Perceived finger orientation is biased towards functional task spaces.

In the absence of visual feedback, the perceived position of the hands is systematically biased towards the plausible manual task space. Here we tested whether perceived orientation of the finger is similarly misperceived in right-handed individuals. Participants' index fingers were passively rotated about the middle joint to a range of test angles, either in the frontoparallel plane (Experiment 1) or the horizontal plane (Experiment 2); they reported perceived orientation of the finger by rotating a visual line presented on a screen optically superimposed on the location of their unseen finger. Perceived finger orientations were biased towards positions that varied across hands and planes. Both hands were biased towards 10° inward in the frontoparallel plane and, in the horizontal plane, the left hand was biased towards 25° inward, whereas the right hand was biased towards 2° inwards. In a third experiment, participants reported finger orientation with respect to non-visual targets: gravitational vertical or straight ahead. Biases in perceived finger orientation to non-visual targets were similar to those found in the visual line task. The asymmetrical nature of biases across hands and planes reflects the typical orientation of the hands while working and supports the theory of a functional rather than anatomical representation of the fingers and hands in space.