Brain location and visual topography of cortical area V6A in the macaque monkey

The brain location, extent and functional organization of the cortical visu al area V6A was investigated in macaque monkeys by using single cell record ing techniques in awake, behaving animals. Six hemispheres of four animals were studied. Area V6A occupies a horseshoe-like region of cortex in the ca udalmost part of the superior parietal lobule. It extends from the medial s urface of the brain, through the anterior bank of the parieto-occipital sul cus, up to the most lateral part of the fundus of the same sulcus. Area V6A borders on areas V6 ventrally, PEc dorsally, PGm medially and MIP laterall y. Of 1348 neurons recorded in V6A, 61% were visual and 39% non-visual in n ature. The visual neurons were particularly sensitive to orientation and di rection of movement of visual stimuli. The inferior contralateral quadrant was the most represented one. Visual receptive fields were also found in th e inferior ipsilateral quadrant and in the upper visual field. Receptive fi elds were on average smaller in the lower visual field than in the upper on e. Both central and peripheral parts of the visual field were represented. Large parts of the visual field were represented in small regions of area V 6A, and the same regions of the visual field were re-represented many times in different parts of this area, without any apparent topographical order. The only reliable sign of retinotopic organization was the predominance of central representation dorsally and far periphery ventrally. The functiona l organization of area V6A is discussed in the view that this area could be involved in the control of reaching out and grasping objects.