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4. SKILL AND ACTION
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4.1 Timing and control of movement (Long, Nimmo-Smith, Wing)
Within the broad and traditional area of APU interest in skill and action, a recent central concern has been with the timing and control of movement. Consider the simple task of tapping with a single finger. It involves two antagonistic phases, moving the finger down and lifting it again on completion of the tap. The two phases could be coordinated by separate though linked peripheral loops, or could be operated by a single central timer. If movement control is based on two separate peripheral components, one would expect the interresponse interval variability to be different for the up and downward phase. Wing (267) has shown that the two movements do not differ in variability, implicating a single central timing system.
An equivalent logic can be applied to the problem of coordinating movements with the two hands, such as would be required when a tray is lifted and kept level. Once again, the evidence suggests a single central timer. It is proposed in the future to apply this technique to the study of patients with difficulties in the coordination of movement.
Long and Nimmo-Smith have studied the response timing involved in the performance of skilled typists. A mathematical model of fluency in performance of this task has been developed and is being explored using an on-line experimental facility whereby a computer records both the time at which the typist strikes each key, and when she shifts direction of gaze amongst source text, keyboard and printout.
4.2 Handwriting (Baddeley, Lewis, Nimmo-Smith, Wing)
Our research on handwriting is concerned with both theoretical and applied questions. Wing (268) has shown that overall size of writing can be adjusted by changing the intervals between reversal of pen direction but that this is not the basis of size differences between letters within a word, for example between £ and Unlike speech, where utterance length affects the time to initiate a response time, it does not take longer to initiate the writing of a series of digits than to initiate a single digit, hence indicating that the subject does not take time to set up a more complex programme before writing a longer series (269). On the other hand, overall speed of repeatedly copying sequences of letters is affected-by number of letters in the sequence (272), a result which is tentatively inter¬preted as reflecting the operation of a short-term memory buffer for the written strokes. Such a concept is also consistent with some of the "slips of the pen" reported in a corpus of writing errors (271).
Applied research in this area has looked at the relative rate of copying and reading cursive and block capital modes of writing (264). Block capitals are read more rapidly, but written substantially more slowly than cursive script. An ongoing project for the Home Office is concerned with the variability in handwriting (273). The data should be of considerable interest to forensic document examiners, as well as providing a useful data base for more fundamental studies of the nature of handwriting. The size of handwritten ticks has been shown to be a sensitive stress measure (270) which has the useful advantage of being both unobtrusive and rapidly and easily collected. Finally, in the clinical field a joint project is planned with Mr. John Gleave of the Addenbrooke's Department of Neurosurgery on the influence on handwriting of Parkinson's Disease. The writing of a large group of patients was sampled before and after stereotactic surgery. It will be analysed using the computer-based system devised by Wing.
4.3 Control systems (McLeod and Poulton, in collaboration with colleagues from RAE Farnborough and the Warren Spring Laboratory)
We have investigated the effects of ship motion on a variety of manual control tasks (142). It was demonstrated that even in comparatively mild sea states there are considerable differences in the amount of degradation that control tasks suffer depending on whether continuous or ballistic hand movements are required by the control, and depending on whether the whole arm or only the hands are involved. The drop in performance is not primarily due to nausea. Since different sorts of tasks are degraded to different degrees by ship motion it appears that a useful human factors gain could be achieved by designing the man/control interface around comparatively motion-resistant tasks. It is hoped to continue this work if a suitable simulator can be made available.
4.4 Visual control of action (McLeod, Wing)
Work on the visual guidance of movement is an area of current rapid development. In addition to the work by Marcel, Hinton and Wilkins described in the Vision section, Wing has developed a computer-based system for recording movements between a starting point and a target. It is proposed to use this to investigate the mechanisms underlying Fitts' Law which relates the time taken to reach a target to the distance and size of the target. In addition to research on normal subjects, joint experiments with Dr. Edgar Miller, Chief Clinical Psychologist at Addenbrooke's Hospital, have begun to explore the question of whether Fitts' Law applies to patients with motor disorders. Patients so far studied include those with Parkinson's Disease and with Wilson's Disease.
In collaboration with Carol Fraser of the Occupational Therapy Department at Addenbrooke's Hospital, Wing has recently started to study the ability of patients to pick up objects of various sizes at varying distances. High-speed cine film records of the movements of the arm and hand are used to study the coordination between the timing of the two constituent elements, transport of the hand toward the object and opening of the hand. The aim is to contrast normal hand control with that achieved by skilled artificial hand users.
A source of dispute in the field of experimental psychology is the adequacy of 'artificial' laboratory tasks for understanding the functioning of the nervous system. Some people believe that the full power of the nervous system can only be demonstrated when it is performing 'natural' tasks which it experiences in everyday life. With this criticism of laboratory-based studies in mind, McLeod has been examining the visual control of the arm movements of professional batsmen playing cricket. It turns out that their performance, in terms of the speed with which they pick up visual information and the accuracy of the timing of their movements, is similar to that obtained by the best subjects in laboratory experiments.
4.5. Attention and Action (McLeod, Shallice)
McLeod has shown that when people carry out two independent tasks simultaneously, performance is restricted by the operation of some limited capacity control device which does not appear to be involved in the performance of the tasks themselves (138; 139; 140). A continuing programme of research is trying to discover what the nature and function of this 'control device' might-be.
A new model for the role of attention in the control of action has been developed by Shallice in collaboration with Dr. D.A. Norman of the University of California (165). It incorporates two levels of control. "Contention scheduling" enables rapid but crude selection to occur. "Supervisory attention control" biasses the operation of the simpler mechanism to deal with more novel or complex contingencies.