ABSTRACT Understanding how α-synuclein misfolding and spreading affect brain systems is central to synucleinopathy research, yet evidence from primate models remains limited. Here, we combined histology, awake structural MRI, awake resting-state fMRI, regional homogeneity, actimetry and touchscreen behavioral testing to longitudinally track the consequences of striatal α-synuclein seeding in the common marmoset. Phosphorylated α-synuclein inclusions were detected from 2 months post-injection and spread progressively to distributed cortical and subcortical regions bilaterally, accompanied by regionally specific structural atrophy. Functional imaging revealed widespread disruption of large-scale networks, particularly within fronto-limbic circuits, alongside reductions in local functional coherence. Despite the absence of overt motor or sleep deficits, animals exhibited a selective impairment in cognitive flexibility, with preserved learning and task engagement. These findings indicate that striatal α-synuclein seeding induces a multi-scale reorganization of brain networks preferentially affecting circuits supporting cognitive flexibility and limbic processing, establishing a primate platform for studying early stages of synucleinopathy.

The lateral frontoparietal (FP) network, also referred to as the central executive network, is critical for goal-directed behavior in primates. Resting-state functional MRI (rs-fMRI) studies have revealed functional homologies between humans and macaques; however, methodological disparities, particularly the use of anesthesia in non-human primates, raise questions about the validity of interspecies comparisons. Anesthetic agents such as isoflurane have been shown to alter functional connectivity (FC), but whether they differentially affect lateral FP subnetworks remains unclear. Here, we investigated the impact of isoflurane on FC patterns in dorsal and ventral FP subnetworks by acquiring rs-fMRI data in awake and anesthetized states from the same macaques. Using anatomically precise seeds and regions of interest defined by sulcal landmarks and known short- and long-range FP connections, we demonstrate that anesthesia reduces FC within short-range lateral FP subnetworks, with preserved connectivity in long-range projections. Notably, we also observed increased FC between lateral frontal and posterior medial cortical regions under isoflurane, suggesting a shift in network dynamics. These findings underscore the non-uniform effects of anesthesia on FP circuitry and offer insights into network reconfigurations associated with unconscious states.

Naturalistic stimuli, such as movies, offer a powerful tool for probing functional brain organization across species. Using movie-driven functional magnetic resonance imaging (md-fMRI), we recorded brain activity in humans and awake marmosets exposed to the same dynamic audiovisual stimulus. We applied tensor independent component analysis (tICA) to identify functional networks in each species, hierarchically cluster them, and examine their within- and between-species temporal correlations to assess functional homologies. We found strong interspecies correspondence in core sensory networks, particularly those involved in visual and auditory processing, suggesting conserved mechanisms for sensory integration. In contrast, networks associated with higher-order cognition, including prefrontal and temporoparietal areas, were observed primarily in humans, highlighting species-specific specializations. These findings demonstrate the value of naturalistic paradigms and data-driven approaches in revealing both shared and divergent brain architectures. By openly sharing our data and pipelines, we aim to advance the marmoset as a model for investigating the evolutionary foundations of brain function. Movie-driven fMRI revealed shared sensory but distinct higher-order brain networks in humans and marmosets, highlighting both conserved and species-specific functional architectures.

The use of the common marmoset (Callithrix jacchus) for neuroscientific inquiry has grown precipitously over the past two decades. Despite windfalls of grant support from funding initiatives in North America, Europe, and Asia to model human brain diseases in the marmoset, marmoset-specific apparatus are of sparse availability from commercial vendors and thus are often developed and reside within individual laboratories. Through our collective research efforts, we have designed and vetted myriad designs for awake or anesthetized magnetic resonance imaging (MRI), positron emission tomography (PET), computed tomography (CT), as well as focused ultrasound (FUS), electrophysiology, optical imaging, surgery, and behavior in marmosets across the age-span. This resource makes these designs openly available, reducing the burden of de novo development across the marmoset field. The computer-aided-design (CAD) files are publicly available through the Marmoset Brain Connectome (MBC) resource (https://www.marmosetbrainconnectome.org/apparatus/) and include dozens of downloadable CAD assemblies, software and online calculators for marmoset neuroscience. In addition, we make available a variety of vetted touchscreen and task-based fMRI code and stimuli. Here, we highlight the online interface and the development and validation of a few yet unpublished resources: Software to automatically extract the head morphology of a marmoset from a CT and produce a 3D printable helmet for awake neuroimaging, and the design and validation of 8-channel and 14-channel receive arrays for imaging deep structures during anatomical and functional MRI.

