PSYC513: Biological and Cognitive Psychology - Lecture 4: Cerebral Asymmetry Notes

Contact Information

• Email: matt.roser@plymouth.ac.uk
• Office: PSQ B207
• Office appointment: Tuesday 10-11am, Thursday 10-11am
• Dr Matt Roser

Lecture 4: Part 1. Cerebral Asymmetry

• Readings:
  • Gage & Baars (2018). Chp 2, section 4.
  • Hugdahl, (2000). Lateralization of cognitive processes in the brain, Acta Psychologica, 105, 211-235.
  • Chapter 4 of Banich and Compton (2012), Cognitive Neuroscience (3rd Ed.).
  • Chapter 11 of Gazzaniga, Ivry, Mangun (2009), Cognitive Neuroscience (3rd Ed.)

Two Hemispheres

• Anatomical and Functional asymmetry
• Connections and Interaction
• Why two hemispheres? (Briefly, more next year)
• Long history of investigation, speculation
• Tan’s Brain (Broca)

Anatomy of the Hemispheres

• Separated by the Longitudinal / Sagittal fissure
• Connected by Commissures
  • Corpus Callosum
  • Anterior Commissure

Hemispheric Connections

• Cortical and Subcortical commissures:
  • Corpus Callosum: > 200 million axons (95% myelinated)
  • Anterior commissure

Hemispheric Connections

• Arrangement in rough topography (anterior origin – anterior crossing)
• 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
• Underlying regional size and myelinisation

Planum Temporale

• Wernicke's area
• Anatomical Asymmetry
• Auditory processing supporting language
• Left auditory cortex
• Planum temporale
• Right auditory cortex

Testing Each Hemisphere

• Historically, unilateral brain damage has revealed much about cerebral asymmetry.
• Newer techniques also are revealing.
• Visual input is predominantly Contralateral
• Test each hemisphere using lateralized visual presentation

Testing Each Hemisphere

• Auditory input is strongest to the Contralateral hemisphere
• Test hemispheric differences with dichotic listening

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

Testing Each Hemisphere

• 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 thoughts, percepts, memories of the other
• Yet split-brained people remain curiously normal in everyday behaviour
• Disconnection syndrome best shown with controlled experiment (resulting in Nobel Prize for R.W. Sperry in 1982)

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

The Split Brain – what can it tell us?

• Control Test each hemisphere in relative independence
• Assess hemispheric integration via commissures:
  • subcortical commissures & limited transfer
  • partial callosotomy & specificity of transfer

Specificity of transfer - partial/staged callosotomy

• Anterior: Semantic
• Central: Motor
• Posterior: Sensory

Functional Asymmetry - Language

• Split-brain disconnection syndrome RVF naming
• Some limited right hemisphere language (lexical not grammatical)
• Unilateral brain damage – Aphasia
• Suggests language centers are predominantly left-hemispheric (97% of right handers; 70% of left handers)

Functional Asymmetry - Language

• Language: The normal brain
• Relative performance differences with side of presentation
• Word – Non-word decision: Right Visual Field or Right Ear/Left hemisphere faster and more accurate
• Semantic Priming – stronger in RVF
• Input from right is most direct

Functional Asymmetry - Visuospatial

• 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
• Right hemisphere extracts higher-level Percepts from the visual array
• Illusory Contours
• Amodal Completion

Functional Asymmetry - Visuospatial

• Unilateral brain damage
• The preferred input type is revealed by experiments using Hierarchical Stimuli
• Left brain damage - disrupts Local representation
• Right brain damage - disrupts Global representation

Functional Asymmetry - Higher Cognition

• Left hemisphere Confabulates and looks for patterns
• The Left-hemisphere Interpreter
• Hemispheric Prediction

Co-operation or Competition?

• 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 (from easy to hard)
• Competition: Motor inhibition via the corpus callosum – each motor cortex inhibits opposite number
• Inhibition of (equipotent) right-hemisphere during language development

TAKE-HOME POINTS

• Most efficient use of cortical space
• Allows for fast (intrahemispheric) processing, necessary for functions like language
• Hemispheric differences are relative & fluid rather than dichotomous, even for domains most commonly characterized as lateralized
• They change with stimulus, task requirements, and task difficulty