Extracting 3D from motion: differences in human and monkey intraparietal cortex

Data-driven analysis of analogous brain networks in monkeys and humans during natural vision

Inferences about functional correspondences between functional networks of human and non-human primates largely rely on proximity and anatomical expansion models. However, it has been demonstrated that topologically correspondent areas in two species can have different functional properties, suggesting that anatomy-based approaches should be complemented with alternative methods to perform functional comparisons. We have recently shown that comparative analyses based on temporal correlations of sensory-driven fMRI responses can reveal functional correspondent areas in monkeys and humans without relying on spatial assumptions. Inter-species activity correlation (ISAC) analyses require the definition of seed areas in one species to reveal functional correspondences across the cortex of the same and other species. Here we propose an extension of the ISAC method that does not rely on any seed definition, hence a method void of any spatial assumption. Specifically, we apply independent component analysis (ICA) separately to monkey and human data to define species-specific networks of areas with coherent stimulus-related activity. Then, we use a hierarchical cluster analysis to identify ICA-based ISAC clusters of monkey and human networks with similar timecourses. We implemented this approach on fMRI data collected in monkeys and humans during movie watching, a condition that evokes widespread sensory-driven activity throughout large portions of the cortex. Using ICA-based ISAC, we detected seven monkey-human clusters. The timecourses of several clusters showed significant correspondences either with the motion energy in the movie or with eye-movement parameters. Five of the clusters spanned putative homologous functional networks in either primary or extrastriate visual regions, whereas two clusters included higher-level visual areas at topological locations that are not predicted by cortical surface expansion models. Overall, our ICA-based ISAC analysis complemented the findings of our previous seed-based investigations, and suggested that functional processes can be executed by brain networks in different species that are functionally but not necessarily anatomically correspondent. Overall, our method provides a novel approach to reveal evolution-driven functional changes in the primate brain with no spatial assumptions.

Evidence for ape and human specializations in geniculostriate projections from VGLUT2 immunohistochemistry

Vesicular glutamate transporters (VGLUTs) reuptake glutamate into synaptic vesicles at excitatory synapses. VGLUT2 is localized in the cortical terminals of neuronal somas located in the main sensory nuclei of the thalamus. Thus, immunolabeling of cortex with antibodies to VGLUT2 can reveal geniculostriate terminal distributions in species in which connectivity cannot be studied with tract-tracing techniques, permitting broader comparative studies of cortical specializations. Here, we used VGLUT2 immunohistochemistry to compare the organization of geniculostriate afferents in primary visual cortex in hominid primates (humans, chimpanzees, and an orangutan), Old World monkeys (rhesus macaques and vervets), and New World monkeys (squirrel monkeys). The New and Old World monkeys had a broad, dense band of terminal-like labeling in cortical layer 4C, a narrow band of labeling in layer 4A, and additional labeling in layers 2/3 and 6, consistent with results from conventional tract-tracing studies in these species. By contrast, although the hominid primates had a prominent layer 4C band, labeling of layer 4A was sparse or absent. Labeling was also present in layers 2/3 and 6, although labeling of layer 6 was weaker in hominids and possibly more individually variable than in Old and New World monkeys. These findings are consistent with previous observations from cytochrome oxidase histochemistry and a very small number of connectivity studies, suggesting that the projections from the parvocellular layers of the lateral geniculate nucleus to layer 4A were strongly reduced or eliminated in humans and apes following their evolutionary divergence from the other anthropoid primates.

An area specifically devoted to tool use in human left inferior parietal lobule

A comparative fMRI study by Peeters et al. (2009) provided evidence that a specific sector of left inferior parietal lobule is devoted to tool use in humans, but not in monkeys. We propose that this area represents the neural substrate of the human capacity to understand tool use by using causal reasoning.

Posterior parietal cortex dynamically ranks topographic signals via cholinergic influence

The hypothesis to be discussed in this review is that posterior parietal cortex (PPC) is directly involved in selecting relevant stimuli and filtering irrelevant distractors. The PPC receives input from several sensory modalities and integrates them in part to direct the allocation of resources to optimize gains. In conjunction with prefrontal cortex, nucleus accumbens, and basal forebrain cholinergic nuclei, it comprises a network mediating sustained attentional performance. Numerous anatomical, neurophysiological, and lesion studies have substantiated the notion that the basic functions of the PPC are conserved from rodents to humans. One such function is the detection and selection of relevant stimuli necessary for making optimal choices or responses. The issues to be addressed here are how behaviorally relevant targets recruit oscillatory potentials and spiking activity of posterior parietal neurons compared to similar yet irrelevant stimuli. Further, the influence of cortical cholinergic input to PPC in learning and decision-making is also discussed. I propose that these neurophysiological correlates of attention are transmitted to frontal cortical areas contributing to the top-down selection of stimuli in a timely manner.

Adaptation to heading direction dissociates the roles of human MST and V6 in the processing of optic flow

The extraction of optic flow cues is fundamental for successful locomotion. During forward motion, the focus of expansion (FoE), in conjunction with knowledge of eye position, indicates the direction in which the individual is heading. Therefore, it is expected that cortical brain regions that are involved in the estimation of heading will be sensitive to this feature. To characterize cortical sensitivity to the location of the FoE or, more generally, the center of flow (CoF) during visually simulated self-motion, we carried out a functional MRI (fMRI) adaptation experiment in several human visual cortical areas that are thought to be sensitive to optic flow parameters, namely, V3A, V6, MT/V5, and MST. In each trial, two optic flow patterns were sequentially presented, with the CoF located in either the same or different positions. With an adaptation design, an area sensitive to heading direction should respond more strongly to a pair of stimuli with different CoFs than to stimuli with the same CoF. Our results show such release from adaptation in areas MT/V5 and MST, and to a lesser extent V3A, suggesting the involvement of these areas in the processing of heading direction. The effect could not be explained either by differences in local motion or by attention capture. It was not observed to a significant extent in area V6 or in control area V1. The different patterns of responses observed in MST and V6, areas that are both involved in the processing of egomotion in macaques and humans, suggest distinct roles in the processing of visual cues for self-motion.

