Behavioral flexibility is associated with changes in structure and function distributed across a frontal cortical network in macaques.

One of the most influential accounts of central orbitofrontal cortex-that it mediates behavioral flexibility-has been challenged by the finding that discrimination reversal in macaques, the classic test of behavioral flexibility, is unaffected when lesions are made by excitotoxin injection rather than aspiration. This suggests that the critical brain circuit mediating behavioral flexibility in reversal tasks lies beyond the central orbitofrontal cortex. To determine its identity, a group of nine macaques were taught discrimination reversal learning tasks, and its impact on gray matter was measured. Magnetic resonance imaging scans were taken before and after learning and compared with scans from two control groups, each comprising 10 animals. One control group learned discrimination tasks that were similar but lacked any reversal component, and the other control group engaged in no learning. Gray matter changes were prominent in posterior orbitofrontal cortex/anterior insula but were also found in three other frontal cortical regions: lateral orbitofrontal cortex (orbital part of area 12 [12o]), cingulate cortex, and lateral prefrontal cortex. In a second analysis, neural activity in posterior orbitofrontal cortex/anterior insula was measured at rest, and its pattern of coupling with the other frontal cortical regions was assessed. Activity coupling increased significantly in the reversal learning group in comparison with controls. In a final set of experiments, we used similar structural imaging procedures and analyses to demonstrate that aspiration lesion of central orbitofrontal cortex, of the type known to affect discrimination learning, affected structure and activity in the same frontal cortical circuit. The results identify a distributed frontal cortical circuit associated with behavioral flexibility.

Viewing ambiguous social interactions increases functional connectivity between frontal and temporal nodes of the social brain.

Social behaviour is coordinated by a network of brain regions, including those involved in the perception of social stimuli and those involved in complex functions like inferring perceptual and mental states and controlling social interactions. The properties and function of many of these regions in isolation is relatively well-understood, but less is known about how these regions interact whilst processing dynamic social interactions. To investigate whether the functional connectivity between brain regions is modulated by social context, we collected functional MRI (fMRI) data from male monkeys (Macaca mulatta) viewing videos of social interactions labelled as "affiliative", "aggressive", or "ambiguous". We show activation related to the perception of social interactions along both banks of the superior temporal sulcus, parietal cortex, medial and lateral frontal cortex, and the caudate nucleus. Within this network, we show that fronto-temporal functional connectivity is significantly modulated by social context. Crucially, we link the observation of specific behaviours to changes in functional connectivity within our network. Viewing aggressive behaviour was associated with a limited increase in temporo-temporal and a weak increase in cingulate-temporal connectivity. By contrast, viewing interactions where the outcome was uncertain was associated with a pronounced increase in temporo-temporal, and cingulate-temporal functional connectivity. We hypothesise that this widespread network synchronisation occurs when cingulate and temporal areas coordinate their activity when more difficult social inferences are being made.SIGNIFICANCE STATEMENT:Processing social information from our environment requires the activation of several brain regions, which are concentrated within the frontal and temporal lobes. However, little is known about how these areas interact to facilitate the processing of different social interactions. Here we show that functional connectivity within and between the frontal and temporal lobes is modulated by social context. Specifically, we demonstrate that viewing social interactions where the outcome was unclear is associated with increased synchrony within and between the cingulate cortex and temporal cortices. These findings suggest that the coordination between the cingulate and temporal cortices is enhanced when more difficult social inferences are being made.

A Putative Multiple-Demand System in the Macaque Brain.

