164 



Lawrence S. Frishkopf and Walter A. Rosenblith 



independent units, each with a probability p of firing, will have a standard 

 deviation of total response proportional to \/Np{\ — p). As a function of 

 /; this quantity has minima at zero and one and has a maximum at/? = ^. The 

 value of p at any stimulus intensity can be obtained from the intensity function. 



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 OU 



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c 

 CM 



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< cr 



_i 



UJ 



tr 



1.00 



0.75 



0.50 



0.25 



1.00 



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60 40 20 60 40 20 60 40 20 



INTENSITY OF FIRST CLICK IN dB BELOW 

 REFERENCE LEVEL 



Fig. 12. 1R2IR2 (see text) as a function of first click intensity. In each block tiiis 

 ratio is plotted for a different interclick interval, as indicated at the lower right. 

 The intensity of the second click was —45 dB throughout. The curves are obtained 

 from the first click intensity function and eq. (1); the parameter ^(At), whose 

 values are given at the lower right, is chosen in each case to give the best 

 fit to the data. After McGill (10). 



Fig. 13. Intensity function (upper) and the corresponding amplitude variance 

 function predicted by the model: (a) for one population; (b) for two disjoint 

 populations. Oq was chosen arbitrarily. Note that a peak of the variability 

 function occurs at the stimulus value at which an intensity function component 

 reaches half its maximum amplitude. 



Fig. 13 shows the kind of variability function obtained by assuming one and two 

 disjoint populations; Cq is the stimulus-independent component of variability 

 arising from biological and non-biological sources. We have shown (II) that 

 instability in stimulus intensity, which would also lead to a peaked variability 

 function, can account for at most three per cent of the observed variability. 

 A detailed study of the shape of the intensity function led us to postulate 



