Notes

The graphs below show individual V1 cells responses measured in monkeys during individual stimulation of both eyes varying the amount of binocular disparity. They labelled positive disparity for objects farther away from the fixation point and negative disparity for objects closer than the fixation point. They find out that there is a number of different cells profiles based on how they respond to different disparity stimuli. For example, the graph on top-right corner shows a cell that strongly response to a positive disparity in the stimulus (i.e., a cell that encodes objects far away from the fixation point), while the bottom-left figure shows a cell that behaves oppositely, i.e., increases firing in the presence of negative disparity. The graph on top-right shows a cell that is perfectly tuned to a specific amount of binocular disparity and strongly responds in such case. The bottom-left graph shows a cell that is again tuned to a specific amount of binocular disparity, but shows an inhibitory effect rather than excitatory.

How are these cells in the cortex tuned for non-zero disparity? Two models have been proposed:

• Position Shift Model: according to this model, the brain features some "disparity-tuned neurons" that are shifted in position such that when the stimulus with binocular disparity arrives to V1 it maximally excites both neurons**.**
• Phase Shift Model: according to this model, the receptive fields of the neurons match in terms of position but differ in terms of internal organization such that they are maximally excited when the stimulus has a binocular disparity.

Binocular Receptive Fields of Disparity Tuned Neurons

In the image above, the receptive fields of disparity tuned neurons have been mapped in a cat cortex. It has been observed that both phase shift and position shift models seem to occur and also a mix between them. The first column of images shows a phase shift of the receptive fields between left and right eye. While in the second column we have both a phase shift and a position shift of the receptive fields between the left and the right eye. (white region = on-region, black region = off-region).