UNLABELLED: In humans, cognitively demanding tasks of many types recruit common frontoparietal brain areas. Pervasive activation of this "multiple-demand" (MD) network suggests a core function in supporting goal-oriented behavior. A similar network might therefore be predicted in nonhuman primates that readily perform similar tasks after training. However, an MD network in nonhuman primates has not been described. Single-cell recordings from macaque frontal and parietal cortex show some similar properties to human MD fMRI responses (e.g., adaptive coding of task-relevant information). Invasive recordings, however, come from limited prespecified locations, so they do not delineate a macaque homolog of the MD system and their positioning could benefit from knowledge of where MD foci lie. Challenges of scanning behaving animals mean that few macaque fMRI studies specifically contrast levels of cognitive demand, so we sought to identify a macaque counterpart to the human MD system using fMRI connectivity in 35 rhesus macaques. Putative macaque MD regions, mapped from frontoparietal MD regions defined in humans, were found to be functionally connected under anesthesia. To further refine these regions, an iterative process was used to maximize their connectivity cross-validated across animals. Finally, whole-brain connectivity analyses identified voxels that were robustly connected to MD regions, revealing seven clusters across frontoparietal and insular cortex comparable to human MD regions and one unexpected cluster in the lateral fissure. The proposed macaque MD regions can be used to guide future electrophysiological investigation of MD neural coding and in task-based fMRI to test predictions of similar functional properties to human MD cortex. SIGNIFICANCE STATEMENT: In humans, a frontoparietal "multiple-demand" (MD) brain network is recruited during a wide range of cognitively demanding tasks. Because this suggests a fundamental function, one might expect a similar network to exist in nonhuman primates, but this remains controversial. Here, we sought to identify a macaque counterpart to the human MD system using fMRI connectivity. Putative macaque MD regions were functionally connected under anesthesia and were further refined by iterative optimization. The result is a network including lateral frontal, dorsomedial frontal, and insular and inferior parietal regions closely similar to the human counterpart. The proposed macaque MD regions can be useful in guiding electrophysiological recordings or in task-based fMRI to test predictions of similar functional properties to human MD cortex.

A neural circuit covarying with social hierarchy in macaques.

Despite widespread interest in social dominance, little is known of its neural correlates in primates. We hypothesized that social status in primates might be related to individual variation in subcortical brain regions implicated in other aspects of social and emotional behavior in other mammals. To examine this possibility we used magnetic resonance imaging (MRI), which affords the taking of quantitative measurements noninvasively, both of brain structure and of brain function, across many regions simultaneously. We carried out a series of tests of structural and functional MRI (fMRI) data in 25 group-living macaques. First, a deformation-based morphometric (DBM) approach was used to show that gray matter in the amygdala, brainstem in the vicinity of the raphe nucleus, and reticular formation, hypothalamus, and septum/striatum of the left hemisphere was correlated with social status. Second, similar correlations were found in the same areas in the other hemisphere. Third, similar correlations were found in a second data set acquired several months later from a subset of the same animals. Fourth, the strength of coupling between fMRI-measured activity in the same areas was correlated with social status. The network of subcortical areas, however, had no relationship with the sizes of individuals' social networks, suggesting the areas had a simple and direct relationship with social status. By contrast a second circuit in cortex, comprising the midsuperior temporal sulcus and anterior and dorsal prefrontal cortex, covaried with both individuals' social statuses and the social network sizes they experienced. This cortical circuit may be linked to the social cognitive processes that are taxed by life in more complex social networks and that must also be used if an animal is to achieve a high social status.

Hierarchical Encoding of Social Cues in Primate Inferior Temporal Cortex.

Faces convey information about identity and emotional state, both of which are important for our social interactions. Models of face processing propose that changeable versus invariant aspects of a face, specifically facial expression/gaze direction versus facial identity, are coded by distinct neural pathways and yet neurophysiological data supporting this separation are incomplete. We recorded activity from neurons along the inferior bank of the superior temporal sulcus (STS), while monkeys viewed images of conspecific faces and non-face control stimuli. Eight monkey identities were used, each presented with 3 different facial expressions (neutral, fear grin, and threat). All facial expressions were displayed with both a direct and averted gaze. In the posterior STS, we found that about one-quarter of face-responsive neurons are sensitive to social cues, the majority of which being sensitive to only one of these cues. In contrast, in anterior STS, not only did the proportion of neurons sensitive to social cues increase, but so too did the proportion of neurons sensitive to conjunctions of identity with either gaze direction or expression. These data support a convergence of signals related to faces as one moves anteriorly along the inferior bank of the STS, which forms a fundamental part of the face-processing network.

Connectivity between the superior colliculus and the amygdala in humans and macaque monkeys: virtual dissection with probabilistic DTI tractography.

