INHIBITION AND OCCLUSION IN CORTICAL NEURONS 



417 



(conditioning) resulted in a facilitatory effect during the early phase and 

 during recovery. The effect in the early phase led us to call this an "occlusive" 

 type of interaction. The late facilitation would not be expected in such an 

 interaction, but may have had its origin in a cortically originating facilitatory 

 effect on lower levels of the sensory system, thus effectively increasing the 

 testing input to the unit. This latter suggestion derives from the observations 

 presented in the subsequent paper of this symposium by Jabbur and Towe 

 (p. 419). When stimulation of digit IV was employed as a conditioning input, 

 the early facilitation of the input from digit II was weaker; the recovery was 

 as expected from an occlusive interaction. Stimulation of digit V effectively 

 diminished the efficacy of stimulation at digit II in exciting the unit. Such an 

 effect could only result from an active inhibition of some neurons in the path- 

 way leading to the unit, and is therefore called an inhibitory interaction. The 

 inhibition is a consequence of stimulation near the boundary of the excitatory 

 receptive field. 



This precise timing analysis, combined with the qualitative observations of 

 Mountcastle and Powefi (1959), leads to the conclusion that a region just 

 outside and overlapping the excitatory receptive field of any given unit has 

 the net effect of reducing the population of neurons excited by stimulation 

 within the excitatory receptive field. The reduction in population is accom- 

 phshed, not by prior activation of some elements in the system, so that they 

 are unresponsive, but by an active inhibition of these units. This "ring" 

 around the excitatory field can be termed an inhibitory receptive field or 

 inhibitory surround. 



A OCCLUSION 



B INHIBITION 



10 20 



C-T (msec) 



10 20 



C-T (msec) 



Fig. 6. Two units isolated simultaneously at 759 ^t below pial surface in p.c.g., 

 showing both the "occlusive" and the "inhibitory" types of interaction. Arrow- 

 head on left of ordinate axis shows conditioning (digit II) Z and p\ on right is 

 testing r and p. a. At 10 msec C — T interval, Z shortened; when p drops at 

 C — r = 20 msec, Z increases, b. Very short duration "inhibitory" interaction. 

 Unit discharged two spikes in each discharge, so changes in both are shown. 



