Biology subjects

Thier, P.

Publications and source records attributed to Thier, P..

At least 19 recordsLinked to original sources

bioRxiv2026

Rule-dependent transformation of observed into self-actions in macaque ventral premotor cortex

The ventral premotor cortex (PMv) has been implicated in both action selection and the perception of observed actions, but it remains unclear whether PMv activity during action observation reflects the observed action itself, or variables related to the observers own action when the observed action becomes behaviourally relevant. Here, we recorded neural activity in macaque PMv during a task that dissociated observed action, rule context, and the subsequently selected self-action. Population activity during observation was already biased toward the upcoming self-action and became increasingly aligned with it. At the level of single neurons, subsets showed modulation by rule and required self-action beyond the observed action. Task-related variables coexisted during observation, rather than being organized into distinct sequential stages. These findings indicate that PMv activity during action observation is not solely determined by the observed action, but instead reflects variables related to the selection of the agents own action, consistent with a transformation from observed action to self-action.

neuroscience↗

bioRxiv2026

Behavioral evidence challenges species-specific ocular morphology as a primary constraint on human gaze-following

Previous research suggests that humans are extremely sensitive to object-directed eye gaze, which effectively guides their attention toward objects of shared interest. This contrasts with non-human primates, who typically require much more salient eye-gaze cues to achieve comparable attentional orienting. However, it remains unclear whether cross-species differences in ocular morphology account for this performance gap. To address this question, we examined humans covert shifts of spatial attention in response to eye-gaze cues provided by either realistic human or rhesus monkey head avatars. Target detection was reliably enhanced on gaze-congruent compared to gaze-incongruent trials, with comparable gaze-cueing effects for both avatar types, despite the fact that monkey eyes lack many of the conspicuous features characteristic of human eyes. Hence, eye morphology alone does not substantially modulate gaze-driven attentional orienting in humans, whereas humans reliable use of monkey eye-gaze cues highlights a clear species difference in perceptual sensitivity to eye gaze signals. Significance StatementEye-gaze-mediated attentional orienting is a conserved ability across primates, yet sensitivity to subtle eye-gaze cues may differ between species. Here, we provide empirical evidence that humans exhibit a quantitatively greater capacity than non-human primates to follow subtle eye-gaze cues. Importantly, we showed that this difference cannot be attributed to species-specific ocular morphology as human participants showed robust and comparable reflexive attentional orienting to both human and rhesus monkey eye-gaze cues. This is striking given the pronounced differences in ocular morphology and coloration/contrast between the two species. These findings suggest that cross-species diversity in extracting spatial information from eye-gaze cues likely reflects differences in perceptual sensitivity rather than bottom-up constraints imposed by species-specific ocular morphology.

neuroscience↗

bioRxiv2026

Behavioral characterization of dynamic facial expression perception in rhesus monkeys (Macaca mulatta) using naturalistic and synthetic stimuli

Facial displays are an important channel of communication for primates, yet it remains unclear based on which criteria monkeys evaluate facial expressions. We trained two rhesus macaques to categorize videos of four facial expression types (neutral, lip-smacking, silent bared-teeth (SBT) and open-mouth threat displays) and then tested generalization to novel individuals and avatars while measuring pupillary responses. The animals were able to categorize facial expressions and generalize their classifications to novel, untrained videos, although performance was not perfect. Misclassifications were not driven solely by visual similarity between expression categories, as demonstrated by a novel automated approach integrating deep learning-based motion tracking with the Macaque Facial Action Coding System to enable objective quantification of facial action units. Instead, open-mouth threats were readily categorized correctly and distinguished from lip-smacking, while also eliciting the strongest arousal responses. By contrast, categorization of SBT displays varied substantially across stimulus identities, influenced by body weight, gaze direction, and coordinated movements of eyebrows and ears. Morphed avatar expressions were categorized according to expression component intensity, demonstrating graded perception. Avatar manipulations revealed that categorization was robust against lack of coherent motion, lack of realism beyond a basic level of texture, and to some extent transcended species form boundaries to a human face. However, human facial expressions elicited no systematic categorizations or differential arousal. Our findings indicate that conspecific facial expressions were perceived as functionally meaningful, context-dependent social signals shaped by all components of facial behavior and signaler characteristics, rather than fixed, mouth movement-defined categories.