Interspecies activity correlations reveal functional correspondence between monkey and human brain areas

Evolution-driven functional changes in the primate brain are typically assessed by aligning monkey and human activation maps using cortical surface expansion models. These models use putative homologous areas as registration landmarks, assuming they are functionally correspondent. In cases where functional changes have occurred in an area, this assumption prohibits to reveal whether other areas may have assumed lost functions. Here we describe a method to examine functional correspondences across species. Without making spatial assumptions, we assess similarities in sensory-driven functional magnetic resonance imaging responses between monkey (Macaca mulatta) and human brain areas by means of temporal correlation. Using natural vision data, we reveal regions for which functional processing has shifted to topologically divergent locations during evolution. We conclude that substantial evolution-driven functional reorganizations have occurred, not always consistent with cortical expansion processes. This novel framework for evaluating changes in functional architecture is crucial to building more accurate evolutionary models.

Acting alters visual processing: flexible recruitment of visual areas by one's own actions

Using functional magnetic resonance imaging, we investigated the effect of motor preparation/execution on the activation of visual cortical areas by action observation. We presented videos of human actors performing several fine manipulative actions (e.g., grasping) with the hand or foot, together with appropriate control stimuli. Subjects either responded in a central fixation task with the hand (A) or foot (B) or viewed the stimuli passively (C). Experimental conditions were arranged according to a 2 × 2 × 3 factorial design with action, effector, and response as factors. Bilateral posterior parietal cortex was more strongly activated for action videos compared with controls during active runs (A or B) contrasted with passive runs (C). Two neighboring regions in the right fusiform gyrus (FG) were activated when the effector employed to respond in the task matched that displayed in the videos (A or B), independently of whether the stimulus was an action or a control. Neighboring regions in the right posterior middle temporal gyrus (MTG) were also activated when the effector observed and that used to respond matched (A or B), but only for action videos, not controls. Our results indicate flexible modulation of visual areas during concurrent action observation and action execution/preparation, which was effector specific in the FG and MTG.

An intracranial event-related potential study on transformational apparent motion. Does its neural processing differ from real motion?

How the brain processes visual stimuli has been extensively studied using scalp surface electrodes and magnetic resonance imaging. Using these and other methods, complex gratings have been shown to activate the ventral visual stream, whereas moving stimuli preferentially activate the dorsal stream. In the current study, a first experiment assessed brain activations evoked by complex gratings using intracranial electroencephalography in 10 epileptic patients implanted with subdural electrodes. These stimuli of intermediate levels of complexity were presented in such a way that transformational apparent motion (TAM) was perceived. Responses from both the ventral and the dorsal pathways were obtained. The response characteristics of visual area 4 and the fusiform cortex were of similar amplitudes, suggesting that both ventral areas are recruited for the processing of complex gratings. On the other hand, TAM-induced responses of dorsal pathway areas were relatively noisier and of lower amplitudes, suggesting that TAM does not activate motion-specific structures to the same extent as does real motion. To test this hypothesis, we examined the activity evoked by TAM in comparison to the one produced by real motion in a patient implanted with the same subdural electrodes. Findings demonstrated that neural response to real motion was much stronger than that evoked by TAM, in both the primary visual cortex (V1) and other motion-sensitive areas within the dorsal pathway. These results support the conclusion that apparent motion, even if perceptually similar to real motion, is not processed in a similar manner.

Functional magnetic resonance imaging of awake monkeys: some approaches for improving imaging quality

Functional magnetic resonance imaging (fMRI) at high magnetic field strength can suffer from serious degradation of image quality because of motion and physiological noise, as well as spatial distortions and signal losses due to susceptibility effects. Overcoming such limitations is essential for sensitive detection and reliable interpretation of fMRI data. These issues are particularly problematic in studies of awake animals. As part of our initial efforts to study functional brain activations in awake, behaving monkeys using fMRI at 4.7 T, we have developed acquisition and analysis procedures to improve image quality with encouraging results. We evaluated the influence of two main variables on image quality. First, we show how important the level of behavioral training is for obtaining good data stability and high temporal signal-to-noise ratios. In initial sessions, our typical scan session lasted 1.5 h, partitioned into short (<10 min) runs. During reward periods and breaks between runs, the monkey exhibited movements resulting in considerable image misregistrations. After a few months of extensive behavioral training, we were able to increase the length of individual runs and the total length of each session. The monkey learned to wait until the end of a block for fluid reward, resulting in longer periods of continuous acquisition. Each additional 60 training sessions extended the duration of each session by 60 min, culminating, after about 140 training sessions, in sessions that last about 4 h. As a result, the average translational movement decreased from over 500 μm to less than 80 μm, a displacement close to that observed in anesthetized monkeys scanned in a 7-T horizontal scanner. Another major source of distortion at high fields arises from susceptibility variations. To reduce such artifacts, we used segmented gradient-echo echo-planar imaging (EPI) sequences. Increasing the number of segments significantly decreased susceptibility artifacts and image distortion. Comparisons of images from functional runs using four segments with those using a single-shot EPI sequence revealed a roughly twofold improvement in functional signal-to-noise-ratio and 50% decrease in distortion. These methods enabled reliable detection of neural activation and permitted blood-oxygenation-level-dependent-based mapping of early visual areas in monkeys using a volume coil. In summary, both extensive behavioral training of monkeys and application of segmented gradient-echo EPI sequence improved signal-to-noise ratio and image quality. Understanding the effects these factors have is important for the application of high field imaging methods to the detection of submillimeter functional structures in the awake monkey brain.

