Delayed Copying of Unfamiliar Outline Images: Does the Decrease in Reaction Time with the Increase in the Delay Reflect a Change in the Internal Representation of the Forthcoming Movement?

A group of 24 adult subjects performed a task consisting of delayed motor reproduction (copying) of unfamiliar outline figures (trajectory templates). Templates were presented for 250 msec and were reproduced in response to an instruction given by sound signal (a brief click), delayed relative to the end of the period of presentation of the trajectory image by time T = 0, 500, 1000, 2000, or 4000 msec. Subjects performed the task in a block experiment in which the delay T was constant within each block of 32 trials. Reaction time (RT) was analyzed, along with the mean duration of movement along one segment of the trajectory (MT) and the duration of the pause between adjacent segments (DT). Reaction time RT was found to show a non-monotonic dependence on delay T, decreasing at T ≤ 1000 msec and increasing at T ≤ 2000 msec. The relationship between RT and T was adequately described by a very simple additive model – the sum of a linearly increasing component and an exponentially decreasing component. The linear increase reflected a decrease in the accuracy of predicting the moment at which the instruction signal would be presented as the delay T increased (foreperiod effect [Niemi and Näätänen, 1981; Meulenbroek and Van Galen, 1988]). The exponentially decreasing component is suggested to reflect transformation of the internal representation of the trajectory template during the period it is held in short-term memory.

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