Nonhierarchical control of rapid movement sequences: a comment on Rosenbaum, Kenny, and Derr

In a recent article, Rosenbaum, Kenny, and Derr (1983) described a hierarchical storage and execution model for a class of repetitive, discrete response sequences. With a few modifications, this model can match the performance of subjects performing sequences from this class. The authors claimed that this provides an "existence proof" for hierarchical control during movement execution, at least for these sequences. My purpose is to show by counterexample that this claim is too strong. I present a logogen activation model for the rapid execution of stored motor sequences which assumes that (a) logogens corresponding to responses are activated via association and repetition; (b) activation decays; and (c) interresponse time is inversely related to activation of the correct response at each position in the sequence. This model can also fit the results of Rosenbaum et al. A much richer data base, designed to discriminate between competing formulations, will be needed to prove the existence of the hierarchical, tree-traversal control process proposed by Rosenbaum et al.