animal behavior and cognition↗

bioRxiv2025

Rhesus monkeys use both eye and head gaze to reallocate covert spatial attention facilitating visual perception

Non-verbal cues, particularly eye gaze, significantly shape human social interactions. Although non-human primates reliably follow head gaze, their capacity to use eye gaze alone for inferring the others focus of attention remains debated. We investigated this question using a realistic rhesus monkey head avatar that directed its gaze toward one of two LEDs (left or right), employing either eye movements alone or combined eye and head movements. After a randomly chosen interval (ranging 50-400 ms) from gaze presentation, one LED transiently increased its luminance to near-threshold levels. Rhesus monkeys were trained to detect and report this luminance change via a saccade to the corresponding LED, independent of the avatars gaze direction, to receive rewards. Our results showed that head-gaze cues robustly directed covert attention toward gaze-congruent targets with short delays, indicative of reflex-like, stimulus-driven orienting. In contrast, eye gaze alone, at comparable amplitudes, did not affect attention shifts. However, increasing the avatars size and eye-gaze amplitude, simulating a close-range interaction, made eye gaze cues effective in guiding attention. These findings demonstrate that rhesus monkeys possess the capacity to use eye-gaze cues to determine conspecifics attentional targets, and validate and underscore the utility of 3D animal models as powerful tools for generating realistic yet precisely controlled stimuli. Our study supports the idea that eye-gaze following is not uniquely human but is an evolutionarily ancient ability, likely shared across Old World monkeys and apes which diverged more than 30 million years ago.

animal behavior and cognition↗

bioRxiv2025

Monkeys integrate facial expressions and direct gaze to modulate gaze-following behavior

For humans, being looked at directly can boost our readiness to follow anothers gaze, but can monkeys invite an observer to engage with them in the same way? We trained three rhesus macaques on a head gaze-following task in which the portrait of a demonstrator monkey would face the viewer before turning to look at a distinct spatial target. The demonstrator could look at the viewer with his eyes opened or closed and display different facial expressions. Unlike in humans, we found that direct gaze alone, devoid of an accompanying specific expression failed to influence the latency of the subsequent gaze-following response. However, when combined with threat, direct gaze significantly accelerated gaze-following. In a second experiment, we show that once turned away the expression associated with prior direct gaze no longer mattered; instead a submissive facial expression accompanying the gaze shift delayed gaze-following. Direct comparison of both experiments reveals that expressions accompanying direct gaze trigger earlier gaze-following responses than the same expressions joining gaze aversion. These results document the pronounced behavioral importance of the valuation of expressions signaling danger, arguably creating heightened alertness in monkey observers, thereby priming their gaze-following to allow for immediate conflict resolution.

animal behavior and cognition↗

bioRxiv2024

Minimally invasive electrocorticography (ECoG) recording in common marmosets

Electrocorticography (ECoG) enables high spatio-temporal resolution recordings of brain activity with excellent signal quality, making it essential for pre-surgical mapping in epilepsy patients and increasingly useful in neuroscience research, including the development of brain-machine interfaces. The relatively minimal cortical convolution in non-human primate (NHP) brains facilitates ECoG recordings, in particular in the common marmoset (Callithrix jacchus), whose lissencephalic (unfolded) brain surface provides broad cortical access. One of the key advantages of ECoG recordings is the ability to study interactions between distant brain regions. The standard approach is the usage of large electrode arrays, which, however, require extended trepanations and a trade-off between size and electrode spacing. This study introduces an alternative ECoG approach for examining interactions between multiple distant cortical areas in marmosets, combining the advantages of circumscribed trepanations with high-density electrode arrays at specific sites of interest. Two adult marmosets underwent ECoG implantation across frontal, temporal, and parietal regions of interest, assessed by circumscribed trepanations to position individual high density electrode arrays. Postoperative monitoring showed rapid recovery and no long-term complications. The animals remained healthy, and stable neural recordings were successfully obtained during behavioral tasks, highlighting the methods effectiveness for long-term cortical monitoring.

