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February 2001

Researchers can examine the differences between the two hemispheres of split brain patients by asking the patient to focus their vision upon a single point, and then flashing an image to one side of the point so quickly that the patient cannot move his/her eyes to look at it, and thus information is presented to one visual field, and thus one hemisphere only. Gazzaniga (1970) reports that when a word is flashed into the right visual field, and thus the left (speaking) hemisphere, the patient can say what the word was. When the word is flashed into the left visual field, and thus the right (mute) hemisphere, the patient says there was only a flash of light, and no word. The left hand (under right hemisphere control) however, can point to what it saw when given a choice of written words. Such odd results have been found in much research, and have led Sperry (1966, p299) to conclude that 'everything we have seen so far indicates that the surgery has left these people with two separate minds, that is, two separate spheres of consciousness.'

There has been further anecdotal evidence about split brain patients which suggests two separate minds, such as the man who hit his wife with one arm whilst simultaneously not wanting to.

If we accept that there is evidence of people having two minds, does this mean that we should accept that in fact these two minds belong to two different people in the same body?

Puccetti (1973) is one philosopher who has argued that there are two persons per brain, and that commissurotomies (splitting the hemispheres) merely make this more evident. One argument Puccetti (1981a) presents in further evidence for his conclusion is that there is a duplication of visual images in the two hemispheres - both hemispheres have visual cortex, each receiving a slightly different viewpoint to it's 'partner'. While visual scientists have claimed that the brain must include a fusing mechanism as we are only ever aware of one image, Puccetti argues that as we have no evidence for such a mechanism, a plausible explanation for our seeing one image only is that each hemisphere houses a separate person, each of whom sees the image on their own visual cortex only.

Puccetti's theory seems plausible. However, it is such a modification of our traditional views of personhood that it must be critically examined. We don't feel as though we are two different persons, so perhaps we are not? This is a point made by Von Eckardt (1981). Puccetti(1981b) retorts that such a notion is comparable to believing a small child does not have two lungs because the child says he/she does not feel like he/she has!

So, have we been fooled all along into thinking we were just one person all our lives? It gives a brand new meaning to the phrase 'I'm in two minds about it'! Well, as you may have noticed, it is not a widely held view.

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