Choosing between movement sequences: A hierarchical editor model.

SUMMARY Research on the control of action has made it clear that the performance of movement sequences is governed by central plans. This article is concerned with the structure of these plans and the course of events that underlies their construction. We conduct choice reaction-time experiments in which subjects choose between sequences of motor responses; the responses are similar to those used in piano playing. The initial goal of the experiments is to distinguish between two models of sequence choice developed to account for results from preliminary experiments on this topic. One model, the motor-program editor model, assumes that subjects prepare for a choice between two sequences by constructing an ordered set of motor subprograms each of which has a list of the motor features shared by the alternative responses at the corresponding serial position; the subsequent choice time is assumed to depend on the number of features to be supplied to the initially readied feature lists. The alternative, hierarchical decisions model, assumes that choosing between sequences of motor responses is achieved by carrying out a series of choices among competing elements at each of a number of distinct functional levels. Five experiments are reported that require subjects to choose between response sequences consisting of one to four button presses. Among the phenomena found are the following: (a) systematic effects of the serial positions of uncertain responses in the alternative sequences; (b) effects of the structural similarity of the two sequences, both on the time to choose between the sequences and on the time to perform responses within them; and (c) effects of the requirement to cancel some responses depending on the structural relationships between those responses and the other responses in the same sequence and in the other sequence. On the basis of these and other results, we propose a hybrid of the motor-program editor model and the hierarchical decisions model. The new, hierarchical editor (HED) model assumes that motor plans are hierarchically structured and that subjects prepare for forthcoming sequence choices by readying responses shared by the two possible sequences. The essential new ideas in the HED model are that the physical production of planned motor sequences is controlled by the successive of nested subprograms and that before this unpacking process begins, it is gone through once in advance to ensure that all uncertain nesting relations are resolved. An important implication of the model is that the performance of movement sequences is achieved through rapid access to symbolic memory stores rather than through linear readout from low-level command stores. Another important implication is that the control of manual response sequences may be based on the same fundamental mechanisms as the control of language production.