Week 4 - Hemispheric Lateralisation Flashcards
Laterality
Dominance or preferential use or superior function of one part of the body
E.g. handedness
Limited to specific function(s)
Focus: language, spatial ability
Other terms, same phenomenon
- Asymmetry of function / structure
- Hemispheric asymmetry
- lateralisation
- hemispheric specialisation
Why does this happen?
- lack of redundancy / unnecessary duplication
- Greater flexibility
- Greater complexity of function
Laterality in animals
Plenty of evidence - birds, apes and other mammals
Lateral dominance of the hypo gloss all nerve conveying messages from the brain to the syrinx
E.g. the numbers of song elements in canaries repertoires reduced when the left branch was cut (left syringeal dominance of song production).
Not as consistently found as in humans
Limited to specific contexts
Issue of assessing in an ecologically valid manner
Laterality in humans
Physical asymmetry (Evolutionary arguments: attractiveness, health)
Neuroanatomical asymmetry (Critical issues: descriptive variability, methods, morphology)
Functional asymmetry (Qualitative & quantitative differences, accurate in general terms).
Specific domains (Language, visuospatial ability, handedness...)
Laterality: Language
Left hemisphere dominance for speech
Processes include: speech production, written word recognition & comprehension, spoken word recognition & comprehension
BUT, right hemisphere dominance for meaning & interpretation (figurative, contextual, connotative)
Processes include: metaphors, irony, prosody, cohesion & repair, discourse, reasoning and logic, linguistic context
Laterality: Visuospatial Ability
Right hemisphere dominance for visuospatial functions
Processes include: mental rotation, spatial relations tests, face recognition, figure-ground discrimination, etc.
Laterality: Attention
Local (left hemisphere) versus global (right hemisphere ) processing
AAAA Left hemisphere damage sees just the big picture (B).
A A Right hemisphere damage just sees the A’s
AAA
A A
AAAA
Laterality: mathematical ability
Left hemisphere lesions
- Alexia (difficulties in reading) for numbers
- Agraphia (difficulties in writing) for numbers
- Anarithmetria (difficulties in retrieving arithmetical information from long-term memory)
Right hemisphere lesions
- spatial Acalculia (difficulties representing numerical information: misreading, omitting signs, decimals)
Laterality: sex differences
Men better at spatial tasks (mental rotation), throwing accuracy and mathematical reasoning
Women better at recall of object location and verbal memory
Attributed to different evolutionary demands on the sexes (hunter-gatherer large scale “cardinal” versus small scale “landmark” navigation mechanisms)
Girls advantage:
Verbal fluency
Rapid math calculations
Memory for spatial positions of objects
Boys advantage: Verbal analogies Rapid math reasoning Memory for layout geometry Mental rotation
Laterality: handedness
Particularly relevant with regard to language
Majority of right handers (90-96%) have LH speech
Most left handers (60-70%) also have left handed speech. The rest have RH speech (4%) or bilateral language representation
Contemporary ideas on what is lateralised
“…two distinct forms of functional lateralisation are present in the left vs. the right cerebral hemisphere
“…the left hemisphere showing a preference to interact more exclusively with itself, particularly for cortical regions involved in language and fine motor coordination.
“…in contrast, right-hemisphere cortical regions involved in visuospatial and attentional processing interact in a more integrative fashion with both hemispheres”.
Interhemispheric communication
Corpus callousum
Callosal agenisis
Commissurotomy - split brain syndrome
Corpus callosum
The major cortical connection between the two hemispheres
250 million nerve fibres - like an information highway between the two hemispheres
There are other connections between the hemispheres (subcortical- hypothalamus, cerebellum, pons) but these are much smaller
All detailed higher-order information passes through the corpus callosum when being transferred from one hemisphere to the other
The transfer time, as measured with ERP, is 5 to 20 ms
Callosal agenesis
Rare congenital disorder - presentation is very varied
Visual impairments, poor muscle tone, poor motor coordination, some cognitive (complex problem solving) and social (face processing) difficulties
Do not show same characteristics of split-brain patients
E.g. Kim peek (photographic memory)
Commisurotomy
Treatment of intractable epilepsy
Corpus callosum is severed / sectioned
Split brain patients
Split brain syndrome
Language- difficulty transferring information about words across hemispheres (shown words to certain side of visual field)
Can see things and think they don’t know what they are but they do
Drawing- split brain patients can draw two separate things at the same time with each hand
E.g. of selective atypical function - normal - “head stone” they’d draw a R.I.P stone. Split brain- cant link the two
