Is a response sequence executed only after the sequence has been fully programmed, as discrete processing models predict, or does execution begin before programming has been completed, as continuous processing models predict? To address this issue, we tested a discrete processing model of human motor performance, the hierarchical editor model of Rosenbaum, Inhoff, and Gordon (1984). This model was developed to account for data from experiments in which people perform one of two possible finger sequences, depending on the identity of a choice signal. The model assumes a hierarchically organized motor program that is first "edited" to resolve any uncertainties and is then "executed" to produce the desired responses. Three experiments reported here show that, contrary to the model's predictions and some well-known motor programming results (Sternberg, Monsell, Knoll, & Wright, 1978), the reaction time to begin a response sequence actually decreases with the length of the sequence under some choice conditions. We account for these results with a model that allows execution to begin while editing is still in progress. A key assumption in the model is that subjects schedule execution so that means and variances of interresponse times are minimized.

This article describes a series of experiments which show that motor performance suffers when responses in a repeated sequence have variable rather than fixed parameter mappings. In speech production, repeated recitation of the beginning of the alphabet is slowed dramatically if the same letters have different stress levels on successive cycles; the same effect holds for vowel-consonant relationships. In keyboard performance, repetitions of fingertapping sequences suffer if the number of consecutive taps by the same finger changes from cycle to cycle; we call these difficult sequences finger fumblers. Finally, similar effects are obtained with violin playing. The model we develop to account for these results says that parameter values mapped to the subprogram for a response item persist after the subprogram has been executed, and that extra processing is required if a new parameter value must be mapped to the subprogram the next time it is called for. This account is consistent with the view that motor programs for forthcoming actions are prepared by editing motor programs for actions that have just been completed. Because our results are similar to interference effects in traditional memory studies, we also suggest that similar mechanisms underlie storage and retrieval of motor responses and symbolic materials.