Organization of orientation and direction selectivity in areas 17 and 18 of cat cerebral cortex

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Abstract
The organization of subunits and sequences subserving preferred stimulus orientation and preferred direction of stimulus motion in cat cerebral cortical areas 17 and 18 was determined by making vertical, tangential, and oblique microelectrode penetrations into those areas. Quantitative measurements of direction selectivity indicated that not all shades of direction selectivity are equally represented in area 17. Peaks in the distribution of direction indices may correspond to the bidirectional, direction biased, and direction selective categories used in qualitative studies. The relationship between preferred direction and location in the visual field was examined for units with receptive fields centered more than 15.degree. from the area centralis. Simple cells had orientation preferences that tended to be parallel to radii extending out from the area centralis. Wide-field complex cells had orientation preferences that tended to be parallel to concentric circles centered on the area centralis; the direction preferences of this group were biased toward motion away from the area centralis. Unit pairs separated by 200 .mu.m or less were 4.2 times as likely to have the same preferred direction as to have opposite preferred directions, indicating that, on average, strings of five neurons have similar direction preferences. Tracks in area 18 showed a similar pattern to those in area 17. In the vertical tracks in area 17 a small proportion (12%) of the units recorded in infragranular layers had preferred orientations that deviated 30.degree. or more from the first unit recorded in the same column. The presence of these cells most likely reflects the relative crowding of columns in infragranular layers, which occurs at the crown of the lateral gyrus. Columns with such large jumps in preferred orientation were not observed in area 18, which occupies a relatively flat region of cortex. In both areas 17 and 18 direction preference in vertical tracks usually reversed at least once, either between supra- and infragranular layers or within infragranular layers. Among these same tracks, orientation preference usually did not change. In tangential tracks, preferred direction and orientation preferences changed together in small increments. Occasionally a large jump in preferred direction would occur with only a small change in preferred orientation. These large jumps were considered to mark the boundaries of the direction sequences. Most frequently these boundaries were separated by 400-600 .mu.m. This value is approximately half the size of a complete set of orientation preferences (700-1,200 .mu.m). Between two and four direction sequences were calculated to be related to a full set of orientation preferences.

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