Mind and Body

Neural Correlates of Consciousness

There are many correlations between sensory experience and neural activity. In the case of vision, for instance, activity in the right hemisphere of the brain is correlated with visual experience in the left visual field, that is, in the field of vision to the left of where one is directly looking. Similarly, activity in the left hemisphere of the brain is correlated with visual experience in the right visual field.

The two hemispheres are normally connected by a large band of neural fibers called the corpus callosum, allowing neural activity to flow back and forth between them. This band has been surgically cut in several patients to treat epilepsy. In an experiment with such patients, one briefly shows a visual image of the phrase key ring so that key appears in the patient's left visual field and ring appears in the right visual field. If patients are asked to say out loud what they saw, they say “ring” and give no evidence of having seen the word key. If asked to draw with their left hand what they saw, they draw a picture of a key, and give no evidence of having seen the word ring.

This striking result is explained as follows. Speech is correlated with activity in the left hemisphere, whereas control of the left hand is correlated with activity in the right hemisphere. The left hemisphere was shown only the word key, and the right hemisphere only the word ring. The severed corpus callosum prevented neural activity from being shared between the two hemispheres. This failure to share neural activity is correlated with failure of the sensory experiences correlated with each hemisphere to be integrated into a single sensory experience. This experiment raises the fascinating question of how many selves (i.e., how many subjects of experience) comprise the patient with a severed corpus callosum. If there are two subjects of experience, were there also two before the corpus callosum was severed?

Philosophical Theories

Philosophical theories of the relation between mind and body come in three basic types: physicalist, substance dualist, and idealist. There are many varieties of these basic types.

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