The effect of withdrawal of visual presentation of errors upon the frequency spectrum of tremor in a manual task

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Abstract
When a subject attempts to exert a steady pressure on a joystick he makes small unavoidable errors which, irrespective of their origin or frequency, may be called tremor. Frequency analysis shows that low frequencies always contribute much more to the total error than high frequencies. If the subject is not allowed to check his performance visually, but has to rely on sensations of pressure in the finger tips, etc., the error power spectrum plotted on logarithmic coordinates approximates to a straight line falling at 6 db/octave from 0. 4 to 9 c/s. In other words the amplitude of the tremor component at each frequency is inversely proportional to frequency. When the subject is given a visual indication of his errors on an oscilloscope the shape of the tremor spectrum alters. The most striking change is the appearance of a tremor peak at about 9 c/s, but there is also a significant increase of error in the range 1-4 c/s. The extent of these changes varies from subject to subject If the 9 c/s peak represents oscillation of a muscle length-servo it would appear that greater use is made of this servo when positional information is available from the eyes than when proprioceptive impulses from the limbs have to be relied on.

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