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

Search Test.

<p>For each stimulus in the Search Test, we plotted the number of neurons that gave the maximal response to that image. The images are ranked from the highest (left) to the lowest number of neurons preferring each particular image.</p

Population analysis Disparity Test.

<p>For each neuron tested we plotted the Object Selectivity Index for the images in congruent stereo mode (3D OSI, defined as (best – worst)/best) as a function of the same index in the absence of disparity (no-stereo mode, 2D OSI). The histograms show the distributions of the 3D and the 2D OSI in our population of AIP neurons.</p

Average responses in the Disparity Test.

<p>(A). Plot showing the average population response to the preferred (dark blue line), the nonpreferred (dark purple line) and the no-stereo mode. For comparison, we also plotted the average monocular responses for each of these conditions (dashed lines). Zero indicates the time of stimulus onset. (B). The average population response is plotted for the congruent stereo mode (dark blue line), the incongruent stereo mode (light blue) and the no-stereo mode (light purple line) for all disparity-selective neurons (N = 63). (C). Histogram showing the number of neurons that preferred congruent (dark grey bars) and incongruent stereo stimuli (light grey bars) for each of the images in the Search Test.</p

Multi-dimensional scaling (MDS) analysis.

<p>(A). Two-dimensional MDS solution using the pair-wise correlations between the images based on the net AIP responses as estimates of the inter-stimulus distances. (B). Two-dimensional MDS solution using the aspect ratio of the images (largest diameter divided by the smallest diameter).</p

Disparity- and contour-selective example neuron.

<p>This neuron also preferred congruent over incongruent stereo stimuli (left), but remained image selective in the absence of disparity (compare the responses to the 2D image of the glasses with those of the vertical branch).</p

Anatomy and stimulus set.

<p>(A). Anatomical magnetic resonance image (MRI) and lateral view of the macaque brain, indicating the reconstructed recording positions in the AIP. (B). Monocular images are illustrated for all 24 images used in the Search Test. A red-green anaglyph version of the apple is shown in the inset (red in front of the left eye = congruent stereo mode).</p

Example neuron not selective for disparity.

<p>PSTH of an example neuron that showed significant image selectivity (preferring the image of a kong over the image of a human hand) in the presence and absence of disparity.</p