Practising a structured continuous key-pressing task : motor chunking or rhythm consolidation?

An experiment is reported on the effects of extensive practice in a task in which a succession of nine keys was pressed with nine separate fingers, each keypress in re-sponse to a corresponding stimulus. The order of the keypresses remained constant over practice. Keypressing cycles followed each other without interruption. A stimulus was usually presented immediately upon depressing the previous key but in the Structured condition a stimulus was preceded at two or three positions by a 750 ms Response Stimulus Interval (RSI). This partitioned the sequence into three response groups for subjects in the 333 condition and into two response groups for subjects in the 45 condi-tion. On occasion all subjects performed in the Unstructured condition in which RSIs were zero. Interkey times in this condition clearly reflected the position of the 750 ms RSIs in the Structured condition. This suggests that motor chunks had developed in the Structured condition which were also used in the Unstructured condition. Rhythm-based control as proposed by Summers (1975) was rejected because group-start/within-group ratios exceeded 2:1 and because rhythm-based control could not predict intervals in the Unstructured 45 condition. Unstructured within-group intervals were slower than Structured within-group intervals which effect was more pronounced in the 333 than in the 45 condition. Also, the initial element of the Unstructured four-key group was faster than the initial element of the Unstructured three-key groups. These and other findings accord with the notion that, in Unstructured sequences, preparing a forthcoming re-sponse group concurs with the execution of the preceding group and suggest that shorter groups are slowed more by concurrent preparation for the forthcoming group than longer groups.