It has been suggested that some cortically blind patients can process the emotional valence of visual stimuli via a fast, subcortical pathway from the superior colliculus (SC) that reaches the amygdala via the pulvinar. We provide in vivo evidence for connectivity between the SC and the amygdala via the pulvinar in both humans and rhesus macaques. Probabilistic diffusion tensor imaging tractography revealed a streamlined path that passes dorsolaterally through the pulvinar before arcing rostrally to traverse above the temporal horn of the lateral ventricle and connect to the lateral amygdala. To obviate artifactual connectivity with crossing fibers of the stria terminalis, the stria was also dissected. The putative streamline between the SC and amygdala traverses above the temporal horn dorsal to the stria terminalis and is positioned medial to it in humans and lateral to it in monkeys. The topography of the streamline was examined in relation to lesion anatomy in five patients who had previously participated in behavioral experiments studying the processing of emotionally valenced visual stimuli. The pulvinar lesion interrupted the streamline in two patients who had exhibited contralesional processing deficits and spared the streamline in three patients who had no deficit. Although not definitive, this evidence supports the existence of a subcortical pathway linking the SC with the amygdala in primates. It also provides a necessary bridge between behavioral data obtained in future studies of neurological patients, and any forthcoming evidence from more invasive techniques, such as anatomical tracing studies and electrophysiological investigations only possible in nonhuman species.

Amygdala lesions disrupt modulation of functional MRI activity evoked by facial expression in the monkey inferior temporal cortex.

We previously showed that facial expressions modulate functional MRI activity in the face-processing regions of the macaque monkey's amygdala and inferior temporal (IT) cortex. Specifically, we showed that faces expressing emotion yield greater activation than neutral faces; we term this difference the "valence effect." We hypothesized that amygdala lesions would disrupt the valence effect by eliminating the modulatory feedback from the amygdala to the IT cortex. We compared the valence effects within the IT cortex in monkeys with excitotoxic amygdala lesions (n = 3) with those in intact control animals (n = 3) using contrast agent-based functional MRI at 3 T. Images of four distinct monkey facial expressions--neutral, aggressive (open mouth threat), fearful (fear grin), and appeasing (lip smack)--were presented to the subjects in a blocked design. Our results showed that in monkeys with amygdala lesions the valence effects were strongly disrupted within the IT cortex, whereas face responsivity (neutral faces > scrambled faces) and face selectivity (neutral faces > non-face objects) were unaffected. Furthermore, sparing of the anterior amygdala led to intact valence effects in the anterior IT cortex (which included the anterior face-selective regions), whereas sparing of the posterior amygdala led to intact valence effects in the posterior IT cortex (which included the posterior face-selective regions). Overall, our data demonstrate that the feedback projections from the amygdala to the IT cortex mediate the valence effect found there. Moreover, these modulatory effects are consistent with an anterior-to-posterior gradient of projections, as suggested by classical tracer studies.

Relationship between functional magnetic resonance imaging-identified regions and neuronal category selectivity.

Functional magnetic resonance imaging (fMRI) has been used extensively to identify regions in the inferior temporal (IT) cortex that are selective for categories of visual stimuli. However, comparatively little is known about the neuronal responses relative to these fMRI-defined regions. Here, we compared in nonhuman primates the distribution and response properties of IT neurons recorded within versus outside fMRI regions selective for four different visual categories: faces, body parts, objects, and places. Although individual neurons that preferred each of the four categories were found throughout the sampled regions, they were most concentrated within the corresponding fMRI region, decreasing significantly within 1-4 mm from the edge of these regions. Furthermore, the correspondence between fMRI and neuronal distributions was specific to neurons that increased their firing rates in response to the visual stimuli but not to neurons suppressed by visual stimuli, suggesting that the processes associated with inhibiting neuronal activity did not contribute strongly to the fMRI signal in this experiment.

Frontopolar cortex shapes brain network structure across prefrontal and posterior cingulate cortex.

