaken for a [b], and [n] was mistaken for a [m]. In general, we observed that confusion with a phoneme often conduces to misunderstanding a consonant in favor of [p] or [b], rather than other consonants of equal frequency of occurrence such as [t]. On another hand we can notice several differences between confusion matrixes: confusions with [b] and [p] are more frequent in condition (N) than in other types of background noise, where all confusions are more widely distributed over all consonants.

Figure 4 .

 4 Figure 4. Confusion matrix in condition N (up) and in condition R8 (down)

  , it is not always the case, as for [z] reaching 50 % in S4 and only 20% in N. Results per phoneme revealed that while the less resistant consonants are [f] and [v] in N, they are [v] and [z] in S4. The more resistant consonants are similar in both background sounds (i.e. [s] and [ʃ]). For vowels the less resistant ones are [e] in N but [o] (followed by [e]) in S4 and the more resistant ones, [a], [i] and [u] in N and [a] only in S4. These results showed that different types of linguistic information can interfere with speech recognition and that several resistances are observed for different phonemes depending on the interfering noise.

Acknowledgements

The authors would like to thank N. Grimault for the help on the cocktail original sounds, C. Grataloup for collecting the data and F. Pellegrino for running the signal analysis. This research was supported by the European Research Council (starting-grant SpiN Project, ERC n°209234).


Marie Bourjade
email: marie.bourjade@univ-amu.fr

Adrien Meguerditchian

Audrey Maille

Florence Gaunet

Jacques Vauclair

Olive baboons, Papio anubis, adjust their visual and auditory intentional gestures to the visual attention of others

Keywords: audience attention, gestural communication, intentionality, language, nonhuman primate Intentional

Although nonhuman primates' gestural communication is often considered to be a likely precursor of human language, the intentional properties in this communicative system have not yet been entirely elucidated. In particular, little is known about the intentional nature of monkeys' gestural signalling and related social understanding. We investigated whether olive baboons can (1) adjust their requesting gestures to the visual attention of the experimenter with special emphasis on the state of the eyes (open versus closed), and (2) flexibly tailor visual and auditory-based gestures to elaborate their communication as a function of whether or not the experimenter can see them. Using a food-requesting paradigm, we found monkeys able to favour either visual or auditory-based requesting gestures to match the experimenter's visual attention. Crucially, when the human was not visually attending, they silenced visual gestures to some extent but performed more attention-getting gestures. This is, to our knowledge, the first report of monkeys elaborating attention-getting signals to compensate for communication breakdown. Gestural communication was also supported by gaze alternation between the experimenter's face and the food, especially when the human was visually attending. These findings offer evidence that olive baboons understand the state of the eyes in others' visual attention and use requesting gestures intentionally. They emphasize that Old World monkeys shift to acoustic communication when the recipient is not visually attending.

In contrast to that of human infants and great apes, this acoustic communication is purely gestural, not vocal.

attentional state into account when gesturing is seen only from around 15 months of age [START_REF] Bates | The acquisition of performatives prior to speech[END_REF][START_REF] Franco | Pointing and social awareness: declaring and requesting in the second year[END_REF][START_REF] Leavens | The whole-hand point: The structure and function of pointing from a comparative perspective[END_REF]. The best evidence of an understanding of attention in children is the coordination with others' attention to external targets, also called 'joint attention' [START_REF] Butterworth | Joint visual attention in infancy[END_REF][START_REF] Scaife | The capacity for joint visual attention in the infant[END_REF]. This ability is considered critical for the development of both language and the ability to attribute mental states to others [START_REF] Camaioni | The role of declarative pointing in developing a theory of mind[END_REF][START_REF] Reddy | Mind knowledge in the first year: Understanding attention and intention[END_REF].

