Responses to two-dimensional shapes in the macaque anterior intraparietal area

Mixed selectivity in monkey anterior intraparietal area during visual and motor processes

Classical studies suggest that the anterior intraparietal area (AIP) contributes to the encoding of specific information such as objects and actions of self and others, through a variety of neuronal classes, such as canonical, motor and mirror neurons. However, these studies typically focused on a single variable, leaving it unclear whether distinct sets of AIP neurons encode a single or multiple sources of information and how multimodal coding emerges. Here, we chronically recorded monkey AIP neurons in a variety of tasks and conditions classically employed in separate experiments. Most cells exhibited mixed selectivity for observed objects, executed actions, and observed actions, enhanced when this information came from the monkey's peripersonal working space. In contrast with the classical view, our findings indicate that multimodal coding emerges in AIP from partially-mixed selectivity of individual neurons for a variety of information relevant for planning actions directed to both physical objects and other subjects.

Effect of viewing distance on object responses in macaque areas 45B, F5a and F5p

To perform tasks like grasping, the brain has to process visual object information so that the grip aperture can be adjusted before touching the object. Previous studies have demonstrated that the posterior subsector of the Anterior Intraparietal area is connected to area 45B, and its anterior counterpart to F5a. However, the role of area 45B and F5a in visually-guided grasping is poorly understood. Here, we investigated the role of area 45B, F5a and F5p in object processing during visually-guided grasping in two monkeys. We tested whether the presentation of an object in near peripersonal space activated F5p neurons more than objects with the same retinal size presented beyond reachable distance and conversely, whether neurons in 45B and F5a—which may encode a purely visual object representation—were less affected by viewing distance when equalizing retinal size. Contrary to our expectations, we found that most neurons in area 45B were object- and viewing distance-selective, and preferred mostly Near presentations. Area F5a showed much weaker object selectivity compared to 45B, with a similar preference for objects presented at the Near position. Finally, F5p neurons were less object selective and frequently Far-preferring. In sum, area 45B—but not F5p– prefers objects presented in peripersonal space.

Temporal dynamics of neural activity in macaque frontal cortex assessed with large-scale recordings

The cortical network controlling the arm and hand when grasping objects consists of several areas in parietal and frontal cortex. Recently, more anterior prefrontal areas have also been implicated in object grasping, but their exact role is currently unclear. To investigate the neuronal encoding of objects during grasping in these prefrontal regions and their relation with other cortical areas of the grasping network, we performed large-scale recordings (more than 2000 responsive sites) in frontal cortex of monkeys during a saccade-reach-grasp task. When an object appeared in peripheral vision, the first burst of activity emerged in prearcuate areas (the FEF and area 45B), followed by dorsal and ventral premotor cortex, and a buildup of activity in primary motor cortex. After the saccade, prearcuate activity remained elevated while primary motor and premotor activity rose in anticipation of the upcoming arm and hand movement. Remarkably, a large number of premotor and prearcuate sites responded when the object appeared in peripheral vision and remained active when the object came into foveal vision. Thus, prearcuate and premotor areas continuously encode object information when directing gaze and grasping objects.

A brief comparative review of primate posterior parietal cortex: A novel hypothesis on the human toolmaker

The primate visual system contains two major cortical pathways: a ventral-temporal pathway that has been associated with object processing and recognition, and a dorsal-parietal pathway that has been associated with spatial processing and action guidance. Our understanding of the role of the dorsal pathway, in particular, has greatly evolved within the framework of the two-pathway hypothesis since its original conception. Here, we present a comparative review of the primate dorsal pathway in humans and monkeys based on electrophysiological, neuroimaging, neuropsychological, and neuroanatomical studies. We consider similarities and differences across species in terms of the topographic representation of visual space; specificity for eye, reaching, or grasping movements; multi-modal response properties; and the representation of objects and tools. We also review the relative anatomical location of functionally- and topographically-defined regions of the posterior parietal cortex. An emerging theme from this comparative analysis is that non-spatial information is represented to a greater degree, and with increased complexity, in the human dorsal visual system. We propose that non-spatial information in the primate parietal cortex contributes to the perception-to-action system aimed at manipulating objects in peripersonal space. In humans, this network has expanded in multiple ways, including the development of a dorsal object vision system mirroring the complexity of the ventral stream, the integration of object information with parietal working memory systems, and the emergence of tool-specific object representations in the anterior intraparietal sulcus and regions of the inferior parietal lobe. We propose that these evolutionary changes have enabled the emergence of human-specific behaviors, such as the sophisticated use of tools.

