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Systems Neurobiology Laboratory, Salk Institute for Biological Studies, La Jolla, California 92037
Submitted 25 September 2002; accepted in final form 5 February 2003
The countermanding paradigm provides a useful tool for examining the mechanisms responsible for canceling eye movements. The key feature of this paradigm is that, on a minority of trials, a stop signal is introduced some time after the appearance of the target, indicating that the subject should cancel the incipient eye movement. If the delay in giving the stop signal is too long, subjects fail to cancel the eye movement to the target stimulus. By modeling this performance as a race between a go process triggered by the appearance of the target and a stop process triggered by the appearance of the stop signal, it is possible to estimate the processing interval associated with canceling the movement. We have now used this paradigm to analyze the canceling of pursuit and saccades. For pursuit, we obtained consistent estimates of the stop process regardless of our technique or assumptions—it took 50–60 ms to cancel pursuit in both humans and monkeys. For saccades, we found different values depending on our assumptions. When we assumed that saccade preparation was under inhibitory control up until movement onset, we found that saccades took longer to cancel (humans: 
110, monkeys: 80 ms) than pursuit. However, when we assumed that saccade preparation includes a final "ballistic" interval not under inhibitory control, we found that the same rapid stop process that accounted for our pursuit results could also account for the canceling of saccades. We favor this second interpretation because canceling pursuit or saccades amounts to maintaining a state of fixation, and it is more parsimonious to assume that this involves a single inhibitory process associated with the fixation system, rather than two separate inhibitory processes depending on which type of eye movement will not be made. From our behavioral data, we estimate that this ballistic interval has a duration of 9–25 ms in monkeys, consistent with the known physiology of the final motor pathways for saccades, although we obtained longer values in humans (28–60 ms). Finally, we examined the effect of trial sequence during the countermanding task and found that pursuit and saccade latencies tended to be longer if the previous trial contained a stop signal than if it did not; these increases occurred regardless of whether the preceding trial was associated with the same or different type of eye movement. Together, these results suggest that a common inhibitory mechanism regulates the initiation of pursuit and saccades.
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