In 12 normal, right-handed male subjects simple reaction time of key-pressing and leverpulling responses to light flashes presented in the right or left visual fields was faster with the arm ipsilateral to the visual stimulus, provided the responses were made unilaterally. Key-pressing responses involved a movement of a single finger, whereas lever-pulling responses involved an integrated movement of the proximal parts of the arm. The mean difference in reaction time between reactions ipsilateral to the stimulus and reactions contralateral to the stimulus was about 2 ms for both key-pressing and lever-pulling responses. When key-pressing and lever-pulling responses to a single lateralized light stimulus were made bilaterally, the advantage in favor of the ipsilateral responses was still present, although it was mainly limited to the right hand, on key-pressing, whereas it was completely absent on lever pulling. The difference between ipsilateral and contralateral reactions on unilateral responding, whether proximal or distal, is attributed to the consistent initiation of the response by the contralateral hemisphere. Given that a lateralized visual stimulus is projected to the opposite hemisphere, ipsilateral responses can be integrated within a single hemisphere, whereas contralateral responses are integrated interhemispherically and therefore require additional time. The reduction or lack of the ipsilateral advantage on bilateral responding is attributed to the engagement of a bilaterally distributed motor control that is preferentially directed to the proximal musculature. © 1980 Springer-Verlag.
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