Default mode of brain function in monkeys

Human neuroimaging has revealed a specific network of brain regions-the default-mode network (DMN)-that reduces its activity during goal-directed behavior. So far, evidence for a similar network in monkeys is mainly indirect, since, except for one positron emission tomography study, it is all based on functional connectivity analysis rather than activity increases during passive task states. Here, we tested whether a consistent DMN exists in monkeys using its defining property. We performed a meta-analysis of functional magnetic resonance imaging data collected in 10 awake monkeys to reveal areas in which activity consistently decreases when task demands shift from passive tasks to externally oriented processing. We observed task-related spatially specific deactivations across 15 experiments, implying in the monkey a functional equivalent of the human DMN. We revealed by resting-state connectivity that prefrontal and medial parietal regions, including areas 9/46d and 31, respectively, constitute the DMN core, being functionally connected to all other DMN areas. We also detected two distinct subsystems composed of DMN areas with stronger functional connections between each other. These clusters included areas 24/32, 8b, and TPOC and areas 23, v23, and PGm, respectively. Such a pattern of functional connectivity largely fits, but is not completely consistent with anatomical tract tracing data in monkeys. Also, analysis of afferent and efferent connections between DMN areas suggests a multisynaptic network structure. Like humans, monkeys increase activity during passive epochs in heteromodal and limbic association regions, suggesting that they also default to internal modes of processing when not actively interacting with the environment.

The extraction of 3D shape in the visual system of human and nonhuman primates

Depth structure, the third dimension of object shape, is extracted from disparity, motion, texture, and shading in the optic array. Gradient-selective neurons play a key role in this process. Such neurons occur in CIP, AIP, TEs, and F5 (for first- or second-order disparity gradients), in MT/V5, in FST (for speed gradients), and in CIP and TEs (for texture gradients). Most of these regions are activated during magnetic resonance scanning in alert monkeys by comparing 3D conditions with the 2D controls for the different cues. Similarities in activation patterns of monkeys and humans tested with identical paradigms suggest that like gradient-selective neurons are found in corresponding human cortical areas. This view gains credence as the homologies between such areas become more evident. Furthermore, 3D shape-processing networks are similar in the two species, with the exception of the greater involvement of human posterior parietal cortex in the extraction of 3D shape from motion. Thus we can begin to understand how depth structure is extracted from motion, disparity, and texture in the primate brain, but the extraction of depth structure from shading and that of wire-like objects requires further scrutiny.

The human brain: rewired and running hot

The past two decades have witnessed tremendous advances in noninvasive and postmortem neuroscientific techniques, advances that have made it possible, for the first time, to compare in detail the organization of the human brain to that of other primates. Studies comparing humans to chimpanzees and other great apes reveal that human brain evolution was not merely a matter of enlargement, but involved changes at all levels of organization that have been examined. These include the cellular and laminar organization of cortical areas; the higher order organization of the cortex, as reflected in the expansion of association cortex (in absolute terms, as well as relative to primary areas); the distribution of long-distance cortical connections; and hemispheric asymmetry. Additionally, genetic differences between humans and other primates have proven to be more extensive than previously thought, raising the possibility that human brain evolution involved significant modifications of neurophysiology and cerebral energy metabolism.

fMRI of cocaine self-administration in macaques reveals functional inhibition of basal ganglia

Disparities in cocaine-induced neurochemical and metabolic responses between human beings and rodents motivate the use of non-human primates (NHP) to model consequences of repeated cocaine exposure in human subjects. To characterize the functional response to cocaine infusion in NHP brain, we employed contrast-enhanced fMRI during both non-contingent injection of drug and self-administration of cocaine in the magnet. Cocaine robustly decreased cerebral blood volume (CBV) throughout basal ganglia and motor/pre-motor cortex and produced subtle functional inhibition of prefrontal cortex. No brain regions exhibited significant elevation of CBV in response to cocaine challenge. Theses effects in NHP brain are opposite in sign to the cocaine-induced fMRI response in rats, but consistent with previous measurements in NHP based on glucose metabolism. Because the striatal ratio of D2 to D1 receptors is larger in human beings and NHP than rats, we hypothesize that the inhibitory effects of D2 receptor binding dominate the functional response in primates, whereas excitatory D1 receptor stimulation predominates in the rat. If the NHP accurately models the human response to cocaine, downregulation of D2 receptors in human cocaine-abusing populations can be expected to blunt cocaine-induced functional responses, contributing to the weak and variable fMRI responses reported in human basal ganglia following cocaine infusion.

