Explicit dialogue between left and right half-systems of split brains

Abstract

Much debate has surrounded the interpretation of the well known ‘split brain’ syndrome, produced by section of the corpus callosum in man [for useful surveys of early studies of the effects of callosal damage (for example by Liepmann (1900), Goldstein (1908) and Bonhoeffer (1914)) see refs 1–3]. On the basis of the observation that left and right half-systems can find themselves in conflict when disparate instructions are flashed to left and right visual hemifields, Sperry⁴has described such patients as having “two free wills inside the same cranial vault”. MacKay^7–9, among others, has argued for a more parsimonious interpretation of the available evidence, suggesting that it demonstrates duality only at the ‘executive’ level where continuous goal-pursuit is administered, rather than at the ‘normative’ supervisory level where the agent's priorities and criteria of evaluation are themselves subject to evaluation and revision. We report here some experiments in which a split-brain patient showed considerable capacity for explicit interhemispheric exchange of information and instructions, yet gave no sign of the duality of supervisory initiative that might have been expected on the hypothesis of ‘two free wills’.