Fluctuations in Neural Thresholds 



155 



Fig. 2 the percentage of responses of a fiber is plotted as a function of stimulus 

 intensity. Again each point is based on 100 stimulus presentations. The total 

 range of thre'shold variation is, on the basis of these data, about seven per cent. 

 The function shown in Fig. 2 approximates the threshold probability function 

 p(S) that was discussed earlier. 



100 



7b 



50 



25 



O 



'/]': 



-.y^: 



97 98 99 X)0 101 102 103 



Fig. 2. Relation between stimulus intensity (abscissa) and the number of respon- 

 ses obtained in 100 presentations at a fixed intensity from a single unit of frog 

 sciatic nerve (see Fig. 1). The interpolated solid line approximates the threshold 

 probability function of a unit. From Pecher (2). 



Fig. 3. Left: ink tracings of simultaneous recordings from two units of frog 

 sciatic nerve to repeated presentations of identical shock stimuli. Units A and B 

 are identified by their latencies. Right: same, but recording from two other 

 units, identified by their amplitudes. After Pecher (2). 



In the left column of Fig. 3 the responses of two different fibers were simul- 

 taneously recorded from a single electrode; the responses arc distinguishable 

 by their latencies. At a fixed level of stimulation all possible combinations of 

 response occur: fiber A responds alone, fiber B responds alone, both respond, 

 neither responds. On the right we see the responses from two other fibers; 

 here the responses are distinguished by their amplitudes. Again, all possible 



