Two minds in one head?
When surgeons cut the connection between the brain's hemispheres, each side began to act as if it had a mind of its own.
- Reading time
- 4 min
- Difficulty
- Intermediate
- Branch
- Identity
- First posed
- Roger Sperry and Michael Gazzaniga, 1960s; philosophical discussion by Thomas Nagel, 1971
The setup
Your brain has two hemispheres, connected by a thick band of around 200 million nerve fibres called the corpus callosum. In the 1940s and especially from the 1960s, surgeons cut this connection in some patients with severe epilepsy, to stop seizures spreading from one side to the other. The operation often worked. The patients recovered and, in everyday life, seemed remarkably normal.
But careful experiments by Roger Sperry and Michael Gazzaniga revealed something strange. In most people, the left hemisphere controls speech, and each hemisphere mainly receives vision from the opposite side of the visual field and controls the opposite hand. If a split-brain patient was shown a word, say 'key', only to the left visual field (and so only to the right hemisphere), they would say they saw nothing. Yet their left hand, controlled by the right hemisphere, could reach under a screen and pick out a key from a pile of objects.
In other experiments, each hemisphere seemed to have its own preferences and plans. Patients sometimes reported one hand undoing what the other had done: buttoning a shirt while the other unbuttoned it. And when the right hemisphere caused an action, the speaking left hemisphere would instantly invent a plausible but false explanation for it, without any awareness that it was guessing.
Why it matters
These real cases bear directly on questions about the self and personal identity. We assume each body houses exactly one mind. Split-brain patients seem to show that this is not guaranteed, and that the unity of consciousness can come apart.
The left hemisphere's habit of confabulating explanations, which Gazzaniga called 'the interpreter', also raises unsettling questions about how much of our own self-explanation is accurate.
Key terms
- Unity of consciousnessThe way all of a person's experiences at one time seem to belong together in a single field, experienced by one subject.
- ConfabulationProducing a confident but false explanation of one's own behaviour or memories without intending to deceive. Split-brain studies offer famous examples.
- Personal identityWhat makes a person at one time the same person as someone at another time.
- SelfThe subject of experience: the 'I' that thinks, feels and acts. Whether it is a thing, a process, a story or an illusion is disputed.
The positions
Two minds
After the operation, each hemisphere supports its own separate stream of consciousness. There are two minds in one head.
One mind
There is still one mind, which has lost some of its integration. Unity is impaired, not doubled.
No determinate number
There is no fact about whether there are one or two minds. Our concept of 'a mind' does not apply cleanly here.
Arguments and objections
Two minds
- Best argument
- Each hemisphere perceives, remembers and acts on information the other lacks. They can pursue different goals at the same time. If two people showed this pattern, we would say there were two minds. Sperry himself concluded that each hemisphere had 'a mind of its own'.
- Strongest objection
- In everyday life, patients behave as one person, with one personality and one set of life plans. The 'two minds' appear only under artificial laboratory conditions that feed information to one side.
One mind
- Best argument
- The hemispheres remain connected below the cortex and share emotions, arousal and much else. Patients do not report feeling like two people. A 2017 study by Yair Pinto and colleagues found that some patients could respond to stimuli in either visual field with either hand, suggesting more unity than the classic results implied.
- Strongest objection
- The patients' reports come from the left hemisphere, which may not know what the right is experiencing. And the classic dissociations are robust and well replicated.
No determinate number
- Best argument
- Thomas Nagel concluded in 1971 that none of the obvious answers fits the evidence, and that 'the countable individual mind' may not be a basic feature of reality. Derek Parfit drew the lesson that personal identity is less deep than we think: what matters is the connections between experiences, and these can come in degrees.
- Strongest objection
- Many find it hard to accept that there could be no fact about how many subjects of experience exist, since each subject seems to know for certain that it exists.
Who argued what
- Two minds Roger Sperry, Roland Puccetti
- One mind Yair Pinto, Tim Bayne
- No determinate number Thomas Nagel, Derek Parfit
Variations
The chicken and the snow
In a famous experiment, Gazzaniga showed a patient's left hemisphere a chicken claw and the right hemisphere a snowy scene. The left hand picked a shovel; the right picked a chicken. Asked why, the patient (speaking from the left hemisphere, which had not seen the snow) answered at once: 'The chicken claw goes with the chicken, and you need a shovel to clean out the chicken shed.' The explanation was confident, fluent and invented.
Hemispheric transplant
Philosophers imagine taking this further. If your two hemispheres were transplanted into two different bodies, and each functioned well, which one would be you? Both have equal claim. See the teleporter for the same problem in another form.
Go deeper
The interpreter in all of us
Gazzaniga argued that the left hemisphere contains an 'interpreter' that constantly builds a coherent story to explain our behaviour, filling gaps with plausible fiction. Psychologists have found similar confabulation in people with intact brains: in classic studies by Richard Nisbett and Timothy Wilson, people gave confident reasons for choices that were actually driven by factors they had not noticed, such as an item's position on a table.
Related ideas
Further reading and sources
- EncyclopediaStanford Encyclopedia of Philosophy: The Unity of Consciousness
- WikipediaSplit-brain
- BookWho's in Charge?, Michael Gazzaniga
- PaperBrain Bisection and the Unity of Consciousness, Thomas Nagel · in Mortal Questions
- BookThe Unity of Consciousness, Tim Bayne
Open questions
- If one hemisphere could feel pain the other did not know about, whose pain would it be?
- How much of your own explanation for your choices is invented after the fact?
- Is 'one person, one mind' a fact of nature, or a habit of thought?