INHIBITION AND OCCLUSION IN CORTICAL NEURONS 



413 



as a function of stimulus intensity and hence of the population of presynaptic 

 terminals activated by the stimulus. Very weak stimuli activate a small number 

 of presynaptic terminals; the unit thus excited discharges with only one spike 

 after a rather long latency. Its response probability at such weak stimulus 

 intensities is usually less than one. Strong stimuH, on the other hand, activate 

 a large population of presynaptic elements; the unit then typically discharges 

 several closely spaced spikes after a short latency. The latency of the unit 

 response gradually passes from one extreme to the other as the stimulus is 

 varied from threshold to supramaximal intensity. Figure 3 shows the pattern 



(msec) 



2--{ 



4 6 8 10 



INTENSITY (threshold units) 



Fig. 3. Unit isolated 680 /t below the pial surface in p.c.g. Intensity of stimulus 

 (0 varied from threshold to twenty-four times threshold intensity (F). When / 

 increased to 2 T, p = \00. The mean number of spikes per discharge (^s/d) at 

 each intensity continued to grow, even when / = 5 T. T shortened rapidly 

 between / = T and i = 2> T, stabilizing for all i = 3 T; the total latency change 

 was about 10 msec. This pattern of Z =/(/) has been observed in nearly every 

 central nervous system neuron studied. 



of these changes. Response probability grows from the threshold value of 

 0-5 to unity as the stimulus intensity is raised from threshold to twice threshold 

 value. Simultaneously, the latency of the initial discharge decreases toward a 

 minimum value; all stimulus intensities greater than about three times 

 threshold fire the unit at the same (minimum) latency. Similarly, the number 

 of spikes in each discharge changes with intensity, growing from one spike 

 per discharge at threshold to several at many times threshold intensity. 

 Increasing the stimulus intensity from threshold to much higher values in- 

 creases the number of fibers activated by the stimulus, and this, in turn, 

 increases the number of active presynaptic terminals on each unit in the system 

 leading to the cortical unit. As the population of active neurons in the system 

 changes, the pattern of unit discharge changes. The proposed mechanism of 



