Multisensory maps in parietal cortex

Abstract

Parietal cortex has long been known to be a site of sensorimotor integration. Recent findings in humans have shown that it is divided up into a number of small areas somewhat specialized for eye movements, reaching, and hand movements, but also face-related movements (avoidance, eating), lower body movements, and movements coordinating multiple body parts. The majority of these areas contain rough sensory (receptotopic) maps, including a substantial multisensory representation of the lower body and lower visual field immediately medial to face VIP. There is strong evidence for retinotopic remapping in LIP and face-centered remapping in VIP, and weaker evidence for hand-centered remapping. The larger size of the functionally distinct inferior parietal default mode network in humans compared to monkeys results in a superior and medial displacement of middle parietal areas (e.g., the saccade-related LIP's). Multisensory superior parietal areas located anterior to the angular gyrus such as AIP and VIP are less medially displaced relative to macaque monkeys, so that human LIP paradoxically ends up medial to human VIP.

Figure 1

Overlapping retinotopic (upper panel) and somatotopic (lower panel) maps in human anterior parietal cortex. The upper row close-ups show 24-subject average polar angle maps from wide-field direct-view fMRI mapping using a moving video wedge (complex-valued surface-based spherical coordinate system averaging method described in [56]). Four lower → middle → upper field traverses are visible in VIP+ in each hemisphere (yellow arrows). The color contours in the lower row show spherically aligned somatotopic whole body mapping data from [24^••] (face data [15]) over the grayed visual data (body part key is at bottom). Top insets show the location of the magnified views on the unfolded, dorsolaterally tilted cortex.

Figure 2

Human parietal eye, hand, face, and body areas. A rough structural (top) and functional (bottom) parcellation of human parietal cortex is shown. Each of the areas is likely to be involved in multiple additional functions beyond those listed here.

Figure 3

The large relative expansion of the inferior parietal component of the default mode network (Def, transparent purple) in humans compared to macaque monkeys results in the medial displacement of LIP+ (nominally, the lateral intraparietal area) to a position medial to VIP+, shown on folded and inflated macaque and human hemispheres. The monkey parietal default mode network component is taken from [57^••]; the human angular gyrus default mode network was defined as the zone bounded by retinotopic, tonotopic, and somatotopic maps in this subject. All cortical surfaces are shown at the same scale.

Figure 4

Quantitative relaxation rate (R₁ = 1/T₁) maps demarcate cortical areas with heavy gray matter myelination [51,52]. Spherical morph average maps of quantitative R₁ values sampled at 50% of cortical thickness are illustrated as positive variation from the mean (ΔR₁, maxima shown are 3-4% above mean). As expected, densely myelinated primary visual, auditory, and somatomotor cortex and early visual areas MT/FST, V3A, and V6 have the largest R₁ values. Parietal area VIP is the next most densely myelinated, as is an extension off the motor strip, PZ, the polysensory zone, that responds to passive visual and face somatosensory stimuli. In medial parietal cortex, reach-related area V6A is also myelinated.