Task dependence of slowing after pyramidal lesions in monkeys

The effect of lesions in a medullary pyramid was studied in two experiments. (a) Monkeys were trained to press a key with a rear-projected circle, presented together with different numbers of keys with ellipses. Two to eight choices were presented in a random sequence. Discrimination was measured with a titration schedule, and short choice reaction times were reinforced selectively. Choice reaction time and the function relating it to the number of choices were unaffected by lesions reducing the area of a transverse section of the pyramid up to 93%. (b) A repetitive response controlled by a fixed-ratio schedule was performed simultaneously with a holding response with the other hand, and the two hands changed function after each food pellet. Lesions reducing the pyramid only 25% increased interresponse times of the phasic response with the contralateral hand. Experiment 1 and previous studies are interpreted to indicate that slowing after pyramidal lesions is dependent on behavioral context. Experiment 2 showed that a simple repetitive movement is affected when the rate is high. The static response was more clearly affected than the phasic response, which indicates a role for the pyramidal tract in posture.