Our current objective is to determine whether pallidal neurons discharge differently during movements that are a part of a learned spatial sequence. We have recorded the activity of more than 250 neurons in the external and internal pallidal segments of 5 monkeys that learned to move to multiple sequences of 2 or 3 peripheral targets under several conditions. These movements could be to targets that became visible at the time of the trigger, either in random order or in a fixed sequence, or they could be made to remembered target locations, either when each was precued with a delay from precue to trigger or when the entire sequence must be remembered (ALL). Whereas the fixed, repeated sequence task might elicit implicit learning, the remembered task requires that the animal explicitly learn the sequence. A large fraction of the pallidal cells studied showed a significant difference between the mean discharge rate prior to the GO signal in the random or fixed task, compared with the precued or remembered task. We found that the fixed task, but not the precued task, elicited explicit memory of the sequence, as demonstrated by the animal's ability to perform the sequence without sequential cues in the ALL block. Thus, memory of the sequence appears to produce a change in background discharge rate. The perimovement discharge may also change, but this is less consistent than the change in the HOLD discharge.
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