Split Brain Studies: One Mind per Hemisphere

#biology #brains #consciousness #free_will #life #multiple_personality_disorder #personality #psychology #psychosurgery #souls

This text begins with a discussion of how the interactions between our left and right hemispheres can raise philosophical concerns and question the idea of free will. We then discuss split-brain patients and the implications that dual-personality phenomenon have for those who stand by a soul-orientated theory of spirituality.

Book CoverThe philosopher Thomas Gilovich explains studies where two different pictures are presented to a split-brain patient. The language centre in the left hemisphere gives responses according to its interpretation of the right's actions. A patient can point to a picture in accordance with what one eye has seen, but the left-hand side of the brain couldn't see, and the person tries to explain their selection of photo in accordance with the wrong input. For example a picture of a snow-filled field is shown to a patient's non-verbal right hemisphere, and they select a shovel from a list of pictures. But the left-hand side of their split-brain doesn't know what input they have seen. When asked, they construct a rational-sounding reason and when quizzed, believe that this made-up reason is the actual reason why they selected it1. The most important philosophical issue is that the person does not realize that they are inventing a reason. It teaches us that our brains interprets our own behaviour and gives us an illusion of agency even when there is none. Further examples and discussion held in "The Illusion of Choice: Free Will and Determinism" by Vexen Crabtree (1999) include cases of hypnotism and other sociological experiments where people give wrong reasons for their own behaviour, revealing that people in general are more like observers than we can admit.2

Psychologists R.W. Sperry and R.E. Myers conducted the most famous split-brain studies in 1953, and it was largely these that earned Sperry his Nobel Prize in 1981. In one series of experiments with cats, they cut the corpus calloseum and trained each half of cat's brains to react differently from each other. After investigations and experiments "Sperry and Myers concluded that the cat now had two 'minds' either of which was capable of learning on its own - and of responding intelligently to changes in the world around it on its own. Subsequent experiments with rats and monkeys gave similar results"3. Evidence from Humans with disorders or brain damage confirms that similar things happen with Humans. In short, there is the potential that our brain contains two minds, one in the left hemisphere and one in the right, with the corpus calloseum serving as the joint between them, the result of which is that only one side controls our movements through the cerebellum.

Split-brain procedures are carried out with serious epileptics, where the attacks are life-threatening. Using 'John Doe' as an example study, doctors have examined 'John Doe Left' and 'John Doe Right':

According to Roger Sperry, psychological tests showed that both John Does had remarkably similar personalities. Except for language ability, they were about as much alike as identical twins. Their attitudes and opinions seemed to be the same; their perceptions of the world were the same; and they woke up and went to sleep at almost the same times. There were differences however. John-Doe-Left could express himself in language and was somewhat more logical and better at [planning...]. John-Doe-Right tended to be somewhat more aggressive, impulsive, emotional - and frequently expressed frustration with what was going on.

"Psychology: The Science of Mind and Behaviour" by Richard Gross (1996)4

Sometimes, the differences in style and character of Left and Right personalities can result in 'rogue' limbs, or cases where the hemispheres no longer co-ordinate control of the limbs, and 'arguments' can ensue between the left and right. The psychologist, R. Sperry, mentioned above, "showed a male patient a complex design and then asked the main to reproduce the pattern by putting colored blocks together. When the patient used his left hand (right hemisphere), he completed the task rapidly. But when the man tried to match the design using his right hand (left hemisphere), he proceeded slowly, clumsily, and made many mistakes. And much to the annoyance of both Sperry and the patient, the man's left hand often tried to "correct" the mistakes the right hand made". This is just one example amongst many.

Book CoverSplit-brain patients have undergone surgery (normally in the treatment of epilepsy) to cut their corpus callosum, which normally joins the two hemispheres and allows an exchange of information from one to the other. While the surgery may relieve the suffering, it has a major side-effect in that the two hemispheres become functionally separate [...]. [In one example experiment involving] sets of photographs of different faces - a beautiful young female model, a podgy-cheeked boy [...]. They were then presented in such a way that the left side of the photo would only be visible to the right hemisphere and vica-versa. [...] Participants were asked to describe what they had seen (the left hemisphere responding): they said 'an old man'. But if asked to point with their left hand to the complete person of the person they had seen (the right hemisphere responding) they would point to the young boy. It seems that two completely separate visual worlds can exist within the same head!

"Psychology: The Science of Mind and Behaviour" by Richard Gross (1996)4

As a result of many such studies in Humans, "the incredible conclusion some scientists draw from the split-brain research, however, is that you have at least two different types of brains and thus two different types of 'mind'"3. R. Gross draws a similar conclusion:

These and many more dramatic experiments led Sperry, Ornstein and others to conclude that each of the separated hemispheres has its own private sensations, perceptions, thoughts, feelings and memories - in short, that they constitute two separate minds, two separate spheres of consciousness.

