Fluctuations in Neural Thresholds 



165 



two populations of neural units, one consisting of 'sensitive' units and one of 

 'insensitive' units. In the three animals tested, variability measurements over 

 the sensitive range are in good agreement with the theory stated above. One 

 case is shown in Fig. 14. The intensity function and the probabilities obtained 

 from it are shown with the derived standard deviation function. Here, Oq is 

 determined from measurements of baseline variability in the absence of a 

 stimulus; TV is chosen to give the best fit to the data. Over the sensitive range 



CLICK INTENSITY (DB RE I.29VACR0SS PHONE) 



Fig. 14. Comparison of the theoretical variability function (with 70 per cent con- 

 fidence limits) and the measured values of cr, over the range of initial growth of the 

 intensity function. Each point represented by a solid circle is based on 100 

 responses; the open circles are based on the first fifty of these responses. The 

 corresponding intensity function, and the probabilities obtained from it, are also 



shown. 



(— lOOdB to — 60 dB) the data fall within the indicated confidence interval 

 approximately seventy per cent of the time, as they should if the model is 

 correct. Over the insensitive range of the intensity function (—60 dB to 

 dB), the standard deviation shows a complex behavior which cannot be simply 

 reconciled with the idea of a single population over that interval. 



The third aspect of this study concerns the masking of the neural responses 

 to clicks by a background noise. Fig. 15 shows the effect of a constant noise 

 level on response amplitude at several stimulus values. In Fig. 16 we have 

 plotted these masked and unmasked intensity functions. The observation 

 was made that a very weak level of continuous noise was sufficient to reduce 

 almost to zero the N^ response to a fairly intense click. A fixed threshold model 

 would predict masking of only the units whose thresholds are below the noise 

 level. If the threshold fluctuates, however, and does so rapidly, nearly all 

 units of a given population will drop below the noise level and fire in a short 



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