Interpretation of perceived motor commands by reference to afferent signals

1. Weights were lifted by flexing the distal joints of the anaesthetized thumbs. A weight-matching task was used to determine the perceived heaviness of a reference weight during control estimates when it was freely lifted, and during experimental estimates when the weight was not lifted, but simply 'gave way' when a reference isometric tension of the same magnitude was attained. 2. Limitation of the afferent input, by anaesthesia of the digit and by replacing lifting with a sudden unloading, did not grossly impair the judgement of heaviness. The sense of heaviness increased when the muscles became fatigued. 3. These results suggest that the central nervous system can interpret the voluntary motor command as an index of heaviness even when provided with only a crude peripheral signal that the weight has been moved.