Primate frontopolar cortex (FPC), occupied by area 10, sits atop a functional hierarchy of prefrontal cortical regions, yet little is known about its involvement in wider cortical networks. Here we examined resting-state-functional-connectivity (rsfc) in rhesus monkeys with intact or lesioned FPC to identify cortical regions associated with FPC. We present a network of FPC-specific regions of interest (ROIs), whose connectivity was affected by lesion of FPC but not by lesion of neighbouring prefrontal cortex (principal sulcus). This network comprised 'core ROIs' with direct anatomical connections to FPC, located in ventrolateral prefrontal cortex, posterior cingulate cortex, and superior temporal gyrus, and 'peripheral ROIs' well connected to the core network. We further show that the principle effect of a lesion to FPC was to cause a profound disturbance of the functional connectivity of posterior cingulate and ventrolateral prefrontal cortex. We therefore suggest that FPC, posterior cingulate and ventrolateral prefrontal cortex comprise a network of interacting cortical areas whose interactions may be critical for mediating the contribution of FPC to decision making.

Perception of emotional expressions is independent of face selectivity in monkey inferior temporal cortex.

The ability to perceive and differentiate facial expressions is vital for social communication. Numerous functional MRI (fMRI) studies in humans have shown enhanced responses to faces with different emotional valence, in both the amygdala and the visual cortex. However, relatively few studies have examined how valence influences neural responses in monkeys, thereby limiting the ability to draw comparisons across species and thus understand the underlying neural mechanisms. Here we tested the effects of macaque facial expressions on neural activation within these two regions using fMRI in three awake, behaving monkeys. Monkeys maintained central fixation while blocks of different monkey facial expressions were presented. Four different facial expressions were tested: (i) neutral, (ii) aggressive (open-mouthed threat), (iii) fearful (fear grin), and (iv) submissive (lip smack). Our results confirmed that both the amygdala and the inferior temporal cortex in monkeys are modulated by facial expressions. As in human fMRI, fearful expressions evoked the greatest response in monkeys-even though fearful expressions are physically dissimilar in humans and macaques. Furthermore, we found that valence effects were not uniformly distributed over the inferior temporal cortex. Surprisingly, these valence maps were independent of two related functional maps: (i) the map of "face-selective" regions (faces versus non-face objects) and (ii) the map of "face-responsive" regions (faces versus scrambled images). Thus, the neural mechanisms underlying face perception and valence perception appear to be distinct.

Rapid event-related, BOLD fMRI, non-human primates (NHP): choose two out of three.

Human functional magnetic resonance imaging (fMRI) typically employs the blood-oxygen-level-dependent (BOLD) contrast mechanism. In non-human primates (NHP), contrast enhancement is possible using monocrystalline iron-oxide nanoparticles (MION) contrast agent, which has a more temporally extended response function. However, using BOLD fMRI in NHP is desirable for interspecies comparison, and the BOLD signal's faster response function promises to be beneficial for rapid event-related (rER) designs. Here, we used rER BOLD fMRI in macaque monkeys while viewing real-world images, and found visual responses and category selectivity consistent with previous studies. However, activity estimates were very noisy, suggesting that the lower contrast-to-noise ratio of BOLD, suboptimal behavioural performance, and motion artefacts, in combination, render rER BOLD fMRI challenging in NHP. Previous studies have shown that rER fMRI is possible in macaques with MION, despite MION's prolonged response function. To understand this, we conducted simulations of the BOLD and MION response during rER, and found that no matter how fast the design, the greater amplitude of the MION response outweighs the contrast loss caused by greater temporal smoothing. We conclude that although any two of the three elements (rER, BOLD, NHP) have been shown to work well, the combination of all three is particularly challenging.

The effect of spatial-temporal audiovisual disparities on saccades in a complex scene.