neuroscience↗

bioRxiv2024

Implied motion guides neither gaze following nor intention attribution

The "gaze beam hypothesis (GBH)" of gaze following posits that the others eyes emit imaginary beams of moving energy travelling to the others object of attention (1), drawing the observers attention to the same object. This idea was initially supported by behavioral experiments showing a motion aftereffect (MAE), indicated by longer reaction times in detecting motion direction after viewing a cartoon face looking at an object in the same direction (2). However, this effect could also be expected if the observer used gaze direction to assume an intentional link between the looker and the object, envisioning directed actions toward the latter (3). To critically compare the two hypotheses, we tested whether an MAE could be induced by having human subjects detect motion direction after viewing various cue images, designed to differentiate the explanatory power of the two. Cues either suggested a connection between an agent and an object through the agents gaze or an object-oriented intention by the presence of equipment in the agents hand, without the agent directly looking at the object. Using Bayesian statistics, our findings provided strong evidence against both hypotheses at the population level, as reaction time modulations did not align with the MAE, leading us to reject motion adaptation as the underlying mechanism for gaze following but also intention attribution. As on an individual level we observed highly diverse effects, with some compatible with one or the other hypothesis, we assume that individual subjects may resort to different perceptual strategies based on different scene interpretations.

neuroscience↗

bioRxiv2024

Gaze and Arrows: does the Gaze Following Patch in the posterior temporal cortex differentiate social and symbolic spatial cues?

The Gaze Following Patch (GFP) is located in the posterior temporal cortex and has been described as a cortical module dedicated to processing other peoples gaze direction in a domain-specific manner. Thus, it appears to be the neural correlate of Baron-Cohens Eye-Direction Detector (EDD) which is one of the core modules in his Mindreading System - a neurocognitive model for the Theory of Mind concept. Inspired by Jerry Fodors ideas on the modularity of the mind, Baron-Cohen proposed that, among other things, the individual modules are domain-specific. In the case of the EDD this means that it exclusively processes eye-like stimuli to extract gaze direction and that other stimuli, that may carry directional information as well, are processed elsewhere. If the GFP is indeed EDDs neural correlate it must meet this expectation. To test this, we compared the GFPs BOLD activity during gaze-direction following with the activity during arrow-direction following. Contrary to the expectation based on the assumption of domain specificity we did not find a differentiation between gaze and arrow-direction following. In fact, we were not able to reproduce the GFP as presented in previous studies. A possible explanation is that in the present study - unlike previous work -, the gaze stimuli did not contain an obvious change of direction that represented a visual motion. Hence, the critical stimulus component responsible for the identification of the GFP in previous experiments might have been visual motion.

neuroscience↗

bioRxiv2023

Is there a brain area dedicated to socially guided spatial attention?

Where we look lets others know what we are interested in and allows them to join our focus of attention. In several studies our group investigated the neuronal basis of gaze following behavior in primates and described a gaze following patch (GFP) as the underlying functional unit. This makes the GFP a promising neurobiological correlate of Simon Baron-Cohens eye direction detector (EDD), an integral part of his mindreading system. With the latter, Baron-Cohen proposed a set of domain-specific neurocognitive modules implementing a Theory of Mind - the attribution of mental states to others. The tenet of domain-specificity requires that the EDD processes exclusively eye-like stimuli. In this preregistered fMRI study, we aimed to critically test if the GFP fulfills this criterion. Contrary to previous studies, we found that this was not the case and that it functionally and anatomically overlapped with the MT+ complex, putting the plausibility of Baron-Cohens mindreading model as a set of domain-specific modules into question. TeaserThe presented results demonstrate that a brain area previously thought to be a domain-specific cortical module specialized in processing other peoples focus of visual attention is, in fact, domain-general. More specifically, we show that it is indistinguishable from cortical areas involved in the processing of visual motion. These findings add to a growing body of evidence arguing against domain-specific accounts in social neuroscience which posit that the functional building blocks of social behaviors are facilitated by dedicated neurocognitive modules.

