Tracing response parameter preparation strategies using transition times

Latencies were measured using a modified four-choice reaction time (4-CRT) test procedure; the responses comprised forward and backward displacements of left- and right-hand levers. Two stimuli were presented, separated by an interval (ISI) of 25, 165, 415 or 815 msec. S1 designated which response should be readied; and on 62% of occasions, S2 indicated that the prepared response should be executed immediately (same trials). During transition trials, S2 specified which one of the alternative responses should be substituted. Same conditions produced significantly shorter reaction times. Transition latencies varied depending on the modification required: changing direction proved easier than changing either hand or hand and direction. This is compatible with a response coding strategy in which limb is designated before direction. Latencies and exchange function analyses that preparations were discontinued after response selection; even at the longest ISI, there was no convincing evidence of preparatory motor programming.