Document Type


Degree Name

Master of Science (MSc)




Faculty of Science

First Advisor

Sukhvinder Singh Obhi

Advisor Role

Thesis Supervisor


When the self paced preparation of an action is interrupted by a stimulus prompting the same motor response there is an increase in the reaction time to the stimulus as compared to an external or simple reaction time (SRT) condition (Obhi & Haggard, 2004). Previous studies have suggested that this cost is not attributable to perceptual or attentional factors. Therefore, to investigate the source of this RT cost we varied the motor demands of movements in Experiments one and two. Results indicated that the level of motor programming demands did not influence the RT cost in these experiments. While RTs for more demanding movements (i.e., bimanual or serial key presses) were slower than those for simple one finger actions (i.e., single key presses), the RT cost was not significantly different for both response types. That is, the RT cost did not increase as a function of motor programming demands. In a control experiment (experiment three), we assimilated the external and truncation conditions through the creation of the external-subvocal condition, to investigate if the source of the RT cost was reflective of the dual task of subvocalization. The results revealed that although the RT cost was reduced, counting subvocally did not eliminate the RT cost. Simply, the RT cost is not explained by the dual task of subvocalization. The ability to modify planned actions is fundamental to everyday life, thus we investigated the time course of modifying planned actions on the basis of an external cue in experiment four. Reaction times to produce modified actions were significantly greater than those to produce unmodified actions. Additionally, it took significantly longer to produce modified actions requiring one less effector than to produce modified actions requiring one more effector. We suggest that two time-consuming processes are involved in switching between internally generated and externally triggered actions that are modified or unmodified: a trigger switch cost when the same action has to be produced in response to an external trigger as opposed to an internal trigger, and a switch cost reflecting changes in the pattern of executed motor commands when modification is necessary. It is suggested that such processes may be mediated by regions of the frontal lobes. Finally, we conducted two experiments to investigate the time course of cancelling a planned action on the basis of an environmental prompt. It was found that an internally prepared action can be cancelled with 300ms, regardless of the motor demands associated with the prepared action, suggesting that we are cancelling the prospective “when” component of the action.

Convocation Year