ABSTRACT
The evolution of the human brain, including the ability to make intentional actions, has resulted in our motor system being activated either via our own intentions or external stimuli. It is important that a balance is maintained between these internal and external drives. It is proposed that humans have an internally generated and an externally triggered motor system. Five ways that these motor systems might be related are posited. The relationship between these two systems was investigated using a novel experimental paradigm, called truncation. This requires participants to prepare an intentional action, which is randomly truncated with a stimulus to which they must react with the same action as they had prepared to make intentionally. Experiment 1 utilized pupil dilation as a physiological measure of the cognitive processes involved in the truncation condition and in control conditions where either internally generated or externally triggered actions occurred alone. Three of the proposed relationships were discounted and alternative objections, such as slower perceptual processing and foreperiod differences, were addressed. Experiment 2 recorded auditory-evoked potentials. This experiment suggested that the reaction time cost incurred in the truncation condition is mainly due to the deactivation of the preparation for impending intentional action, rather than the activation of the reactive response. Experiment 3 evaluated the locus of the interference between the two tasks. The results suggested that there was an incompatibility between intentional and reactive preparation for a single action. We conclude that: (1) The large and positive reaction time cost of truncation, which was consistently found when comparing a truncation condition with a simple reaction time condition, is due to switch costs between the hypothetical internally generated and externally triggered motor systems. (2) These two systems are not normally simultaneously active. The findings are discussed within the context of executive function.
