Week 4 - Hemispheric specialisation

Structural assymmetries

Differences in brain size and structure across hemispheres

• Enlargement of planum temporale in left hemisphere

• Enlargement of anterior portion of right hemisphere

• Parts of thalamus are larger on left side

• Left hemisphere neurons tend to have longer dendrites than right hemisphere neurons

Functional assymetries

Differences in task specialisation across hemispheres

• Language, speech, and major problem solving = left

• Spatial ability, emotion, musical ability = right

Geshwid & Levitsky (1968)

• Post-mortem study of 100 right-handers

• Anatomical differences in planum temporale (centre of Wernicke’s area)

• Larger in left hemisphere in 65% of ppts

• First study to show lateralisation of language processing

Vanderauwera et al (2016)

Planum temporale symmetrical in children with dyslexia, so their language difficulties may stem from a lack of a specialised left hemisphere

Micro-level assymmetries of language-associated areas between hemispheres

• Differences in anterior (Broca’s) and posterior (Wernicke’s) language association cortex

• Neurons in left and right hemisphere have different distribution of dendritic branch orders

• Columns spaced farther apart in left wernicke’s area

• Greater long range connectivity in language associated areas of left hemisphere

Left hemisphere = speech?

90% of population show left hemisphere dominance for speech

Corpus callosum

• Largest white matter structure in the brain

• Made up of 250 million axonal fibres that cross from one hemisphere into the other

• Primary communication highways between hemispheres

Homotopic connections

Connect corresponding regions across hemispheres

Heterotopic connections

Travel to a different region in the other hemisphere

Synchronous processing

Communication across corpus callosum enables integration of visual info from both visual fields

Inhibitory processing

When a course of action is decided, inhibitory processing may halt incompatible motor behaviour

Anterior commissure

• Small band of tissue connecting hemispheres

• 1/10 size of corpus callosum

• Connects regions of temporal lobes inc. amydala and fibres of olfactory tract

Posterior commissure

• Even smaller than anterior

• Carries some interhemispheric fibres

• Above cerebral aqueduct at junction at junction of third ventricle

Split brain procedure

• Surgically severing the corpus callosum

• Myers & Sperry did first split brain experiments on cats, monkeys and chimps

van Wagenen

Severed the corpus callosum of patients with severe, untreatable epilepsy and they got better with little to no side effects

Method for testing the split-brain procedure

• Primarily use visual stimuli as visual system is more lateralised than other sensory functions

• Ability to communicate to one hemisphere is based on the anatomy of the optic nerve

• Experimenter quickly (>200ms) presents stimulus to one visual field

• Other eye can’t help because duration is too short (eye movements take 200ms)

Anatomy of the optic nerve

• Info from right visual field hits left side of retina in both eyes

• Visual field stays separated as it travels up optic nerve of each eye

• Optic nerve divides in half at optic chiasm

• Fibres carrying visual info from medial portion of retina cross and project to visual cortex of opposite hemisphere

• So all info from left visual field ends up in right hemisphere

Methodological concerns for testing split-brain procedure

• Identifying prior neurological damage

• Accurately evaluating the extent of the sectioning

• Paying attention to experimental design to eliminate cross-cuing

Cross-cuing

Occurs when one hemisphere initiates a behaviour and the other hemisphere detects that behaviour by an external cue and helps the other hemisphere

Callosal funtion specificity

• Splenium is most posterior portion of corpus callosum and when sectioned, the transfer of visual, tactile and auditory sensory info is severely distrupted

• Anterior part of callosum is involved in transfer of higher-order semantic info

Patient WJ

• WW2 veteran who had severe seizures because of a head injury

• When stimuli were flashed in right visual field, WJ verbally reported the stimuli, but not in left visual field

• His hemispheres were working independently

Grammar

Rule-based systems we have for organising words in a sentence

Generative syntax

Using the rules of grammar to create a near infinite number of sentences

Lexicon

The mind’s dictionary containing all the words we know

Language and speech

• Speech production is 96% left-localised

• Studies of WJ show that the left brain (not the right) can understand rule-based relationships between words

