Lateralisation and Sperry's Split-Brain Study
Two Halves, One Brain
Look at your own body: your right hand writes, your left hand steadies the page, and somehow it all feels like one continuous ‘you’. But the two halves of your brain are not identical twins doing the same job. Each one specialises in different tasks — a phenomenon called lateralisation. This lesson explores what each hemisphere does, how they communicate, and the split-brain research that first revealed how independent they can be.
Asymmetrical Function
The brain shows asymmetrical function: the left and right hemispheres are not mirror images in terms of what they do. Each hemisphere also controls the opposite side of the body — the left hemisphere controls movement and sensation on the right side of the body, and the right hemisphere controls the left side. This crossover is one reason psychologists became interested in whether the hemispheres might also differ psychologically, not just physically.
Lateralisation means the two hemispheres of the brain specialise in different functions. This is called asymmetrical function — the hemispheres are not identical in what they do, even though they look similar in structure.
The Left Hemisphere
In most people, the left hemisphere is dominant for language. It plays the main role in speech production, understanding language, reading, and logical, analytical and sequential thinking — working through problems step by step. Because most people rely on the left hemisphere for verbal skills, damage to the left hemisphere is more likely to produce language problems than damage to the right.
The Right Hemisphere
The right hemisphere specialises in different skills: spatial awareness, recognising faces, processing emotion, and creative or holistic thinking — seeing the ‘whole picture’ rather than working through it in steps. The right hemisphere generally has very limited language ability, which is exactly why split-brain research produced such striking results.
The Corpus Callosum
The two hemispheres are not sealed off from each other. They are connected by the corpus callosum, a thick bundle of nerve fibres running between them. In a normal, intact brain the corpus callosum constantly passes information back and forth, so the two hemispheres work together and we experience a single, unified stream of consciousness rather than two separate minds.
The left hemisphere is dominant for language, logic and step-by-step thinking. The right hemisphere is dominant for spatial awareness, facial recognition and holistic thinking. The corpus callosum connects the two hemispheres so they can share information.
Some psychologists have used lateralisation to try to explain claimed psychological differences between males and females, such as suggestions that males show stronger verbal-spatial specialisation between hemispheres while females show a more even spread of language processing across both hemispheres. This has sometimes been linked to average differences in corpus callosum structure.
This explanation has clear weaknesses. Findings on hemisphere and corpus callosum sex differences are inconsistent between studies and have not always been replicated, so the evidence base is shaky. The explanation is also biologically deterministic: it risks reducing complex sex differences in behaviour down to brain structure alone, ignoring social and cultural explanations such as upbringing, expectations and experience. This makes it a reductionist account of a much more complex picture.
Sperry’s Split-Brain Study (1968)
The clearest evidence for lateralisation comes from Sperry’s study of patients who had undergone a split-brain operation to control severe epilepsy. This surgery cut the corpus callosum, meaning the two hemispheres could no longer share information directly. Sperry’s aim was to find out what happens to perception and awareness when the hemispheres are disconnected.
In the procedure, split-brain patients stared at a fixed point while an image was flashed briefly to only one side of their visual field. Because of how the visual system is wired, an image in the left visual field is sent only to the right hemisphere, and an image in the right visual field is sent only to the left hemisphere. In an intact brain the corpus callosum would share this information across — but in these patients it could not.
Sperry found that when an object was flashed to the right visual field (left hemisphere), patients could name it easily, because the language-dominant left hemisphere had processed it. But when an object was flashed to the left visual field (right hemisphere), patients could not say what they had seen — yet they could correctly select the matching object by touch using their left hand, which is controlled by the right hemisphere. Sperry concluded that the two hemispheres can operate independently, with different specialised functions, and that language is strongly localised to the left hemisphere.
Sperry showed that split-brain patients could name objects flashed to the right visual field (processed by the left, language-dominant hemisphere) but not objects flashed to the left visual field (processed by the right hemisphere) — even though they could identify the object by touch with their left hand.
A strength of Sperry’s study is that it used a highly controlled, objective procedure — images were flashed for a fixed, brief time to a specific visual field, so the researcher could be confident about which hemisphere had received the information. This makes the findings scientific and replicable in principle.
A key weakness is the sample: split-brain patients are rare, so Sperry’s sample was small, and every participant had a history of severe epilepsy and major brain surgery. This makes it difficult to generalise the findings to people with typical, intact brains — their brains may have already adapted differently because of years of epilepsy before the operation. The study can also be criticised for low ecological validity, since flashing images to a fixed point in a lab is very different from everyday perception.
Sperry’s research remains one of the most influential studies in neuropsychology because it gave direct, physical evidence that the two hemispheres are specialised for different jobs — something earlier researchers could only infer indirectly from brain damage cases.