Caudal Intraparietal Sulcus and three-dimensional vision: A combined functional magnetic resonance imaging and single-cell study

The visual representation of 3D orientation in macaque areas STPp and VPS

In the current study, we investigated the neural mechanisms underlying the representation of three-dimensional (3D) surface orientation within the posterior portion of the superior temporal polysensory area (STPp) and the visual posterior Sylvian area (VPS) in the macaque brain. Both areas are known for their integration of visual and vestibular signals, which are crucial for visual stability and spatial perception. However, it remains unclear how exactly these areas represent the orientation of 3D surfaces. To tackle this question, we used random dot stereograms (RDS) to present 3D planar stimuli defined by slant and tilt, with depth via binocular disparity. Through this method, we examined how STPp and VPS encode this information. Our results suggest that both regions encode the orientation and depth of 3D surfaces, with interactions among these parameters influencing neural responses. Additionally, we investigated how motion cues affect the perception of 3D surface orientation. STPp consistently encoded plane orientation information regardless of motion cue, whereas VPS responses showed less stability. These findings shed light on the distinct processing mechanisms for 3D spatial information in different cortical areas, offering insights into the neural basis of visual stability and spatial perception. KEY POINTS: Both STPp and VPS can encode 3D surface orientation. Slant is encoded independently from tilt and disparity in STPp and VPS areas. TDD neurons shift their depth preferences based on tilt in STPp and VPS areas. STPp maintains stable 3D orientation encoding under motion conditions, while VPS shows less stability with changes in tilt and disparity preferences.

Functional MRI for stereoscopic vision analysis: an experimental design

PURPOSE

The aim was to establish a functional MRI protocol for analyzing human stereoscopic vision in clinical practice. The feasibility was established in a cohort of 9 healthy subjects to determine the functional cortical areas responsible for virtually relief vision.

METHODS

Nine healthy right-handed subjects underwent orthoptic examination and functional MRI. The activation paradigms used were based on a block sequence with the projection of static and dynamic 2D and 3D test patterns during three experiments. The test patterns were projected through two separate eyepieces to create stereoscopic vision. SPM software was used for post-processing and data analysis.

RESULTS

Among the three different test patterns used, the second, which corresponded to a static high-relief image of a billiard, appeared to be significant for identifying cortical area activation during stereoscopy. In the group analysis, only areas V3A and V6 showed statistically significant activation. Individual analysis revealed activation of the rostral IPS and V5/MT+.

CONCLUSION

More data is needed to determine the precise cortical area of activation for stereoscopy. This study proposes a useful and accessible method for functional MRI analysis of stereoscopy.

Two Disparity Channels in Human Visual Cortex With Different Contrast and Blur Sensitivity

Purpose

Our goal is to describe the contrast and blur sensitivity of multiple horizontal disparity subsystems and to relate them to the contrast and spatial sensitivities of their monocular inputs.

Methods

Steady-state visual evoked potential (SSVEP) amplitudes were recorded in response to dynamic random dot stereograms (DRDSs) alternating at 2 Hz between zero disparity and varying magnitudes of crossed disparity for disparity plane and disparity grating stimuli. Half-image contrasts ranged between 2.5% and 80% and over a range of Gaussian blurs from 1.4 to 12 arcmin. Separate experiments measured contrast and blur sensitivity for the monocular half-images.

Results

The first and second harmonics disparity responses were maximal for disparity gratings and for the disparity plane condition, respectively. The first harmonic of the disparity grating response was more affected by both contrast and blur than was the second harmonic of the disparity plane response, which had higher contrast sensitivity than the first harmonic.

Conclusions

The corrugation frequency, contrast, and blur tuning of the first harmonic suggest that it reflects activity of neurons tuned to higher luminance spatial frequencies that are selective for relative disparity, whereas the second harmonic reflects the activity of neurons sensitive to absolute disparity that are driven by low monocular spatial frequencies.

Translational Relevance

SSVEPs to DRDSs provide two objective neural measures of disparity processing, the first harmonic-whose stimulus preferences are similar to those of behavioral stereoacuity-and the second harmonic that represents an independent disparity-specific but not necessarily stereoscopic mechanism.

