Parallel Response Selection after Callosotomy

Hazeltine, Eliot; Weinstein, Andrea J.; Ivry, Richard B. · 2008 · Journal of Cognitive Neuroscience

DOI: 10.1162/jocn.2008.20.3.526

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Summary

This study investigates whether dual-task costs in split-brain patients arise from fundamental architectural constraints or strategic adaptations. Previous research using the psychological refractory period (PRP) paradigm showed that split-brain patients exhibited robust dual-task costs similar to neurologically intact controls, despite the lack of interhemispheric communication. However, those studies prioritized one task, potentially encouraging participants to adopt a strategy of withholding the second response until the first was completed. The authors hypothesized that if stimuli are presented simultaneously without priority instructions, split-brain patients might demonstrate parallel response selection, revealing that previous costs were strategic rather than structural. The researchers tested one split-brain patient (J.W.) and five age-matched controls using a bimanual response task. Stimuli were presented simultaneously to the left and right visual fields, requiring responses with the corresponding hands. Unlike standard PRP tasks, neither task was prioritized, and participants were instructed to respond as quickly as possible. Trials were categorized as anatomically compatible, spatially compatible, or incompatible based on the mapping of stimuli to responses. The study measured reaction times (RTs) and response coupling to assess dual-task interference and compatibility effects. The results revealed a stark contrast between the patient and controls. Control participants strongly coupled their responses, producing them within 50 milliseconds of each other on nearly all trials, and exhibited significant dual-task costs (increased RTs on bimanual trials compared to unimanual trials). These costs were mediated by compatibility effects, with incompatible mappings causing substantial delays. In contrast, J.W. showed weak response coupling and minimal dual-task costs. His RTs on bimanual trials were comparable to, or even faster than, his unimanual baseline. Furthermore, J.W. displayed negligible compatibility effects, lacking the interference seen in controls. Statistical analysis confirmed that J.W.’s dual-task costs were several standard deviations below the mean of the control group. The findings suggest that split-brain patients can select responses for two tasks in parallel when not constrained by prioritization instructions. The robust dual-task costs observed in previous PRP studies with split-brain patients likely reflected a strategic decision to serialize responses to avoid conflict, rather than a mandatory bottleneck in response selection. This implies that the cognitive architecture of the bisected brain allows for independent, parallel processing of lateralized tasks, and that dual-task interference in intact brains may be partly driven by strategic control mechanisms rather than solely by structural limitations.

Key finding

Split-brain patients show greatly reduced dual-task costs and compatibility effects when stimuli are presented simultaneously without priority instructions, indicating that previous dual-task deficits were likely strategic rather than architectural.

Methodology

lab_experiment

Sample size: 6

Provenance

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Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.

Topics

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• dual task multitasking