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The two studies undertaken investigated how reaction times are affected by preknowledge concerning hand and direction parameters, and the order in which these data are disclosed. In the first experiment, preknowledge was manipulated by selecting 2-CRT pairs which had either a common hand or direction component. Reaction signal information content (as defined by the number of unknown response parameters) was controlled, but no parameter differences emerged. When parameter order was manipulated by sequential precuing, the ‘hand then direction’ order conferred a significant advantage. Motor preparation and response selection explanations for this effect are considered. Various difficulties with the paradigms used are discussed, including those of distinguishing between response selection and motor programming contributions, controlling and inferring the strategies adopted by subjects, and defining task difficulty. The prospects for independently fostering either response identification or motor preparation by means of an appropriate selection of precues are also discussed.

Latencies were measured using a modified four-choice reaction time (4-CRT) test procedure; the responses comprised forward and backward displacements of left- and right-hand levers. Two stimuli were presented, separated by an interval (ISI) of 25, 165, 415 or 815 msec. S1 designated which response should be readied; and on 62% of occasions, S2 indicated that the prepared response should be executed immediately (same trials). During transition trials, S2 specified which one of the alternative responses should be substituted. Same conditions produced significantly shorter reaction times. Transition latencies varied depending on the modification required: changing direction proved easier than changing either hand or hand and direction. This is compatible with a response coding strategy in which limb is designated before direction. Latencies and exchange function analyses that preparations were discontinued after response selection; even at the longest ISI, there was no convincing evidence of preparatory motor programming.

The accuracy with which people execute wrist and elbow movements were measured using three main conditions: (i) single -jointed (wrist or elbow) movements to targets, (ii) dual -jointed (wrist and elbow) movements to targets, and (iii) components of dual-jointed movements to targets, when the task for the subject was to perform the elbow or wrist constituent of the action in isolation, without displacing the second joint. Elbow precision was significantly worse under component than dual conditions, which is compatible with the notion that wrist and elbow activity are conjugately, rather than independently, programmed when a dual-jointed action is performed. The pattern of wrist accuracy was divergent, but possible reasons for this were discussed. In all cases, error was measured in terms of deviation from perfect posture; using this index, the hypothesis that incorporating more moving joints into an action serves to increase movement complexity and jeopardise precision was tested, but the results were ambiguous. Discussion also centered on the problems of using performance data to infer changes in motor programming, and the need for rigorous conceptualisation and research in this area.

Subjects were trained to execute rhythmical forward and backward arm swings, with consistent timing and displacement characteristics. A DC servomotor was used to introduce brief perturbations, opposing elbow extension. Pilot studies indicated that: (1) Weak disturbances tended to foreshorten the voluntary action, while larger perturbations (strong enough to reverse the ongoing action) caused positional overshooting. (2) When prolonged ( > 500 msec) perturbations were investigated, there was no evidence that EMG responses within the first 100 msec provided much force to counter the disturbance. (3) Subjects were unable to learn to curb the steep rises in velocity observed when strong perturbations were released, despite extensive feedback experience. The main study confirmed that electromyographic and movement parameters reflect the magnitude of the perturbation. The actions of cerebral palsied subjects were more disrupted by perturbations, abnormal antagonist activity and clonus were recorded. Models of compensation are discussed.

The choice reaction times of dual (elbow and wrist) and single (elbow) jointed movements were measured using ‘mixed’ (dual and single) and ‘matched’ (both dual or both single) response pairings. Although subjects found it more difficult to execute dual jointed actions (as shown by error rate), there was no concomitant prolongation of latencies for acceptable responses. In the light of these findings, the utility of CRT as an index of motor programming complexity, and the claim that pre-programming of responses is precluded with CRT testing, were discussed.

Choice reaction times for short (Dit) and long (Dah) Morse key presses were measured using a number of different left hand/right hand Dit/Dah 2-response combinations. When Dit and Dah responses were made with the same hand, Dits were initiated significantly faster, which confirms the finding of Klapp et al. (1974). The absolute sizes of Dit and Dah CRT's, however, varied significantly from one test situation to another; and the Dit:Dah difference was shown to be intransitive. Such context effects cast doubt on the extrapolative power of CRT findings, and on the usefulness of CRT as a measure of movement complexity. Latencies were reduced when subjects knew in advance which hand would be used; but the usefulness of response duration pre-knowledge was not unambiguously demonstrated. Preparation of responses is, therefore, not precluded by CRT testing; and possible explanations for these savings in terms of response selection and motor preprogramming were discussed. On balance, the hypothesis that the longer reaction times of Dah responses are due to their greater movement complexity (Klapp et al. 1974) was rejected, in favour of the explanation that subjects adopt a strategy of preferentially preparing Dit responses.