Vertical meridian representation on the prelunate gyrus in area V4 of macaque.

The representation of the lower quadrant in area V4 is presently thought to extend along the prelunate gyrus from a foveal representation laterally all the way to the dorsal end of the superior temporal sulcus. However, several studies suggest the possibility of a more complex organization. To see if the visuotopic organization on the crown of the gyrus was relatively homogeneous or instead contained inhomogeneities indicative of more complex organization, we recorded from a grid of points over the prelunate gyrus. Receptive-field size and scatter in the region are large, making it difficult to infer topography from simple inspection of receptive-field sequences. We developed an averaging procedure using data from all recording sites to detect an inhomogeneity in topography with respect to the vertical meridian. With this procedure, we found a vertical meridian representation just medial to the medial end of the lateral sulcus. We also found a significant difference in the incidence of orientation sensitivity on either side of the meridian representation. The results show that the crown of the prelunate gyrus cannot be described as a single homogeneous region, but instead contains at least two different sub-regions adjoining along a shared representation of the vertical meridian.