neuroscience↗

bioRxiv2022

Nonhuman primates exploit the prior assumption that the visual world is vertical

When human subjects tilt their heads in dark surroundings, the noisiness of vestibular information impedes precise reports on objects orientation with respect to earths vertical axis. This difficulty is mitigated if a vertical visual background is available. Tilted visual backgrounds induce feelings of head tilt in subjects who are in fact upright. This is often explained as a result of the brain resorting to the prior assumption that natural visual backgrounds are vertical. Here, we tested whether monkeys show comparable perceptual mechanisms. To this end we trained two monkeys to align a visual arrow to a vertical reference line that had variable luminance across trials, while including a large, clearly visible background square whose orientation changed from trial to trial. On around 20% of all trials, the vertical reference line was left out to measure the subjective visual vertical (SVV). When the frame was upright, the monkeys SVV was aligned with the gravitational vertical. In accordance with the perceptual reports of humans, however, when the frame was tilted, it induced an illusion of head tilt as indicated by a bias in SVV towards the frame orientation. Thus all primates exploit the prior assumption that the visual world is vertical.

neuroscience↗

bioRxiv2022

Poor matching of action codes challenges 'mirroring' in macaque F5 mirror neurons

The discharge of F5 mirror neurons of a monkey observing another individual performing an action has been interpreted as a motor representation, stimulated by sensory input, ultimately serving action understanding. This hypothesis requires mirror neurons to exhibit an action tuning that is matched between action observation and execution. However, the evidence for this is insufficient, mainly because previous work has suffered from a limited exploration of action space tuning and an object vs. action vision confound. To overcome these limitations, we conducted an experiment in which identical objects had to be manipulated in three different ways in order to serve distinct action goals. We show that the population of F5 mirror neurons can dissociate the three actions almost perfectly, during both observation and execution. However, the population code for action execution was only poorly matched to the code for observation. Just a few neurons exhibited only matched codes, yet, always confined to short, varying segments of the overall action. These findings challenge the hypothesis that an observer understands the actions of others by the activation of motor representations of the observed actions. Rather they support the alternative notion that F5 might use information on observed actions to select behavioral responses.

neuroscience↗

bioRxiv2022

Dissociable Roles of FEF Neurons in Initiating Saccades

The frontal eye field (FEF) plays a key role in initiating saccades. To explain how single neurons in this region may enable the initiation of saccades, Jeffery Schall proposed the "variable rate model" (Hanes and Schall, 1996) which assumes that the discharge of a neuron must reach a fixed activation threshold for a saccade to be evoked. However, the validity of this model has been questioned as results from later work testing saccades in different behavioral contexts seemed to require the assumption of variable thresholds. Moreover, even when sticking to the original behavioral context, not all types of frontal eye field neurons seemed to behave according to a variable rate model. In an attempt to reconsider the viability of the variable rate model avoiding limitations of previous research, we studied single neurons recorded from the FEF of two rhesus monkeys while they performed a memory-guided saccade task. We evaluated the degree to which each type of FEF neurons complied with the variable rate model by quantifying how precisely their discharge predicted an imminent saccade based on their immediate presaccadic activity. In addition, we asked whether there might be a hierarchical relation between visuomotor and motor neurons with only the latter responsible for the release of saccades and therefore satisfying the assumption of the variable rate model. We show that decoders trained on single motor neurons presaccadic activity performed better than decoders trained on visuomotor neurons in predicting an imminent saccade, in line with the assumption of a top position in a putative processing hierarchy. While this position might have suggested significant resilience to perturbation of the visual input to the FEF, we found quite the opposite that motor neurons but not visuomotor neurons were susceptible to perturbation. As a matter of fact, they lost the ability to predict the initiation of a saccade based on their immediate pressacadic discharge rate. Our results support the notion that motor neurons and visuomotor neurons have dissociable functions in information processing for saccades with motor neurons more closely related to saccade initiation, in line with the tenets of the variable rate model. Yet, they also clearly indicate that even these supposedly top layer neurons exhibit an unexpected degree of dependence on afferent input questions the viability of the assumption of a simple processing hierarchy.