Auditory deficits are a well-known symptom in neuropsychiatric disorders such as schizophrenia. The non-competitive N-methyl-D-aspartate receptor antagonist ketamine has been used to model sensory and cognitive deficits in nonhuman primates, but its whole-brain effects remain largely unknown. Here we employed ultra-high-field fMRI at 9.4T in awake male and female marmoset monkeys ( Callithrix jacchus ) to compare brain activations to conspecific vocalizations, scrambled vocalizations, and non-vocal sounds following the administration of a subanesthetic dose of ketamine. Our findings reveal a broad suppression of activations across auditory regions following ketamine compared to saline. Additionally, we observed differential effects depending on the type of sound, with notable changes in the mediodorsal thalamus and anterior cingulate cortex, particularly during the processing of vocalizations. These findings suggest a potential overlap between the effects of ketamine and neural disruptions observed in schizophrenia, particularly affecting vocalization processing. Significant Statement This study explores the effects of ketamine, a compound known for its psychotomimetic effects that mimic those of neuropsychiatric disorders like schizophrenia, on auditory processing in common marmosets using ultra-high-field fMRI. We reveal a global suppression of neural activity across auditory regions under ketamine, with varying effects depending on the sound type in certain regions. Notably, the mediodorsal thalamus showed significant susceptibility in processing socially relevant sounds. These findings suggest parallels between ketamine's impact and auditory processing disruptions seen in schizophrenia.

We recently identified voice-selective patches in the marmoset auditory cortex, but whether these regions specifically encode conspecific vocalizations over heterospecific ones-and whether they share a similar functional organization with those of humans and macaques-remains unknown. In this study, we used ultra-high-field functional magnetic resonance imaging (fMRI) in awake marmosets to characterize the cortical organization of vocalization processing and directly compare it with prior human and macaque data. Using an established auditory stimulus set designed for cross-species comparisons-including conspecific, heterospecific (macaque and human), and non-vocal sounds-we identified voice-selective patches showing preferential responses to conspecific calls. Robust responses were found in three temporal voice patches (anterior, middle, and posterior) and in the pregenual anterior cingulate cortex (pgACC), all showing significantly stronger responses to conspecific vocalizations than to other sound categories. A key finding was that, while the temporal patches also showed weak responses to heterospecific calls, the pgACC responded exclusively to conspecific vocalizations. Representational similarity analysis (RSA) revealed that dissimilarity patterns across these patches aligned exclusively with the marmoset-specific categorical model, indicating species-selective representational structure. Cross-species RSA comparisons revealed conserved representational geometry in the primary auditory cortex (A1) but species-specific organization in anterior temporal areas. These findings highlight shared principles of vocal communication processing across primates.

The common marmoset (Callithrix jacchus) is known for its highly vocal nature, displaying a diverse range of calls. Functional imaging in marmosets has shown that the processing of conspecific calls activates a brain network that includes fronto-temporal areas. It is currently unknown whether different call types activate the same or different networks. In this study, nine adult marmosets (four females) were exposed to four common vocalizations (phee, chatter, trill, and twitter), and their brain responses were recorded using event-related functional magnetic resonance imaging at 9.4 T. We found robust activations in the auditory cortices, encompassing core, belt, and parabelt regions, and in subcortical areas like the inferior colliculus, medial geniculate nucleus, and amygdala in response to these calls. Although a common network was engaged, distinct activity patterns were evident for different vocalizations that could be distinguished by a 3D convolutional neural network, indicating unique neural processing for each vocalization. Our findings also indicate the involvement of the cerebellum and medial prefrontal cortex in distinguishing particular vocalizations from others.

