INHIBITION AND OCCLUSION IN CORTICAL NEURONS 



411 



areas of skin on the limb or trunk. Not only is current spread into adjacent 

 digits negligible, but the fibers innervating the skin of the distal portion of one 

 digit ramify within that digit, and not into adjacent digits. 



Tiie gradual decline in effectiveness of the stimulus near the margin of the 

 excitatory receptive field suggests that some effect on the unit should be demon- 

 strable by stimulation a little beyond the boundary. Indeed, the standard 

 conditioning-testing procedure clearly shows that stimulation just outside 

 the excitatory field alters the excitability of the unit and of the system leading 

 toit(Amassian, 1952, 1953; Toweand Amassian, 1958). Surprisingly enough, 

 this change takes the form of depressing the responsiveness of the unit to 

 stimulation within the excitatory receptive field (Fig. 1). This depression is 



Fig. 1. Interaction in single unit isolated 1050 /< below the pial surface in the 

 postcentral gyrus (p.c.g.) of the monkey. C, conditioning stimulus to digit IV of 

 forepaw failed to discharge unit. T, testing stimulus to digit II of forepaw fired 

 unit after 17-5 msec mean latency. (1) Digit IV stimulated 1 msec before digit II; 

 unit fired after 181 msec mean latency. (2) Digit IV stimulated 14 msec before 

 digit II; unit failed to discharge. (3) Testing stimulus alone still fired unit at same 



mean latency. 



sufficiently powerful to render the unit inexcitable for about a tenth of a second, 

 and less excitable than normal for another tenth of a second. Both the 

 duration and intensity of the depression can be diminished by weakening 

 the conditioning stimulus or by applying the conditioning stimulus farther 

 away from the boundary of the excitatory receptive field (Towe and Amassian, 



