Spatial S-R compatibility with orthogonal stimulus-response relationship

Spatial stimulus-response (S-R) compatibility with unimanual two-finger choice reactions was investigated under conditions in which the spatial orientation of response keys was either parallel to or perpendicular to the orientation of the stimuli. Subjects responded to green or red lights in the left or right visual field (irrelevant stimulus location). The response keys were oriented horizontally on the left or right side of the body midline parallel to the stimuli, and were pressed with the palms facing down (Condition A), or were oriented orthogonally to the stimuli in the midsaggital plane, either horizontally and pressed with palms facing down (B) or facing up (C), or vertically and pressed with palms facing the body (D). The results for Condition A demonstrate the usual spatial S-R compatibility effect between field of stimulation and spatial position of responding finger. For Conditions B and D, a strong reaction time advantage still obtained for those stimulus-finger pairings that are compatible under Condition A. Condition C revealed an RT advantage for the opposite pairings. This shift of the compatibility effect from Condition B to Condition C indicates that the left/right distinction of fingers does not follow a simple, fixed spatio-anatomical mapping rule. The results are discussed within the framework of a hierarchical model of spatial S-R compatibility, with spatial coding and spatio-anatomical mapping as factors.