Understanding how the brain encodes temporal order in communication is central to explaining how complex interactions are perceived as coherent events. In humans, disrupting the sequence of words or scenes abolishes characteristic activity in higher-order networks, but whether similar mechanisms exist in nonhuman primates remains unknown. Here we used ultra–high-field fMRI (9.4 T) in awake marmosets to test how the marmoset brain evaluates temporal structure in natural conspecific vocalizations. Animals heard vocal sequences from three social contexts (angry, conversational, food-related) presented in intact, reversed, or randomized order, with call identity held constant. Disrupting sequence order altered responses across a distributed cortical–subcortical network. Contrast to reversed order, intact sequences drove stronger activation in prefrontal, cingulate, parietal, and somatosensory regions, whereas randomization produced the most widespread disruptions, additionally recruiting motor, insular, hippocampal, and thalamic territories. Uni- and multivariate analyses revealed a core network—including prefrontal area 8, cingulate areas 24/32, somatosensory cortex, and parietal Tpt—consistently sensitive to temporal coherence, with broader recruitment under severe disruption. Network-level dynamics further varied by context: conversation elicited earlier sensitivity to sequence disruptions, angry peaked later, and food built more gradually. These findings provide the first whole-brain evidence that marmosets engage hierarchically organized, context-sensitive networks to evaluate multi-agent vocal sequence structure, establishing a cross-species bridge to human narrative processing. ### Competing Interest Statement The authors have declared no competing interest.

The functional organization of the frontal lobe is a source of debate, focusing on broad functional subdivisions, large-scale networks, or local refined specificities. Multiple neurocognitive models have tried to explain how functional interactions between cingulate and lateral frontal regions contribute to decision making and cognitive control, but their neuroanatomical bases remain unclear. We provide a detailed description of the functional connectivity between cingulate and lateral frontal regions using resting-state functional MRI in rhesus macaques. The analysis focuses on the functional connectivity of the rostral part of the cingulate sulcus with the lateral frontal cortex. Data-driven and seed-based analysis revealed three clusters within the cingulate sulcus organized along the rostro-caudal axis: the anterior, mid, and posterior clusters display increased functional connectivity with, respectively, the anterior lateral prefrontal regions, face-eye lateral frontal motor cortical areas, and hand lateral frontal motor cortex. The location of these clusters can be predicted in individual subjects based on morphological landmarks. These results suggest that the anterior cluster corresponds to the anterior cingulate cortex, whereas the posterior clusters correspond to the face-eye and hand cingulate motor areas within the anterior midcingulate cortex. These data provide a comprehensive framework to identify cingulate subregions based on functional connectivity and local organization.

Numerous task-based functional magnetic resonance imaging (fMRI) studies have demonstrated that complex neural functions such as language processing, action observation, face recognition, and motor coordination are governed by widespread, intricate networks that span both cortical and subcortical areas. Nonhuman primate models are indispensable for advancing our understanding of the evolution of these networks and provide unique opportunities for experimental interventions that are not feasible in humans. In this study, we utilized movie-driven fMRI (md-fMRI) to investigate and delineate homologous functional networks in the common marmoset ( Callithrix jacchus ). Both marmosets and human subjects watched the same movie which incorporated a variety of visual and auditory stimuli. This method enabled the identification of potential homologues of large-scale functional networks involved in visual, auditory, cognitive, motor, and limbic functions in marmosets, offering new insights into the shared neurofunctional architecture across species. ### Competing Interest Statement The authors have declared no competing interest.

Understanding the brain circuitry involved in vocal processing across species is crucial for unraveling the evolutionary roots of human communication. While previous research has pinpointed voice-sensitive regions in primates, direct cross-species comparisons using standardized protocols are limited. This study utilizes ultra-high field fMRI to explore vocal processing mechanisms in humans and marmosets. By employing voice-sensitive regions of interest (ROIs) identified via auditory localizers, we analyzed response time courses to species-specific vocalizations and non-vocal sounds using a dynamic auditory-stimulation paradigm. This approach gradually introduced sounds into white noise over 33 seconds. Results revealed that both species have responsive areas in the temporal, frontal, and cingulate cortices, with a distinct preference for vocalizations. Significant differences were found in the response time courses between vocal and non-vocal sounds, with humans displaying faster responses to vocalizations than marmosets. We also identified a shared antero-ventral auditory pathway in both species for vocal processing, originating from the superior temporal gyrus. Conversely, a posterior-dorsal pathway was more prominent in humans, whereas in marmosets, this pathway processed both sound types similarly. This comparative study sheds light on both conserved and divergent auditory pathways in primates, providing new insights into conspecific vocalization processing. ### Competing Interest Statement The authors have declared no competing interest.