Nonhuman primates do communicate with gestures too. A communicative gesture has recently been defined as 'any non-vocal bodily action directed to a recipient that is mechanically ineffective and represents a meaning, beyond itself, that is in part manifested by others of the social group' (Scott & Pika, 2012, p. 158; but see [START_REF] Perlman | A mother gorilla's variable use of touch to guide her infant: Insights into iconicity and the relationship between gesture and action[END_REF] for an alternative view of mechanical effectiveness). Great apes and cercopithecines produce these communicative signals, and so far research has mostly emphasized their use, function and language-like properties [START_REF] Pika | Developments in primate gesture research[END_REF]. Indeed, this gestural system of communication is often considered to be the most likely precursor of human language [START_REF] Corballis | From mouth to hand: Gesture, speech, and the evolution of righthandedness[END_REF][START_REF] Pollick | Ape gestures and language evolution[END_REF][START_REF] Vauclair | Lateralization of communicative signals in nonhuman primates and the hypothesis of the gestural origin of language[END_REF] owing to shared similarities such as the flexible and voluntary use of gestures [START_REF] Liebal | The origins of non-human primates' manual gestures[END_REF][START_REF] Meguerditchian | From gesture to language: Ontogenetic and phylogenetic perspectives on gestural communication and its cerebral lateralization[END_REF], or the brain specialization for gesturing [START_REF] Corballis | From mouth to hand: Gesture, speech, and the evolution of righthandedness[END_REF][START_REF] Hopkins | Evolution of behavioral and brain asymmetries in primates[END_REF]. However, whether nonhuman primates gesture with the genuine intent to modify their recipient's behaviour, attention or knowledge has not yet been entirely elucidated [START_REF] Gómez | Pointing behaviors in apes and human infants: A balanced interpreta tion[END_REF]. Although there is solid evidence that great apes are sensitive to their partner's attentional state when gesturing, little is known about the intentio nal nature of monkeys' gestural signalling and related social understanding [START_REF] Call | The gestural communication of apes and monkeys[END_REF][START_REF] Scott | A call for conformity: Gesture studies in human and nonhuman primates[END_REF]. Specifically, for both great apes and monkeys it is not clear whether the relevant cues to attention of the recipient are the eyes or more general indicators such as head and body orientation [START_REF] Emery | The eyes have it: The neuroethology, function and evolution of social gaze[END_REF]Povinelli & Eddy, 1996;[START_REF] Povinelli | What young chimpanzees know about seeing[END_REF]; but see [START_REF] Kaminski | Body orientation and face orientation: two factors controlling apes' begging behavior from humans[END_REF].

Deictic gestures that refer to external targets are used by nonhuman primates to perform requests [START_REF] Gómez | Requesting gestures in captive monkeys and apes: Conditioned responses or referential behaviours?[END_REF][START_REF] Pika | Gestures of apes and pre-linguistic human children: Similar or different?[END_REF]. To be considered as intentional, they must fulfil several criteria used for prelinguistic children's pointing [START_REF] Bates | The acquisition of performatives prior to speech[END_REF][START_REF] Leavens | Manual deixis in apes and humans[END_REF]:

(1) the gesture is goal-oriented and the signal persists or is completed with other signals until the desired outcome is reached; (2) the gesture is adjusted in accordance to the attentional state of the audience, whose attention can be regained by the use of additional attentiongetting behaviours; and (3) the gesture is supported by visual orienting behaviours alternating between the recipient and the distal object of interest (gaze alternation). Evidence is accumulating that great apes use visual gestures only if the recipient is visually attending (e.g. bonobos, Pan paniscus: [START_REF] Pika | Gestural communication of apes[END_REF]orang-utans, Pongo pygmaeus: Liebal, Pika, & Tomasello, 2006;gorillas, Gorilla gorilla: Genty, Breuer, Hobaiter, & Byrne, 2009;chimpanzees, Pan troglodytes: Hobaiter & Byrne, 2011) and persist with (e.g. [START_REF] Genty | Why do gorillas make sequences of gestures?[END_REF]Liebal, Call, & Tomasello, 2004) or elaborate [START_REF] Cartmill | Orangutans modify their gestural signaling according to their audience's comprehension[END_REF][START_REF] Leavens | Intentionality as measured in the persistence and elaboration of communication by chimpanzees (Pan troglodytes)[END_REF] their gestures until they achieve a certain goal. However, attempts to determine which cues to attention are used by apes and monkeys to adjust their communication have led to mixed results. While it is often not possible to characterize the state of the eyes of individuals in naturalistic settings (e.g. [START_REF] Emery | The eyes have it: The neuroethology, function and evolution of social gaze[END_REF][START_REF] Genty | Gestural communication of the gorilla (Gorilla gorilla): repertoire, intentionality and possible origins[END_REF], experimental studies have further demonstrated that nonhuman primates generally use body orientation (e.g. great apes [START_REF] Hostetter | Differential use of vocal and gestural communication by chimpanzees (Pan troglodytes) in response to the attentional status of a human (Homo sapiens)[END_REF][START_REF] Kaminski | Body orientation and face orientation: two factors controlling apes' begging behavior from humans[END_REF]Povinelli et al., 1996;monkeys: Hattori, Kuroshima, & Fujita, 2010;[START_REF] Meunier | Olive baboons communicate intentionally by pointing[END_REF] or face orientation (e.g. great apes [START_REF] Tempelmann | Focus on the essential: all great apes know when others are being attentive[END_REF]monkeys: Maille, Engelhart, Bourjade, & Blois-Heulin, 2012) as an indicator of a human's attention, although they may sometimes use face orientation only when the human's body is oriented towards them (e.g. chimpanzees: [START_REF] Kaminski | Body orientation and face orientation: two factors controlling apes' begging behavior from humans[END_REF].

