Differential control of the digits of the human hand: evidence from digital anaesthesia and weight matching.

The present study assessed changes in perceived heaviness of weights lifted by the thumb, index, ring and little fingers during anaesthesia of the "lifting" digit. Subjects (n = 19) lifted weights solely by flexion of the distal joint of the digit, using a bilateral weight-matching paradigm. Changes in perceived heaviness during anaesthesia differed in both their sign and magnitude for the different digits. Anaesthesia of the thumb and index finger produced large increases in perceived heaviness (median, thumb: +41%; index finger: +13%), whereas anaesthesia of the ring finger decreases perceived heaviness (-14%). Anaesthesia of the little finger also reduced perceived heaviness (median, -21%; n = 6). Because perceived heaviness is biased by signals of motor commands, these findings suggest a net facilitatory reflex effect of the digital nerve inputs from the thumb and index fingers onto their respective flexor motoneurones with the corresponding effect for the ring and little fingers being inhibitory.