3. Localisation theory

• Hemispheres of the brain

  • Cerebrum is divided into the left and right hemisphere
  • Activity on the left side of the body controlled by the right hemisphere and activity on the right hand side of the body is controlled by the left hemisphere
  • This is known as lateralisation
• Cerebral cortex

  • Outer layer of both hemispheres and subdivided into four centres
  • Motor area - back of the frontal lobe and voluntarily controls movement in the body, damage includes loss of control over fine movements
  • Somatosensory area - front of the parietal lobes, sensory info processed here and receptors occupy over half of the area. Damage includes numbness and tingling
  • Visual area - occipital lobe, receives and processes visual info, damage includes blindness
  • Auditory area - temporal lobes, analyses speech based info, damage may produces partial hearing loss
• Language centres of the brain

  • Language is restricted to the left side of the brain
  • Broca's area - area in the left frontal lobe and damage causes Broca's aphasia characterised by slow speech and lacks fluency
  • Wernicke's area - left temporal lobe being responsible for language understanding and results in Wernicke's aphasia when damaged. People produced neologisms
• Strength - evidence from neurosurgery

  • Provides research to support the localisation of function in the brain
  • Dougherty et al. studied patients with very severe mental illnesses who sometimes need to have surgery as a last resort, such as a cingulotomy for severe OCD in which a region called the cingulate gyrus in the brain is isolated during surgery. It was found in a sample of 44 pps that at post surgery follow up at 32 weeks, 30% had a successful response to the surgery and 14% had a partial response
  • Links to prefrontal cortex and supports localisation theory
• Limitation - animal research

  • Lashley removed between 10 - 50% of the cortex in rats and measured their ability to learn routes through a maze. They were not able to find that any area of the brain was more important than any other in learning these mazes, but that it seemed to need every part of the cortex to learn it effectively
  • Cannot be generalised to humans due to different brain structures and biology - removal should have more detrimental effect
  • This implies that brain function may be more holistic for more complex cognitive abilities and not as localised as other research suggests
• Limitation - language localisation questioned

  •  A review by Dick and Tremblay found that only 2% of researchers actually think that language is entirely controlled  to the very specific regions of Broca's and Wernicke's areas. More recent scanning techniques such as fMRI seem to suggest that lots of areas of the brain are involved in the processing of language. Eg. language streams in brain activity were found across other parts of the cortex, including regions in the right hemisphere.
  • Suggests that language function at least is distributed much more holistically
• Strength - Phineas Gage study

  • A metre length pole was hurled through his left cheek, passing behind his left eye, exiting the skull from the top of his head, removing his left frontal lobe
  • His personality completely changed and went from being calm and reserved before the accident to angry and rude
  • Supports localisation theory because of this