neuroscience↗

bioRxiv2022

The scotopic band: primate detailed scotopic vision and perceptual uncertainty

Primates perceive detailed images in photopic (daylight) vision via a preferred-processing, high-acuity pathway, made up of a small photopic center (the fovea), and dedicated eye movements, whose function is to shift images of target objects to the fovea and keep them while foveal cones sense the image. Thus, the preferred processing pathway processes images serially, not in parallel. No such pathway is known for scotopic (night-light) vision or mesopic (twilight) vision - though details are informative in dim light, even vital; it remains unclear whether and how scotopic vision of details is accomplished. Here we show that primates do have a scotopic preferred-processing pathway. It consists of a scotopic center, located on a scotopic band, and eye movements that shift target images not to the fovea but directly to the scotopic center. In contrast to the stationary fovea, the scotopic center can relocate over the scotopic band. The scotopic center relocates to match changing visual conditions and contextual needs. Ambient light intensity is mapped monotonically onto the scotopic band, with mesopic vision at one end. The dimmer the light is, the more dorsal the scotopic center relocates on the scotopic band. Importantly, the scotopic center relocates (or set) not only in passive response to ambient light but also actively, driven by internal factors. In near-threshold conditions the scotopic center relocates to a more dorsal band location than in salient conditions. That the same relocation of the scotopic center can be instigated both by dimming the ambient light and by reducing the detectability of targets indicates that, at the core, the longitudinal axis of the scotopic band encodes the level of perceptual uncertainty.

neuroscience↗

bioRxiv2022

Multidimensional cerebellar computations for flexible kinematic control of movements

Both the environment and our body keep changing dynamically. Hence, ensuring movement precision requires adaptation to multiple demands occurring simultaneously. Here we show that the cerebellum performs the necessary multi-dimensional computations for the flexible control of different movement parameters depending on the prevailing context. This conclusion is based on the identification of a manifold-like activity in both mossy fibers (MF, network input) and Purkinje cells (PC, output), recorded from monkeys performing a saccade task. Unlike MFs, the properties of PC manifolds developed selective representations of individual movement parameters. Error feedback-driven climbing fiber input modulated the PC manifolds to predict specific, error type-dependent changes in subsequent actions. Furthermore, a feed-forward network model that simulated MF-to-PC transformations revealed that amplification and restructuring of the lesser variability in the MF activity is a pivotal circuit mechanism. Therefore, flexible control of movement by the cerebellum crucially depends on its capacity for multi-dimensional computations.

neuroscience↗

bioRxiv2021

A prosocial character of head-gaze aversion in common marmosets

Gaze aversion is a behavior adopted by several mammalian and non-mammalian species in response to eye contact and usually interpreted as reaction to perceived threat. Unlike many other primates, common marmosets (Callithrix jacchus) are thought to have high tolerance for direct gaze, barely exhibiting gaze avoidance towards conspecifics and humans. Here we show that this does not hold for marmosets interacting with a familiar experimenter who suddenly establishes eye contact in a playful interaction ("peek-a-boo"). In video footage synchronously recorded from the two agents, we found that our monkeys consistently alternated between eye contact and head-gaze aversion. The striking similarity with the gaze aversions dynamics exhibited by human infants interacting with their caregivers suggests a shared behavioral strategy to disengage temporarily from overwhelming social stimulation, in order to prepare for a new round of rewarding, affiliative face-to-face interaction. The potential of our finding for a marmoset model of autism is discussed.