Neuronal Representations Supporting Three-Dimensional Vision in Nonhuman Primates

The visual system must reconstruct the dynamic, three-dimensional (3D) world from ambiguous two-dimensional (2D) retinal images. In this review, we synthesize current literature on how the visual system of nonhuman primates performs this transformation through multiple channels within the classically defined dorsal (where) and ventral (what) pathways. Each of these channels is specialized for processing different 3D features (e.g., the shape, orientation, or motion of objects, or the larger scene structure). Despite the common goal of 3D reconstruction, neurocomputational differences between the channels impose distinct information-limiting constraints on perception. Convergent evidence further points to the little-studied area V3A as a potential branchpoint from which multiple 3D-fugal processing channels diverge. We speculate that the expansion of V3A in humans may have supported the emergence of advanced 3D spatial reasoning skills. Lastly, we discuss future directions for exploring 3D information transmission across brain areas and experimental approaches that can further advance the understanding of 3D vision.

Cortical Deficits are Correlated with Impaired Stereopsis in Patients with Strabismus

In this study, we explored the neural mechanism underlying impaired stereopsis and possible functional plasticity after strabismus surgery. We enrolled 18 stereo-deficient patients with intermittent exotropia before and after surgery, along with 18 healthy controls. Functional magnetic resonance imaging data were collected when participants viewed three-dimensional stimuli. Compared with controls, preoperative patients showed hypoactivation in higher-level dorsal (visual and parietal) areas and ventral visual areas. Pre- and postoperative activation did not significantly differ in patients overall; patients with improved stereopsis showed stronger postoperative activation than preoperative activation in the right V3A and left intraparietal sulcus. Worse stereopsis and fusional control were correlated with preoperative hypoactivation, suggesting that cortical deficits along the two streams might reflect impaired stereopsis in intermittent exotropia. The correlation between improved stereopsis and activation in the right V3A after surgery indicates that functional plasticity may underlie the improvement of stereopsis. Thus, additional postoperative strategies are needed to promote functional plasticity and enhance the recovery of stereopsis.

Parallel processing, hierarchical transformations, and sensorimotor associations along the 'where' pathway

Visually guided behaviors require the brain to transform ambiguous retinal images into object-level spatial representations and implement sensorimotor transformations. These processes are supported by the dorsal ‘where’ pathway. However, the specific functional contributions of areas along this pathway remain elusive due in part to methodological differences across studies. We previously showed that macaque caudal intraparietal (CIP) area neurons possess robust 3D visual representations, carry choice- and saccade-related activity, and exhibit experience-dependent sensorimotor associations (Chang et al., 2020b). Here, we used a common experimental design to reveal parallel processing, hierarchical transformations, and the formation of sensorimotor associations along the ‘where’ pathway by extending the investigation to V3A, a major feedforward input to CIP. Higher-level 3D representations and choice-related activity were more prevalent in CIP than V3A. Both areas contained saccade-related activity that predicted the direction/timing of eye movements. Intriguingly, the time course of saccade-related activity in CIP aligned with the temporally integrated V3A output. Sensorimotor associations between 3D orientation and saccade direction preferences were stronger in CIP than V3A, and moderated by choice signals in both areas. Together, the results explicate parallel representations, hierarchical transformations, and functional associations of visual and saccade-related signals at a key juncture in the ‘where’ pathway.

Using High-Density Electroencephalography to Explore Spatiotemporal Representations of Object Categories in Visual Cortex

Visual object perception involves neural processes that unfold over time and recruit multiple regions of the brain. Here, we use high-density EEG to investigate the spatiotemporal representations of object categories across the dorsal and ventral pathways. In , human participants were presented with images from two animate object categories (birds and insects) and two inanimate categories (tools and graspable objects). In , participants viewed images of tools and graspable objects from a different stimulus set, one in which a shape confound that often exists between these categories (elongation) was controlled for. To explore the temporal dynamics of object representations, we employed time-resolved multivariate pattern analysis on the EEG time series data. This was performed at the electrode level as well as in source space of two regions of interest: one encompassing the ventral pathway and another encompassing the dorsal pathway. Our results demonstrate shape, exemplar, and category information can be decoded from the EEG signal. Multivariate pattern analysis within source space revealed that both dorsal and ventral pathways contain information pertaining to shape, inanimate object categories, and animate object categories. Of particular interest, we note striking similarities obtained in both ventral stream and dorsal stream regions of interest. These findings provide insight into the spatio-temporal dynamics of object representation and contribute to a growing literature that has begun to redefine the traditional role of the dorsal pathway.