In a previous study we quantified the effect of multisensory integration on the latency and accuracy of saccadic eye movements toward spatially aligned audiovisual (AV) stimuli within a rich AV-background (Corneil et al. in J Neurophysiol 88:438-454, 2002). In those experiments both stimulus modalities belonged to the same object, and subjects were instructed to foveate that source, irrespective of modality. Under natural conditions, however, subjects have no prior knowledge as to whether visual and auditory events originated from the same, or from different objects in space and time. In the present experiments we included these possibilities by introducing various spatial and temporal disparities between the visual and auditory events within the AV-background. Subjects had to orient fast and accurately to the visual target, thereby ignoring the auditory distractor. We show that this task belies a dichotomy, as it was quite difficult to produce fast responses (<250 ms) that were not aurally driven. Subjects therefore made many erroneous saccades. Interestingly, for the spatially aligned events the inability to ignore auditory stimuli produced shorter reaction times, but also more accurate responses than for the unisensory target conditions. These findings, which demonstrate effective multisensory integration, are similar to the previous study, and the same multisensory integration rules are applied (Corneil et al. in J Neurophysiol 88:438-454, 2002). In contrast, with increasing spatial disparity, integration gradually broke down, as the subjects' responses became bistable: saccades were directed either to the auditory (fast responses), or to the visual stimulus (late responses). Interestingly, also in this case responses were faster and more accurate than to the respective unisensory stimuli.

Preserved extrastriate visual network in a monkey with substantial, naturally occurring damage to primary visual cortex.

Lesions of primary visual cortex (V1) lead to loss of conscious visual perception with significant impact on human patients. Understanding the neural consequences of such damage may aid the development of rehabilitation methods. In this rare case of a Rhesus macaque (monkey S), likely born without V1, the animal's in-group behaviour was unremarkable, but visual task training was impaired. With multi-modal magnetic resonance imaging, visual structures outside of the lesion appeared normal. Visual stimulation under anaesthesia with checkerboards activated lateral geniculate nucleus of monkey S, while full-field moving dots activated pulvinar. Visual cortical activation was sparse but included face patches. Consistently across lesion and control monkeys, functional connectivity analysis revealed an intact network of bilateral dorsal visual areas temporally correlated with V5/MT activation, even without V1. Despite robust subcortical responses to visual stimulation, we found little evidence for strengthened subcortical input to V5/MT supporting residual visual function or blindsight-like phenomena.

Activity in the superior colliculus reflects dynamic interactions between voluntary and involuntary influences on orienting behaviour.

Performance in a behavioural task can be influenced by both bottom-up and top-down processes such as stimulus modality and prior probability. Here, we exploited differences in behavioural strategy to explore the role of the intermediate and deep layers of the superior colliculus (dSC) in covert orienting. Two monkeys were trained on a predictive cued-saccade task in which the cue predicted the target's upcoming location with 80% validity. When the delay between cue and target onset was 250 ms, both monkeys showed faster responses to the uncued (Invalid) location. This was associated with a reduced target-aligned response in the dSC on Valid trials for both monkeys and is consistent with a bottom-up (i.e. involuntary) bias. When the delay was increased to 650 ms, one monkey continued to show faster responses to the Invalid location whereas the other monkey showed faster responses to the Valid location, consistent with a top-down (i.e. voluntary) bias. This latter behaviour was correlated with an increase in activity in dSC neurons preceding target onset that was absent in the other monkey. Thus, using the information provided by the cue shifted the emphasis towards top-down processing, while ignoring this information allowed bottom-up processing to continue to dominate. Regardless of the selected strategy, however, neurons in the dSC consistently reflected the current bias between the two processes, emphasizing its role in both the bottom-up and top-down control of orienting behaviour.

Methods matter: A primer on permanent and reversible interference techniques in animals for investigators of human neuropsychology.

The study of patients with brain lesions has contributed greatly to our understanding of the biological bases of human cognition, but this approach also has several unavoidable limitations. Research that uses animal models complements and extends human neuropsychology by addressing many of these limitations. In this review, we provide an overview of permanent and reversible animal lesion techniques for researchers of human neuropsychology, with the aim of highlighting how these methods provide a valuable adjunct to behavioural, neuroimaging, physiological, and clinical investigations in humans. Research in animals has provided important lessons about how the limitations of one or more techniques, or differences in their mechanism of action, has impacted upon the understanding of brain organisation and function. These cautionary tales highlight the importance of striving for a thorough understanding of how any intereference technique works (whether in animal or human), and for how to best use animal research to clarify the precise mechanisms underlying temporary lesion methods in humans.

Functional reorganisation and recovery following cortical lesions: A preliminary study in macaque monkeys.

