Human muscle spindle afferent activity in relation to visual control in precision finger movements.

1. Impulse activities of muscle spindle afferents from the finger extensor muscles were recorded in the radial nerve of human subjects. In addition to single unit activity, surface EMG was recorded as well as finger joint position and angular velocity. 2. All units were studied under two conditions of voluntary finger movements. In the visual condition, the subject tracked ramp and hold sequences at a single metacarpophalangeal joint. In the non-visual condition the subject was asked to produce the same movement while visual control was denied altogether. 3. With sixteen units, detailed statistical analyses failed to reveal significant differences in muscle spindle afferent activity between the visual and the non-visual task. However, with two group Ia units, impulse rate was marginally but significantly higher in the visual task even when differences in average movement velocity, velocity variability and EMG level had been factored out. 4. The findings suggested that access to visual information for movement control did not produce any large-scale differences in spindle afference, although a small effect of an increased and independent gamma-activation emerged in the statistical analysis in 11% of the units.