Visual motion serves but is not under the purview of the dorsal pathway

Visual motion processing is often attributed to the dorsal visual pathway despite visual motion's involvement in almost all visual functions. Furthermore, some visual motion tasks critically depend on the structural integrity of regions outside the dorsal pathway. Here, based on numerous studies, I propose that visual motion signals are swiftly transmitted via multiple non-hierarchical routes to primary motion-dedicated processing regions (MT/V5 and MST) that are not part of the dorsal pathway, and then propagated to a multiplicity of brain areas according to task demands, reaching these regions earlier than the dorsal/ventral hierarchical flow. This not only places MT/V5 at the same or even earlier visual processing stage as that of V1, but can also elucidate many findings with implications to visual awareness. While the integrity of the non-hierarchical motion pathway is necessary for all visual motion perception, it is insufficient on its own, and the transfer of visual motion signals to additional brain areas is crucial to allow the different motion perception tasks (e.g. optic flow, visuo-vestibular balance, movement observation, dynamic form detection and perception, and even reading). I argue that this lateral visual motion pathway can be distinguished from the dorsal pathway not only based on faster response latencies and distinct anatomical connections, but also based on its full field representation. I also distinguish between this primary lateral visual motion pathway sensitive to all motion in the visual field, and a much less investigated optic flow sensitive medial processing pathway (from V1 to V6 and V6A) that appears to be part of the dorsal pathway. Multiple additional predictions are provided that allow testing this proposal and distinguishing between the visual pathways.

Similarity-Based Fusion of MEG and fMRI Reveals Spatio-Temporal Dynamics in Human Cortex During Visual Object Recognition

Every human cognitive function, such as visual object recognition, is realized in a complex spatio-temporal activity pattern in the brain. Current brain imaging techniques in isolation cannot resolve the brain's spatio-temporal dynamics, because they provide either high spatial or temporal resolution but not both. To overcome this limitation, we developed an integration approach that uses representational similarities to combine measurements of magnetoencephalography (MEG) and functional magnetic resonance imaging (fMRI) to yield a spatially and temporally integrated characterization of neuronal activation. Applying this approach to 2 independent MEG-fMRI data sets, we observed that neural activity first emerged in the occipital pole at 50-80 ms, before spreading rapidly and progressively in the anterior direction along the ventral and dorsal visual streams. Further region-of-interest analyses established that dorsal and ventral regions showed MEG-fMRI correspondence in representations later than early visual cortex. Together, these results provide a novel and comprehensive, spatio-temporally resolved view of the rapid neural dynamics during the first few hundred milliseconds of object vision. They further demonstrate the feasibility of spatially unbiased representational similarity-based fusion of MEG and fMRI, promising new insights into how the brain computes complex cognitive functions.

The neural representation of objects formed through the spatiotemporal integration of visual transients

Oftentimes, objects are only partially and transiently visible as parts of them become occluded during observer or object motion. The visual system can integrate such object fragments across space and time into perceptual wholes or spatiotemporal objects. This integrative and dynamic process may involve both ventral and dorsal visual processing pathways, along which shape and spatial representations are thought to arise. We measured fMRI BOLD response to spatiotemporal objects and used multi-voxel pattern analysis (MVPA) to decode shape information across 20 topographic regions of visual cortex. Object identity could be decoded throughout visual cortex, including intermediate (V3A, V3B, hV4, LO1-2,) and dorsal (TO1-2, and IPS0-1) visual areas. Shape-specific information, therefore, may not be limited to early and ventral visual areas, particularly when it is dynamic and must be integrated. Contrary to the classic view that the representation of objects is the purview of the ventral stream, intermediate and dorsal areas may play a distinct and critical role in the construction of object representations across space and time.