Bayesian modeling of perceived surface slant from actively-generated and passively-observed optic flow

We measured perceived depth from the optic flow (a) when showing a stationary physical or virtual object to observers who moved their head at a normal or slower speed, and (b) when simulating the same optic flow on a computer and presenting it to stationary observers. Our results show that perceived surface slant is systematically distorted, for both the active and the passive viewing of physical or virtual surfaces. These distortions are modulated by head translation speed, with perceived slant increasing directly with the local velocity gradient of the optic flow. This empirical result allows us to determine the relative merits of two alternative approaches aimed at explaining perceived surface slant in active vision: an "inverse optics" model that takes head motion information into account, and a probabilistic model that ignores extra-retinal signals. We compare these two approaches within the framework of the bayesian theory. The "inverse optics" bayesian model produces veridical slant estimates if the optic flow and the head translation velocity are measured with no error; because of the influence of a "prior" for flatness, the slant estimates become systematically biased as the measurement errors increase. The bayesian model, which ignores the observer's motion, always produces distorted estimates of surface slant. Interestingly, the predictions of this second model, not those of the first one, are consistent with our empirical findings. The present results suggest that (a) in active vision perceived surface slant may be the product of probabilistic processes which do not guarantee the correct solution, and (b) extra-retinal signals may be mainly used for a better measurement of retinal information.

Diffusion-weighted imaging tractography-based parcellation of the human parietal cortex and comparison with human and macaque resting-state functional connectivity

Despite the prominence of parietal activity in human neuroimaging investigations of sensorimotor and cognitive processes, there remains uncertainty about basic aspects of parietal cortical anatomical organization. Descriptions of human parietal cortex draw heavily on anatomical schemes developed in other primate species, but the validity of such comparisons has been questioned by claims that there are fundamental differences between the parietal cortex in humans and other primates. A scheme is presented for parcellation of human lateral parietal cortex into component regions on the basis of anatomical connectivity and the functional interactions of the resulting clusters with other brain regions. Anatomical connectivity was estimated using diffusion-weighted magnetic resonance image (MRI)-based tractography, and functional interactions were assessed by correlations in activity measured with functional MRI at rest. Resting-state functional connectivity was also assessed directly in the rhesus macaque lateral parietal cortex in an additional experiment, and the patterns found reflected known neuroanatomical connections. Cross-correlation in the tractography-based connectivity patterns of parietal voxels reliably parcellated human lateral parietal cortex into 10 component clusters. The resting-state functional connectivity of human superior parietal and intraparietal clusters with frontal and extrastriate cortex suggested correspondences with areas in macaque superior and intraparietal sulcus. Functional connectivity patterns with parahippocampal cortex and premotor cortex again suggested fundamental correspondences between inferior parietal cortex in humans and macaques. In contrast, the human parietal cortex differs in the strength of its interactions between the central inferior parietal lobule region and the anterior prefrontal cortex.

The selectivity of neurons in the macaque fundus of the superior temporal area for three-dimensional structure from motion

Motion is a potent cue for the perception of three-dimensional (3D) shape in primates, but little is known about its underlying neural mechanisms. Guided by recent functional magnetic resonance imaging results, we tested neurons in the fundus of the superior temporal sulcus (FST) area of two macaque monkeys (Macaca mulatta, one male) using motion-defined surface patches with various 3D shapes such as slanted planes, saddles, or cylinders. The majority of the FST neurons (>80%) were selective for stimuli depicting specific shapes, and all the surfaces tested were represented among the selective FST neurons. Importantly, this selectivity tolerated changes in speed, position, size, or between binocular and monocular presentations. This tolerance demonstrates that the 3D structure-from-motion (3D-SFM) selectivity of FST neurons is a higher-order selectivity, which cannot be reduced to a lower-order speed selectivity. The 3D-SFM selectivity of FST neurons was unaffected by removal of the opposed-motion cue that supplemented the speed gradient cue in the standard stimuli. When tested with the same standard stimuli, fewer neurons in the middle temporal/visual 5 (MT/V5) area were selective than FST neurons. In addition, selective MT/V5 neurons represented fewer types of surfaces and were less tolerant of stimulus changes than FST neurons. Overall, these results indicate that FST neurons code motion-defined 3D shape fragments, underscoring the central role of FST in processing 3D-SFM.

The retinotopic organization of the human middle temporal area MT/V5 and its cortical neighbors

Although there is general agreement that the human middle temporal (MT)/V5+ complex corresponds to monkey area MT/V5 proper plus a number of neighboring motion-sensitive areas, the identification of human MT/V5 within the complex has proven difficult. Here, we have used functional magnetic resonance imaging and the retinotopic mapping technique, which has very recently disclosed the organization of the visual field maps within the monkey MT/V5 cluster. We observed a retinotopic organization in humans very similar to that documented in monkeys: an MT/V5 cluster that includes areas MT/V5, pMSTv (putative ventral part of the medial superior temporal area), pFST (putative fundus of the superior temporal area), and pV4t (putative V4 transitional zone), and neighbors a more ventral putative human posterior inferior temporal area (phPIT) cluster. The four areas in the MT/V5 cluster and the two areas in the phPIT cluster each represent the complete contralateral hemifield. The complete MT/V5 cluster comprises 70% of the motion localizer activation. Human MT/V5 is located in the region bound by lateral, anterior, and inferior occipital sulci and occupies only one-fifth of the motion complex. It shares the basic functional properties of its monkey homolog: receptive field size relative to other areas, response to moving and static stimuli, as well as sensitivity to three-dimensional structure from motion. Functional properties sharply distinguish the MT/V5 cluster from its immediate neighbors in the phPIT cluster and the LO (lateral occipital) regions. Together with similarities in retinotopic organization and topological neighborhood, the functional properties suggest that MT/V5 in human and macaque cortex are homologous.

