Week 8 - Lateralisation, Language and the Split Brain — Quick Notes

Cerebral lateralisation = differences in function between the left and right hemispheres; Lateralisation of function = bias in processing across hemispheres.
Left hemisphere (LH): typically dominant for language; contralateral control of movement (controls the right side); acts as an "interpreter" of actions and moods.
Right hemisphere (RH): typically dominant for spatial abilities, emotion processing, non-language sounds/music, non-verbal memory tasks.
Handedness link: language dominance is in the LH for most people; often related to handedness differences.
Key caveats: many functions show only small hemispheric biases; both hemispheres contribute with different processing styles.
Common myths:
- Left-brained vs right-brained is a simplification; not supported as a strict dichotomy.
- Individual differences exist (e.g., sex differences, atypical lateralisation in certain disorders).
Basic measures of lateralisation include neuroanatomy, behavioural tests, and imaging techniques.

Contralateral (crossed) brain organisation: LH controls movement and sensation on the right side; RH controls the left.
Corpus callosum and commissures enable interhemispheric communication.
Key anatomical asymmetries:
- Planum temporale often larger in the LH (language region).
- Frontal operculum and other language-related regions show asymmetries.
- Primary auditory cortex (Heschl's gyrus) shows asymmetries related to language processing.
Major connections: corpus callosum (genu, body, splenium); cortical connections can be homotopic, heterotopic, or ipsilateral.

LH language dominance is ~ $ilde{92}\%$ in right-handers and ~ $ilde{69}\%$ in left-handers.
Sex differences: males may show greater lateralisation on average; differences reported in language vs spatial tasks.
Atypical lateralisation observed in certain conditions (e.g., schizophrenia, autism, dyslexia).
Overall message: lateralisation is a tendency, not a universal rule; individual variation is common.

Methods include:
- Unilateral brain damage (lesion studies) to infer lateralised functions.
- Split-brain procedure (commissurotomy) to isolate hemispheres.
- Electrical stimulation mapping (e.g., Penfield) during surgery.
- Wada (sodium amytal) test to temporarily inactivate one hemisphere.
- Functional brain imaging (fMRI, PET, ERP, MEG).
General findings:
- LH damage often causes aphasia (language comprehension/production) and apraxia (movement planning).
- RH damage often results in non-verbal/spatial deficits and visuospatial processing changes.
- Cerebral dominance for language is typically LH-dominant.

Commissurotomy/severing the corpus callosum used to limit seizure spread.
Myers and Sperry (1953) classic studies in split-brain patients revealed:
- Stimulus in the right visual field (RVF) is processed by the LH and can be verbally reported; left hand may pick the correct object.
- Stimulus in the left visual field (LVF) is processed by the RH; patients may not verbally identify it but can pick the correct object with the left hand.
Language and speech patterns:
- Language production is typically LH-dominant; many split-brain patients speak from LH while RH recognizes emotional prosody.
- RH is skilled at recognizing emotional prosody and certain nonverbal cues.
Visuospatial processing:
- RH specialized for mental rotation, spatial matching, and some visuospatial tasks.
The LH interpreter:
- LH tends to interpret actions and moods of the RH, often generating coherent explanations for unilateral actions.
Implications:
- Each hemisphere can learn independently; tasks can be performed in parallel by each hemisphere, but verbal report may reveal only one side’s output.
- Cross-cueing and facial feedback can help communicate across hemispheres.

Theories of the evolution of cerebral asymmetry:
- Analytic–synthetic theory (Harris, 1978): LH analytic, RH synthetic; argued to have evolved with two cognitive modes; often considered vague/untestable.
- Motor theory: LH controls fine movements; speech is a type of fine movement; damage to LH → language and motor deficits.
- Linguistic theory: LH specialization for language; evidence from cases where language and gestures show distinct patterns.
When did lateralisation evolve?
- Lateralisation likely present early in vertebrates (~ $4\times 10^{8}$ to $5\times 10^{8}$ ) years ago.
- Right-handedness may have evolved from right-side body usage for feeding.
- Left-hemisphere dominance for communication observed in species predating humans (birds, dogs, monkeys).
Survival advantages:
- Increased neural efficiency by concentrating function in one hemisphere.
- Facilitates parallel processing if different processes are lateralised to the same hemisphere.

The Wernicke–Geschwind model proposed that language is localised in a network involving:
- Broca's area (inferior frontal gyrus): language production; damage → expressive aphasia.
- Wernicke's area (posterior superior temporal gyrus): language comprehension; damage → receptive aphasia.
- Arcuate fasciculus: pathway connecting Broca's and Wernicke's areas; damage → conduction aphasia (impaired repetition).
- Angular gyrus: involved in reading/writing; damage → alexia and agraphia.
Critiques and evidence:
- Early lesion studies showed that damage to Broca's or Wernicke's areas often did not produce the expected pure deficits.
- More widespread or subcortical damage frequently underlies aphasia; isolated damage to classical WG areas is rare.
- Structural imaging shows aphasias with damage beyond WG areas, and some intact language abilities with WG-area damage.
- Electrical stimulation during surgery disrupted language at many sites outside WG areas; individual organisation of language varies.
Current status:
- Two elements are supported: Broca's and Wernicke's areas play important roles in language.
- Anterior (production) vs posterior (comprehension) damage relationships exist, but precise one-to-one predictions are not robust.
- Pure, isolated lesions producing classic WG aphasias are rare; language is distributed across networks.

Premise: specific cognitive processes involved in language have neural substrates that may be distributed and partly independent of language use.
Core language processes (orthographic, phonological, grammatical, semantic):
- Orthographic: visual word form processing.
- Phonological: auditory/phonemic processing for speech.
- Grammatical: syntactic structure processing.
- Semantic: meaning extraction.
Functional imaging findings:
- Reading tasks show activation in multiple regions with patchy, individual variability; involvement of classic language areas is not uniform across individuals.
- Damasio (1996) PET study on naming showed naming engages left temporal regions beyond classic Wernicke’s area and that activation varies with category.
Takeaway:
- Language is supported by a distributed network; cognitive neuroscience emphasizes functional specialization within broader networks rather than strict, fixed modular localization.