Primate communication relies on multimodal cues, such as vision and audition, to facilitate the exchange of intentions, enable social interactions, avoid predators, and foster group cohesion during daily activities. Understanding the integration of facial and vocal signals is pivotal to comprehend social interaction. In this study, we acquire whole-brain ultra-high field (9.4 T) fMRI data from awake marmosets ( Callithrix jacchus ) to explore brain responses to unimodal and combined facial and vocal stimuli. Our findings reveal that the multisensory condition not only intensifies activations in the occipito-temporal face patches and auditory voice patches but also engages a more extensive network that includes additional parietal, prefrontal and cingulate areas, compared to the summed responses of the unimodal conditions. By uncovering the neural network underlying multisensory audiovisual integration in marmosets, this study highlights the efficiency and adaptability of the marmoset brain in processing facial and vocal social signals, providing significant insights into primate social communication.

Over the course of evolution, the amygdala (AMG) and medial frontal cortex (mPFC) network, involved in behavioral adaptation, underwent structural changes in the old-world monkey and human lineages. Yet, whether and how the functional organization of this network differs remains poorly understood. Using resting-state functional magnetic resonance imagery, we show that the functional connectivity (FC) between AMG nuclei and mPFC regions differs between humans and awake macaques. In humans, the AMG-mPFC FC displays U-shaped pattern along the corpus callosum: a positive FC with the ventromedial prefrontal (vmPFC) and anterior cingulate cortex (ACC), a negative FC with the anterior mid-cingulate cortex (MCC), and a positive FC with the posterior MCC. Conversely, in macaques, the negative FC shifted more ventrally at the junction between the vmPFC and the ACC. The functional organization divergence of AMG-mPFC network between humans and macaques might help understanding behavioral adaptation abilities differences in their respective socio-ecological niches.

The observation of others’ actions activates a network of temporal, parietal and premotor/prefrontal areas in macaque monkeys and humans. This action-observation network (AON) has been shown to play important roles in social action monitoring, learning by imitation, and social cognition in both species. It is unclear whether a similar network exists in New-World primates, which separated from Old-Word primates ~35 million years ago. Here we used ultra-high field fMRI at 9.4 T in awake common marmosets ( Callithrix jacchus ) while they watched videos depicting goal-directed (grasping food) or non-goal-directed actions. The observation of goal-directed actions activates a temporo-parieto-frontal network, including areas 6 and 45 in premotor/prefrontal cortices, areas PGa-IPa, FST and TE in occipito-temporal region and areas V6A, MIP, LIP and PG in the occipito-parietal cortex. These results show overlap with the humans and macaques’ AON, demonstrating the existence of an evolutionarily conserved network that likely predates the separation of Old and New-World primates.

The correct identification of facial expressions is critical for understanding the intention of others during social communication in the daily life of all primates. Here we used ultra-high-field fMRI at 9.4 T to investigate the neural network activated by facial expressions in awake New World common marmosets from both male and female sex, and to determine the effect of facial motions on this network. We further explored how the face-patch network is involved in the processing of facial expressions. Our results show that dynamic and static facial expressions activate face patches in temporal and frontal areas (O, PV, PD, MD, AD, and PL) as well as in the amygdala, with stronger responses for negative faces, also associated with an increase of the respiration rates of the monkey. Processing of dynamic facial expressions involves an extended network recruiting additional regions not known to be part of the face-processing network, suggesting that face motions may facilitate the recognition of facial expressions. We report for the first time in New World marmosets that the perception and identification of changeable facial expressions, vital for social communication, recruit face-selective brain patches also involved in face detection processing and are associated with an increase of arousal. SIGNIFICANCE STATEMENT Recent research in humans and nonhuman primates has highlighted the importance to correctly recognize and process facial expressions to understand others' emotions in social interactions. The current study focuses on the fMRI responses of emotional facial expressions in the common marmoset (Callithrix jacchus), a New World primate species sharing several similarities of social behavior with humans. Our results reveal that temporal and frontal face patches are involved in both basic face detection and facial expression processing. The specific recruitment of these patches for negative faces associated with an increase of the arousal level show that marmosets process facial expressions of their congener, vital for social communication.

