Lateralisation and Split Brain Research

Question 1

Hemispheric Lateralisation

Answer 1

• Hemispheric lateralisation is the theory that
  each hemisphere of the brain has specific
  functions.
• There is a large body of evidence supporting
  these claims – eg. The ‘language centres’,
  Broca’s area and Wernicke’s area appear only to
  exist in the left hemisphere.
• The hemispheres of the brain are connected
  by the corpus callosum - which sends
  messages to and from the separate
  hemispheres across the brain.

Answer 2

• Information from our right visual field (RVF) is processed in our left hemisphere.
• Information from our left visual field is processed in the right hemisphere.
• Hemispheres control opposite sides of the body
• The two hemispheres communicate via the Corpus Callosum.

Question 3

Corpus Callosum

Answer 3

• The Corpus Callosum connects the two hemispheres of the brain and allows us to use both sides together.
• Eg. Talking about things (speech = left) that are experienced in the right hemisphere.

Question 4

Split-Brain Research (Sperry)

Answer 4

• Patients who have had their CC severed to treat severe epilepsy.
• Such research provides strong evidence for the case that our brains
  are lateralised (each hemisphere has specific functions) and we are
  able to function effectively (with a ‘whole’ brain) because of the
  Corpus Callosum.
  -If the Corpus Callosum was cut, this would mean that information from each hemisphere couldn’t pass between the two.
• This would disable some functions that are only available in one hemisphere, such as speech.

Question 5

Split-Brain Research (Sperry) - Procedure

Answer 5

Procedure:
- 11 split-brain patients
- Performance on tasks compared with people without split-brain
- This involved asking the participant to fixate on a point in the centre
of a screen. The researchers would then project a stimulus on either
the left or right hand side of the fixation point for less than 1/10 of a
second.
- As language is processed in the left hemisphere, when a stimulus is
presented to the left visual field of a split-brain patient they should
not be able to name the stimulus. WHY?

Question 6

Split-Brain Research (Sperry) - 3 key findings

Answer 6

• Patients can verbalise an image shown to their right visual field (because the left hemisphere contains the speech centres).
• Patients cannot verbalise an image shown to their left visual field
  (because the right hemisphere processes this, but contains no speech centre) BUT they can draw it with their left hand (the right hemisphere has dominant motor function).
• If a patient is shown an image in both visual fields, they will say they’ve seen the one in the Right visual field (processed by left hemisphere), but draw the image they’ve seen in the LVF, with their left hand.

Question 7

Evaluation of Lateralisation - Sperry’s Controlled Methodology

Answer 7

Sperry’s research was carried out in a lab, was very well designed with standardised procedures and high levels of control.
His procedure could be replicated so his findings could be validated.
BUT
He used small samples and all participants had epilepsy so we can’t generalise. The split-brain procedure is rarely carried out these days, therefore patients who have had this procedure are rarely encountered in sufficient numbers to be useful for research.

Question 8

Evaluation of Lateralisation - Lateralisation Changes with Age

Answer 8

• Lateralisation of function appears not to stay exactly the same
  throughout an individual’s lifetime, but changes with normal ageing.
• Across many types of tasks and many brain areas, lateralised patterns found in younger individuals tend to switch to bilateral patterns in healthy older adults.
• Szaflarski et al (2006) found that language became more lateralised to the left hemisphere with increasing age in children and adolescents, but after the age of 25, lateralisation decreased with each decade of life.

Question 9

Evaluation of Lateralisation - Issue/debate

Answer 9

• Nomothetic nature of biopsychology theories -
• Assumption that all human brains function in the same way…
• Would benefit from an interactionist approach with more idiographic evidence from individuals…