314 Information Storage and Neural Control 



to cope with a greater overload, enabling it to follow at much 

 higher rates than would otherwise be possible. Our findings are 

 in harmony with others who have worked in this field. 



In order to measure the maximum information transmission 

 capacity of a nerve fiber which employs pulse-interval coding, 

 we must be able to stimulate the neuron with an input source 

 which can deliver trains of two or more pulses at diff^erent intervals. 

 This follows from information theory, since in an evenly spaced 

 pulse train there is no uncertainty about the time of arrival of the 

 next pulse, and hence no information. Maximum uncertainty is 

 available only in a random source in which the pulse is equally 

 likely to occur at any time. It is also necessary to determine the 

 minimum interpulse interval which can be discriminated by the 

 neural system. We can determine this by measuring the standard 

 deviation of the latent period in a fiber, or its "jitter." 



We proceeded, therefore, to use an electronic timer, accurate 

 to 1 microsecond, to measure the jitter of single sciatic nerve 

 fibers of the frog, studying variation as tiie time between two ad- 

 jacent pulses was reduced. This turned out to be of the order of 

 2-5 microseconds, and adding a third pulse before the other two 

 did not aff^ect this value. Using a mathematical model developed 

 by Rapoport and Horvath (16), we were able to calculate the 

 curve of maximum channel capacity of such a neuron at various 

 input rates. We found that the output increased as a function of 

 the input up to 4,000 bits per second (an astonishingly high 

 capacity for such a small system — assuming optimal pulse-interval 

 coding); then leveled off" and decreased, thereafter, as the input 

 rate increased. This performance curve is shown in Figure 1. 



As for adjustment processes, the skipping of pulses which we 

 found at the higher input rates was, of course, omission. The 

 lower output intensities could be called erroneous processing if 

 they were not intense enough to cross the threshold of the neuron 

 on the other side of the synapse. That threshold, incidentally, 

 can be considered a sort of filtering. For other neuronal adjustment 

 processes to information overload, we have no evidence. 



Organ Research 



We used the same electronic timer to stimulate the optic nerve 

 of the white rat, recording the output from a macroelectrode on 