Coding Observed Motor Acts: Different Organizational Principles in the Parietal and Premotor Cortex of Humans

Understanding actions of conspecifics is a fundamental social ability depending largely on the activation of a parieto-frontal network. Using functional MRI (fMRI), we studied how goal-directed movements (i.e., motor acts) performed by others are coded within this network. In the first experiment, we presented volunteers with video clips showing four different motor acts (dragging, dropping, grasping, and pushing) performed with different effectors (foot, hand, and mouth). We found that the coding of observed motor acts differed between premotor and parietal cortex. In the premotor cortex, they clustered according to the effector used, and in the inferior parietal lobule (IPL), they clustered according to the type of the observed motor act, regardless of the effector. Two subsequent experiments in which we directly contrasted these four motor acts indicated that, in IPL, the observed motor acts are coded according to the relationship between agent and object: Movements bringing the object toward the agent (grasping and dragging) activate a site corresponding approximately to the ventral part of the putative human AIP (phAIP), whereas movements moving the object away from the agent (pushing and dropping) are clustered dorsally within this area. These data provide indications that the phAIP region plays a role in categorizing motor acts according to their behavioral significance. In addition, our results suggest that in the case of motor acts typically done with the hand, the representations of such acts in phAIP are used as templates for coding motor acts executed with other effectors.

Space representation for eye movements is more contralateral in monkeys than in humans

Contralateral hemispheric representation of sensory inputs (the right visual hemifield in the left hemisphere and vice versa) is a fundamental feature of primate sensorimotor organization, in particular the visuomotor system. However, many higher-order cognitive functions in humans show an asymmetric hemispheric lateralization--e.g., right brain specialization for spatial processing--necessitating a convergence of information from both hemifields. Electrophysiological studies in monkeys and functional imaging in humans have investigated space and action representations at different stages of visuospatial processing, but the transition from contralateral to unified global spatial encoding and the relationship between these encoding schemes and functional lateralization are not fully understood. Moreover, the integration of data across monkeys and humans and elucidation of interspecies homologies is hindered, because divergent findings may reflect actual species differences or arise from discrepancies in techniques and measured signals (electrophysiology vs. imaging). Here, we directly compared spatial cue and memory representations for action planning in monkeys and humans using event-related functional MRI during a working-memory oculomotor task. In monkeys, cue and memory-delay period activity in the frontal, parietal, and temporal regions was strongly contralateral. In putative human functional homologs, the contralaterality was significantly weaker, and the asymmetry between the hemispheres was stronger. These results suggest an inverse relationship between contralaterality and lateralization and elucidate similarities and differences in human and macaque cortical circuits subserving spatial awareness and oculomotor goal-directed actions.

Genes and brain sex differences

Throughout development, numerous biological events occur that differentially affect males and females. Specifically, sex-determining genes that are triggered by the sex-chromosome complement initiate a series of events that determine an organism's sex and lead to the differentiation of the body in sex-specific ways. Such events contribute to many unique sex differences, including the susceptibility to different diseases. Although it was believed that sex hormones singularly differentiated the brain and body, there is emerging research showing that genes also play a direct role. In this chapter, we review this line of work and focus on the use of a unique mouse model that separates the effect of gonadal hormones and sex chromosomes. As genetic technology continues to advance, our understanding of the role that hormones and genes play in sex differences can be used to advance the physical and mental health of both men and women.

The extraction of depth structure from shading and texture in the macaque brain

We used contrast-agent enhanced functional magnetic resonance imaging (fMRI) in the alert monkey to map the cortical regions involved in the extraction of 3D shape from the monocular static cues, texture and shading. As in the parallel human imaging study, we contrasted the 3D condition to several 2D control conditions. The extraction of 3D shape from texture (3D SfT) involves both ventral and parietal regions, in addition to early visual areas. Strongest activation was observed in CIP, with decreasing strength towards the anterior part of the intraparietal sulcus (IPS). In the ventral stream 3D SfT sensitivity was observed in a ventral portion of TEO. The extraction of 3D shape from shading (3D SfS) involved predominantly ventral regions, such as V4 and a dorsal potion of TEO. These results are similar to those obtained earlier in human subjects and indicate that the extraction of 3D shape from texture is performed in both ventral and dorsal regions for both species, as are the motion and disparity cues, whereas shading is mainly processed in the ventral stream.

Motion on Numbers: Transcranial Magnetic Stimulation on the Ventral Intraparietal Sulcus Alters Both Numerical and Motion Processes

It has often been proposed that there is a close link between representation of number and space. In the present work, single-pulse transcranial magnetic stimulation (TMS) was applied to the ventral intraparietal sulcus (VIPS) to determine effects on performance in motion detection and number comparison tasks. Participants' reaction times and thresholds for perception of laterally presented coherent motion in random dot kinematograms increased significantly when the contralateral VIPS was stimulated in contrast to the interhemispheric sulcus (Experiment 1) and to the ipsilateral VIPS (Experiment 2). In number comparison tasks, participants compared the magnitude of the laterally presented numbers 1–9 with the number 5. Again, reaction times significantly increased when TMS was applied to the contralateral VIPS in contrast to control sites. The finding that VIPS-directed TMS results in impaired efficiency in both motion perception and number comparison suggests that these processes share a common neural substrate.

The neuroscience of vision-based grasping: a functional review for computational modeling and bio-inspired robotics

The topic of vision-based grasping is being widely studied in humans and in other primates using various techniques and with different goals. The fundamental related findings are reviewed in this paper, with the aim of providing researchers from different fields, including intelligent robotics and neural computation, a comprehensive but accessible view on the subject. A detailed description of the principal sensorimotor processes and the brain areas involved is provided following a functional perspective, in order to make this survey especially useful for computational modeling and bio-inspired robotic applications.

