Sensitivity of human visual cortical areas to the stereoscopic depth of a moving stimulus

Many fMRI studies have documented motion-sensitivity in the human occipital cortex and several have examined sensitivity to binocular disparity. However, selectivity to the stereo-defined depth of a moving luminance-defined stimulus has not been examined with fMRI. We used an fMRI adaptation paradigm to examine such selectivity. On each trial of an event-related design, two brief rotating dot patterns were presented sequentially. These had either the same or opposite directions of motion and were presented in either the same or different depth planes (+/-1 deg disparity). There were no monocular cues to depth. Attention was controlled by a demanding task at fixation; in addition, control trials in which stimulus salience was manipulated confirmed that there was no modulation by attention. In MT and MST, the compound response was smaller (adapted) when the two had the same depth than when they were different. This suggests the presence of separate neural populations sensitive to near and far motion, consistent with physiological results. Selectivity for motion direction was also seen. The magnitude of the depth effect was similar to that of direction in MT/MST, suggesting equally pronounced tuning. Visual areas V1-V4 also showed strong selectivity for near and far depth planes, whereas direction sensitivity was weaker overall and was measurable only in V3 and beyond.