"Psychology: The Science of Mind and Behaviour" by Richard Gross (1996)5

Split-brain studies show that in some situations, it is clear that our brain can contain two minds, two personalities, "two separate spheres of consciousness". Cases of multiple-personality can also result in very different personalities existing in the same brain. And some more extreme conclusions can be drawn: it is possible that all people have two consciousnesses but that each is only aware of itself. I will, however, only discuss here the fact that sometimes, the same brain contains two minds. This clearly has implications for studies of souls.

If 'souls' exist, it must be true that a soul can encompass two consciousnesses, with two different personalities, memories and skills. And if souls survive bodily death... which consciousness is it that survives? In Christian mythology, a bodily resurrection will occur, and the saved will ascend to heaven... in the case of split-brain patients, how are two separate consciousnesses reborn in the same heavenly body, which cannot contain the same biopsychical dysfunctions as the imperfect Earthly body did? It makes a nonsense of the simplistic theologies of the afterlife if we hold that 'souls' survive death, given the evidence of split-brain cases. This doesn't show that souls don't exist, merely that popular opinions about souls simply cannot account for all the possibilities that the biology of consciousness provides. It is probably closer to the truth, and certainly a correct implementation of occam's razor, to conclude that consciousness is a result of purely biological factors, and that no such things as 'souls' exist, rather than try to reconcile them with split-brain and multiple-personality studies.

If this is possible, how come split brain patients don't become debilitated by the confusing contradictions arising in their hemispheres? Well this brings us to some other oddities of our conscious life: we constantly patch over irregularities in our perceptions of the world, and much of our justifications for our own actions are derived from our actions, and do not precede them.

The lure of certainty is built into the brain at a very basic level. This is most poignantly demonstrated by split-brain patient [experiments]. The patient is then shown various images and asked to pick out the one that is most closely associated with what he or she has just seen. In a tragicomic display of indecisiveness, the split-brain patient's hands point to two different objects. The right hand points to a chicken (this matches the chicken claw that the left hemisphere witnessed), while the left hand points to a shovel (the right hemisphere wants to shovel the snow). The conflicting reactions of the patient reveals the inner contradictions of each of us. The same brain has come up with two very different answers.

But something interesting happens when scientists ask a split-brain patient to explain the bizarre response: the patient manages to come up with an explanation. 'Oh, that's easy,' one patient said. 'The chicken claw goes with the chicken, and you need a shovel to clean out the chicken shed.' Instead of admitting that his brain was hopelessly confused, the patient wove his confusion into a plausible story.

"The Decisive Moment: How the Brain Makes Up Its Mind" by Jonah Lehrer (2009)6

Some similar subconscious excuses arise when people are hypnotised into doing something and then asked why they done it. The reasons they invent are imaginative and fluent, and genuinely rational, except for the simple fact that the person saying them doesn't even know that the reason isn't true. See The Illusion of Choice: Free Will and Determinism for some examples. Our brains trick us into a false sense that we are a single, coherent and conscious being. It's not quite that simple!

By Vexen Crabtree 2006 Dec 31
(Last Modified: 2015 Jun 06)
Parent page: Soul Theory and Skepticism: Science Versus Spirituality

References: (What's this?)

Crabtree, Vexen
(1999) "The Illusion of Choice: Free Will and Determinism" (1999). Accessed 2017 Feb 17.

Dennett, Daniel C.
(1996) Kinds of Minds. Hardback book. Science Masters Edition.

Gilovich, Thomas
(1991) How We Know What Isn't So: The Fallibility of Human Reason in Everyday Life. Paperback book. 1993 edition. Published by The Free Press, NY, USA.

Gross, Richard
(1996) Psychology: The Science of Mind and Behaviour. Paperback book. 3rd edition. Published by Hodder & Stoughton, London UK.

Lehrer, Jonah
(2009) The Decisive Moment: How the Brain Makes Up Its Mind. Hardback book. Published by Canongate Books, Edinburgh.

McConnel, James V.
(1986) Understanding Human Behavior. Hardback book. 5th edition. Originally published 1974. Current version published by CBS College Publishing, Holt Rinehart and Winston, New York, USA.

Toates, Mackintosh & Nettle
(2004) Emotions and Mind. Published by The Open University, Milton Keynes, UK. A neurology textbook by Frederick Toates, Bundy Mackintosh and Daniel Nettle.


  1. Gilovich (1991) p22. Added to this page on 2007 Dec 04.^
  2. Frederick Toates in Toates, Mackintosh and Nettle (2004) chapter 4 "Consciousness revisited" p119-120.^
  3. McConnel (1986) p41-42.^
  4. Gross (1996) p61.^
  5. Gross (1996) p62.^
  6. Lehrer (2009) p202. Added to this page on 2015 Jun 06.^

© 2017 Vexen Crabtree. All rights reserved.