animal behavior and cognition↗

bioRxiv2020

Differential effect of value and salience on saccadic reaction times

Express saccades, a mode of visually guided saccades, distinguished from regular saccades by extremely short reaction times, are triggered by inserting a temporal gap between the fixation dot and the saccade target. It is usually assumed that they are produced by a specific pathway in which the superior colliculus plays a key role. Whether and how this pathway deals with information on the subjective value of a saccade target is unknown. We, therefore, studied the influence of varying reward expectancies and compared it with the impact of the presence and absence of a temporal gap between the disappearance of the fixation dot and the appearance of the target on the visually guided saccades of two rhesus macaques (Macaca mulatta). We observed that the introduction of a gap shifted the entire saccadic reaction time distribution to shorter latencies while increasing the probability of express saccades. On the other hand, promoting the monkeys reward expectancy shortened reaction times and increased peak velocities of regular saccades, and increased the probability of express saccades. Importantly, we observed that the reaction time and peak velocity of express saccades were not sensitive to the value of the saccade target, suggesting that the express pathway does not have access to information on value. We propose a new model on express saccades that treats the salience of visual objects in the scene differently from the subjective value assigned to them.

neuroscience↗

bioRxiv2019

Reflexive gaze following in common marmoset monkeys

The ability to extract the direction of the others gaze allows us to shift our attention to an object of interest to the other and to establish joint attention. By mapping ones own expectations, desires and intentions on the object of joint attention, humans develop a Theory of (the others) Mind (TOM), a functional sequence possibly disrupted in autism. Although old world monkeys probably do not possess a TOM, they follow the others gaze and they establish joint attention. Gaze following of both humans and old world monkeys fulfills Fodors criteria of a domain specific function and is orchestrated by very similar cortical architectures, strongly suggesting homology. Also new world monkeys, a primate suborder that split from the old world monkey line about 35 million years ago, have complex social structures. One member of this group, the common marmoset (Callithrix jacchus), has received increasing interest as a potential model in studies of normal and disturbed human social cognition. Marmosets are known to follow human head-gaze. However, the question is if they use gaze following to establish joint attention with conspecifics. Here we show that this is indeed the case. In a free choice task, head-restrained marmosets prefer objects gazed at by a conspecific and, moreover, they exhibit considerably shorter choice reaction times for the same objects. These findings support the assumption of an evolutionary old domain specific faculty shared within the primate order and they underline the potential value of marmosets in studies of normal and disturbed joint attention.\n\nHIGHLIGHTSO_LICommon marmosets follow the head gaze of conspecifics in order to establish joint attention.\nC_LIO_LIBrief exposures to head gaze are sufficient to reallocate an animals attention.\nC_LIO_LIThe tendency to follow the others gaze competes with the attractional binding of the conspecifics face\nC_LI

animal behavior and cognition↗

bioRxiv2019

Decoding of the other’s focus of attention by a temporal cortex module

Faces attract the observers attention towards objects and locations of interest for the other, thereby allowing the two agents to establish joint attention. Previous work has delineated a network of cortical \"patches\" in the macaque cortex, processing faces, eventually also extracting information on the others gaze direction. Yet, the neural mechanism that links information on gaze direction, guiding the observers attention to the relevant object has remained elusive. Here we present electrophysiological evidence for the existence of a distinct \"gaze-following patch (GFP)\" with neurons that establish this linkage in a highly flexible manner. The others gaze and the object, singled out by the gaze, are linked only if this linkage is pertinent within the prevailing social context. The properties of these neurons establish the GFP as a key switch in controlling social interactions based on the others gaze.\n\nOne Sentence SummaryNeurons in a \"gaze-following patch\" in the posterior temporal cortex orchestrate the flexible linkage between the others gaze and objects of interest to both, the other and the observer.

neuroscience↗