Studies on primate extrastriate visual cortex. I. The interhemispheric connections of visual cortex in the owl monkey, Aotus trivergatus, and the bushbaby, Galago senegalensis. II. A functional localization of neuronal response properties in extrastriate cortex of the owl monkey, Aotus trivirgatus

Anatomical techniques have been used to map within visual cortex the pattern of degenerating axonal terminals produced by surgical section of the selenium of the corpus callosum in the owl monkey, Aotus trivirgatus, and the bushbaby, Galago senegalensis: To the extent to which degenerating callosal terminals correspond to physiological representations of the visual vertical meridian, the pattern of interhemispheric connections can be used as a powerful aid in locating visual area boundaries. The goals of this study have been 1) to assess the degree of correspondence between degenerating callosal terminals and anatomically identifiable vertical meridian representations, and 2) to gain information concerning the existence and organization of as yet unknown extrastriate visual areas. In both the owl monkey and the bushbaby, a discrete band of degeneration corresponds precisely to the vertical meridian representation at the V1-V2 border, and a less precise increase in the density of axonal terminals corresponds to the vertical meridian representation of extrastriate area MT. A well-defined band of degeneration on the ventral surface of the owl monkey's cerebral hemisphere corresponds to a previously unknown vertical meridian representation which is shared by two newly defined extrastriate visual areas. Over much of areas DL and DI in the owl monkey, and in the region where the center-of-gaze representations of V2, DL and MT are in close proximity in both the owl monkey and the bushbaby, the pattern of callosal terminals is complex and has little value for determining precise areal boundaries. The response properties of 275 single neurons from four extrastriate visual areas have been quantitatively studied in chronically prepared, sedated owl monkeys. During experimental sessions, computer controlled visual stimuli were presented to the animal while the computer recorded the responses of single cortical neurons. The responses were stored on magnetic tape for later analysis. The major result of this study is the demonstration of a striking localization of direction-selective neurons to extrastriate area MT of the owl monkey. MT neurons responded maximally to stimuli tuned to a single optimal direction of motion whereas neurons of the medial third tier areas (DM, DI and M) responded maximally to oriented stimuli moving in either of two directions perpendicular to the optimal axis of orientation. Other differences were noted between neurons of MT and neurons of the medial third tier areas. A systematic difference exists in the tightness of tuning to the optimal direction of motion. Neurons in MT tended to be less well tuned to the optimal direction of motion than did neurons of the medial third tier areas. Neurons in MT also exhibited a different pattern of preferences for stimulus velocity than did neurons of the medial third tier areas. A distinct group of MT cells preferred velocities of 10-25 degrees/second, while neurons of the medial third tier areas had a broad distribution of preferred velocities ranging from 10-100 degrees/second. There was no systematic difference in tuning to preferred velocity for the two regions