However, there is little evidence that nonhuman primates adjust their signals to the open and directed state of the recipient's eyes (but see [START_REF] Hattori | Tufted capuchin monkeys (Cebus apella) show understanding of human attentional states when requesting food held by a human[END_REF][START_REF] Hostetter | Now you see me, now you don't: evidence that chimpanzees understand the role of the eyes in attention[END_REF]. Instead, many studies have failed to demonstrate that subjects tailor their gestural signals as a function of the state of the experimenter's eyes [START_REF] Kaminski | Body orientation and face orientation: two factors controlling apes' begging behavior from humans[END_REF]Povinelli et al., 1996;[START_REF] Theall | Do chimpanzees tailor their gestural signals to fit the attentional states of others?[END_REF]. Although chimpanzees have been reported to move into someone's visual field before starting to gesture rather than using auditory or tactile signals to regain attention [START_REF] Liebal | To move or not to move: How apes adjust to the attentional state of others[END_REF], two studies showed that chimpanzees favoured the modality of communication that best fitted the experimenter's visual attention [START_REF] Leavens | Tactical use of unimodal and bimodal communication by chimpanzees, Pan troglodytes[END_REF], using auditory signals specifically when the experimenter could not see them [START_REF] Hostetter | Now you see me, now you don't: evidence that chimpanzees understand the role of the eyes in attention[END_REF]. While this may constitute the best evidence so far that great apes can finely tune their gestures to the level of attention of the recipient, there is no such evidence for monkeys, to which this stringent paradigm remains to be applied.

We addressed this question in olive baboons using a food-requesting paradigm.

Baboons use two distal threat gestures in their natural communication, i.e. 'slapping ground' and 'rubbing ground' [START_REF] Estes | The behavior guide to African mammals: Including hoofed mammals, carnivores, primates[END_REF][START_REF] Kummer | Social organization of hamadryas baboons: A field study[END_REF], usually performed towards an obviously attending partner [START_REF] Meguerditchian | Baboons communicate with their right hand[END_REF][START_REF] Meguerditchian | From gesture to language: Ontogenetic and phylogenetic perspectives on gestural communication and its cerebral lateralization[END_REF]. They are further known to rely on the use of gaze cues by conspec ifics for soliciting help in conflicts [START_REF] Packer | Reciprocal altruism in Papio anubis[END_REF] and for deceptive communication (Whiten & Byrne, 1988). In experimental settings baboons gestured more towards a human facing them than one oriented away [START_REF] Meunier | Olive baboons communicate intentionally by pointing[END_REF], but no study has disambiguated which cues to attention they relied on.

We manipulated the experimenter's visual attention by varying the orientation of the experimenter's whole body, including head (front/back), and the state of her eyes (open/closed). We then addressed whether baboons (1) adjust their requesting gestures to the visual attention of the experimenter with special emphasis on the state of her eyes, and (2) flexibly tailor visual and auditory signals to elaborate their communication as a function of whether or not the experimenter can see them. If baboons are able to use the state of the eyes as a cue to visual attention, they should produce more requests when the experimenter's eyes are open than when they are closed. If they not only use the state of the eyes as a cue to attention, but also understand the role of open eyes as an attentional state that is specific to their visual behaviour, baboons should tailor their gestural communication to the visual attention of the experimenter, and therefore produce more auditory-based gestures than visual gestures when the experimenter cannot see them compared to when she can. However, if baboons rely on more general cues to attention such as body orientation, they should produce more requests when the experimenter is facing them than when the experime nter is oriented away.

Methods
Subjects

The experiments took place in the Primate Station of the Centre National de la Recherche Scientifique (UPS 846, Rousset, France; Agreement number for conducting experiments on vertebrate animals: D13-087-7). Sixteen baboons, 10 males and six females, ranging in age from 6 to 16 years were tested between August 2011 and March 2012 (see Appendix Table A1). All subjects lived in reproductive social groups comprising one adult male, two to five adult females and their immature offspring (up to 2 years old). Groups had free access to 14 m² outdoor areas connected to 12 m² indoor areas. The enclosures were enriched by wooden platforms and vertical structures of different heights, in both the outdoor and indoor areas. All monkeys were fed four times a day with industrial monkey pellets, seed mixture, fresh vegetables and fruits. Water was available ad libitum and subjects were never deprived of food or water during testing. Subjects were tested in their outdoor area, and only females were partly isolated from dominant individuals (which were kept inside) during testing. The experimental procedure complied with the current French laws and the European directive 86/609/CEE. According to Article 3 (definitions) of the current European directive, this experiment does not qualify as an experimental procedure and therefore does not require institutional ethics approval.

