Visual receptive fields in the lateral suprasylvian area (Clare-Bishop area) of the cat

Single units were recorded from the visual area of the lateral suprasylvian gyrus (LSSA or Clare-Bishop area) in 20 unanesthetized cats. Most LSSA units were poorly responsive to stationary visual stimuli, but they responded vigorously to moving visual stimuli. Their receptive fields appeared to be constituted of a large activating region (discharge area) often surrounded by inhibitory flanks. Relating unit behavior to changes of stimulus length, the LSSA neurons could be subdivided into 5 categories. The first category (22 out of 95 units tested, 23.16%) consisted of units showing summation inside the discharge area. Expanding the stimulus outside the discharge area did not affect the response. The second category (7.37%) was formed by units which showed summation inside the discharge area and inhibition when the stimulus was extended outside the discharge area. The third category (21.05%) consisted of units largely insensitive to the stimulus length inside the discharge area, but surrounded by inhibitory flanks. The fourth category (41.05%) consisted of units which showed inhibition of the response when the stimulus, well inside the discharge area, became longer than a certain optimal lenght. They were surrounded by inhibitory flanks. The fifth category (7.37%) was formed by units insensitive to variations of the stimulus length inside as well as outside the discharge area. Almost all units, independent of their category, were directionally specific, that is their response could be decreased 50% or more by varying the direction of movement away from that which gave the maximal response (preferred direction). Typically the response was halved when the stimulus was moved +/- 50 degrees from the preferred direction. Among the directionally specific units, 71% showed the minimal response 180 degrees away from the preferred direction (direction specificity curve type 1), 20% had the minimal response 90 degrees from the preferred direction (direction specificity curve type 2); the remaining could not be classified in this respect. Of LSSA units, 87% (all those of type 1 and many of those of type 2) were directionally selective, that is their response to movement in the preferred direction was at least double that in the opposite direction. The LSSA units usually preferred stimuli moving at rather high speeds. The optimal speed for 71% of units was 20 degrees/sec or greater. Almost all units responded over a wide range of speeds, many of them from 5-10 degrees/sec to over 100 degrees/sec. Most neurons had a low spontaneous activity and some of them remained completely silent for seconds.