A double-response paradigm to study stimulus intensity effects upon the motor system in simple reaction time experiments

A double response paradigm is utilized to study stimulus intensity effects upon the motor system in simple visual and auditory reaction times (RT). Subjects had to respond with both hands simultaneously upon detection of a stimulus. The RT difference of both hands is thereby of special interest, because it is considered that this variable does not contain any sensory latency and therefore allows one to study those processes that follow stimulus detection. It was found that the RT-difference distribution varies with stimulus intensity, which questions the general view that stimulus intensity affects only very early sensory stages in the processing chain. In particular, it was found that the variance of RT difference diminished with increasing stimulus intensity. This finding supports the notion of speeding up the motor process by increasing stimulus intensity. A generalization of a stochastic model by Meijers and Eijkman (1974) and Meijers, Teulings, and Eijkman (1976) is advanced to account for the findings. The central assumption is that more units (e.g., motoneurons) are activated if stimulus intensity is increased. The model’s qualitative predictions are confirmed.