INHIBITION AND OCCLUSION IN CORTICAL NEURONS 



415 



Table 1. Facilitatory interaction at 

 somesthetic cortex 



The response latency to digit V is significantly shorter than to digit IV, 

 when Student's /, with pooled estimate of error, is used. The response latency 

 to the combined input is significantly shorter than to digit IV or V alone. 



inputs at a small time interval. The initial response latency is earlier than that 

 to either input alone, and the variation in latency on successive trials is much 

 reduced. Such a change might, as suggested by Figs. 3 and 4, result from an 

 increased population of active neurons in the system leading to the cortical 

 unit; it is hke the change produced by interacting stimuli to adjacent digits 

 (Fig. 2). In fact, this sort of facihtatory interaction is observed whenever both 

 the conditioning and testing stimuli are apphed well within the boundaries of 

 the unit excitatory receptive field. 



When a conditioning input which itself fails to discharge the unit is appUed 

 a few milliseconds before the testing input, the latency of the test response 

 increases and the number of spikes in each discharge decreases. Like the 

 facilitatory interaction, this latter effect can be found at all levels within the 

 sensory pathway. Table 2 illustrates this phenomenon in a unit from the 



Table 2. Inhibitory interaction at unit in 



CUNEATE nucleus 



Stimulation of digit III 12 msec before digit II was stimulated changed the 

 response to stimulation at digit II. Both the change in latency (DL\ note size of 

 standard deviations, s) and interspike interval (//) were highly significant. 



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