Apparatus

Prior to each test session, we placed inside the cage a concrete block perpendicularly to the mesh, at about 90 cm from the ground so that subjects could gesture at about the height of a person. The mesh was equipped with a 10x60 cm opening through which the baboons could freely pass their arms. During testing, a Plexiglas panel of 80x35 cm with two 10x15 cm holes separated by 25 cm from centre to centre was fixed to the mesh over the opening (see Supplementary Videos S1-S4). This panel was devised to facilitate subsequent recording of baboons' gestures on video footage. Baboons were hence allowed to beg through the holes towards an experimenter standing 1 m in front of the cage. Two video cameras were placed 2 m in front of the cage on both sides of the experimenter at an angle of 45° to the subject's midline. All sessions were videotaped at a rate of 30 frames/s.
Test Procedure All subjects were previously trained to beg through one of the holes of the Plexiglas panel to request the food reward held in the experimenter's hand (see the Appendix for the full procedure). Baboons were then tested for their requesting behaviour in four conditions. In the control condition, the condition Out, the experimenter deposited a piece of banana (4 cm long throughout the study) on the ground, 1 m in front of the cage, and left the test area (see Supplementary Video S1). In the other three conditions, the test conditions, the experimenter stood 1 m in front of the cage holding a piece of banana in one hand always in sight of the subject: ( 1) Eyes open: the experimenter faced and looked at the subject (see Supplementary Video S2); (2) Eyes closed: the experimenter faced the subject but kept her eyes closed (see Supplementary Video S3); (3) Back turned: the experimenter was oriented away from the subject but held the food behind her back (see Supplementary Video S4). Note that the experimenter did not stare at the baboons in the Eyes open condition but rather looked alternately to the eyes and the upper part of the nose so as to avoid possible fear reactions.

Each test session comprised four 30 s experimental trials alternated with eight motivation trials in which the experimenter offered the subject the food as soon as it requested it. At the end of each 30 s experimental trial, the experimenter gave the subject the piece of banana regardless of its behaviour during the trial. Each baboon received four test sessions (one per day), each experimental condition being presented once per session. The order of exposure to the four conditions was counterbalanced between subjects and sessions; four distinct random orders of conditions were presented to four groups of four subjects using a Latin square procedure so as to control for possible habituation to the procedure (see full details in Appendix Table A2).

Data Scoring and Reliability

Two different types of manual gestures were observed during the study and scored on the videos for further analysis. Begging gestures were visual gestures consisting of extending one or two arm(s) with fingers and hand(s) being in line with the arm(s) (Fig. 1). Attention-getting gestures were auditory-based gestures consisting of banging the Plexiglas panel. Visual orienting behaviour that took the form of gaze alternation bouts between the experimenter's face and the food was also recorded. A first main observer coded all occurrences of begging gestures and attention-getting behaviours at 30 frames/s using a VLC media player. A begging gesture started when the wrist crossed the mesh and ended with the partial or complete withdrawal of the arm. A new occurrence was scored whenever the subject brought its arm back, with the elbow being inside the cage, and extended it again. A new occurrence of attention-getting gesture was scored each time the subject banged the Plexiglas panel producing distinct sounds. A second main observer coded all occurrences of visual orienting behaviour frame by frame using Avidemux 2.5 (32-bit). Gaze alternation bouts were recorded based on the conservative number of four consecutive looks alternating between the experimenter's face and the piece of banana. For reliability p urposes, 15% of the video material was randomly assigned to two novel observers who were naïve to the experiment. This resulted in a total of 40 experimental trials, each of 30 s, in 10 different test sessions.

One novel observer coded the begging and attention-getting gestures while the other coded gaze alternation bouts. Reliability was assessed within pairs of main and novel observers and was high for both gestures (Cohen's k = 0.82) and gaze alternation bouts (Cohen's k = 0.76).

Statistical analysis

We used an approach of multimodel inference to determine which cues to attention most affected the responses of the subjects [START_REF] Burnham | Multimodel inference understanding AIC and BIC in model selection[END_REF]. We processed the numbers of begging gestures, attention-getting gestures and gaze alternation bouts produced by the 16 subjects over all test sessions across experimental conditions. Missing data occurred for one subject (Tulie) in the last test session and were considered as such in the models. We followed a three-step procedure: (1) we fitted several models varying the nature of cues to attention as fixed effects (Table 1); (2) we selected the models that best fitted the observed data; and (3) we performed tests of significance on the retained models.

Model fitting

As the frequency distribution of all dependent variables was not normal, we selected a Poisson family with a log link function adapted to count data for fitting generalized linear mixed models with 'condition' as fixed effect (i.e. Main Models). Pseudoreplication caused by repeated observations of the same individual was taken into consideration by adding the individual as a random effect. Second, we examined the possible variation of behavioural responses over time (habituation) by fitting models with the interaction between 'condition' and 'block' of test sessions as fixed effects and 'individual' and 'block' as random effects (i.e. Time Models). The first two test sessions were pooled as block 1 and the last two test sessions as block 2. Third, we tested which postural cues to attention had the strongest effect on dependent variables by performing nested models of the parameter 'condition' (i.e. Nested Models). This procedure allowed us to weight the relative influence of the different cues to attention (e.g. state of the eyes, body orientation) 'nested' in the parameter 'condition', and

Model selection

For each dependent variable we proceeded to select the best fitting models on the basis of the lowest AICc (i.e. Akaike information criterion corrected, Table 1), which applies a secondorder correction adapted to small samples [START_REF] Burnham | Multimodel inference understanding AIC and BIC in model selection[END_REF].

