Is "left" always where the thumb is right?: stimulus-response compatibilities as a function of posture and location...

To determine hand-centered coordinate systems for left-right discriminations in representational space, including scenes imagined behind one's head ("backspace").

Thirty-two healthy men responded with the index or ring finger of their right (dominant) hand to numbers presented in the center of a screen. These indicated hours on the right or left half of a clock face. A decision "earlier or later than six o'clock?" was required. There were 4 response conditions: (1) pronated and (2) supinated hand in frontspace and (3) pronated and (4) supinated hand in backspace.

"Left" and "right" were reliably and differentially associated with the 2 different fingers for all conditions except the pronated hand in backspace (conflict between body-centered and hand-centered reference frames). When backspace trials were analyzed as a function of individual strategies of the localization of the imaginary clockface (ie, around the stimulus number in frontspace versus around the response hand in backspace), it was found that the 2 strategies were associated with opposite right/left assignments.

Together, these results indicate relatively universal left/right assignments to the fingers of a hand held in canonical postures and interindividually variable (but intraindividually consistent) assignments for less canonical postures. The task appears suitable for the quantification of representational neglect, especially in backspace.