In the correlated and to a greater extent in the anticorrelated activation maps, there is a region of out-of-phase activity clustered in the foveal region of areas V1, V2, and V3. The most likely explanation of this is a response to zero disparity. In both disparity stimuli, the ‘active’ region is contrasted with a zero disparity region. If a larger proportion of voxels in a given areas of cortex are sensitive to zero or near-zero disparity, a greater response may be evoked by the zero-disparity region than the active quadrant. Since neurons close to the fovea tend to be sensitive to disparities close to zero (Prince, Cumming, & Parker,
2002), it is not surprising that this out-of-phase activity is clustered in the foveal region of cortex. It is now widely accepted that disparity selective neurons show a continuum of tuning shape (Cumming & DeAngelis,
2001; DeAngelis & Newsome,
1999; LeVay & Voigt,
1988; Prince, Pointon, Cumming, & Parker,
2002) rather than the discrete classes originally proposed (Poggio & Fischer,
1977; Poggio, Gonzalez, & Krause,
1988). However, Cumming and DeAngelis (
2001) have shown that the distribution of disparity tuning curves changes from striate to extrastriate visual areas. In V1, the tuning is clustered around symmetric tuning, V2 shows a fairly even distribution, while in extrastriate cortex (MT and MST) there is a greater preponderance of odd symmetric tuning, that is, near and far cells. Near and far cells are activated more by non-zero disparities, so the activation to the zero disparity background would be lower in extrastriate areas.