Outline and evaluate research into lateralisation and/or the split brain.
The term lateralisation refers to the idea that the two halves of the human brain are not exactly the same, and that some functions might only be found in one hemisphere. In human beings, the left hemisphere is usually responsible for language and the right hemisphere is usually responsible for visual motor tasks. Lateralisation was first discovered in the 1800’s by physicians such as Broca and Wernicke who performed autopsies on patients who had several language difficulties before their deaths.
The part of the brain named Broca’s area was named after Paul Broca, a French neurosurgeon who treated a patient he referred to as ‘Tan’ because that was the only syllable he could express. Although Tan could understand language, he was unable to express his own thoughts. After studying 8 other patients who also had similar language deficits to Tan, Broca found that there was a ‘language centre’ in the posterior portion of the frontal lobe of the left hemisphere. Broca believed that this area was critical for speech production as those who had damage in this area of their brain were unable to produce speech.
Shortly after Broca had discovered a speech production area in the brain, Carl Wernicke, a German neurologist, discovered another important part of the brain. He named this part ‘Wernicke’s area’ and it was responsible for speech comprehension. It was located just behind Broca’s area in the posterior portion of the frontal lobe. Patients who had damage to their brain in Wernicke’s area, were able to produce speech but unable to understand language. Wernicke proposed that language involves separate motor and sensory regions located in different cortical regions. The motor region, located in Broca’s area, is close to the region that controls the mouth, tongue and vocal cords. The sensory region which is located in Wernicke’s area is close to regions of the brain responsible for auditory and visual input.
Although Broca and Wernicke showed that the parts of the brain responsible for speech production and speech comprehension were on the left hemisphere, many psychologists questioned how we can talk about things located in the right hemisphere. They found that the answer to this was the fact that the two hemispheres were connected by the corpus callosum; a bundle of nerve fibres that joined the two halves. The corpus callosum allows information received from one hemisphere to be sent to the other hemisphere. Psychologists wanted to study the abilities of the two hemispheres, and found the chance to do so when they were treating patients with severe epilepsy. In order to treat patients with severe epilepsy, surgeons cut through the bundle of nerve fibres that formed the corpus callosum. The aim of this procedure was to prevent the violent activity that accompanied epileptic seizures crossing from one hemisphere to the other. The patients who had this procedure done to them were referred to as ‘split-brain patients’.
One of the first studies on split brain research was carried out by Roger Sperry and Michael Gazzaniga (1967). They studied the capabilities of the separated hemispheres by presenting information to just one hemisphere at a time. As part of their study, they took advantage of the fact that information received on one side of the visual fields went to the opposite hemisphere. This is because it meant that if the corpus callosum was cut in split-brain patients, the information presented to one hemisphere had no way of travelling to the other hemisphere, and could only be processed in the hemisphere that received it. This would therefore show the abilities of each hemisphere. In a typical study, the split brain patient would concentrate on a dot in the centre of the screen while information was presented to either the left or right visual field. They would then be asked to make responses with either their left hand (controlled by the right hemisphere), or verbally (controlled by the right hemisphere). Sperry and Gazzaniga found that the left hemisphere of the brain was dominant in terms of speech and language, and the right hemisphere was dominant in terms of visual motor tasks. This provided evidence for the idea of lateralisation.
There are also other studies that support the idea of lateralisation. For example; Turk et al (2002) investigated facial recognition with a patient called JW, who had also undergone a commissurotomy (severing of the corpus callosum), due to his epilepsy. He was presented with faces that had been morphed together from either his own face or psychologist Michael Gazzaniga. The results showed that JW’s left hemisphere was important for self-recognition, and the right hemisphere was important for recognising other familiar people.