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Neuropsychologia. 2009 Feb;47(3):933-7. doi: 10.1016/j.neuropsychologia.2008.12.005. Epub 2008 Dec 13.

Spatial attention and interhemispheric visuomotor integration in the absence of the corpus callosum.

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1
Department of Psychology, University of California, Los Angeles, Los Angeles, CA 90095, United States. mooshage@ninds.nih.gov

Abstract

In the lateralized simple reaction time (SRT) task with unimanual responses (Poffenberger paradigm), reaction times (RTs) are faster with ipsilateral (uncrossed) than with contralateral (crossed) response hand-target hemifield combinations. The difference between crossed and uncrossed responses (CUD) has typically been interpreted to reflect callosal transfer time. Indeed, acallosal subjects and split-brain subjects have longer CUDs than control subjects. However, a few recent studies have demonstrated that, contrary to classical findings, the CUD is also affected by non-anatomical factors. Here we show that the CUD is also affected by non-anatomical factors in patients with agenesis of the corpus callosum and complete commissurotomy where interhemispheric transfer must be subcallosal. We tested acallosal subject M.M. and split brain patient A.A. on a lateralized SRT task with their arms alternately uncrossed (natural arms position) or crossed (unnatural arms position) across blocks of trials. The results revealed a significant effect of arms crossing on the size and direction of the CUD as previously found in normal subjects [Mooshagian, E., Iacoboni, M., & Zaidel, E. (2008). The role of task history in simple reaction time to lateralized light flashes. Neuropsychologia, 46(2), 659-664]. This suggests that non-anatomical factors that modulate interhemispheric visuomotor integration may occur in absence of the corpus callosum. Anterior commissure and interhemispheric cortico-subcortical pathways are likely implicated in these effects.

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