Date of Award


Degree Type


Degree Name

Master of Science




Dr. Brian Comeil


The countermanding task requires subjects to cancel an impending movement in the presence of an imperative stop signal. The outcome of a given countermanding trial has been modeled as a race between stochastically independent GO and STOP processes and is largely dependent on the amount of time between target and stop signal presentation (the stop signal delay - SSD). Here, we investigated the effect of fixation condition on the GO and STOP processes using human subjects in eye-only, hand-only and eye-hand co-ordinated countermanding tasks. In Experiment 1, we found that duration of the STOP process, estimated through the derivation of the stop signal reaction time (SSRT), was ~20 ms shorter on trials with a 200 ms gap between fixation point removal and target presentation compared to when the fixation point remained illuminated (an “overlap” condition). Similarly, we found SSRTs were ~10 ms shorter on trials with simultaneous fixation point removal and target presentation compared to the overlap condition. However, in Experiment 2, this priming of movement inhibition due to fixation condition disappeared, when the stop signal delay for each trial was determined dynamically, based on subject performance on previous trials. Overall, we suggest the disappearance of fixation condition effects in Experiment 2 was due to smaller gap effects on reaction times and a greater variance in fixation condition effects on the STOP process in Experiment 2. We believe these differences arose due to a higher percentage of trials in Experiment 2 where the GO and STOP processes approached threshold at approximately the same time. Therefore we postulate that such trials produce a greater amount of conflict between movement generation and inhibition systems, causing both systems to rely less on fixation cues.



To view the content in your browser, please download Adobe Reader or, alternately,
you may Download the file to your hard drive.

NOTE: The latest versions of Adobe Reader do not support viewing PDF files within Firefox on Mac OS and if you are using a modern (Intel) Mac, there is no official plugin for viewing PDF files within the browser window.