The detection of human finger movement is not facilitated by input from receptors in adjacent digits

These experiments were designed to determine whether cutaneous input from a digit provides a general facilitation of the detection of movements applied to an adjacent digit. The ability to detect passive movements at the proximal interphalangeal joint of the right index finger was measured when cutaneous (and joint) input was removed (using local anaesthesia) from the tip of one or both digits adjacent to the test finger (16 subjects). The same parameter was also measured when input was artificially increased by stimulation of the adjacent digits at three intensities: below, above and at perceptual threshold (PT; 15 subjects). Detection of flexion or extension movements was not altered by anaesthesia of one or both adjacent digits. Since it was possible that too few tonically active afferents in the hand had been blocked to reveal an effect, the median nerve was blocked, with movements applied to the little finger, causing no measurable impairment in acuity (three subjects). Simultaneous electrical stimulation of the tips of the adjacent digits at intensities above PT impaired movement detection, but had no effect when delivered at or below PT. To test whether the effect of detectable electrical stimuli was due to a specific interaction between the artificial input and the input evoked by moving the digit, or due to mental distraction, stimuli were delivered above PT to either the left or right little finger, or the test index finger during movement of the index finger. Electrical stimulation of the index finger significantly reduced detection by approximately 50%, but stimulation of the remote little fingers did not. Electrical stimulation is a non-natural stimulus, so a "natural" stimulus was applied by continuously stroking the tips of the adjacent digits with a brush (10 subjects). The natural stimulus also significantly reduced movement detection by approximately 50%. Together, these findings suggest that tonic inputs from digital nerve afferents adjacent to, or more remote from the passively moved finger do not facilitate movement detection. However, the reduced detection during stimulation of the adjacent digits shows that there is nevertheless some interaction between the various proprioceptive inputs from the digits.