Fluctuations in Neural Thresholds 



157 



Pecher also reported that the latencies of responses to identical shock stimuli 

 exhibit variability of a sort that cannot be attributed to stimulus instability. 

 The variability is greatest for stimuli near threshold. 



RosENBLiTH (4) has obtained a threshold probability function for single 

 units in the auditory system of the anesthetized cat. These results are shown 

 in Fig. 4. Responses to repeated clicks were recorded by means of a micro- 

 electrode from a unit in the cochlear nucleus. The ratio of the number of 

 responses to the number of stimuli is plotted for various stimulus values; 

 the range of threshold variability is about 1 5 dB. 



Lloyd and McIntyre (5) have investigated the variability in the responses 

 of single ventral root motoneurons [triceps surae) to identical shock stimuli 



60 50 40 30 20 10 

 dB BELOW CLICK REFERENCE LEVEL 



Fig. 4. Percentage of clicks eliciting a response from a single element in the 



cochlear nucleus of the cat as a function of click intensity. Each point is based 



on 10 to 40 click presentations. The interpolated solid line approximates the 



threshold probability function of a unit. 



delivered to the gastrocnemius nerve in decapitated cats. Here, the impulse 

 traverses a single synapse in the spinal cord. It was found that at every stimulus 

 level there were neurons that sometimes responded and sometimes failed to 

 respond. Some neurons always responded; others never responded. By raising 

 the stimulus level, the latter could be brought into the range of partial response 

 and, in some cases, of certain response. Different motoneurons receive different 

 amounts of transsynaptic stimulation when a shock is applied to the sensory 

 bundle. The strength of the effective stimulus is said, in this terminology, to 

 depend on the 'transmitter potential' of the synapse. The 'firing index' of a 

 motoneuron is defined as the percentage of trials in which it responds. Lloyd 

 and McIntyre measured firing indices for 110 motoneurons under a variety 

 of stimulus conditions. A histogram showing, for a constant stimulus, the 

 number of motoneurons in each firing index interval is seen in Fig. 5. For the 

 purpose of this histogram, units with firing indices of zero and 100 were not 

 counted. 



An appreciable change in stimulus strength changes the firing index of a 

 particular motoneuron but affects the histogram very little. From this we can 

 conclude that the distribution of motoneurons with respect to the effective 

 stimulus level is approximately uniform. The situation may be visualized with 

 the help of Fig. 6. Each vertical line represents the effective stimulus or 'synaptic 

 drive' to one motoneuron; all motoneurons in this idealization are assumed 

 to be identical, but subject to different effective stimuli. The curve represents 



