158 



Lawrence S. Frishkopf and Walter A. Rosenblith 



the threshold probability distribution common to all of the neurons. Units 

 with synaptic drive to the left of the distribution have firing indices of zero ; 

 those to the right have firing indices of 100; and units with synaptic drive in 

 the range of the distribution have intermediate firing indices. As the stimulus 

 level is raised, the synaptic drive to every unit is shifted to the right by the 



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 FIRING INDICES 



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Fig. 5. Histogram showing the number of spinal motoneurons (triceps surae) 

 within each firing index interval ; responses were obtained by delivering repeated 

 shocks to the gastrocnemius nerve. The firing index of a unit is the percentage of 

 total stimulus presentations to which the unit responds. Units with firing indices 

 of zero and 100 are not included in this diagram. From Lloyd and McIntyre (5). 



same amount; thus some units with a firing index of zero will be shifted into 

 the intermediate range ; some with intennediate firing indices will be shifted into 

 the range of firing index 100. But because the units are uniformly distributed 

 the same number will move into the intermediate range as move out of it, 

 and the distribution of intermediate firing indices will remain unchanged. 



Fig. 6. Idealized relation between the threshold probability distribution of a 

 motoneuron and the levels of synaptic drive to diff"erent motoneurons of a 



population (see text). 



The particular choice of a bell-shaped probability distribution will lead 

 to the U-shaped histogram of Fig. 5. For it is clear that if we divide the abscissa 

 in such a way that equal areas under the distribution are subtended, those 

 intervals will be largest near the tails of the distribution (firing indices near 

 and 100) and smallest at the center of the distribution (firing index near 50). 

 Since the density of units along the abscissa is uniform, this means that many 

 more motoneurons will have firing indices between and 10 than between 

 45 and 55. 



