Organisation of keying skills : the effect of motor complexity and number of units

The psychological processes involved in initiating and executing a rapid sequence of movements were investigated. Subjects were required to read a sequence of digits, and then on presentation of a visual cue, key the whole sequence as fast as possible. Motor pattern, in terms of ease of organisation, and number of key strokes in the sequence were experimentally manipulated. Times to initiate and execute the sequences were a function of motor pattern as well as number of keys, in ways which depended on the position of a keystroke relative to an hierarchical organisation in an output motor store. On the basis of these results, models which postulate a fixed time to load and/or execute any distinct psychological motor unit are decisively rejected. This includes the simplest model of Stenberg et al. (1978), where the motor units are single keystrokes, and the more complex model of Rosenbaum et al. (1983), where subjects perform a binary search through high level units in a motor buffer. Models such as the spreading activation model of Rumelhart and Norman (1982), which do not allow for organisation in the output motor store were also refuted. An adequate model must have mechanisms for hierarchical organisation in the output motor store which depend on cognitive as well as physical constraints, and allow for active structuring by performers of a skill.