Hemispheric Lateralisation

• Broca's and Wernicke's Area

  
• Hemispheres

  • The left and the right hemispheres of the brain are very similar.  
  • One difference however is the presence of the language areas, which are only found on the left hand side.
  • Broca's area - speech production
  • Wernicke's area - speech comprehension
  • Damage to either of these areas would result in aphasia - impaired ability to produce / understand speech. Broca’s aphasia / Wernicke's aphasia 
• Paul Broca was a neurosurgeon who treated a patient he referred to as ‘Tan’ as that was the only syllable he could express. Tan had the ability to understand spoken language but was unable to speak nor express his thoughts in writing. After Tan died, Broca conducted a post-mortem autopsy on this patient to figure out what part of the brain caused this. The autopsy revealed damage to a specific area of the left frontal lobe, not far from area of the motor cortex that controls the mouth, tongue and vocal cords area important in producing language. This area became known as Broca’s area in 1865.
• Carl Wernicke, a German neurologist, discovered an area of the brain that was involved in interpreting language in 1874. He did this by recording behaviour of stroke patients who had issues with understanding speech. After the patients died he conducted a post-mortem examination to locate the brain damage. The area he discovered damaged amongst these individuals was located in the left temporal lobe very close to the auditory cortex. Wernicke believed that this area was important in interpreting and understanding language. This area became known as Wernicke’s area.
• Flinker et al - speech production isn't strictly localised to Broca's area.

  Schwarz et al - used a task where pps were given the phrase “The dancer applauds the clown” and had to link it to the correct image. Schwarz found that People with Broca’s aphasia struggle with this task more than neurotypical people – Suggests speech comprehension isn’t strictly localised to Wernicke’s area. 
• Lateralised means a function is controlled by one hemisphere, localised refers to the specific location that controls that function. 
• Hemisphere controls the opposite side of the body

  
• • Epilepsy - Disorders associated with uncontrolled seizures.  
  • Split brain operation - The procedure that stops the two hemispheres communicating which is used to treat severe epilepsy.
  • Corpus Callosum - The part of the brain that connects the right and left hemisphere that is surgically cut in a split brain operation.
• Severed corpus callosum causes your two hemispheres to be unable to communicate meaning if you saw something with your left eye, which would then move to your right hemisphere, you would not be able to express what you saw.

  
• Sperry research

  • Sperry wanted to use patients who had their corpus callosum severed to investigate the effects of disconnecting the two hemispheres.
  • Sperry tested people who had hemispheric disconnection compared with a control group that hadn't.
  • He measured their language and motor performance on different visual and tactile tasks.
  • 11 participants with hemispheric disconnection / all men, right handed and had epilepsy.
• Sperry Research

  • Task for language ability - Blindfolded one of the pps eyes. Asked to fixate with the seeing eye on a point in the middle of a screen. Stimulus word projected on the left or right hand side of the fixation point for less than 0.01s (only to one visual field). Then asked what they saw.
  • Task for motor ability - Step 1-3 as above. Pps put one hand behind the screen and asked to select the object they saw.
  • Task for language ability - Pps place hands underneath a screen so they cannot see them. An object is placed in one of the hands of the ppt. Pps asked to name the object. 
• Sperry Findings

  Pps would ONLY be able to name the word if it was presented to their right visual field. Pps would only be able to pick an object up with their left hand if the word was displayed to their left visual field and only their right hand if the word was displayed to their right visual field. Pps would ONLY be able to name an object they were touching if it was placed in their right hand. This shows that the left hemisphere is dominant for language and that each hemisphere controls the movement, sensation and vision of the opposite side of the body. 
• Sperry Evaluation

  • RWA - split brain research can help us develop effective therapy for stroke patients and has lead to the development of our understanding of hemispheric lateralisation.
  • Artifical Data - in real life situations a severed corpus callosum can be compensated for by the use of two eyes / the data was produced in an experimental setting.
• Sperry Evaluation

  • Individual Differences - some pps had experienced drug therapy for much longer than others / the disconnection between the hemispheres was greater in some pps / samples were very small.
  • Invalid Comparison Groups - The control groups often contained people with no history of epileptic seizures so we cannot establish cause and effect as it is possible the performance of the pps may be due to their epilepsy and not their hemispheric disconnection.