Fluctuations in Neural Thresholds 



167 



interval preceding the click; this assumes, of course, that the noise level lies 

 within the range of threshold fluctuations of the unit. 





By a quantitative treatment based on these qualitative notions we have 

 been able to show (a) that the hypothesis of a fixed threshold does not account 

 for the observed data and (b) that over the sensitive range of the intensity 



i25r 



100 



75 



50 



I 25 



^^^_ NO NOISE BACKGROUND 



(COMPOSITE OF 3 FUNCTIONS) 

 NOISE BACKGROUN0(OB RE 1.29V ACROSS 

 PHONE lUNFILTEF 



-92 

 -82 



-67 



-80 -70 -60 -50 -40 -30 



CLICK INTENSITY (06 RE 1.29V ACROSS PHONE) 



Fig. 1 6. Intensity functions for clicks, with an(d without noise backgroun(i ; noise 

 levels —92, —82 and —67 dB. Each point of the masked functions represents the 

 average A''i amplitude of ten responses to identical stimuli. The upper curve was 

 obtained by averaging the three unmasked functions which correspond to the 

 masked functions shown ; thus each point represents the average A''i amplitude of 

 thirty responses to identical stimuli. Typical data on which these curves are 



based are shown in Fig. 15. 



function a single population of units making threshold 'jumps' at a rate of 

 about 2000 times per second can account for the data. In addition, it is observed 

 that low level noise has little effect on the intensity function over the insensitive 

 range, except to reduce it by the constant contribution of the sensitive popu- 

 lation. The need for a division of units into at least two populations is thus 

 confinned. When the noise level is raised into the insensitive range the observed 

 effect is not nearly so marked, implying either that more than one population is 

 involved in that interval or that the rate of threshold fluctuation is considerably 

 slower than for the sensitive units. 



It is noteworthy that population analyses based on two very diff'erent 

 experiments, variability and masking, have a great deal in common. 



REFERENCES 



1. E. A. Blair and J. Erlanger: A comparison of the characteristics of axons through 

 their individual electrical responses. Anier. J. Physiol. 106, 524-564 (1933). 



