An experiment is reported in which the number of sources from which a signal may emanate is related to simple reaction time. Responses to signals from all sources are the same. The performance of younger subjects does not deteriorate seriously until the number of signal sources is increased to eight, whilst that of older subjects deteriorates when the number of alternative sources is increased from one to two. The result is provisionally interpreted in terms of the uncertainty produced by extending the signal source, the "threshold" for uncertainty seemingly being lower for older than for younger subjects. Attention is drawn to the possible interpretation of this by employing the concept of increasing "neural noise.".