Lecture 4: Part 1:

Cerebral asymmetry:

Two hemispheres:

• Anatomical and functional asymmetry

• Connections and ineraction

• Why two hemispheres?

• Long history of investigation, speculation.

Anatomy of the hemipheres:

• Separated by the longitudinal/sagittal fissure.

• Connected by commissures including corpus callosum, anterior commissure.

Hemispheric connections:

• Cortical and subcortical commissures:

  • Corpus callosum is less than 200 million axons (95% myelinated).

  • Anterior commissure is much smaller and connects the anterior temporal lobes.

• Associative cortex connections predominate.

• Homotopic, heterotopic connections.

• Homotopy strictest between primary cortex (midline fusion).

Anatomical asymmetry:

• Anterior right hemisphere and posterior left hemisphere overlap midline.

• Sylvian fissure: Ascends more anteriorly in the right hemisphere, longer in the left hemisphere.

• Gross asymmetry is related to the underlying regional size and myelinization.

Asymmetric regions:

• Planum Temporale (The temporal plane):

  • Top surface of the temporal lobe.

  • Encompasses Wernicke’s area and areas for auditory processing supporting language.

  • Around 30% larger in the left hemisphere.

Testing each hemisphere:

• Historically, unilateral brain damage has revealed much about cerebral asymmetry.

• Newer techniques are also revealing.

• Visual input is exclusively contralateral

• Test each hemisphere using lateralized visual presentation.

Dichotic listening:

1. Syllable presented to right ear alone, participant repeats syllable.

2. Syllable presented to left ear alone; participant repeats syllable.

3. Different syllable presented to both ears simultaneously; participant repeats only right ear syllable.

Testing each hemisphere:

• fMRI can reveal lateralisation of the main brain regions involved in cognitive processes.

• Activity in the two hemispheres can be compared and the difference plotted in a lateralisation map.

The split brain:

• Commissurotomy: Section of the interhemispheric commissures.

• Callosotomy: Section of corpus callosum.

The disconnection syndrome:

• Prevents spread of seizure activity from one side of the brain to the other.

• But creates “disconnection syndrome”

• Each cerebral hemisphere disconnected from the other at the cortical level.

• Neither side has access to thought, percept's, memories of the other.

• Yet split brained people remain curiously normal in everyday behaviour.

Testing the split brain:

• Each side of visual space projects to opposite side of the brain.

• Technique is to flash information very quickly to one or other side before eye movements can occur.

• This allows properties of each side of the brain to be assessed.

What can the split brain tell us:

• It allows us to test each hemisphere in relative independence.

• Assess hemispheric integration via commissures:

  • Subcortical commissures and limited transfer.

  • Partial callosotomy and specificity of transfer.

Functional asymmetry:

• Language abilities:

• Unilateral brain damage: Aphasia

  • Suggests language centres are predominantly left-hemispheric.

  • 97% of right handers, 70% of left handers.

• Split brain disconnection syndrome:

  • Some limited right hemisphere language (lexical not grammatical)

• Language: The normal brain:

  • Right visual field or right ear/left hemisphere faster and more accurate.

  • Input from right is most direct.

Functional asymmetry:

• Visuospatial abilities:

• Block construction: Split brain patients best with left hand.

• Simple processes bilateral: Sophisticated processes draw on the right hemisphere.

• Right hemisphere superiority for construction, detecting offset, orientation, mirror reversal and perceiving degraded stimuli.

Unilateral brain damage:

• The preferred input type is revealed by experiments using hierarchical stimuli. (A large stimulus is constructed from many small stimuli).

• Left brain damage: Disrupts local representation.

• Right brain damage: Disrupts global representation.

Functional asymmetry:

• Higher cognition:

• The left hemisphere confabulates and looks for patterns.

• Hemispheric prediction task: Will the target appear at the top or at the bottom?

• 80% of the time the stimulus appears at the top and 20% of the time appears at the bottom.

• The right hemisphere controlling the left hand would just choose the top in hope that you get 80%right as they are randomised.

• But the left hemisphere is more likely to do the opposite and try to find a pattern.

Do the hemispheres cooperate or compete:

• Co-operation:

• Increased task difficulty leads to a bilateral advantage: When we have to do hard tasks performance is better when stimuli are displayed bilaterally than when they are displayed unilaterally.

• This is an example of hemispheric asymmetry and interaction changing as the task changes.

Do the hemispheres cooperate or compete:

• Competition:

• Motor inhibition via the corpus callosum: Each motor cortex inhibits the opposite motor cortex to allow for unilateral action.

• The left hemisphere inhibits the right hemisphere during language development over the early years of life.

Why have a divided brain?

• The most efficient use of cortical space.

• Newly evolved functions could be supported by the hemisphere without the loss of other functions.

• Allows for fast (intrahemispheric) processing, necessary for functions like language.

Take home points:

• Hemispheric differences are relative and fluid rather than dichotomous, even for domains most commonly characterised as lateralised.

• They change with stimulus, task requirements and task difficulty.

• They are fundamental to our evolved nervous system.