SUMMARY Understanding the default-mode network (DMN) in the common marmoset (Callithrix jacchus) has been challenging due to inconsistencies with human and marmoset DMNs. By analyzing task-negative activation in fMRI studies, we identified medial prefrontal cortical areas, rostral auditory areas, entorhinal cortex, posterior cingulate cortex area 31, hippocampus, hypothalamus, and basomedial amygdala as marmoset DMN components. Notable, medial and posterior parietal areas that were previously hypothesized to be part of the DMN were activated during visual task blocks. Seed analysis using resting-state fMRI showed strong connectivity between task-negative areas, and tracer data supported a structural network aligning with this functional DMN. These findings challenge previous definition of the marmoset DMN and reconcile many inconsistencies with the DMNs observed in humans, macaque monkeys, and even rodents. Overall, these results highlight the marmoset as a powerful model for DMN research, with potential implications for studying neuropsychiatric disorders where DMN activity and connectivity are altered.

The common marmoset (Callithrix jacchus) is gaining attention in the field of cognitive neuroscience. The development of an effective protocol for fMRI data acquisition in awake marmosets is a key factor in developing reliable comparative studies. Here, we describe a protocol to obtain fMRI data in awake marmosets using auditory and visual stimulation. We describe steps for surgical and anesthesia procedures, MRI training, and positioning the marmosets within an MRI-compatible body restraint. We then detail fMRI scanning and preprocess-ing of functional images. For complete details on the use and execution of this protocol, please refer to Jafari et al. (2023).1

Theory of Mind (ToM) refers to the ability to ascribe mental states to other individuals. This process is so strong that it extends even to the attribution of mental states to animations depicting interacting simple geometric shapes, such as in the Frith-Happé animations in which two triangles move either purposelessly (Random condition), or as if one triangle is reacting to the other triangle’s mental state (ToM condition). Currently, there is no evidence that nonhuman primates attribute mental states to moving abstract shapes. Here we investigated whether highly social marmosets ( Callithrix jacchus ) process ToM and Random Frith-Happé animations differently. Our results show that marmosets and humans (1) follow more closely one of the triangles during the observation of ToM compared to Random animations, and (2) activate large and comparable brain networks when viewing ToM compared to Random animations. These findings indicate that marmosets, like humans, process ToM animations differently from Random animations.

Vocalizations play an important role in the daily life of primates and likely form the basis of human language. Functional imaging studies have demonstrated that listening to voices activates a fronto-temporal voice perception network in human participants. Here, we acquired whole-brain ultrahigh-field (9.4 T) fMRI in awake marmosets (Callithrix jacchus) and demonstrate that these small, highly vocal New World primates possess a similar fronto-temporal network, including subcortical regions, that is activated by the presentation of conspecific vocalizations. The findings suggest that the human voice perception network has evolved from an ancestral vocalization-processing network that predates the separation of New and Old World primates.

Abstract We constantly face situations involving interactions with others that require us to automatically adjust our physical distances to avoid discomfort or anxiety. A previous case study has demonstrated that the integrity of both amygdalae is essential to regulate interpersonal distances. Despite unilateral lesion to the amygdala, as to other sectors of the medial temporal cortex, are known to also affect social behavior, their role in the regulation of interpersonal distances has never been investigated. Here, we sought to fill this gap by testing three patients with unilateral temporal lesions following surgical resections, including one patient with a lesion mainly centered on the amygdala and two with lesions to adjacent medial temporal cortex, on two versions of the stop distance paradigm (i.e. in a virtual reality environment and in a real setting). Our results showed that all three patients set shorter interpersonal distances compared to neurotypical controls. In addition, compared to controls, none of the patients adjusted such physical distances depending on facial emotional expressions, despite they preserved ability to categorize them. Finally, patients' heart rate responses differed from controls when viewing approaching faces. Our findings bring compelling evidence that unilateral lesions within the medial temporal cortex, not necessarily restricted to the amygdala, are sufficient to alter interpersonal distance, thus shedding new light on the neural circuitry regulating distance in social interactions.