Manual Asymmetries in Aimed Movements

Three hypotheses for the right-hand advantage in aiming movements were examined in these experiments: (1) the right-hand system is more efficient at processing visual information during the movement; (2) subjects make more use of visual information prior to movement initiation when using the right hand; (3) the right hand is less variable in generating force in initiating the pointing response as force demands increase. In the first experiment subjects pointed at a target located directly in front of them from two starting positions which defined short (25-cm) and long (35-cm) movements. The movements were made in three movement times, fast (150 to 249 msec), medium (250 to 349 msec) and slow (350 to 449 msec), under three vision conditions—full vision, and no vision (lights out) with immediate or delayed movement initiation. Performance was measured in movement time and accuracy in amplitude of movement. The results did not completely support any of the hypotheses regarding the right-hand advantage, although the left hand was generally more variable than the right. Also, variability increased with increases in movement length and decreases in movement time. The second experiment was designed to examine further the hypotheses regarding the right-hand advantage. In this experiment the same three visual conditions were used; however, subjects made only fast (<250-msec) movements. Also six rather than two starting positions were used. The increased variability of the left hand was observed again here. Further pointing accuracy with the left hand was more adversely affected in the no-vision delay condition. The implications of these results were discussed as they pertain to understanding the processes involved in visual aiming and the observed manual asymmetries.