The role of task history in simple reaction time to lateralized light flashes

Department of Psychology, University of California, Los Angeles, Los Angeles, CA 90095, United States.
Neuropsychologia (Impact Factor: 3.3). 02/2008; 46(2):659-64. DOI: 10.1016/j.neuropsychologia.2007.09.008
Source: PubMed


In lateralized simple reaction time (SRT) tasks with unimanual responses, 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) is typically interpreted to reflect callosal transfer time. Indeed, split brain patients have much longer CUDs than control subjects. However, while many studies have supported the hypothesis that the CUD reflects callosal transmission time, a few studies have suggested that the CUD may be affected by non-anatomical factors. We investigated the nature of these inconsistent results in two experiments. In the first, we asked half of our subjects to cross their arms while performing the task. The CUD was not affected by arms crossing, supporting the anatomical model of the CUD. In the second experiment, however, all subjects were asked to cross their arms in half of the trials. In this experiment, arms crossing significantly affected the CUD, thus showing that spatial attention modulates the CUD. These latter results cannot be readily explained by a simple callosal relay interpretation of the CUD. Rather, the CUD seems to reflect a mix of anatomical and non-anatomical factors produced by task history. Thus, the seemingly inconsistent results of previous studies can be reconciled by taking into account differences in task history across studies.

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    • "Our results suggest that the effect is triggered by adaptation (no such effect is observed during the familiarization phase, see Fig. 6), develops gradually and has a slow wash-out—at least, slower than that of force field adaptation, as it is not completely canceled at the end of the wash-out phase. This finding (interaction increases while adaptation develops) is in apparent contrast with other reports that hand crossing interference is reduced by training (Craig and Belser 2006); see also (Mooshagian et al. 2008). However, in those experimental protocols hand interference has a minor behavioral relevance, thus it makes sense that the effect fades away with training. "
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    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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Jul 21, 2014