Damage following traumatic brain injury or stroke can often extend beyond the boundaries of the initial insult and can lead to maladaptive cortical reorganisation. On the other hand, beneficial cortical reorganisation leading to recovery of function can also occur. We used resting state FMRI to investigate how cortical networks in the macaque brain change across time in response to lesions to the prefrontal cortex, and how this reorganisation correlated with changes in behavioural performance in cognitive tasks. After prelesion testing and scanning, two monkeys received a lesion to regions surrounding the left principal sulcus followed by periodic testing and scanning. Later, the animals received another lesion to the opposite hemisphere and additional testing and scanning. Following the first lesion, we observed both a behavioural impairment and decrease in functional connectivity, predominantly in frontal-frontal networks. Approximately 8 weeks later, performance and connectivity patterns both improved. Following the second lesion, we observed a further behavioural deficit and decrease in connectivity that showed little recovery. We discuss how different mechanisms including alternate behavioural strategies and reorganisation of specific prefrontal networks may have led to improvements in behaviour. Further work will be needed to confirm these mechanisms.

Reply to Vinken and Vogels.

In their comment, Vinken and Vogels [1] take issue with our claim [2] that "IT neurons encode long-term, latent probabilistic information about stimulus occurrence". They offer a biologically plausible model of our findings, which they argue is based on neuronal fatigue. However, like our account, their model includes latent variables that are modulated slowly with stimulus probability; models without such latent processes, such as those based on temporally local fatigue effects, cannot explain our findings. Although we share their desire for more clarity about the mechanisms underlying visual expectation, and appreciate their thoughtful critique, we argue here that their comment mostly restates our findings with a more complex model and alternative terminology.

Encoding of Stimulus Probability in Macaque Inferior Temporal Cortex.

Optimal perceptual decisions require sensory signals to be combined with prior information about stimulus probability. Although several theories propose that probabilistic information about stimulus occurrence is encoded in sensory cortex, evidence from neuronal recordings has not yet fully supported this view. We recorded activity from single neurons in inferior temporal cortex (IT) while monkeys performed a task that involved discriminating degraded images of faces and fruit. The relative probability of the cue being a face versus a fruit was manipulated by a latent variable that was not revealed to the monkeys and that changed unpredictably over the course of each recording session. In addition to responding to stimulus identity (face or fruit), population responses in IT encoded the long-term stimulus probability of whether a face or a fruit stimulus was more likely to occur. Face-responsive neurons showed reduced firing rates to expected faces, an effect consistent with "expectation suppression," but expected stimuli were decoded from multivariate population signals with greater accuracy. These findings support "predictive coding" theories, whereby neural signals in the mammalian visual system actively encode and update predictions about the local sensory environment.

Contrasting Roles for Orbitofrontal Cortex and Amygdala in Credit Assignment and Learning in Macaques.

Recent studies have challenged the view that orbitofrontal cortex (OFC) and amygdala mediate flexible reward-guided behavior. We trained macaques to perform an object discrimination reversal task during fMRI sessions and identified a lateral OFC (lOFC) region in which activity predicted adaptive win-stay/lose-shift behavior. Amygdala and lOFC activity was more strongly coupled on lose-shift trials. However, lOFC-amygdala coupling was also modulated by the relevance of reward information in a manner consistent with a role in establishing how credit for reward should be assigned. Day-to-day fluctuations in signals and signal coupling were correlated with day-to-day fluctuation in performance. A second experiment confirmed the existence of signals for adaptive stay/shift behavior in lOFC and reflecting irrelevant reward in the amygdala in a probabilistic learning task. Our data demonstrate that OFC and amygdala each make unique contributions to flexible behavior and credit assignment.

Uncovering the visual "alphabet": advances in our understanding of object perception.

The ability to rapidly and accurately recognize visual stimuli represents a significant computational challenge. Yet, despite such complexity, the primate brain manages this task effortlessly. How it does so remains largely a mystery. The study of visual perception and object recognition was once limited to investigations of brain-damaged individuals or lesion experiments in animals. However, in the last 25years, new methodologies, such as functional neuroimaging and advances in electrophysiological approaches, have provided scientists with the opportunity to examine this problem from new perspectives. This review highlights how some of these recent technological advances have contributed to the study of visual processing and where we now stand with respect to our understanding of neural mechanisms underlying object recognition.