Session Four (Brain Lateralisation) Flashcards
What is meant by the term Brain Lateralisation?
- The idea that certain functions are located in certain areas (specifically, the sides) of the brain.
- Commonly, people think of the Left hemisphere as the ‘verbal’ hemisphere and the Right hemisphere as the ‘visa-spatial’ hemisphere.
- In reality its likely no such clear distinction exists, only a bias of one particular side to particular tasks
- No such thing as ‘being more left or right brained’, if cerebral dominance exists it will be for particular tasks NOT as part of someone’s personality.
The idea of brain lateralisation emerges from observations made in the 19th century, what were they?
- Dax noted that most stroke patients who developed language issues had L hemisphere damage on autopsy
- Broca had 2 patients could clearly understand verbal communication but could not articulate it, on post-mortem identified Broca’s area as the site of the damage causing this non-fluent aphasia.
- Wernicke had 10 patients who could speak fluently but who’s speed made no sense (word salad). Deficit was in comprehension/meaning rather than speech output. Post-mortem identified Wernicke’s area as responsible for the cause of fluent aphasia.
- Since both B and W areas are in the L hemisphere, people assumed L side = language centre.
What neuroanatomical asymmetries exist between the brain hemispheres?
- Generally Left side has more grey matter but Right is heavier
- The right frontal lobe extends further forward and is wider than the left.
- Left occipital lobe is wider and goes further back
- Notable differences in the Sylvian fissures on either side.
- Right side has two Heschi’s gyri, whereas Left only has a single gyrus (these are primary auditory complexes.
- This might be to fit the Left side’s significantly larger Planum Temporale (another important area for language)
- (N.B. this difference is especially large in musicians, were the Planum Temporale is huge)
What explanations are given for some of the asymmetry seen between the brain hemispheres.
- Differences in synaptogenesis, grey matter or white matter development may explain some of the differences observed between the hemispheres.
- These processes can lead to differences between the hemispheres but also between different people, e.g. musicians show greater development of certain auditory areas.
What patients and methods are used to investigate functional asymmetry and brain lateralisation?
Participants:
- Patients with lateralised lesions
- Patients with hemispherectomies
- Patients with commissurotomies (split brain syndrome)
Methods:
- Neuropsychological assessment
- Visual-field presentation techniques
- fMRI
What are some examples of lateralised brain lesions?
- Strokes
- Epilepsy
- Tumours
- Surgeries (where one lobe has been removed on one side)
If brain lateralisation theory is true, what would we expect to see in lateralised lesion patients?
What do we actually see?
We would expect to see DOUBLE DISSOCIATION:
- Lesion to a certain brain region causes disruption of cognitive function A but not B
- Lesion to brain region n the other side would cause loss of cognitive function B but not A.
We actually see RELATIVE DISSOCIATION:
- Damage to one hemisphere leads to relatively greater impairment in a cognitive function than damage to the other hemisphere.
- e.g. Patients with left hemisphere lesions are more impaired across the range of language functions than patients with right hemisphere lesions, but they aren’t totally impaired.
What are the language functions of the brain. associated with the left hemisphere?
- Speech
- Writing
- Reading
- Comprehension
What is the advantage of using lesion studies over, for example, fMRI when determining function of a brain area?
fMRI shows us what parts of the brain activate when we perform a certain task, telling us what brain regions are INVOLVED.
Lesion studies tell us whether or not a function is possible once a region has been knocked out, telling us what brain regions are NECESSARY for a function.
Briefly distinguish the presentations of patients with Left and Right brain lesions?
Left:
- Decreased linguistic functions
- Production of language, following of commands, reading, writing all likely affected.
Right:
- Decreased Visuospatial perception
- e.g. Figure ground discrimination, Face recognition, Position discrimination, Line orientation, VS construction
- Can be tested using complex drawing task (house)
- Spatial memory tasks (e.g. maze learning) and Musical cognition (discriminating between tones and melodies) often also affected.
- VS neglect syndrome is a common presentation
What lesion is most commonly the cause of Visuospatial neglect syndrome?
Right inferior parietal lesions. Most commonly caused by MCA strokes.
Can be a temporary or permanent sequelae.
What side is most commonly neglected in VS neglect and why?
Left side.
Left sided symptoms indicate right sided brain pathology. Since the right side is the more competent VS side it makes sense that VS functions are markedly reduced when damaged.
Rarely see right sided VS neglect as would imply left sided brain damage which:
a) is more likely to cause language issues and
b) theoretically the right brain should be able to compensate for the left sides VS difficulties.
How does VSN syndrome contrast to say losing sight in one eye?
- Sort of similar in the sense that they are only aware of half of their eyesight
- However obviously not a visual problem.
- More to do with attention to the space on their left and their ability to represent the space around them.
- Main distinction is that people with VSN aren’t aware of the deficit as much, when people with hemianopia will tile their heads to look around them VSN patients will not.
- Technically their brain is seeing that side, they just aren’t consciously aware of it.
- Very difficult for this patient to learn of the deficit and adjust to it, they tend to not be able to comprehend anything is wrong.
- Completely unaware of that side of space and tend to think everything is fine.
How can we test for VS neglect syndrome?
These patients neglect people, objects, sounds, tactile sensation on symptomatic side, therefore they are tested through:
- Drawing tests, will miss left side of picture.
- Visual search tests, will omit targets on left.
- Line drawing test, when told to bisect a line through the middle they will usually deviate heavily to the right.
What did Pulsifer’s 2004 study into Rasmussen syndrome patients tell us about brain lateralisation?
- RS = unilateral paediatric encephalitis, that causes permanent brain damage and potentially life threatening, intractable seizures.
- Often treated through hemispherectomy, total removal of one hemisphere of the brain
- Pulsifer wanted to follow patients who’d had the procedure to look for any issues with expressing language (L function) or Visuomotor ability (R function) as well as general IQ
- Theoretically if true brain lateralisation is exist the procedure should totally knock out one of these functions
- Found that there were very minor changes in expressive language post-surgery, but NONE in VM function or IQ
- Importantly, found no notable distinction between those who’d had surgery on the R or L.
- Generally, language and sensorimotor functions were preserved whichever hemisphere was removed.
- (N.B. patients did have issues of coordination and cognition, but these are more general brain functions)
- This suggests that while some bias may be true, the brain isn’t anywhere near as lateralised as previously thought.
- However, these were children, so their neuro-plasticity may have helped them recover from the surgery. Results may not be the same in an adult.
- Also confusing the results is the fact that these kids have just had a severe brain disease,e so some of the small deficiencies seen in their scores might be due to the effect of that. Unclear how results relate to fully healthy patients.
