154 Lawrence S. Frishkopf and Walter A. Rosenblith 



II. SUMMARY OF STUDIES OF OTHER WORKERS 



The class of phenomena that we have been discussing was first observed by 

 Blair and Erlanger (1). They reported that an electric stimulus, repeatedly 

 presented to a single sciatic nerve fiber of the frog, will for most stimulus values 

 either always produce or always fail to produce a response. The transition 

 between these two situations, however, is not sharp. Upon raising the shock 

 intensity, a value is reached at which the fiber sometimes responds and some- 

 times fails to respond to repeated stimulation. In order to obtain a response 

 every time it is necessary to raise the shock intensity an additional two per 

 cent, far in excess of the uncontrollable variation in the stimulus. Moreover, 

 Blair and Erlanger were able, on occasion, to record simultaneously from 

 two fibers whose potentials could be distinguished by their difference in latencies. 

 On repeated testing with a near-threshold stimulus, sometimes both would 

 respond, sometimes one, sometimes the other, and sometimes neither. Such a 

 result cannot be accounted for on the basis of stimulus instability alone. 



The most complete study of this kind that has been published to date was 

 made by Charles Pecher (2) in 1939. Using a technique similar to that of 



A- A- 

 J^ .^ 

 ^ J\^ 



Fig. 1 . Left : ink tracings of recordings from single units of frog sciatic nerve, 

 showing occurrence and failure of response to repeated presentations of identical 

 shock stimuli. Right: same, with amplitude of pulse producing the shock raised 

 4 per cent. Each series shown is part of a longer sequence of 100 presentations. 

 Thirty-five responses were obtained with the weaker stimulus (left); 85 responses 

 were obtained with the stronger stimulus (right). After Pecher (2). 



Blair and Erlanger, he also found a stimulus range in which a fiber sometimes 

 responded and sometimes failed to respond to a constant stimulus. Some of 

 his data appear in Fig. 1. In the column on the left we see the responses to 

 successive identical stimuh, of which some produce a response and some fail 

 to do so. In the second column the intensity was raised four per cent. In 



