Split-brain subjects—people whose cerebral hemispheres were surgically disconnected by sectioning the corpus callosum—behave in controlled experiments as though each hemisphere had a mind of its own. Schechter (2015) articulated the conceptual basis of this 'duality intuition': the two hemispheres' mental states interact only indirectly, via effection (overt behavior) and transduction (sensory perception)—the same channels through which two separate people's minds interact—so the split-brain subject houses two minds, labeled R and L. Two questions are left open by the paper's account and treated as open by the author's subsequent work: the empirical premise remains unsettled (subcortical and cerebellar pathways survive surgery, and patients differ in surgical extent, so it is unknown for which subjects and under what conditions hemispheric interaction is exclusively effection/transduction-mediated), and while the paper individuated minds rather than persons, whether R and L qualify as distinct persons with the attendant moral and rational agency remains unresolved. Both questions remain open: the 2020 collective review of the field records a consensus that the evidence is insufficient, subsequent empirical work (2021–2025) documents residual interhemispheric communication and extreme subject heterogeneity while explicitly calling for further studies, and the personhood question remains an actively contested philosophical debate with live positions on all sides.

Background

The corpus callosum is the large band of nerve fibers connecting the two cerebral hemispheres of the human brain. In a surgical treatment for severe epilepsy pioneered in the 1960s—called commissurotomy or callosotomy—this tract (and sometimes other commissures) is cut, producing a 'split-brain' subject. Classic experiments following this surgery (Gazzaniga, Bogen and Sperry, 1962) showed striking dissociations: an object shown only to the left visual field (processed by the right hemisphere) cannot be named by the speaking left hemisphere, and the two hands can act at cross purposes. Yet in daily life split-brain subjects behave remarkably normal.

These findings fueled a famous debate in philosophy of mind. Nagel (1971) asked whether brain bisection divides consciousness and whether such a subject is one mind or two; Puccetti (1973, 1981) argued the split-brain subject literally contains two minds—and indeed two persons—while skeptics replied that no principled criterion for counting minds independently of bodies had ever been supplied.

Schechter (2015) filled this gap by reconstructing what she called the duality intuition. Her key conceptual move: a single mind is characterized by the capacity of its mental states to interact directly—two percepts can be combined in experience without anything having to be done and observed first. In split-brain subjects, by contrast, task-relevant information passes between the hemispheres only through effection (overt behavior, such as a hand movement or gaze shift) followed by transduction (the other hemisphere perceiving that behavior through the senses)—exactly the channels through which two distinct people coordinate. Classic experiments with subject L.B. on bimanual coordination, tactile word naming, and cross-cueing (subtly using one's own gaze or movements to inform the other hemisphere) illustrate the pattern: when these behavioral and sensory channels are blocked, interhemispheric transfer fails. On this basis she labeled the two candidate thinkers R (the subject considered as everything but the left hemisphere) and L (everything but the right hemisphere) and concluded the split-brain subject has two minds.

Two issues are left open by that paper's account, and they are connected. First, the empirical premise is not established across the split-brain population: subcortical structures (for example the superior colliculus, which integrates visual input from both hemifields) and cortico-cerebellar routes (connections from the cerebral cortex to the cerebellum that cross through the pons, a brainstem relay structure) survive the surgery, patients differ in how much commissural tissue was actually cut, and ipsilateral (same-side) motor control plus learned cross-cueing can mimic interhemispheric unity—so it remains unknown for which subjects, and under what conditions, hemispheric mental interaction is exclusively indirect. Second, the paper individuated minds, not persons: persons are the bearers of moral standing and rational agency, and whether R and L qualify as distinct persons—the question Puccetti had answered affirmatively—requires an account of the relationship between the criteria for individuating minds and those for individuating persons, which the paper did not provide. Later work sharpened both issues: Pinto and colleagues (2017) found that a completely callosotomized patient showed divided perception but unified action control, and argued this shows undivided consciousness—prompting an exchange in Brain over whether preserved unity reflects subcortical transfer, ipsilateral motor control, or cross-cueing—while Schechter's own later book defended a one-person, two-minds verdict on contested grounds. The unresolved target is the scope of the split-brain subject's psychological duality—duality of minds only, or of persons as well; de Haan et al. (2020) leave intermediate conceptualizations explicitly on the table.

Problem Statement

Determine the scope of the split-brain subject's psychological duality by answering two linked questions left open by Schechter's (2015) effection/transduction account of mind individuation:

(1) The empirical premise. For which split-brain subjects, and under what conditions—task domain, time since surgery, and extent of commissural section—do the mental states of the two hemispheres interact exclusively via effection (overt behavior) and transduction (sensory perception)? This requires characterizing the extent, pathways, and conditions of residual direct interhemispheric interaction after commissurotomy: distinguishing residual subcortical and cortico-cerebellar communication from ipsilateral motor control and learned cross-cueing, and establishing for which subjects the two-minds criterion's exclusivity premise holds and for which it fails.

(2) Personhood. Do R and L qualify as distinct persons, with the attendant moral and rational agency? This requires making explicit the relationship between the criteria for individuating minds and the criteria for individuating persons (such as first-personal self-consciousness and self-distinction, rational agency, and moral status), and applying them to the split-brain case in light of the empirical facts about residual interaction and unified everyday behavior.

The objective is an argued, evidence-responsive resolution of these two linked questions concerning the scope of the split-brain subject's psychological duality: for (1), whether the exclusivity premise is supported, refuted, or undetermined for the subjects and conditions on record rather than a single verdict for the class; for (2), whether R and L are distinct persons, the criteria for individuating minds and for individuating persons on which the answer rests, and what the answer to (1) implies for whether (2) still demands its own answer.

References

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