Is "left" always where the thumb is right?: stimulus-response compatibilities as a function of posture and location of the responding hand.
OBJECTIVE
To determine hand-centered coordinate systems for left-right discriminations in representational space, including scenes imagined behind one's head ("backspace").


METHODS
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.


RESULTS
"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.


CONCLUSIONS
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.
Leuthard
Jürg
J
Department of Neuropsychology Unit, University Hospital Zurich, CH-8091 Zurich, Switzerland.
Bächtold
Daniel
D
Brugger
Peter
P
D000328:Adult; D005385:Fingers / Q000502:physiology; D007839:Functional Laterality / Q000502:physiology*; D006225:Hand / Q000502:physiology*; D006801:Humans; D007092:Imagination / Q000502:physiology; D008297:Male; D011187:Posture / Q000502:physiology*; D011597:Psychomotor Performance / Q000502:physiology; D013028:Space Perception / Q000502:physiology; D013933:Thumb / Q000502:physiology*