Perspective Cues Make Eye-specific Contributions to 3-D Motion Perception

Robust 3-D visual perception is achieved by integrating stereoscopic and perspective cues. The canonical model describing the integration of these cues assumes that perspective signals sensed by the left and right eyes are indiscriminately pooled into a single representation that contributes to perception. Here, we show that this model fails to account for 3-D motion perception. We measured the sensitivity of male macaque monkeys to 3-D motion signaled by left-eye perspective cues, right-eye perspective cues, stereoscopic cues, and all three cues combined. The monkeys exhibited idiosyncratic differences in their biases and sensitivities for each cue, including left- and right-eye perspective cues, suggesting that the signals undergo at least partially separate neural processing. Importantly, sensitivity to combined cue stimuli was greater than predicted by the canonical model, which previous studies found to account for the perception of 3-D orientation in both humans and monkeys. Instead, 3-D motion sensitivity was best explained by a model in which stereoscopic cues were integrated with left- and right-eye perspective cues whose representations were at least partially independent. These results indicate that the integration of perspective and stereoscopic cues is a shared computational strategy across 3-D processing domains. However, they also reveal a fundamental difference in how left- and right-eye perspective signals are represented for 3-D orientation versus motion perception. This difference results in more effective use of available sensory information in the processing of 3-D motion than orientation and may reflect the temporal urgency of avoiding and intercepting moving objects.

Common functional localizers to enhance NHP & cross-species neuroscience imaging research

Functional localizers are invaluable as they can help define regions of interest, provide cross-study comparisons, and most importantly, allow for the aggregation and meta-analyses of data across studies and laboratories. To achieve these goals within the non-human primate (NHP) imaging community, there is a pressing need for the use of standardized and validated localizers that can be readily implemented across different groups. The goal of this paper is to provide an overview of the value of localizer protocols to imaging research and we describe a number of commonly used or novel localizers within NHPs, and keys to implement them across studies. As has been shown with the aggregation of resting-state imaging data in the original PRIME-DE submissions, we believe that the field is ready to apply the same initiative for task-based functional localizers in NHP imaging. By coming together to collect large datasets across research group, implementing the same functional localizers, and sharing the localizers and data via PRIME-DE, it is now possible to fully test their robustness, selectivity and specificity. To do this, we reviewed a number of common localizers and we created a repository of well-established localizer that are easily accessible and implemented through the PRIME-RE platform.

Unique Neural Activity Patterns Among Lower Order Cortices and Shared Patterns Among Higher Order Cortices During Processing of Similar Shapes With Different Stimulus Types

We investigated the neural mechanism of the processing of three-dimensional (3D) shapes defined by disparity and perspective. We measured blood oxygenation level-dependent signals as participants viewed and classified 3D images of convex-concave shapes. According to the cue (disparity or perspective) and element type (random dots or black and white dotted lines), three types of stimuli were used: random dot stereogram, black and white dotted lines with perspective, and black and white dotted lines with binocular disparity. The blood oxygenation level-dependent images were then classified by multivoxel pattern analysis. To identify areas selective to shape, we assessed convex-concave classification accuracy with classifiers trained and tested using signals evoked by the same stimulus type (same cue and element type). To identify cortical regions with similar neural activity patterns regardless of stimulus type, we assessed the convex-concave classification accuracy of transfer classification in which classifiers were trained and tested using different stimulus types (different cues or element types). Classification accuracy using the same stimulus type was high in the early visual areas and subregions of the intraparietal sulcus (IPS), whereas transfer classification accuracy was high in the dorsal subregions of the IPS. These results indicate that the early visual areas process the specific features of stimuli, whereas the IPS regions perform more generalized processing of 3D shapes, independent of a specific stimulus type.

Effective Connectivity Reveals an Interconnected Inferotemporal Network for Three-Dimensional Structure Processing

Disparity-defined 3D shape is processed in both the ventral and the dorsal visual stream. The network of cortical areas that is activated during the processing of disparity-defined 3D shape includes, in addition to parietal and premotor areas, three clearly distinct regions in inferotemporal cortex (ITC). To investigate the connectivity of the latter regions, we combined electrical stimulation with fMRI in male macaque monkeys. Electrical stimulation of each of the 3D-structure nodes in ITC mainly elicited increased fMRI activations in the other 3D-structure nodes and more variably in other parts of ventral visual cortex. Importantly, no increased activation was found in parietal areas, nor in PFC, whereas microstimulation in posterior parietal cortex did activate the ITC. Our results indicate that 3D-structure nodes in ITC form a strongly interconnected network, receiving input from parietal areas implicated in 3D-structure processing.SIGNIFICANCE STATEMENT Previous studies combining electrical microstimulation with functional imaging showed an interconnected set of regions in the ventral stream processing faces or bodies, but is has been unclear whether the same is true for other visual categories. Here the authors show that there is a connected system of stereo-selective regions in inferotemporal cortex, receiving input from parietal areas in the dorsal stream.