Test of significance

We used chi-square tests of the log-likelihood ratios to test whether the retained models fitted the observed data significantly better than a hypothetical null model in which no fixed effect had been implemented [START_REF] Brown | Applied mixed models in medicine[END_REF]. All tests were two tailed and were performed with R 2.10.1 software (http://cran.r-project.org) with level of significance set at 0.050.

Results
Recognition of Recipient's Visual Attention

The baboons adjusted their begging behaviour to the visual attentional state of the experimenter (Fig. 2). The experimental condition most affected the number of begging gestures (Table 1, Main Model). Baboons gestured more when the experimenter had her eyes open than in the other three conditions, Eyes closed (Wald test: z = -2.28, P = 0.023), Back turned (Wald test: z = -9.30, P < 0.001) and Out (Wald test: z = -11.64, P < 0.001). Body orientation by itself (Table 1, Front/Back Model) and the state of the experimenter's eyes alone (Table 1, Can see/Cannot see Model) were not better predictors of subjects' responses than experimental conditions mixing both cues (Table 1), suggesting that both played a role in the understanding of attentional state by baboons. In a transfer test performed by novel experimenters so as to exclude possible conditioned responses driven by the sight of the main experimenter, baboons showed very similar responses (see the Appendix and Fig. A1).

The baboons displayed significantly more gaze alternation bouts (Table 1, Main Model: Fig. 3) when the experimenter had her eyes open than when her eyes were closed (Wald test: z = -2.13, P = 0.033) or when her back was turned (Wald test: z = -6.41, P < 0.001). Body orientation by itself (Table 1, Front/Back model) and the state of the experimenter's eyes alone (Table 1, Can see/Cannot see model) were not better predictors of subjects' responses than experimental conditions mixing both cues (Table 1) suggesting that both played a role in the understanding of attentional state by baboons.

Attraction of Recipient's Visual Attention

Of the 16 subjects, 14 displayed attention-getting gestures, i.e. auditory-based gestures consisting of banging the apparatus. Banging was more frequent when the experimenter was present than when she was absent (Wald test: z = -4.22, P < 0.001), and when the experimenter could not see the subject than when she could (Wald test: z = 0.029, P = 0.029), during the first two test sessions only (Table 1, Can See/Cannot See Nested Model).

Restricting our analysis to these two sessions in which no habituation to the procedure was likely to occur, we found that baboons performed more banging when the experimenter could not see them than when she could (one-sample permutation test: t = 2.09, P = 0.021; Fig. 4).

Body orientation of the experimenter either alone (Table 1, Front/Back Model) or in combination with the state of her eyes (Table 1, Main Model) were not better predictors of the subjects' banging than being seen or not by the experimenter (Table 1).

Adjustment of Gestures to Recipient's Visual Attention

Considering the first two test sessions, we investigated whether subjects favoured visual requests (food-begging gestures) over auditory-based gestures (banging) when the experimenter could see them compared to when she could not. Gesture types produced by the baboons were affected by the possibility of being seen by the experimenter (Fisher's exact probability test: P < 0.001). Baboons made more visual requests when the experimenter could see them than when she could not. Conversely, they banged more when the experimenter could not see them than when she could (Fig. 5).

Discussion

Three novel findings resulted from this study. First, baboons tailored communicative signals from different modalities as a function of a human's visual attention based on the state of the eyes. Second, gestures were accompanied by gaze alternation between the human's face and the food. Third, monkeys spontaneously elaborated attention-getting signals when there was a communication breakdown. Until now, this latter ability was considered as a feature unique to communication of humans and great apes. Collectively, these findings provide solid evidence that baboons understand the state of the eyes in others' visual attention and use requesting gestures intentionally.

The primate brain contains neurons that are selectively responsive to eye direction, head orientation and body orientation, possibly as part of a hierarchical process for determining the direction of another's attention (see [START_REF] Emery | The eyes have it: The neuroethology, function and evolution of social gaze[END_REF] for a review). In baboons, the eye region is the primary focus of attention during processing of both humans' and conspecifics' faces [START_REF] Martin-Malivel | Do humans and baboons use the same information when categorizing human and baboon faces[END_REF] and is essential for face recognition [START_REF] Kyes | Baboon (Papio hamadryas) visual preferences for regions of the face[END_REF]. Monkeys also distinguish directed from averted gazes from both a conspecific and a human [START_REF] Keating | Visual scan patterns of rhesus monkeys viewing faces[END_REF] and they follow the gaze direction of other individuals [START_REF] Tomasello | Five primate species follow the visual gaze of conspecifics[END_REF], sometimes relying on eye gaze direction only (e.g. in rhesus macaques, Macaca mulatta: [START_REF] Lorincz | Visual cues for attention following in rhesus monkeys[END_REF]in baboons: Vick, Bovet, & Anderson, 2001). Furthermore, monkeys use humans' state of the eyes as a cue to adjust behaviour in competitive situations (e.g. [START_REF] Flombaum | Rhesus monkeys attribute perceptions to others[END_REF][START_REF] Vick | Use of human visual attention cues by olive baboons (Papio anubis) in a competitive task[END_REF]. It is therefore puzzling that sensitivity to others' state of the eyes has hardly ever been evidenced in a communicative context, except in a few studies [START_REF] Hattori | Tufted capuchin monkeys (Cebus apella) show understanding of human attentional states when requesting food held by a human[END_REF][START_REF] Hostetter | Now you see me, now you don't: evidence that chimpanzees understand the role of the eyes in attention[END_REF]). Here, we found that baboons performed virtually no gestural behaviour when the human was absent, but produced most visual gestures when the human was facing them with her eyes open. This suggests that their requesting behaviour was not merely driven by the sight of the food. Instead, it appears to be genuine communication motivated by the presence of the human partner. Moreover, while the information provided by the head and body orientation may be sufficient for interpreting direction of attention in quadrupedal species [START_REF] Emery | The eyes have it: The neuroethology, function and evolution of social gaze[END_REF], this study shows that baboons also use open eyes as a cue when it is available in a communicative context.

