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Abstract:
The goal of this study is to determine whether the primary
mechanism responsible for the inhibition of oculomotor movements in
a countermanding task is top-down or bottom-up in nature. In
countermanding, subjects perform saccadic eye movements to an
eccentric target unless presented with an infrequent stop signal
which requires them to maintain gaze centrally. We used auditory
and visual stop signals (and their combination) to examine the
effect of non-localized and localized signals on subjects'
inhibitory control. We measured both saccadic reaction time and
stop-signal accuracy and used both to calculate a stop signal
reaction time (SSRT), a measure of time required to inhibit an eye
movement for each subject. SSRTs to different modalities help
quantify top-down and bottom-up oculomotor control of movement.
Five subjects generated 8000 control trials and 4800 stop-signal
trials. Analysis yielded mean SSRTs of 258 ms, 121 ms, and 108 ms
for auditory, visual, and combined stop signals, respectively. This
shows a strong bias in favour of a fovea-aligned visual signal
suggesting that a bottom-up reflexive mechanism is responsible for
the inhibition of oculomotor movements in the presence of a visual
stop signal localized on the fovea. An alternative mechanism of
top-down control requiring more time was responsible for inhibiting
oculomotor movements in the presence of a non-localized auditory
stop signal.
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