Receptive field properties of neurons in the macaque anterior intraparietal area

Visual object information is necessary for grasping. In primates, the anterior intraparietal area (AIP) plays an essential role in visually guided grasping. Neurons in AIP encode features of objects, but no study has systematically investigated the receptive field (RF) of AIP neurons. We mapped the RF of posterior AIP (pAIP) neurons in the central visual field, using images of objects and small line fragments that evoked robust responses, together with less effective stimuli. The RF sizes we measured varied between 3°(2)and 90°(2), with the highest response either at the fixation point or at parafoveal positions. A large fraction of pAIP neurons showed nonuniform RFs, with multiple local maxima in both ipsilateral and contralateral hemifields. Moreover, the RF profile could depend strongly on the stimulus used to map the RF. Highly similar results were obtained with the smallest stimulus that evoked reliable responses (line fragments measuring 1-2°). The nonuniformity and dependence of the RF profile on the stimulus in pAIP were comparable to previous observations in the anterior part of the lateral intraparietal area (aLIP), but the average RF of pAIP neurons was located at the fovea whereas the average RF of aLIP neurons was located parafoveally. Thus nonuniformity and stimulus dependence of the RF may represent general RF properties of neurons in the dorsal visual stream involved in object analysis, which contrast markedly with those of neurons in the ventral visual stream.

Effects of Microstimulation in the Anterior Intraparietal Area during Three-Dimensional Shape Categorization

The anterior intraparietal area (AIP) of rhesus monkeys is part of the dorsal visual stream and contains neurons whose visual response properties are commensurate with a role in three-dimensional (3D) shape perception. Neuronal responses in AIP signal the depth structure of disparity-defined 3D shapes, reflect the choices of monkeys while they categorize 3D shapes, and mirror the behavioral variability across different stimulus conditions during 3D-shape categorization. However, direct evidence for a role of AIP in 3D-shape perception has been lacking. We trained rhesus monkeys to categorize disparity-defined 3D shapes and examined AIP's contribution to 3D-shape categorization by microstimulating in clusters of 3D-shape selective AIP neurons during task performance. We find that microstimulation effects on choices (monkey M1) and reaction times (monkey M1 and M2) depend on the 3D-shape preference of the stimulated site. Moreover, electrical stimulation of the same cells, during either the 3D-shape-categorization task or a saccade task, could affect behavior differently. Interestingly, in one monkey we observed a strong correlation between the strength of choice-related AIP activity (choice probabilities) and the influence of microstimulation on 3D-shape-categorization behavior (choices and reaction time). These findings propose AIP as part of the network responsible for 3D-shape perception. The results also show that the anterior intraparietal cortex contains cells with different tuning properties, i.e. 3D-shape- or saccade-related, that can be dynamically read out depending on the requirements of the task at hand.

Shape representations in the primate dorsal visual stream

The primate visual system extracts object shape information for object recognition in the ventral visual stream. Recent research has demonstrated that object shape is also processed in the dorsal visual stream, which is specialized for spatial vision and the planning of actions. A number of studies have investigated the coding of 2D shape in the anterior intraparietal area (AIP), one of the end-stage areas of the dorsal stream which has been implicated in the extraction of affordances for the purpose of grasping. These findings challenge the current understanding of area AIP as a critical stage in the dorsal stream for the extraction of object affordances. The representation of three-dimensional (3D) shape has been studied in two interconnected areas known to be critical for object grasping: area AIP and area F5a in the ventral premotor cortex (PMv), to which AIP projects. In both areas neurons respond selectively to 3D shape defined by binocular disparity, but the latency of the neural selectivity is approximately 10 ms longer in F5a compared to AIP, consistent with its higher position in the hierarchy of cortical areas. Furthermore, F5a neurons were more sensitive to small amplitudes of 3D curvature and could detect subtle differences in 3D structure more reliably than AIP neurons. In both areas, 3D-shape selective neurons were co-localized with neurons showing motor-related activity during object grasping in the dark, indicating a close convergence of visual and motor information on the same clusters of neurons.

