156 



Lawrence S. Fpushkopf and Walter A. Rosenblith 



combinations occur. Such a result can only be explained as a result of spon- 

 taneous variation in fiber threshold. If threshold were fixed and the stimulus 

 unstable, then only three of the four combinations could occur. That combina- 

 tion would be excluded in which the fiber with higher threshold fires alone. 



When responses from two fibers can be distinguished, an opportunity is 

 offered to test the degree of correlation of threshold fluctuation among different 

 fibers. If fluctuations occur independently in two fibers, the probability of both 

 firing to a single stimulus would be the product of their probabilities of firing 

 separately. Any correlation in threshold variations would alter the probability 

 of joint firing. These probabilities can be approximated by counting the number 

 of times that fiber A fires, that fiber B fires, and that both fire, and dividing 

 each by A'^. In the table below the results of such measurements by Pecher 



Table I 



are given for nine different fiber-pairs. In all of these instances the computed 

 and observed frequencies of joint occurrence are in good agreement. The 

 hypothesis of independent fluctuations is thus supported by this experiment. 



Pecher tried to determine whether or not for a single fiber the 'response 

 no-response' pattern to a sequence of periodic stimuli can be accounted for 

 by the hypothesis that successive responses occur with equal and independent 

 probability p. He chose a criterion of independence that relates the variables 

 r and n^, where n^ is the number of times that a sequence of r successive responses 

 (bounded at each end by the absence of a response) occurs in a sample of 

 length A^(r<A^): 



/• + ^ In nj. = K 



where k and K depend on/) and A'^ (3). On the basis of samples of 1000 to 2000, 

 Pecher concluded that within statistical limits a linear relation exists between 

 r and In n^ for rates of stimulation less than one per second. At higher rates 

 the criterion was no longer satisfied. This is not a sufficient test for independence, 

 since one could construct sequences which satisfy this relation and yet contain 

 strong internal regularities. 



