Split-brain
Explanation
In the 1960s, Roger Sperry and Michael Gazzaniga studied patients who had undergone commissurotomy: surgery that severed the corpus callosum (the bundle of nerve fibres connecting the two cerebral hemispheres) as a treatment for intractable epilepsy. The results of their studies revolutionised neuroscience and the philosophy of mind.
After surgery, the patients seemed normal in everyday life, but careful experiments revealed something unsettling: each hemisphere could act as a semi-independent agent, with its own perceptions, knowledge and decisions, without reciprocal access. The patient, with his or her single apparent self, seemed to harbour two centres of processing.
The paradigmatic experiments involved presenting stimuli selectively to a single hemisphere (via the contralateral visual field or unilateral touch). The left hemisphere (where language is housed) could verbally describe what it saw. The right could not describe it, but could point to correct answers with the left hand. When different stimuli were presented to each hemisphere, the patients produced inconsistent or contradictory responses.
Especially fascinating was the phenomenon of confabulation. When the patient was asked to explain an action carried out by the right hemisphere (based on information accessible only to it), the verbal left hemisphere produced a rational but incorrect explanation. It would not acknowledge not knowing: it post-hoc invented a coherent story. This phenomenon would have enormous implications for understanding normal confabulation.
Philosophically, split-brain patients raise deep questions. Does the patient have one consciousness or two? Is the self divided? Sperry, Nobel laureate in 1981, suggested that each hemisphere had its own consciousness. Gazzaniga, his disciple, has since qualified this: the apparent unity of normal consciousness may be illusory, a narrative construction by the interpreter of the left hemisphere.
Split-brain studies have influenced philosophy (Dennett and others on the self as narrative), psychology (importance of the hemispheres), neuropsychology (hemiagnosias, hemicorruption phenomena) and popular culture (the creative right brain / logical left brain dichotomy, simplified to the point of cliché). They remain central evidence in debates about the unity of consciousness and the nature of the self.
Strengths
- Decisive empirical evidence on dissociations of consciousness.
- The 'interpreter' concept theoretically and intuitively fertile.
- Foundation for narrative and confabulationist theories of the self.
- Profound ethical and philosophical implications.
Main critiques
- Some recent studies question the 'complete disconnection' of consciousnesses.
- Generalisation of the dual model to normal brains is problematic.
- Hemispheric asymmetry sometimes overstated (McGilchrist qualifies it).
- Interpretation of dissociation is debated.