The dorsomedial cortical visual area: a third tier area in the occipital lobe of the owl monkey (Aotus trivirgatus).

In the owl monkey, microelectrode mapping of Brodmann's area 19 indicates that this region contains part or all of at least 5 separate representations of the visual field, each of which adjoins the anterior border of V II and collectively are termed the third tier of cortical visual areas (V I is the first tier; V II is the second tier). Described in detail in this report is one of the third tier areas which is located on the dorsal surface and the adjacent medial wall of the occipital lobe and corresponds to a densely myelinated zone of cortex. In this dorsomedial area (DM), the representation of the horizontal meridian is partially split, and thus, like V II (see ref. 4) and the dorsolateral crescent5, DM is a second order transformation of the visual hemifield. In one abnormal owl monkey, a portion of the upper quadrant was represented twice in DM. This abnormal case may provide some clues as to how the normal pattern of visuotopic organization is established in the developing brain.