Receptor-driven, multimodal mapping of cortical areas in the macaque monkey intraparietal sulcus

The intraparietal sulcus (IPS) is structurally and functionally heterogeneous. We performed a quantitative cyto-/myelo- and receptor architectonical analysis to provide a multimodal map of the macaque IPS. We identified 17 cortical areas, including novel areas PEipe, PEipi (external and internal subdivisions of PEip), and MIPd. Multivariate analyses of receptor densities resulted in a grouping of areas based on the degree of (dis)similarity of their receptor architecture: a cluster encompassing areas located in the posterior portion of the IPS and associated mainly with the processing of visual information, a cluster including areas found in the anterior portion of the IPS and involved in sensorimotor processing, and an ‘intermediate’ cluster of multimodal association areas. Thus, differences in cyto-/myelo- and receptor architecture segregate the cortical ribbon within the IPS, and receptor fingerprints provide novel insights into the relationship between the structural and functional segregation of this brain region in the macaque monkey.

Stereomotion Processing in the Nonhuman Primate Brain

The cortical areas that process disparity-defined motion-in-depth (i.e., cyclopean stereomotion [CSM]) were characterized with functional magnetic resonance imaging (fMRI) in two awake, behaving macaques. The experimental protocol was similar to previous human neuroimaging studies. We contrasted the responses to dynamic random-dot patterns that continuously changed their binocular disparity over time with those to a control condition that shared the same properties, except that the temporal frames were shuffled. A whole-brain voxel-wise analysis revealed that in all four cortical hemispheres, three areas showed consistent sensitivity to CSM. Two of them were localized respectively in the lower bank of the superior temporal sulcus (CSMSTS) and on the neighboring infero-temporal gyrus (CSMITG). The third area was situated in the posterior parietal cortex (CSMPPC). Additional regions of interest-based analyses within retinotopic areas defined in both animals indicated weaker but significant responses to CSM within the MT cluster (most notably in areas MSTv and FST). Altogether, our results are in agreement with previous findings in both human and macaque and suggest that the cortical areas that process CSM are relatively well preserved between the two primate species.

Optimized but Not Maximized Cue Integration for 3D Visual Perception

Reconstructing three-dimensional (3D) scenes from two-dimensional (2D) retinal images is an ill-posed problem. Despite this, 3D perception of the world based on 2D retinal images is seemingly accurate and precise. The integration of distinct visual cues is essential for robust 3D perception in humans, but it is unclear whether this is true for non-human primates (NHPs). Here, we assessed 3D perception in macaque monkeys using a planar surface orientation discrimination task. Perception was accurate across a wide range of spatial poses (orientations and distances), but precision was highly dependent on the plane's pose. The monkeys achieved robust 3D perception by dynamically reweighting the integration of stereoscopic and perspective cues according to their pose-dependent reliabilities. Errors in performance could be explained by a prior resembling the 3D orientation statistics of natural scenes. We used neural network simulations based on 3D orientation-selective neurons recorded from the same monkeys to assess how neural computation might constrain perception. The perceptual data were consistent with a model in which the responses of two independent neuronal populations representing stereoscopic cues and perspective cues (with perspective signals from the two eyes combined using nonlinear canonical computations) were optimally integrated through linear summation. Perception of combined-cue stimuli was optimal given this architecture. However, an alternative architecture in which stereoscopic cues, left eye perspective cues, and right eye perspective cues were represented by three independent populations yielded two times greater precision than the monkeys. This result suggests that, due to canonical computations, cue integration for 3D perception is optimized but not maximized.

Functional links between sensory representations, choice activity, and sensorimotor associations in parietal cortex

Three-dimensional (3D) representations of the environment are often critical for selecting actions that achieve desired goals. The success of these goal-directed actions relies on 3D sensorimotor transformations that are experience-dependent. Here we investigated the relationships between the robustness of 3D visual representations, choice-related activity, and motor-related activity in parietal cortex. Macaque monkeys performed an eight-alternative 3D orientation discrimination task and a visually guided saccade task while we recorded from the caudal intraparietal area using laminar probes. We found that neurons with more robust 3D visual representations preferentially carried choice-related activity. Following the onset of choice-related activity, the robustness of the 3D representations further increased for those neurons. We additionally found that 3D orientation and saccade direction preferences aligned, particularly for neurons with choice-related activity, reflecting an experience-dependent sensorimotor association. These findings reveal previously unrecognized links between the fidelity of ecologically relevant object representations, choice-related activity, and motor-related activity.