Effective connectivity of depth-structure-selective patches in the lateral bank of the macaque intraparietal sulcus

Extrastriate cortical areas are frequently composed of subpopulations of neurons encoding specific features or stimuli, such as color, disparity, or faces, and patches of neurons encoding similar stimulus properties are typically embedded in interconnected networks, such as the attention or face-processing network. The goal of the current study was to examine the effective connectivity of subsectors of neurons in the same cortical area with highly similar neuronal response properties. We first recorded single- and multi-unit activity to identify two neuronal patches in the anterior part of the macaque intraparietal sulcus (IPS) showing the same depth structure selectivity and then employed electrical microstimulation during functional magnetic resonance imaging in these patches to determine the effective connectivity of these patches. The two IPS subsectors we identified-with the same neuronal response properties and in some cases separated by only 3 mm-were effectively connected to remarkably distinct cortical networks in both dorsal and ventral stream in three macaques. Conversely, the differences in effective connectivity could account for the known visual-to-motor gradient within the anterior IPS. These results clarify the role of the anterior IPS as a pivotal brain region where dorsal and ventral visual stream interact during object analysis. Thus, in addition to the anatomical connectivity of cortical areas and the properties of individual neurons in these areas, the effective connectivity provides novel key insights into the widespread functional networks that support behavior.

Choice-related Activity in the Anterior Intraparietal Area during 3-D Structure Categorization

The anterior intraparietal area (AIP) of macaques contains neurons that signal the depth structure of disparity-defined 3-D shapes. Previous studies have suggested that AIP's depth information is used for sensorimotor transformations related to the efficient grasping of 3-D objects. We trained monkeys to categorize disparity-defined 3-D shapes and examined whether neuronal activity in AIP may also underlie pure perceptual categorization behavior. We first show that neurons with a similar 3-D shape preference cluster in AIP. We then demonstrate that the monkeys' 3-D shape discrimination performance depends on the position in depth of the stimulus and that this performance difference is reflected in the activity of AIP neurons. We further reveal correlations between the neuronal activity in AIP and the subject's subsequent choices and RTs during 3-D shape categorization. Our findings propose AIP as an important processing stage for 3-D shape perception.

Grasping Execution and Grasping Observation Activity of Single Neurons in the Macaque Anterior Intraparietal Area

Primates use vision to guide their actions in everyday life. Visually guided object grasping is known to rely on a network of cortical areas located in the parietal and premotor cortex. We recorded in the anterior intraparietal area (AIP), an area in the dorsal visual stream that is critical for object grasping and densely connected with the premotor cortex, while monkeys were grasping objects under visual guidance and during passive fixation of videos of grasping actions from the first-person perspective. All AIP neurons in this study responded during grasping execution in the light, that is, became more active after the hand had started to move toward the object and during grasping in the dark. More than half of these AIP neurons responded during the observation of a video of the same grasping actions on a display. Furthermore, these AIP neurons responded as strongly during passive fixation of movements of a hand on a scrambled background and to a lesser extent to a shape appearing within the visual field near the object. Therefore, AIP neurons responding during grasping execution also respond during passive observation of grasping actions and most of them even during passive observation of movements of a simple shape in the visual field.

Coding of shape features in the macaque anterior intraparietal area

The exquisite ability of primates to grasp and manipulate objects relies on the transformation of visual information into motor commands. To this end, the visual system extracts object affordances that can be used to program and execute the appropriate grip. The macaque anterior intraparietal (AIP) area has been implicated in the extraction of affordances for the purpose of grasping. Neurons in the AIP area respond during visually guided grasping and to the visual presentation of objects. A subset of AIP neurons is also activated by two-dimensional images of objects and even by outline contours defining the object shape, but it is unknown how AIP neurons actually represent object shape. In this study, we used a stimulus reduction approach to determine the minimum effective shape feature evoking AIP responses. AIP neurons responding to outline shapes also responded selectively to very small fragment stimuli measuring only 1-2°. This fragment selectivity could not be explained by differences in eye movements or simple orientation selectivity, but proved to be highly dependent on the relative position of the stimulus in the receptive field. Our findings challenge the current understanding of the AIP area as a critical stage in the dorsal stream for the extraction of object affordances.

