Plan decoding and response timing during execution of movement sequences

Abstract

An experiment is reported which examined the timing characteristics of finger-tapping sequences that consisted of segments of repeated and alternating elements. Eight musically-trained subjects performed the experiment. They tapped the sequences on a set of response keys that allowed identification of the onset and offset of finger to key contact. Inter-tap intervals were measured from onset to onset, and contact durations from onset to offset. The sequential position of errors and unusually long (outlier) intervals suggest that the sequences were organized on a higher level than the single tap. The structure of inter-tap intervals did not reflect however such an organization. The transition between supposedly higher-order units such as segments and half-sequences was not marked by systematic lenthening of the transition interval. Decreasing the size of the segments that could be described by a single production rule (repeat or alternate) resulted in a slowing down of execution. These findings are consistent with the joint assumptions that the sequential plan is decoded in segments rather than element by element, and that execution of a segment and preparation of the next segment may overlap. The basic rhythm of execution appears to be selected with reference to expected processing demands.