Visuospatial processing

• Right hemisphere good at detecting upright faces, discriminating among similar faces, and recognising faces of familiar others

• Left hemisphere better at recognising one’s own face

Hemispheres and facial expressions

• Only left hemisphere can trigger voluntary facial expressions

• Both hemispheres can trigger involuntary expressions

Double dissociation experiment with WJ

• Showed WJ faces morphed between himself and Gazziniga

• Had to state whether it was himself or Gazziniga

• Left hemisphere was biased toward self-perception

• Right hemisphere more biased toward close others perception (but less so than left hemisphere bias)

Emotion

• Right hemisphere and emotional prosody

• Good at picking up emotional context of speech

Robinson et al (1988)

• Asked patients with lesions to left or right hemisphere to complete Navon Letters Task (local target = small letters, global target = big letters)

• Left lesions (right dominance) were slow to identify local targets

• Right lesions (left dominance) were slow to identify global targets

Right hemisphere and cognitive tasks

Right hemisphere has limited integrative capacity for cognitive tasks

Theory of mind

• Ability to attribute mental states to oneself and others

• The TOM network is activated bilaterally

• Right TPJ most importance for TOM and seems specialised for TOM functions

Schurz et al (2014)

5 brain regions in the TOM network consistently activated in past neuroimaging studies:

1. Medial prefrontal cortex

2. Precuneus

3. Temporo-parietal junction

4. Superior temporal sulcus

5. Inferior frontal gyrus

The interpreter

• Left hemisphere appears to have specialised ability to make causal inferences and form hypotheses

• This ability, known as the interpreter, seeks to explain internal and external events to produce appropriate responses to behaviour

• It never admits ignorance about behaviour of the right hemisphere, it always makes up a story to fit the behaviour

Patient MSG

• Negatively arousing stimuli was showed to right hemisphere

• Patient denied seeing anything but was visibly upset

• Left hemisphere felt the autonomic response to the emotional stimulus but had no idea what caused it

Gazziniga & LeDoux (1978)

• Split brain patient shown two pictures, one to left and one to right

• Asked to choose which pictures were associated with pictures lateralised to right and left side of brain

• Chicken claw flashed to left, snow scene flashed to right

• Chose shovel with left hand and chicken with right

• When asked, said needed a shovel to clean out a chicken shed

• Left hemisphere had no knowledge about why he picked the shovel, so interpreted it to produce a response consistent with the context it knew

Woolford et al (2000)

• Probability guessing paradigm

• Left hemisphere used frequency matching strategy - guessing one 75% of the time and the other 25% of the time

• Right hemisphere maximised - guessing the same every time

• Right hemisphere outperformed the left as approached task in simplest manner

• Left hemisphere has tendency to search for causal events

Dichotic listening task

• Used to compare hemispheric specialisation in auditory perception

• Competing messages are presented, one to left ear and one to right ear

• People tend to report more words presented to the right ear, consistent with left brain = language

Attention and hemispheres

• Some forms of attention are integrated at the subcortical level

• Other forms act independently in the separated hemispheres

• Split-brain patients can use either hemisphere to direct attention to positions in either the left or right visual field

Deduplicative paramnesia

Delusional belief that a place has been duplicated, exists in two different locations at the same time, or has been moved to a different location

Limitations of laterality research on healthy ppts

• Effects are small and inconsistent because intact corpus callosum

• Bias in scientific review process toward publishing papers that find significant differences

Brain’s modular architecture

• Brain organised into modules of functionally interconnected regions

• Independent, specialised networks that perform unique functions

Advantages of modules

• More economical - saves energy because the distances it sends electrical impulses over it shorter

• Multiple modules can work in parallel

• Makes it easier to acquire new skills

Analytic-holistic dichotomy

• Left hemisphere seen as analytic and sequential

• Right seen as holistic and parallel