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Abstract:
We studied how the saccadic reaction time (SRT) on each trial
was affected by the sequence of saccades made on previous trials in
a simple choice SRT task. Although the saccadic target was
randomized between two mirror image locations relative to an
initial central fixation point, monkeys had shorter SRTs when the
current saccade was a repetition of the saccade on the previous
trial. This repetition effect was cumulative in that SRTs became
progressively shorter as the number of preceding repetitions
increased. Recently, saccade-related neurons in the monkey superior
colliculus (SC) have been shown to modulate their level of
pretarget motor preparatory activity in accordance with changes in
response probability and the level of this preparatory activity is
inversely correlated to the concomitant SRTs. In the present task,
extracellular recordings from the same class of SC neurons reveals
that monkeys are preparing movements in advance of target
presentation for repeated saccadic targets over movements to
non-repeated saccadic targets. This study suggests that motor
behaviour is shaped by subjective expectancy, that is, as subjects
develop hypotheses to respond to a constantly changing world, they
weigh events that have occurred recently more heavily than past
events even when there is no statistical basis for this
bias.
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