Does the middle temporal area carry vestibular signals related to self-motion?

Recent studies have described vestibular responses in the dorsal medial superior temporal area (MSTd), a region of extrastriate visual cortex thought to be involved in self-motion perception. The pathways by which vestibular signals are conveyed to area MSTd are currently unclear, and one possibility is that vestibular signals are already present in areas that are known to provide visual inputs to MSTd. Thus, we examined whether selective vestibular responses are exhibited by single neurons in the middle temporal area (MT), a visual motion-sensitive region that projects heavily to area MSTd. We compared responses in MT and MSTd to three-dimensional rotational and translational stimuli that were either presented using a motion platform (vestibular condition) or simulated using optic flow (visual condition). When monkeys fixated a visual target generated by a projector, half of MT cells (and most MSTd neurons) showed significant tuning during the vestibular rotation condition. However, when the fixation target was generated by a laser in a dark room, most MT neurons lost their vestibular tuning whereas most MSTd neurons retained their selectivity. Similar results were obtained for free viewing in darkness. Our findings indicate that MT neurons do not show genuine vestibular responses to self-motion; rather, their tuning in the vestibular rotation condition can be explained by retinal slip due to a residual vestibulo-ocular reflex. Thus, the robust vestibular signals observed in area MSTd do not arise through inputs from area MT.

Subsecond timing in primates: comparison of interval production between human subjects and rhesus monkeys

This study describes the psychometric similarities and differences in motor timing performance between 20 human subjects and three rhesus monkeys during two timing production tasks. These tasks involved tapping on a push-button to produce the same set of intervals (range of 450 to 1,000 ms), but they differed in the number of intervals produced (single vs. multiple) and the modality of the stimuli (auditory vs. visual) used to define the time intervals. The data showed that for both primate species, variability increased as a function of the length of the produced target interval across tasks, a result in accordance with the scalar property. Interestingly, the temporal performance of rhesus monkeys was equivalent to that of human subjects during both the production of single intervals and the tapping synchronization to a metronome. Overall, however, human subjects were more accurate than monkeys and showed less timing variability. This was especially true during the self-pacing phase of the multiple interval production task, a behavior that may be related to complex temporal cognition, such as speech and music execution. In addition, the well-known human bias toward auditory as opposed to visual cues for the accurate execution of time intervals was not evident in rhesus monkeys. These findings validate the rhesus monkey as an appropriate model for the study of the neural basis of time production, but also suggest that the exquisite temporal abilities of humans, which peak in speech and music performance, are not all shared with macaques.

Modulation of the contrast response function by electrical microstimulation of the macaque frontal eye field

Spatial attention influences representations in visual cortical areas as well as perception. Some models predict a contrast gain, whereas others a response or activity gain when attention is directed to a contrast-varying stimulus. Recent evidence has indicated that microstimulating the frontal eye field (FEF) can produce modulations of cortical area V4 neuronal firing rates that resemble spatial attention-like effects, and we have shown similar modulations of functional magnetic resonance imaging (fMRI) activity throughout the visual system. Here, we used fMRI in awake, fixating monkeys to first measure the response in 12 visual cortical areas to stimuli of varying luminance contrast. Next, we simultaneously microstimulated subregions of the FEF with movement fields that overlapped the stimulus locations and measured how microstimulation modulated these contrast response functions (CRFs) throughout visual cortex. In general, we found evidence for a nonproportional scaling of the CRF under these conditions, resembling a contrast gain effect. Representations of low-contrast stimuli were enhanced by stimulation of the FEF below the threshold needed to evoke saccades, whereas high-contrast stimuli were unaffected or in some areas even suppressed. Furthermore, we measured a characteristic spatial pattern of enhancement and suppression across the cortical surface, from which we propose a simple schematic of this contrast-dependent fMRI response.

3D surface perception from motion involves a temporal-parietal network

Previous research has suggested that three-dimensional (3D) structure-from-motion (SFM) perception in humans involves several motion-sensitive occipital and parietal brain areas. By contrast, SFM perception in nonhuman primates seems to involve the temporal lobe including areas MT, MST and FST. The present functional magnetic resonance imaging study compared several motion-sensitive regions of interest including the superior temporal sulcus (STS) while human observers viewed horizontally moving dots that defined either a 3D corrugated surface or a 3D random volume. Low-level stimulus features such as dot density and velocity vectors as well as attention were tightly controlled. Consistent with previous research we found that 3D corrugated surfaces elicited stronger responses than random motion in occipital and parietal brain areas including area V3A, the ventral and dorsal intraparietal sulcus, the lateral occipital sulcus and the fusiform gyrus. Additionally, 3D corrugated surfaces elicited stronger activity in area MT and the STS but not in area MST. Brain activity in the STS but not in area MT correlated with interindividual differences in 3D surface perception. Our findings suggest that area MT is involved in the analysis of optic flow patterns such as speed gradients and that the STS in humans plays a greater role in the analysis of 3D SFM than previously thought.

