EVIDENCE OF A LIMITED VISUO-MOTOR MEMORY USED IN PROGRAMMING WRIST MOVEMENTS

Human subjects can pre-program movements on the basis of visual cues. Experience in a particular task leads to the storage of appropriate co ntrol parameters which are used in programming subsequent movements, v ia a short-term motor memory. The form, duration and usage of this mem ory are, however, uncertain. Repetitive wrist flexion and extension mo vements were measured in four subjects. Three were neurologically norm al men; the fourth subject had a peripheral large-fibre sensory neurop athy, depriving him of proprioceptive information about wrist movement . Subjects made alternating 45 degrees wrist movements between two vis ual targets; visual feedback of wrist position was provided for the fi rst part of each trial. After 10 s of tracking, the subjects paused fo r an interval of 0-24 s before resuming tracking without visual feedba ck of wrist position. The positional accuracy of subsequent movements was analysed with respect to pause interval. Movement accuracy was red uced by the removal of visual feedback in all four subjects: movements after the pause interval were less accurate than those before the pau se. Errors also accumulated within each sequence of movements made wit hout visual feedback. Analysis of the first movement in each trial aft er the pause indicated a clear relationship between movement accuracy and pause interval. In all four subjects, movement accuracy decayed wi th longer pause intervals. In the deafferented subject, manipulation o f the visual inputs (requiring visual fixation, rather than normal pur suit of the target; or direct viewing of the hand instead of viewing a cursor on a computer screen) affected the relationship between pause interval and subsequent movement accuracy. We propose that the memory used when producing these movements is a short-lasting visuo-motor sig nal, lasting a few seconds, which is derived from visual knowledge of previous movements, rather than a memory of a particular motor output. This visuo-motor signal is used to scale the amplitude of subsequent wrist movements. The brevity of the visuo-motor memory and the resulta nt inaccuracy of this deafferented subject and of our neurologically n ormal subjects implies that human feedforward control of the amplitude and position of wrist movements is severely limited.