Exploring the Split Brain
Whether one subscribes to a dualistic distinction between or a monistic equivocation of the brain and the self, the human brain plays a central role in the mysteries of human experience. As such, neuroscience is a pivotal interlocutor in conversations on phenomenology and consciousness. For example, one may consider the experiments performed on split-brain patients by Michael Gazzaniga and Roger W. Sperry, whose findings have exhibited a tremendous influence on not only neuroscience but research into human consciousness more broadly.
Fifteen years after Myers and Sperry’s experiments in severing the optic chiasm of various animals, Gazzaniga (1967) summarized his and Sperry’s own investigations into the results of split-brain operations on humans. Myers and Sperry found that severing the optic chiasm in an animal’s brain significantly affected that animal’s ability to perform certain tasks; whether the animal could perform a task successfully depended on which eye it was allowed to use, and therefore which hemisphere of the brain received visual data. Similarly, to treat chronic epileptic seizures, surgeons had severed the corpus callosum in human patients. Though the human patients exhibited no apparent changes in personality after their surgeries, each patient appeared to be more responsive to stimulation on the right side of the body than on the left side.
Gazzaniga and Sperry hypothesized that severing the corpus callosum of a human patient, isolating the hemispheres of the brain, would have a similar effect to severing the optic chiasm in an animal. To investigate this possibility, they gathered a sample of split-brain patients and gave them a number of visual tests, displaying a series of lights, words, and objects to each patient’s left and right visual fields. Gazzaniga and Sperry found that if a stimulus was presented to the patient’s right visual field, the patient could verbally confirm and describe said stimulus. Conversely, if a stimulus was presented to the patient’s left visual field, the patient was unable to say anything about the stimulus; however, the patient was able to perform non-verbal gestures to indicate whether they had noticed a stimulus in their left visual field and to identify that stimulus.