Letter versus Dot Stimuli as Tools for “Splitting the Normal Brain with Reaction Time”

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Abstract
Filbey and Gazzaniga (1969) found simple dot-present or -absent reports averaged 35 ms slower for the left than for the right visual field. Other data suggests that verbal processing efficiency differences between the cerebral hemispheres, rather than transcallosal transfer time alone, must be tapped to obtain half-field differences as large as 35 ms. Three experiments were conducted. The first failed to show any half-field differences in vocal RT for dot detection; the second replicated previous reports of significant right field superiority of vocal RT to letter stimuli for right handers, and also showed a substantially smaller half-field difference for left handers; the third experiment, utilizing the fixation control procedure of the second experiment, again failed to show half-field differences for the dot detection paradigm. Differences between the Filbey and Gazzaniga and present results probably reflect important procedural differences. We conclude that transcallosal transfer time for simple dot information is much smaller than assumed by Filbey and Gazzaniga and that the letter report-time task taps hemispheric asymmetries of verbal processing efficiency.