The Individual as an Information Processing System 313 



of performance. We realized that at each level we would encounter 

 a complex statistical problem if we used limited sequences of 

 inputs. We also met other problems in calculating" bits, particularly, 

 in knowing what code was employed at the cell and organ levels, 

 and in knowing the exact size of the implicit ensemble at all levels. 

 We hope our methods at least begin to cope with these issues. 



Cellular Research 



A stimulator was constructed which could administer pulses to 

 a neuron at various average rates, and at various intensities at 

 each of these. A single fiber in the sciatic nerve of the frog was 

 isolated by microdissection, and was stimulated at the rates of 

 100, 200, 400, 600, 800, and 1,000 pulses per second, using four 

 different values of stimulus voltage (1, 5, 2, 0, 2.5, and 3.0 times 

 the threshold value). We recorded the output of the fiber thus 

 stimulated from microelectrodes in the same cell and across a 

 synapse in the next cell. 



As the input rate was increased, the fiber eventually ceased to 

 follow every input and started missing some. Among the fibers 

 which we have studied, three different types of responses have 

 been observed. Some fibers, when they reach the point at which 

 they can no longer follow every stimulus, start skipping every 

 other stimulus. As the rate is further increased they respond only 

 to every third or fourth stimulus in a regular fashion. Other fibers 

 skip in a perfectly random manner, so that at a given rate the 

 number of pulses skipped will have a Poisson distribution. Still 

 other fibers transmit several adjacent stimuli and then fail to 

 transmit any stimuli at all for a long period, after which they 

 again fire repeatedly. Sometimes all three types of functions are 

 found in the same fiber at different times and at different rates of 

 stimulation. 



Two other phenomena were also noted. As the rate of stimu- 

 lation was increased, there was a fall in the amplitude of the 

 response and a decrease in the lag between the occurrence of the 

 input and the start of the response pulse. The amplitude decrease 

 is probably related to the energetics of membrane recovery; the 

 lower recovery time leads to a lower potential. The decrease in 

 latency must have a similar explanation; it makes the fiber able 



