Astor, Katharine Tamsin; (2002) Intention and reactivity. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The evolution of the human brain, including the ability to make intentional actions, has resulted in our motor system being activated via either our own intentions or external stimuli. It is important that a balance is maintained between these internal and external drives. This thesis investigates the relationship between these two systems. Evidence supporting a distinction between them is considered and hypotheses are examined, specifically: are they independent, unitary, facilitatory, inhibitory, or is there a switching mechanism? Intentional action is studied using (i) objective paradigms: the generation of intentional actions and how this interacts with stimulus-driven actions, and (ii) subjective paradigms: the perceived time of stimuli and actions. A novel experimental paradigm investigated the relationship between the putative internally-generated and externally-triggered motor systems. The "truncation" paradigm involves subjects preparing an intentional action, which is interrupted by a randomly occurring stimulus requiring a response. Truncation, therefore, involves a balance between intention and reactivity. Seven experiments produced a reliable difference (averaging 51 ms) in reaction time between simple RT and truncated reaction time (TRT). This difference was termed the RT cost of intention. The neural basis of the process responsible for this RT cost was investigated using two physiological techniques (pupil dilation and electroencephalography) in two separate experiments. Alternative causes of the RT cost, such as delayed stimulus processing in truncation, were examined and rejected. The conclusions are that (i) the RT cost of intention is due to switch costs between the hypothetical internally-generated and externally-triggered motor systems; (ii) these two systems are not normally simultaneously active; and (iii) at a subjective level, people are aware of the response delay incurred by the switch process.

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