In one study very similar to ours, [START_REF] Kaminski | Body orientation and face orientation: two factors controlling apes' begging behavior from humans[END_REF] pointed out a hierarchical use of cues to attention by chimpanzees. The apes responded primarily to body orientation and secondarily to face orientation only when the experimenter's body was oriented towards them. The present study did not allow us to distinguish between the possibly hierarchical contribution of head and body cues. However, the baboons responded more with visual signals to the Eyes closed than the Back turned conditions, and neither body orientation by itself nor the state of the experimenter's eyes was a better predictor of the subjects' begging for food than the Eyes open condition which mixed both cues. This suggests that not only the state of the eyes but also body and head orientation were relevant cues to others' visual attention for olive baboons. However, it remains possible that baboons respond to the state of the eyes only when the human's body is oriented toward s them.

If our findings seem contradictory to certain previous studies in which food was deposited on a platform (e.g. [START_REF] Kaminski | Body orientation and face orientation: two factors controlling apes' begging behavior from humans[END_REF]Povinelli & Eddy, 1996), they do corroborate the findings from other studies in which food was held in the experimenter's hand (e.g. [START_REF] Hattori | Tufted capuchin monkeys (Cebus apella) show understanding of human attentional states when requesting food held by a human[END_REF][START_REF] Hostetter | Now you see me, now you don't: evidence that chimpanzees understand the role of the eyes in attention[END_REF].This slight methodological difference may therefore deserve further discussion. As previously stressed for great apes, body orientation, but not face orientation, may convey information about the experimenter's physical ability to give food rather than information about her ability to perceive a visual signal [START_REF] Kaminski | Body orientation and face orientation: two factors controlling apes' begging behavior from humans[END_REF][START_REF] Tempelmann | Focus on the essential: all great apes know when others are being attentive[END_REF]. We suggest that holding food in the hands may increase and keep constant the disposition of the human to give food regardless of body orientation. Under such circumstances, it is possible that subjects process more subtle cues to attention such as the open versus closed state of the experimenter's eyes when begging for food. In a similar experiment, capuchin monkeys, Cebus apella, successfully adjusted their requesting gestures to the attentional state of a human holding food in one hand, but failed to adjust their requesting gestures to the attentional state of a human when gestures had to be directed at food deposited on a table [START_REF] Hattori | Tufted capuchin monkeys (Cebus apella) show understanding of human attentional states when requesting food held by a human[END_REF]. While both gestures are communicative, pointing towards food on a table appears to be a rather difficult task for monkeys (e.g. [START_REF] Hattori | Tufted capuchin monkeys (Cebus apella) show understanding of human attentional states when requesting food held by a human[END_REF]. More research is hence needed to understand whether pointing towards an external target and begging from an experimenter require differential cognitive means for attracting the partner's attention.

Wild baboons have been reported to use visual orienting behaviour to attract a partner's visual attention. For instance, they solicit help in conflicts by looking alternately to an opponent and a solicited helper [START_REF] Packer | Reciprocal altruism in Papio anubis[END_REF]. Here, we report evidence of gaze alternation supporting gestural communication that was tuned to the visual attention of the recipient. Gaze alternation has long been considered as a cornerstone of the developme nt of intentional communication in human infants [START_REF] Bates | The acquisition of performatives prior to speech[END_REF][START_REF] Camaioni | The role of declarative pointing in developing a theory of mind[END_REF][START_REF] Franco | Pointing and social awareness: declaring and requesting in the second year[END_REF]. In line with a previous study [START_REF] Meunier | Olive baboons communicate intentionally by pointing[END_REF], our baboons displayed visual orienting behaviour that was related not only to the locations of the social partner and the object of interest, but also to the state of the eyes of the experimenter. This suggests that baboons understand others' visual attention as a prerequisite for coordinating their own attention with that of others toward s an external target. This is reminiscent of children developing joint visual attention [START_REF] Butterworth | Joint visual attention in infancy[END_REF].

