W9 - Hemispheric laterality

lateralisation of function

unequal representation of various psychological functions in the 2 hemispheres of the brain

have 2 of pretty much everything

how is lateralisation studied

split-brain patients

cortical localisation of functions through:

• wada test

• imaging methods (fMRI)

• dichotic listening test (DLT)

early evidence for cortical localisation of functions - aphasia

deficit in language comprehension or production due to brain damage

• usually on the left

Broca’s area

• left inferior prefrontal cortex

• damage leads to expressive aphasia

early evidence for cortical localisation of functions - apraxia

difficulty performing movements when asked to so out of context

also a consequence of damage on the left

• primary motor cortex (Liepmann, 1900)

led to the theory of hemispheric dominance

split brains in cats

Myers and Sperry (1953)

studied function of the corpus callosum

they cut the corpus callosum and optic chiasm in one group of cats

• these are the two routes through which information from each eye crosses over the opposite hemisphere

catts were trained on a visual discrimination task when one eye was blindfolded

all cats (in the experimental and control group) first showed learning of the task and the same level

the blindfold was then transferred to the second eye

control cats performed the task at the same level, but the experimental group cats had to relearn the task

• because of no transfer of information between hemispheres for experimental cats

• shows the role of the corpus callosum in transferring information between the 2 hemispheres

left hemisphere

receives sensory input from right visual field and right hand

controls right hand movements

language 

right hemisphere

receives sensory input from left visual field left hand

controls left hand movements

cannot speak

split brain in humans

replicated cat study with humans

person stares at fixation cross

• word flashes on screen - visual acuity

  • for tenth of a second so no time to move eyes

objects out of sight - comprehension

brains already split, but optic chiasm intact as that can create blindness

something on right side of screen encoded in left hemisphere = language, so they say what they saw

• controls right hand movements too so identify what was seen in right visual field

something on left side of screen encoded in right hemisphere, which cannot speak, so say they saw nothing

• its the left side of brain answering which hasn’t seen anything in right visual field

• controls left hand movements, so can identify the object

testing cerebral lateralisation - wada test

sodium amytal test

lateralisation of language

sodium amytal is inserted into the carotid artery on one side of the neck

anesthetize one hemisphere and check for a specific function (e.g., language) in the other

• when the left hemisphere is anesthetized, production of speech is affected and when the ability returns this is associated with errors.

testing cerebral lateralisation - functional brain imaging methods

fMRI/PET can be used to see which half is more active when doing a language-related activity eg. reading

• expensive and not everyone can go in one

testing cerebral lateralisation - dichotic listening test (DLT)

involves the simultaneous presentation of two different acoustic signals to the right and left ears

• participants better at reporting stimuli presented to the right ear if more left hemisphere dominant (Kimura, 1967; 2011)

because when both ears are simultaneously stimulated, uncrossed pathways are suppressed

• right ear input directly reaches LH

• but left ear input travels via RH

wada vs fMRI - Bauer et al. (2014)

conducted a meta-analysis and systematic review of studies comparing the Wada test with fMRI

• 81% of patients were correctly classified

• of the 19% of patients where technique disagreed, most had atypical lateralization

of those correctly classified:

• 94% of patients were typical (left-hemispheric lateralization)

• 51% of patients were atypical language lateralization (right side)

wada vs DLT - Staruss et al. (1987)

epileptic patients whose speech dominance was ascertained using the Wada test were tested using a DLT

• patients with typical LH speech dominance showed greater right-ear-advantage

  • laterality index score +37%, p < .05

• no advantage towards either ear was seen for patients RH/atypical speech dominance

  • LI = -12%, ns

fMRI vs DLT - Hund-Georgiadis et al. (2001)

assessed language lateralisation using DLT and fMRI

the two methods showed very strong agreement (97.1%)

the lateralisation indices differed by more than 40% in 11/34 participants

greater variability in lateralisation in LH participants

conclusions of ways to test lateralisation

wada test = most valid and reliable

• but most invasive

fMRI = non-invasive alternative

• with lower validity and reliability than Wada

DLT = simple to administer with good convergent validity

fMRI and DLT more accurate in diagnosing typical language lateralisation

functional differences between the hemispheres

vision

• left = words, letters

• right = faces, geometric patterns, emotional expressions

audition

• left = language sounds

• right = music

touch

• right = tactile patterns, braille

movement

• left = complex and ipsilateral movements

• right = movements in spatial patterns

memory

• left = verbal memory

• right = perceptual aspects in memory

language

• left = speech, reading, writing

• right = emotional content

spatial ability

• right = mental rotation of shapes, direction, distance

where functional differences do exist, they tend to be slightly biased in favour of one hemisphere - not absolute differences

for example - language lateralisation is far from being absolute

• language related activity also takes place in the right hemisphere of the brain

• the right ear advantage in individuals with left language dominance is only 55%

anything other than language is very small

hemisphere functional differences

left hemisphere

• plays superior role in controlling ipsilateral movements

• damage to the left hemisphere can influence production or fine motor movements of the left compared to the right hand

right hemisphere

• plays superior role in controlling spatial tasks/abilities (eg. hemispatial neglect) and emotions

how should we interpret functional laterization?

all cognitive processes associated with a broad functional category might not be lateralised to the same hemisphere

• eg. speech perception and understanding word meaning involves the right hemisphere

anatomical asymmetries of the brain

the link between the anatomical and functional asymmetries of the brain is mixed

these language structures are different in the hemispheres:

• frontal operculum

• Heschi’s gyrus

• Planum temporale

  • due to more recent evolution and adaptation to language

theories of cerebral asymmetry / evolution of lateralisation

analytic-synthetic theory

motor theory

linguistic theory

analytic-synthetic theory

2 modes of thinking:

• analytic (left)

  • solves problems

• synthetic (right)

  • creativity

vague and essentially untestable

motor theory

left hemisphere controls fine movements

• speech is just a category of movement

does not explain why motor function would have become lateralised

linguistic theory

primary role of left hemisphere is language

damage to left hemisphere affects American sign language speakers’ language

• but not charade gestures/movements

oversimplification