Motor command in the ventrolateral thalamic nucleus: neuronal variability can be overcome by ensemble average.

The variability and stationarity of single unit activity and reaction times in a simple reaction time task were investigated by means of unit recordings from the ventrolateral thalamic nucleus in the cat. The study was carried out on eight neurons recorded over many trials and displaying an increase in activity correlated with the performed reaction time. The stationarity of the correlation appeared to be better than that of the unit activity or reaction time taken alone. In this stationary process the neuronal variability is considerable, and the investigator has to use time averages over many trials to be able to extract information from the single unit data. Since the nervous system does not required repeated presentations of a stimulus to trigger a movement, it must be using parallel processing of information throught ensemble averages. A simulation of an ensemble average based upon the data from these eight neurons showed that such a process can be efficient. An evaluation of the number of neurons required to obtain a quasi deterministic neuronal command of the reaction time value, gave the speculative figure of 70. This figure could represent the dimension of the set of neurons with the same characteristics involved in the motor command.