Human fMRI reveals that delayed action re-recruits visual perception

Behavioral and neuropsychological research suggests that delayed actions rely on different neural substrates than immediate actions; however, the specific brain areas implicated in the two types of actions remain unknown. We used functional magnetic resonance imaging (fMRI) to measure human brain activation during delayed grasping and reaching. Specifically, we examined activation during visual stimulation and action execution separated by a 18-s delay interval in which subjects had to remember an intended action toward the remembered object. The long delay interval enabled us to unambiguously distinguish visual, memory-related, and action responses. Most strikingly, we observed reactivation of the lateral occipital complex (LOC), a ventral-stream area implicated in visual object recognition, and early visual cortex (EVC) at the time of action. Importantly this reactivation was observed even though participants remained in complete darkness with no visual stimulation at the time of the action. Moreover, within EVC, higher activation was observed for grasping than reaching during both vision and action execution. Areas in the dorsal visual stream were activated during action execution as expected and, for some, also during vision. Several areas, including the anterior intraparietal sulcus (aIPS), dorsal premotor cortex (PMd), primary motor cortex (M1) and the supplementary motor area (SMA), showed sustained activation during the delay phase. We propose that during delayed actions, dorsal-stream areas plan and maintain coarse action goals; however, at the time of execution, motor programming requires re-recruitment of detailed visual information about the object through reactivation of (1) ventral-stream areas involved in object perception and (2) early visual areas that contain richly detailed visual representations, particularly for grasping.

Three-dimensional Shape Coding in Grasping Circuits: A Comparison between the Anterior Intraparietal Area and Ventral Premotor Area F5a

Depth information is necessary for adjusting the hand to the three-dimensional (3-D) shape of an object to grasp it. The transformation of visual information into appropriate distal motor commands is critically dependent on the anterior intraparietal area (AIP) and the ventral premotor cortex (area F5), particularly the F5p sector. Recent studies have demonstrated that both AIP and the F5a sector of the ventral premotor cortex contain neurons that respond selectively to disparity-defined 3-D shape. To investigate the neural coding of 3-D shape and the behavioral role of 3-D shape-selective neurons in these two areas, we recorded single-cell activity in AIP and F5a during passive fixation of curved surfaces and during grasping of real-world objects. Similar to those in AIP, F5a neurons were either first- or second-order disparity selective, frequently showed selectivity for discrete approximations of smoothly curved surfaces that contained disparity discontinuities, and exhibited mostly monotonic tuning for the degree of disparity variation. Furthermore, in both areas, 3-D shape-selective neurons were colocalized with neurons that were active during grasping of real-world objects. Thus, area AIP and F5a contain highly similar representations of 3-D shape, which is consistent with the proposed transfer of object information from AIP to the motor system through the ventral premotor cortex.

The role of binocular disparity in stereoscopic images of objects in the macaque anterior intraparietal area

Neurons in the macaque Anterior Intraparietal area (AIP) encode depth structure in random-dot stimuli defined by gradients of binocular disparity, but the importance of binocular disparity in real-world objects for AIP neurons is unknown. We investigated the effect of binocular disparity on the responses of AIP neurons to images of real-world objects during passive fixation. We presented stereoscopic images of natural and man-made objects in which the disparity information was congruent or incongruent with disparity gradients present in the real-world objects, and images of the same objects where such gradients were absent. Although more than half of the AIP neurons were significantly affected by binocular disparity, the great majority of AIP neurons remained image selective even in the absence of binocular disparity. AIP neurons tended to prefer stimuli in which the depth information derived from binocular disparity was congruent with the depth information signaled by monocular depth cues, indicating that these monocular depth cues have an influence upon AIP neurons. Finally, in contrast to neurons in the inferior temporal cortex, AIP neurons do not represent images of objects in terms of categories such as animate-inanimate, but utilize representations based upon simple shape features including aspect ratio.

Cortical regions involved in the observation of bimanual actions

Although we are beginning to understand how observed actions performed by conspecifics with a single hand are processed and how bimanual actions are controlled by the motor system, we know very little about the processing of observed bimanual actions. We used fMRI to compare the observation of bimanual manipulative actions with their unimanual components, relative to visual control conditions equalized for visual motion. Bimanual action observation did not activate any region specialized for processing visual signals related to this more elaborated action. On the contrary, observation of bimanual and unimanual actions activated similar occipito-temporal, parietal and premotor networks. However, whole-brain as well as region of interest (ROI) analyses revealed that this network functions differently under bimanual and unimanual conditions. Indeed, in bimanual conditions, activity in the network was overall more bilateral, especially in parietal cortex. In addition, ROI analyses indicated bilateral parietal activation patterns across hand conditions distinctly different from those at other levels of the action-observation network. These activation patterns suggest that while occipito-temporal and premotor levels are involved with processing the kinematics of the observed actions, the parietal cortex is more involved in the processing of static, postural aspects of the observed action. This study adds bimanual cooperation to the growing list of distinctions between parietal and premotor cortex regarding factors affecting visual processing of observed actions.