Hemispheric Lateralisation And Split Brain

• •This is the idea that the two halves (hemispheres) of the brain are functionally different and that certain mental processes and behaviours are mainly controlled by (specialised to) one hemisphere rather than the other. i.e. they have functional specialisations.•
• -Lateralisation is about one half (hemisphere) of the brain-
• localisation is about one particular part of one hemisphere.
• Some functions like language can be localised (e.g. to Broca’s and Wernicke’s areas) and lateralised (to the left hemisphere
• Hemispheric Lateralisation

  •This is the idea that the two halves (hemispheres) of the brain are functionally different and that certain mental processes and behaviours are mainly controlled by (specialised to) one hemisphere rather than the other. i.e. they have functional specialisations.
• Corpus Collosum

  •The corpus callosum is a thick bundle of nerves which connects the two hemispheres of the brain.•It allows the two sides of the brain to communicate with each other so that the whole brain can work as one complete organ i.e. information received by one hemisphere can be sent to the other hemisphere.•In this way, the two hemispheres retain their own roles while working together to control behaviour in the whole body.
• Sperry and Gazzaniga's research procedure

  1. Studied 11 individuals who had undergone surgical procedure to cut corpus callosum and other tissues connecting the two hemispheres to control epileptic seizures

  2. Presented stimuli (e.g. words, pictures) to one visual field using a tachistoscope

  3. Participants fixated on a dot or cross in the centre of a screen whilst stimuli were presented to one half of the visual field so briefly (about 1/10 of a second) that the participant didn't have time to move their eyes so that both eyes saw it, meaning it only went to one hemisphere and couldn't go to the other hemisphere since the corpus callosum was cut

  4. Compared to a control group who had no history of epilepsy
• •Anything in the left visual field goes to the right hemisphere.•Anything in the right visual field goes to the left hemisphere.
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• Describing what you see

  •A picture of an object was shown to either the left or right visual field and the participant was asked to name the object.•If the object was shown to the right visual field, it could be named easily, but if it was shown to the left visual field, the patient couldn’t describe it and typically reported nothing being there.•This suggests that language is processed in the left hemisphere.•
• Recognition by touch

  •Objects were shown to the participants’ left visual field and they were asked to select a matching object from a grab-bag of different objects using their left hand. The objects were placed behind a screen so as to not be seen. Participants were sometimes asked to select a related object rather than a matching one.•The left hand was able to select a matching object and a related object (e.g. ashtray when shown a picture of a cigarette), but they could not describe the object. They could ‘understand’ the object and respond accordingly.
• Composite words

  •Two words were presented one on either side of the visual field (e.g. ‘key’ on the left and ‘ring’ on the right). Participants were asked to say what they saw and draw it if they couldn’t say it.•
• •The patient could draw the a key with their left hand, and say the word ring.•This suggests that language is processed in the left hemisphere and the right hemisphere is dominant in terms of drawing tasks.
• Matching faces

  •Participants were shown an image of a face (different faces to the left and right visual field) and then were asked to match it to one from a series of other faces.•For the first task, the picture processed by the right hemisphere (left visual field) was consistently selected, whilst the picture presented to the left hemisphere was consistently ignored.
• •This suggests that facial recognition is lateralised to the right hemisphere.
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  •Sperry and Gazzaniga’s work into the split-brain has produced an impressive body of research findings, the main conclusion of which appears to be that the left hemisphere is more geared towards analytic and verbal tasks, whilst the right is more adept at performing spatial tasks and music.•This a key contribution to our understanding of brain processes.
• Split-brain research

  • Used highly specialised and standardised procedures
  • Presented visual information to one hemispheric field at a time
  • Fixation point/ flashing 1/10 of a second, meaning the patient would not have time to move their eye across the image and so spread the information across both sides of the visual field and consequently both hemispheres of the brain
• The procedure developed was very useful and well-controlled
• The research supports the internal validity of the theory of hemispheric lateralisation
• Sample size

  • Very small, only 11 participants who took part in all variations of the basic procedure, all with a history of epileptic seizures
• Small sample size


  May have caused unique changes in the brain that influenced the findings
• Participants

  • Some had experienced more disconnection of the two hemispheres as part of their surgical procedure than others
  • Some had experienced drug therapy for much longer than others
• Control group

  • May have been inappropriate because they were people who had no history of epileptic seizures
• Eval

  •Pop-psychological literature overemphasises and oversimplifies the functional distinction between the hemispheres on the basis of split-brain research.•Modern neuroscientists would argue the actual distinction is less clear-cut and much more messy than the basic ‘verbal’ –‘non-verbal’ distinction.••Therefore, hemispheric lateralisation may be more complex than originally stated and the brain may work more holistically.