However, the fact that the state of the eyes is not always used as a cue by nonhuman primates to infer attention direction (e.g. [START_REF] Kaminski | Body orientation and face orientation: two factors controlling apes' begging behavior from humans[END_REF][START_REF] Maille | To beg, or not to beg? That is the question: mangabeys modify their production of requesting gestures in response to human's attentional states[END_REF]Povinelli & Eddy, 1996), or when it is used it does not necessarily supplant head and body cues (e.g. this study), has led some to contrast simple learning of cues to attention with actual understanding of visual attention [START_REF] Gómez | Assessing theory of mind with nonverbal procedures: Problems with training methods and an alternative procedure[END_REF]Povinelli & Eddy, 1996). In this respect, it is likely that, owing to explicit training, baboons discriminated cues to the Eyes open condition as cues that increased the likelihood of getting the reward. However, neither visual orientating nor attention-getting behaviour was explicitly trained in the present study, yet it was flexibly adjusted to the visual attention of the experimenter. Baboons produced more visual gestures and visual orienting behaviours, but fewer attention-getting gestures when the experimenter could see them than when she could not. Whether baboons had implicitly learned these cues to attention during training or through prior experience, which may result in implicit knowledge of others' visual attention, is not possible to disentangle here. Whatever the operating process, it most probably led to an increased understanding of the conditions under which their communicative signals can be effective.

The use of acoustic communication (i.e. including vocalizations, nonvoiced sounds or bimodal communication such as visual/auditory-based gestures) as a means of attracting the attention of an otherwise inattentive partner has been reported in chimpanzees [START_REF] Hopkins | Chimpanzees differentially produce novel vocalizations to capture the attention of a human[END_REF][START_REF] Hostetter | Now you see me, now you don't: evidence that chimpanzees understand the role of the eyes in attention[END_REF][START_REF] Leavens | Tactical use of unimodal and bimodal communication by chimpanzees, Pan troglodytes[END_REF], although not found in all studies [START_REF] Tempelmann | Focus on the essential: all great apes know when others are being attentive[END_REF][START_REF] Theall | Do chimpanzees tailor their gestural signals to fit the attentional states of others?[END_REF]. To our knowledge, our results are the first report of monkeys producing gestures as a means of elaborating communication that failed to elicit the desired outcome. We thus propose that baboons possess flexible communicative means that they can use with the same intent, although the present study did not systematically manipulate the expected outcome of the communicative exchange (but see [START_REF] Leavens | Intentionality as measured in the persistence and elaboration of communication by chimpanzees (Pan troglodytes)[END_REF]. In contrast to a previous study that found baboons banged the cage as a result of frustration [START_REF] Meunier | Olive baboons communicate intentionally by pointing[END_REF], the behavioural pattern observed here does not result from thwarted communicative bids only, as evidenced by differential responses as a function of condition (owing to the fact that all experimental trials lasted for only 30 s and were systematically rewarded). Baboons produced more auditorybased gestures but fewer visual gestures when the experimenter could not see them, suggesting they might have used auditory communication as a substitute for visual communication to capture the attention of the experimenter.

This study brings critical insight to the interplay between intentional communication and social understanding through the primate lineage. Human infants [START_REF] Liszkowski | Infants' visual and auditory communication when a partner is or is not visually attending[END_REF] and chimpanzees [START_REF] Hopkins | Chimpanzees differentially produce novel vocalizations to capture the attention of a human[END_REF][START_REF] Hostetter | Now you see me, now you don't: evidence that chimpanzees understand the role of the eyes in attention[END_REF][START_REF] Leavens | Tactical use of unimodal and bimodal communication by chimpanzees, Pan troglodytes[END_REF] are known to use vocalizations as a means of recruiting their partner's attention. Here, we emphasize that Old World monkeys are also capable of shifting to acoustic communication when the recipient is not visually attending. In contrast to human infants and chimpanzees, this acoustic communication is purely gestural, not vocal. This finding questions the evolutionary emergence of vocal inte ntional communication in the primate lineage. Intentional acoustic communication might have been 'scaffolded' onto the special intent to attract others' attention (see [START_REF] Falk | Prelinguistic evolution in early hominins: Whence motherese?[END_REF], initially through gestural communication in Old World monkeys and progressively through both gestural and vocal communication in great apes, before turning out predominantly vocal in early humans. Future research may address this topical question of whether acoustic intentional communication might have appeared in evolution concomitantly to the understanding of another's attention.

Baboons showed similar behavioural trends when they were tested with novel experimenters over two test sessions (see Appendix Fig. A1). They adjusted their begging behaviour to the visual attentional state of the experimenter (Main Model: AIC = 156.4; chisquare tests for the log-likelihood ratios: Main Model -Null Model: P < 0.001). Baboons produced significantly more gestures in the Eyes open than in the Back turned (Wald test: z = -4.20, P < 0.001) and Out (Wald test: z = -6.47, P < 0.001) conditions, but not in the Eyes closed condition (Wald test: z = -1.14, P = 0. 253).

