Enhancing the ecological validity of tests of lateralization and hemispheric interaction: Evidence from fixated displays of letters or symbols of varying complexity

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Two experiments expand upon behavioural evidence of interactions among lateralization, hemispheric interaction, and task complexity with findings from an ecologically valid procedure. Target displays of letters or symbols were presented at fixation in go/no-go matching tasks of physical or categorical identity. Simultaneously with the target, a distractor appeared in the left visual field or right visual field to weight processing of the target to the hemisphere ipsilateral to the distractor, or the distractor did not appear at all. Comparison of the respective distractor-present trials with distractor-absent trials measures the relative costs or benefits of hemispheric interaction.

Both experiments found that 3-item displays were processed faster and more accurately than displays of 5 items, suggesting they are relatively simple. Accuracy to the simple tasks showed left-hemisphere lateralization in the lexical task, right-hemisphere lateralization in the spatial task, a cost of hemispheric interaction compared to the advantaged hemisphere, and a benefit of hemispheric interaction compared to the less-advantaged hemisphere, suggesting that the contributions of the less-advantaged hemisphere interfere with processing, and that the advantaged hemisphere controls the lion's share. In contrast, 5-item displays for physical match in both experiments showed a significant benefit to accuracy of hemispheric interaction compared to the left hemisphere, an insignificant benefit compared to the right hemisphere, no lateralization, no cost of hemispheric interaction, and a consequence to performance that was more costly to the hemisphere that had been advantaged in simple tasks, suggesting that the advantaged hemisphere relinquishes control as tasks become more complex and complementary processing results from both increased collaboration and decreased lateralization between the hemispheres. The findings expand upon behavioural evidence, converge with imaging evidence, and suggest future directions for brain mapping.

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