NEUR 4.1 - Brain Laterality and Split Brains

• Functions that map to contralateral areas in the brain

  • Motor
  • Sensory
  • Visual
• The right hemisphere is generally unable to produce verbal spoken responses
• The left hemisphere can verbally report what it has seen


  The right hemisphere can only show it with the left hand
• Lateralised functions


  Some brain functions rely more on one side of the brain than the other

  
• Functions lateralised to left hemisphere

  • Language
  • Speech
• Functions lateralised to right hemisphere

  • Tone of voice (Prosody)
  • Face perception
  • Perceptual grouping
• Crossed (contralateral) functions


  Movement, Sensation, and Vision
• Left hemisphere controls


  • Right body movement and sensation
  • Right side vision

  
• Right hemisphere controls


  • Left body movement
  • Left sensation
  • Left vision

  
• Broca's Area


  Speech Production
• In 1861, Paul Broca described a patient who was unable to speak after damage to the left frontal lobe (Broca's area)
• Language


  Lateralised to the left hemisphere in most people
• Language functions in left hemisphere


  • Language comprehension
  • Speech
  • Reading
  • Speech production
• Language and hand dominance


  No overall "dominant hemisphere"
• Language lateralisation in right-handed people

  • 95% have language in Left Hemisphere
  • 5% have language in right hemisphere
• Language lateralisation in left-handed people

  • 70% have language in Left Hemisphere
  • 30% have language in right hemisphere
• Contralateral function


  Opposite side
• Ipsilateral function


  Same side
• Primary Motor and Sensory Cortex

  Connect to contralateral (opposite) side of body
• Contralateral connections


  • Right hemisphere to left side of body
  • Left hemisphere to right side of body
• Contralateral function: Vision


  Each side of visual space mapped to contralateral visual cortex (opposite side of body)
• Left side of vision to Right hemisphere, Right side of vision to Left hemisphere
• Input to each half of retina of each eye is split so that left vision from both eyes goes to right hemisphere, and right vision from both eyes to left hemisphere
• Corpus Callosum


  Connects the left and right hemispheres
• Axons of neurons (nerve fibres)


  Crossing to the opposite (contralateral) hemisphere

  
• Neurons


  Send their axons via the corpus callosum to connect with neurons in the opposite hemisphere
• Inter-hemispheric communication


  Allows transfer of information between the two hemispheres
• Vision goes to contralateral hemisphere
• Stimuli on left of screen go to Right Hemisphere, must cross to Left Hemisphere for language to report what object was

  
• Left Hemisphere for language to report what object was, Left Hemisphere for right hand to point to object
• "Split Brain"


  Surgical treatment for very severe epilepsy where corpus callosum is severed to stop seizure activity from spreading to the other hemisphere
• In Split Brain: Images flashed to left or right of screen are "seen" by only right or left contralateral hemisphere
• in split brain: People can reach under screen to touch and feel objects – find them by feel. Patient cannot say what their left hand is doing
• in split brain: Right hemisphere can "read" and understand words, but no speech (so no verbal report)
• Hemispheres can function independently - Left hemisphere can tell what it has seen, Right hemisphere can only show it (via the left hand)