Body orientation by itself (Front/Back Model: AIC = 155.7) may consequently be a better predictor of subjects' responses than experimental conditions mixing both cues, although the two models did not differ significantly (chi-square tests for the log-likelihood ratios: Main Model -Front/Back Model: P = 0.251). However, the state of the experimenter's eyes was not an accurate predictor of the subjects' responses (Can see/Cannot see Model: AIC = 164.5; chi-square tests for the log-likelihood ratios: Main Model -Can see/Cannot see Model: P < 0.010).

These findings support the proposal that baboons' gestural communication is driven not by the sight of the food reward alone nor by the sight of the main experimenter who could have been associated with the delivery of the reward. We propose that baboons' begging gestures should be interpreted as genuine communicative attempts motivated by the presence of a partner whose cooperation is required to get the reward. Further testing is, however, needed to find out whether baboons processed well-known and novel faces differently and whether such differential treatment may explain why they did not rely on the state of the novel experimenter's eyes to adjust their communicative behaviour in this experiment.

  post hoc comparisons. To test whether the effect of the state of the eyes could be stronger than the effect of head and body orientation, we pooled the Eyes closed and Back turned conditions into condition Cannot see to compare with condition Can see (i.e. Eyes open). To test whether the effect of head and body orientation could be stronger than the effect of the state of the eyes, we pooled the Eyes open and Eyes closed conditions into condition Front to compare with condition Back (i.e. Back turned).

Figure captions Figure 1 .

 captions1 Figure captions

Figure 2 .

 2 Figure 2. Mean rate/min ± SEM of begging gestures for each experimental condition (N = 16

Figure 3 .

 3 Figure 3. Mean rate/min ± SEM of gaze alternation bouts for each experimental condition (N

Figure 4 .

 4 Figure 4. Mean rate/min ± SEM of attention-getting gestures depending on experimenter's

Figure 5 .

 5 Figure 5. Percentages of visual and auditory-based gestures depending on experimenter's

Figure A1 .

 A1 Figure A1. Mean rate/min ± SEM of begging gestures towards novel experimenters for each

Table 1 .

 1 Summary of the models fitted for each dependent variable

	628
	Model name	Fixed effect	Random effect	AICc ΔAICc Significance
	Dependent variable: number of begging gestures
	Null Model	None	Individual	812.10 510.11	***
	Main Model	Condition	Individual	301.99 0.00	/
	Time Model	Block, condition, block:condition Individual:block 308.70 6.71	NS
	Front/back Nested Model	Condition	Individual	305.19 3.20	*
	Can see/cannot see Nested Model Condition	Individual	356.19 54.20	***
	Dependent variable: number of gaze alternation bouts
	Null Model	None	Individual	281.70 45.21	***
	Main Model	Condition	Individual	236.49 0.00	/

Acknowledgme nts

This research was supported by French National Research Agency (ANR) grants; references ANR-08-BLAN-0011_01 and ANR-12-PDOC-0014. We thank B. Rosay and C. Canteloup for coding the video materials, V. Dufour and B. Dauvier for helpful discussions, and several anonymous referees for their valuable comments on the manuscript.

Suppleme ntary Material

Supplementary material is available, in the online version of this article, at doi

Appendix

Training of the subjects

All subjects took part in training trials. The procedure comprised three steps in which the experimenter stood in front of the cage of the focal subject holding a raisin in her open palm in front of the subject, while progressively increasing the distance to the cage. In the first step, the raisin was kept within the reach of the subject who extended one arm to grasp it in the experimenter's hand. In the second step, the distance increased up to the limit of being out of reach and the experimenter anticipated the attempt of the subject to reach the food in giving the subject the raisin each time the subject initiated an arm extension out of the cage. In the third step, the experimenter stood out of the subject's arm reach and went on giving the subject the raisin immediately after each initiation of arm extension. For the arm extensions being considered as begging gestures, we set postural criteria ensuring that manual actions were no longer mechanically effective: (1) the subject had not to try to grasp the raisin by rotating its shoulder so as to go further through the wire mesh; (2) the subject's fingers had to be extended in line with the hand and the arm. Subjects had to reach the criterion of 80% of valid gestures across three consecutive 10-trials sessions administered once a day.

Replication with novel experimenters

Two extra test sessions were performed with novel experimenters so as to exclude possible conditioned responses driven by the sight of the main experimenter. Baboons were presented once to a novel woman and once to a novel man in a 2 by 2 design relying on the experimenter's novelty (main experimenter versus novel experimenters first) and experimenter's sex (novel woman versus novel man first). Experimental procedure and data analysis were similar to those for the main experiment.

Interactions between two effects are represented by colons. Bold characters indicate the retained model for each dependent variable. AICc: Akaike information criterion with second-order correction; ΔAICc: difference between the AIC of model i and the AIC of the retained model.

Chi-square tests for the log-likelihood ratios: *P < 0.05, ***P < 0.001.