Visual field map clusters in macaque extrastriate visual cortex

The macaque visual cortex contains >30 different functional visual areas, yet surprisingly little is known about the underlying organizational principles that structure its components into a complete "visual" unit. A recent model of visual cortical organization in humans suggests that visual field maps are organized as clusters. Clusters minimize axonal connections between individual field maps that represent common visual percepts, with different clusters thought to carry out different functions. Experimental support for this hypothesis, however, is lacking in macaques, leaving open the question of whether it is unique to humans or a more general model for primate vision. Here we show, using high-resolution blood oxygen level-dependent functional magnetic resonance imaging data in the awake monkey at 7 T, that the middle temporal area (area MT/V5) and its neighbors are organized as a cluster with a common foveal representation and a circular eccentricity map. This novel view on the functional topography of area MT/V5 and satellites indicates that field map clusters are evolutionarily preserved and may be a fundamental organizational principle of the Old World primate visual cortex.

Dorsal-ventral integration in the recognition of motion-defined unfamiliar faces

The primate visual system is organized into two parallel anatomical pathways, both originating in early visual areas but terminating in posterior parietal or inferior temporal regions. Classically, these two pathways have been thought to subserve spatial vision and visual guided actions (dorsal pathway) and object identification (ventral pathway). However, evidence is accumulating that dorsal visual areas may also represent many aspects of object shape in absence of demands for attention or action. Dorsal visual areas exhibit selectivity for three-dimensional cues of depth and are considered necessary for the extraction of surfaces from depth cues and can carry out cognitive functions with such cues as well. These results suggest that dorsal visual areas may participate in object recognition, but it is unclear to what capacity. Here, we tested whether three-dimensional structure-from-motion (SFM) cues, thought to be computed exclusively by dorsal stream mechanisms, are sufficient to drive complex object recognition. We then tested whether recognition of such stimuli relies on dorsal stream mechanisms alone, or whether dorsal-ventral integration is invoked. Results suggest that such cues are sufficient to drive unfamiliar face recognition in normal participants and that ventral stream areas are necessary for both identification and learning of unfamiliar faces from SFM cues.

The functional role of the inferior parietal lobe in the dorsal and ventral stream dichotomy

Current models of the visual pathways have difficulty incorporating the human inferior parietal lobe (IPL) into dorsal or ventral streams. Some recent proposals have attempted to integrate aspects of IPL function that were not hitherto dealt with well, such as differences between the left and right hemisphere and the role of the right IPL in responding to salient environmental events. However, we argue that these models also fail to capture adequately some important findings regarding the functions of the IPL. Here we critically appraise existing proposals regarding the functional architecture of the visual system, with special emphasis on the role of this region, particularly in the right hemisphere. We review evidence that shows the right IPL plays an important role in two different, but broadly complementary, aspects of attention: maintaining attentive control on current task goals as well as responding to salient new information or alerting stimuli in the environment. In our view, findings from functional imaging, electrophysiological and lesion studies are all consistent with the view that this region is part of a system that allows flexible reconfiguration of behaviour between these two alternative modes of operation. Damage to the right IPL leads to deficits in both maintaining attention and also responding to salient events, impairments that contribute to hemineglect, the classical syndrome that follows lesions of this region.

Role of a lateralized parietal-basal ganglia circuit in hierarchical pattern perception: evidence from Parkinson's disease

The role of corticostriatal circuits in hierarchical pattern perception was examined in Parkinson's disease. The hypothesis was tested that patients with right-side onset of motor symptoms (RPD, left hemisphere dysfunction) would be impaired at local level processing because the left posterior temporoparietal junction (TP) emphasizes processing of local information. By contrast, left-side onset patients (LPD; right hemisphere dysfunction) would show impaired global processing because right TP emphasizes global processing. Participants identified targets at local or global levels without and with attention biased toward those levels. Despite normal attentional control between levels, LPD patients showed a single dissociation, demonstrating abnormal global level processing under all conditions, whereas RPD patients showed abnormal local level processing mainly when attention was biased toward the local level. These findings link side of motor symptom onset to visuospatial cognitive abilities that depend upon the contralateral TP, highlighting that side of onset can predict visuospatial impairments, and provide evidence that an inferior parietal-basal ganglia pathway involving the caudate head and the hemispherically asymmetrical TP region is necessary for hierarchical pattern perception.

The processing of three-dimensional shape from disparity in the human brain

Three-dimensional (3D) shape is important for the visual control of grasping and manipulation and for object recognition. Although there has been some progress in our understanding of how 3D shape is extracted from motion and other monocular cues, little is known of how the human brain extracts 3D shape from disparity, commonly regarded as the strongest depth cue. Previous fMRI studies in the awake monkey have established that the interaction between stereo (present or absent) and the order of disparity (zero or second order) constitutes the MR signature of regions housing second-order disparity-selective neurons (Janssen et al., 2000; Srivastava et al., 2006; Durand et al., 2007; Joly et al., 2007). Testing the interaction between stereo and order of disparity in a large cohort of human subjects, revealed the involvement of five IPS regions (VIPS/V7*, POIPS, DIPSM, DIPSA, and phAIP), as well as V3 and the V3A complex in occipital cortex, the posterior inferior temporal gyrus (ITG), and ventral premotor cortex (vPrCS) in the extraction and processing of 3D shape from stereo. Control experiments ruled out attention and convergence eye movements as confounding factors. Many of these regions, DIPSM, DIPSA, phAIP, and probably posterior ITG and ventral premotor cortex, correspond to monkey regions with similar functionality, whereas the evolutionarily new or modified regions are located in occipital (the V3A complex) and occipitoparietal cortex (VIPS/V7* and POIPS). Interestingly, activity in these occipital regions correlates with the depth amplitude perceived by the subjects in the 3D surfaces used as stimuli in these fMRI experiments.

