414 



A. L. TOWE 



this dependency on stimulus intensity has been presented elsewhere (Morse 

 and Towe, 1959) and will not be here; it suffices to indicate that the initial 

 latency of the discharge of the unit, the number of spikes per discharge and 

 the interspike intervals are functions of the size and shape of the afferent 

 volley impinging upon the unit. 



The latency-intensity relationship just described can be obtained by stimu- 

 lation anywhere within the unit excitatory receptive field. However, the mini- 

 mum latency attained at strong stimulus intensities is a function of the position 

 of the stimulus within the field. Figure 4 illustrates this dependency for a unit 



™ DIGITS 



Fig. 4. Change in T with change in stimulus intensity and locus. The surface was 

 constructed by finding a best-fit curve of i = /(/) for each of the five digits, and 

 inserting some representative intensities; twenty-one different intensities were 

 tested for each digit. The center of this unit's excitatory receptive field was 

 between digits II and III, probably closer to III. Intensity in potentiometric 



units. 



with the center of its excitatory receptive field near the third digit; stimulation 

 of this digit yielded the largest surface primary response in the cortex just 

 overlying the unit. Each of the five digits was stimulated at various intensities 

 from threshold to many times threshold. The mean initial latency of discharge 

 was plotted for each digit as a function of stimulus intensity. As the site of 

 stimulation is moved progressively out from the center of the excitatory 

 receptive field, the minimum latency attained progressively becomes longer, 

 until the unit fails to discharge entirely. Hence, when interaction studies are 

 carried out, some attention must be given to the position of both inputs with 

 respect to the receptive field of the unit under observation. When the condi- 

 tioning input is apphed just prior to the testing input, the latter might 

 effectively excite the unit before the former, or the two inputs might affect the 

 unit simultaneously. Table 1 reveals the results of interacting two excitatory 