Choice-Related Activity during Visual Slant Discrimination in Macaque CIP But Not V3A

Creating three-dimensional (3D) representations of the world from two-dimensional retinal images is fundamental to visually guided behaviors including reaching and grasping. A critical component of this process is determining the 3D orientation of objects. Previous studies have shown that neurons in the caudal intraparietal area (CIP) of the macaque monkey represent 3D planar surface orientation (i.e., slant and tilt). Here we compare the responses of neurons in areas V3A (which is implicated in 3D visual processing and precedes CIP in the visual hierarchy) and CIP to 3D-oriented planar surfaces. We then examine whether activity in these areas correlates with perception during a fine slant discrimination task in which the monkeys report if the top of a surface is slanted toward or away from them. Although we find that V3A and CIP neurons show similar sensitivity to planar surface orientation, significant choice-related activity during the slant discrimination task is rare in V3A but prominent in CIP. These results implicate both V3A and CIP in the representation of 3D surface orientation, and suggest a functional dissociation between the areas based on slant-related choice signals.

Single-cell responses to three-dimensional structure in a functionally defined patch in macaque area TEO

Both dorsal and ventral visual pathways harbor several areas sensitive to gradients of binocular disparity (i.e., higher-order disparity). Although a wealth of information exists about disparity processing in early visual (V1, V2, and V3) and end-stage areas, TE in the ventral stream, and the anterior intraparietal area (AIP) in the dorsal stream, little is known about midlevel area TEO in the ventral pathway. We recorded single-unit responses to disparity-defined curved stimuli in a functional magnetic resonance imaging (fMRI) activation elicited by curved surfaces compared with flat surfaces in the macaque area TEO. This fMRI activation contained a small proportion of disparity-selective neurons, with very few of them second-order disparity selective. Overall, this population of TEO neurons did not preserve its three-dimensional structure selectivity across positions in depth, indicating a lack of higher-order disparity selectivity, but showed stronger responses to flat surfaces than to curved surfaces, as predicted by the fMRI experiment. The receptive fields of the responsive TEO cells were relatively small and generally foveal. A linear support vector machine classifier showed that this population of disparity-selective TEO neurons contains reliable information about the sign of curvature and the position in depth of the stimulus. NEW & NOTEWORTHY We recorded in a part of the macaque area TEO that is activated more by curved surfaces than by flat surfaces at different disparities using the same stimuli. In contrast to previous studies, this functional magnetic resonance imaging-defined patch did not contain a large number of higher-order disparity-selective neurons. However, a linear support vector machine could reliably classify both the sign of the disparity gradient and the position in depth of the stimuli.

Towards a unified perspective of object shape and motion processing in human dorsal cortex

Although object-related areas were discovered in human parietal cortex a decade ago, surprisingly little is known about the nature and purpose of these representations, and how they differ from those in the ventral processing stream. In this article, we review evidence for the unique contribution of object areas of dorsal cortex to three-dimensional (3-D) shape representation, the localization of objects in space, and in guiding reaching and grasping actions. We also highlight the role of dorsal cortex in form-motion interaction and spatiotemporal integration, possible functional relationships between 3-D shape and motion processing, and how these processes operate together in the service of supporting goal-directed actions with objects. Fundamental differences between the nature of object representations in the dorsal versus ventral processing streams are considered, with an emphasis on how and why dorsal cortex supports veridical (rather than invariant) representations of objects to guide goal-directed hand actions in dynamic visual environments.

More than Action: The Dorsal Pathway Contributes to the Perception of 3-D Structure

An evolving view in cognitive neuroscience is that the dorsal visual pathway not only plays a key role in visuomotor behavior but that it also contributes functionally to the recognition of objects. To characterize the nature of the object representations derived by the dorsal pathway, we assessed perceptual performance in the context of the continuous flash suppression paradigm, which suppresses object processing in the ventral pathway while sparing computation in the dorsal pathway. In a series of experiments, prime stimuli, which were rendered imperceptible by the continuous flash suppression, still contributed to perceptual decisions related to the subsequent perceptible target stimuli. However, the contribution of the prime to perception was contingent on the prime's structural coherence, in that a perceptual advantage was observed only for targets primed by objects with legitimate 3-D structure. Finally, we obtained additional evidence to demonstrate that the processing of the suppressed objects was contingent on the magnocellular, rather than the parvocellular, system, further linking the processing of the suppressed stimuli to the dorsal pathway. Together, these results provide novel evidence that the dorsal pathway does not only support visuomotor control but, rather, that it also derives the structural description of 3-D objects and contributes to shape perception.