Camouflage and visual perception

How does an animal conceal itself from visual detection by other animals? This review paper seeks to identify general principles that may apply in this broad area. It considers mechanisms of visual encoding, of grouping and object encoding, and of search. In most cases, the evidence base comes from studies of humans or species whose vision approximates to that of humans. The effort is hampered by a relatively sparse literature on visual function in natural environments and with complex foraging tasks. However, some general constraints emerge as being potentially powerful principles in understanding concealment--a 'constraint' here means a set of simplifying assumptions. Strategies that disrupt the unambiguous encoding of discontinuities of intensity (edges), and of other key visual attributes, such as motion, are key here. Similar strategies may also defeat grouping and object-encoding mechanisms. Finally, the paper considers how we may understand the processes of search for complex targets in complex scenes. The aim is to provide a number of pointers towards issues, which may be of assistance in understanding camouflage and concealment, particularly with reference to how visual systems can detect the shape of complex, concealed objects.

Parietal regions processing visual 3D shape extracted from disparity

Three-dimensional (3D) shape is important for the visual control of grasping and manipulation. We used fMRI to study the processing of 3D shape extracted from disparity in human parietal cortex. Subjects stereoscopically viewed random-line stimuli portraying a 3D structure, a 2D structure in multiple depth planes or a 2D structure in the fixation plane. Subtracting the second from the first condition yields depth-structure sensitive regions and subtracting the third from the second position-in-depth sensitive regions. Two anterior intraparietal sulcus (IPS) regions, the dorsal IPS medial (DIPSM) and the dorsal IPS anterior (DIPSA) regions, were sensitive to depth structure and not to position in depth, while a posterior IPS region, the ventral IPS (VIPS) region, had a mixed sensitivity. All three IPS regions were also sensitive to 2D shape, indicating that they carry full 3D shape information. Finally DIPSM, but not DIPSA was sensitive to a saccade-related task. These results underscore the importance of anterior IPS regions in the processing of 3D shape, in agreement with their proximity to grasping-related regions. Moreover, comparison with the results of Durand, J.B., Nelissen, K., Joly, O., Wardak, C., Todd, J.T., Norman, J.F., Janssen, P., Vanduffel, W., Orban, G.A., 2007. Anterior Regions of Monkey Parietal Cortex Process Visual 3D Shape. Neuron 55, 493-505 obtained in the monkey indicates that DIPSA and DIPSM may represent human homologues for the posterior part of AIP and the adjoining part of LIP respectively.

Adaptive estimation of three-dimensional structure in the human brain

Perceiving the three-dimensional (3D) properties of the environment relies on the brain bringing together ambiguous cues (e.g., binocular disparity, shading, texture) with information gained from short- and long-term experience. Perceptual aftereffects, in which the perception of an ambiguous 3D stimulus is biased away from the shape of a previously viewed stimulus, provide a sensitive means of probing this process, yet little is known about their neural basis. Here, we investigate 3D aftereffects using psychophysical and functional MRI (fMRI) adaptation paradigms to gain insight into the cortical circuits that mediate the perceptual interpretation of ambiguous depth signals. Using two classic bistable stimuli (Mach card, kinetic depth effect), we test aftereffects produced by 3D shapes defined by binocular (disparity) or monocular (texture, shading) depth cues. We show that the processing of ambiguous 3D stimuli in dorsal visual cortical areas (V3B/KO, V7) and posterior parietal regions is modulated by adaptation in line with perceptual aftereffects. Similar behavioral and fMRI adaptation effects for the two types of bistable stimuli suggest common neural substrates for depth aftereffects independent of the inducing depth cues (disparity, texture, shading). In line with current thinking about the role of adaptation in sensory optimization, our findings provide evidence that estimation of 3D shape in dorsal cortical areas takes account of the adaptive context to resolve depth ambiguity and interpret 3D structure.

Neural representations of faces and body parts in macaque and human cortex: a comparative FMRI study

Single-cell studies in the macaque have reported selective neural responses evoked by visual presentations of faces and bodies. Consistent with these findings, functional magnetic resonance imaging studies in humans and monkeys indicate that regions in temporal cortex respond preferentially to faces and bodies. However, it is not clear how these areas correspond across the two species. Here, we directly compared category-selective areas in macaques and humans using virtually identical techniques. In the macaque, several face- and body part-selective areas were found located along the superior temporal sulcus (STS) and middle temporal gyrus (MTG). In the human, similar to previous studies, face-selective areas were found in ventral occipital and temporal cortex and an additional face-selective area was found in the anterior temporal cortex. Face-selective areas were also found in lateral temporal cortex, including the previously reported posterior STS area. Body part-selective areas were identified in the human fusiform gyrus and lateral occipitotemporal cortex. In a first experiment, both monkey and human subjects were presented with pictures of faces, body parts, foods, scenes, and man-made objects, to examine the response profiles of each category-selective area to the five stimulus types. In a second experiment, face processing was examined by presenting upright and inverted faces. By comparing the responses and spatial relationships of the areas, we propose potential correspondences across species. Adjacent and overlapping areas in the macaque anterior STS/MTG responded strongly to both faces and body parts, similar to areas in the human fusiform gyrus and posterior STS. Furthermore, face-selective areas on the ventral bank of the STS/MTG discriminated both upright and inverted faces from objects, similar to areas in the human ventral temporal cortex. Overall, our findings demonstrate commonalities and differences in the wide-scale brain organization between the two species and provide an initial step toward establishing functionally homologous category-selective areas.