HEMISPACE ASYMMETRIES AND LATERALITY EFFECTS IN ARM POSITIONING

Hemispace asymmetries and laterality effects were examined on an arm p ositioning reproduction task. Sixteen male subjects were asked to repr oduce both abductive and adductive positioning movements with the left or right arm within either the left or the right hemispace. Hemispace was manipulated using a 90 degrees head-rotation paradigm. A left hem ispace advantage in positioning accuracy was predicted for both left a nd right arm movements on the grounds that the perceptual-motor contro l of positioning movements made in left hemispace is primarily mediate d by the right hemisphere which is known to be advantageous for tasks which are spatial in nature (Heilman, Bowers, & Watson, 1984). No arm laterality effects were predicted to occur because the proximal muscul ature involved in the control of arm movements is innervated from both contralateral and ipsilateral cerebral hemispheres (Brinkman & Kuyper s, 1973). Results showed that the predicted left hemispace advantage w as evident for the right arm on the positioning variability measure al one, whereas it was absent for all other possible conditions on all er ror measures. Laterality (arm) effects were absent as predicted. The e xperiment also demonstrated a greater degradation of reproduction perf ormance under the ''crossed'' arm-hemispace conditions than under the ''uncrossed'' conditions. A plausible explanation for the uncrossed ad vantage for the task is that under normal conditions, a single hemisph ere is primarily responsible for both controlling the contralateral ar m and directing attention to the contralateral hemispace, and conseque ntly potential interhemispheric interference is minimized. A clear res ponse bias effect in movement reproduction was also evident as a funct ion of the direction of concurrent arm movement and head rotation. Arm movements made in the same direction as head rotation were systematic ally undershot in reproduction to a much greater degree than arm movem ents made in the opposite direction to head rotation. (C) 1995 Academi c Press, Inc.