PY2507.3 ~ {Laterisation + Aphasia}
hemispheric laterization
it is the doctrine that the two hemispheres [of the brain] are specialized for different tasks and/or mode of processing - by Paul Broca 1865
what did Marc Dax discover
that language problems [particularly aphasia] are related to disease of the left hemisphere [so essentially recognized that language functions are localized in the LH as well]
where is language typically localized
in the LH, in areas like Broca's area + Wernicke's area which are crucial for speech production & comprehension [ so Dax's work has shown that in most right handed individuals LH plays dominant role in language]
Franz Joseph Gall (1790s)
founder of phrenology [now outdated & discredited] - theory that linked specific mental faculties & personality traits to the shape & bumps of the skull [even though his work was flawed had an influence on developent of modern neuroscience]
Gall's phrenological theory - what did he belief & discover
studied anatomy of brain through dissection + brain is organized into regions, each responsible for diff functions + the more developed a mental capacity was the larger the corresponding area of the brain would be + shape of skull reflected underlying brain structure & character + mental faculties
seat of parental love [according to Gall]
a pic of a woman who loved her children vs one who killed them - so then faculty of parental love was placed in the back of cerebrum [major diff between the 2 pictures]
language in FL [according to Gall]
he had this guess because alledgedly those those that read poems or were very literate had bulging or protruding eyes
Paul Brocas developments
did analysis of patients who had lesions in the brain and discovered : 1861 - patient Tan [global aphasia] who had difficulties in speech production from lesions in PFL [foot of the 3rd convolution]
1863 - 8 more cases presented
1865 - discovery of speaking using LH
Carl Wernicke (1974)
lesions to posterior temporal gyrus [on the left] result in speech comprehension + production deficits [of a diff kind ]
what evidence was presented after Broca and Wernicke's discoveries
RH dominant for visual spatial processing
language is represeneted diff in right & left handers
language localisation & hemispheres in the brain
whereby language is localized in the LH for 95% of right handers, 85% of ambidextrous and 70% of left handers [30% left half have bilateral dominance and other half have language in the RH]
Wada test aka ISAP
Intracarotid Sodium Amobarbital Procedure - used primarily to determine which hemisphere of the brain controls certain functions [esp in patients undergoing surgery for conditions like epilepsy]
how did Wada test work
a barbiturate [usually sodium amobarbital] is injected into one of carotid arteries to temporarily put to slee one hemisphere of the brain - then pateunts ability to do various tasks is tested [which would show the inactive heispheres role in those functions]
how did studies explore diffs between the 2 hemispheres
anatomical diff + brain assymetries in other species + behaviour of split brain patients + behaviour of normal subjects in laterization experiments + effects of unilateral lesions
anatomical diff between 2 hemispheres
RH - slightly heavier + larger than left but has less grey matter
slyvian fissure longer in the left + slope is gentler - planom temporales [tucked into SF] larger on left [the diff can be appreciated in left handers] + RFL extends further forward & wider than left whilst LOL extends further backward & wider
diff with other species: primates
primates can develop complex communication systems + can learn quite large vocab of words but they don't have language the way humans have so they can't combine symbolic units following rules is limited
diff with other species: primates [synchronization]
assymetry synchronization
dogs wag tail on the left when encoutering a new dog that they don't know + frog jump to left when escaping but right when feeding + parrots pick food with left foot
evoluntionary advantages of laterization
having diff behaviours controlled by diff sides if the brain may maximize efficiency - for animals being a minority might help disorient your enemy or provide new solutions where majority can't
bio asymmetries
consistent occur in all individuals of a species - e.g left-right asymmetry in organs like the heart or liver + brain
behavioural assymetries
while most individuals show a certain behavioural pattern there is a subset of them who show opp asymetry - e.g. laterization of brain function
human language being combinatorial
subset of symbols are combined in diff orders following rules [grammar] to convey meaning [the units can be phonemes, words, phrases, sentences] and combinations can be described as hierachical & recursive
when combinations are hierarchical & recursive
larger structures are created by combination of smaller structures which replicate the structure of larger units [ the woman ate the cake - that the child that spooke to the woman - bought
split-brain patients
patients who have undergone a commisurotomy [ a surgery that has severed the main tracts of fibres connecting the 2 hemispheres e.g. corpus callosum & anterior commissure]
why was commisurotomy
carried out in patients who have severe epileptic seizures [life threatening] could not be controlled using medication - so the idea was that by splitting the brain the epilepsy could not be transferred or one side could not communicate with the other to reduce the effects
corpus callosum
thick bundle of fibres connecting 2 hemispheres - so essentially responsible for communication between the 2 [splenium-posterior part + genu-anterior part]
the corpus callosum is made up of 3 parts
splenium + body + genu
what do laterization experiments take advantage of [in terms of anatomical organization]
input to the brain to all of us is crossed, so our pathways of sensory input are crossed. so what is presented to you on the right periphery of visual field arises on left side of brain.
laterization experiments in split-brain patients
once you direct input selectively to one hemisphere, it is unable to cross over so you can then investigate the capacity of one hemisphere in isolation
cross-over pathway of visual input
to be crossed, info needs to be presented briefly [e.g. 100ms] on far periphery + as soon as you move your eyes & fixate a stimulus, both hemispheres will receive info
example of cross-over pathway [visual field edition]
present an apple briefly on right stimulus will present on temporal retina [LH] + nasal retina [RH], neurons from TR will go directly to right brain crossing level of optic chiasm as well as left brain.
tachistoscopic presentation
very brief representation [for crossing to work] usually less than 100ms before being removed or masked - the goal is to present stimuli so briefly that participant doesn't have enough time to consciously process it
auditory input crossing pathways
info from one ear to contralateral hemisphere is stronger, carried by more fibres + info fromeach ear connects to btoh hemispheres + through experiments we learn that people tend to remember stimuli presented to right ear [goes to left part of brain] - so not really studied with split-brain patients
tactile input
completely crossed + what you feel on right end goes on LH first and vice versa + so in a split-brain patient in the absence of visual info what is touched by one hand will be known only by contralateral hemisphere
contralateral hemisphere
the opp hemisphere of brain that controls or processes info from one side of body + croos-ver control is known as contralateral control
findings from split-brain research
confirmed that LH is dominant for language + given us insights into linguistic abilities of RH + RH is not completely illiterate [can't speak, understanf abstract words or grammar BUT can understand common, concrete words & simple sentences even if written]
chimeric face test
to study hemispheric laterization + how brain processes emotioal info from faces + you combine 2 diff faces
chimeric face test - patients vs control
RH is dominant in processing faces & recogition of emotional expression + split brain patient will tend to recognizze face corresponding to left part of chimera face
limitations of split brain research
long-lasting protracted epileptic seizures can sometimes lead to changes in how brain functions + surgery may cause brain damage
why do split-brain ppatients undergo corpus callosotomy
done to treat severe epilepsy when seizures cannot be controlled by other means
lesions to LH
aphasia + dyslexia & dysgraphia + agnosia + apraxia + poor STM & LTM for verbal materials
lesions to RH
difficultues processing music & reading in Braille & with directions and spatial orientation + agnosia + apraxia + poor STM & LTM for visuo-spatial materials
aphasia
lesion with LH + difficulties with spoken language
Dyslexia & dysgraphia
difficulties with reading & spelling
agnosia [LH]
difficulties recognizing objects
apraxia [LH]
difficulties with object-appropriate actions
agnosia [RH]
difficulties recognizing faces & complex spatial objects
apraxia [RH]
difficulties with movememts in spatial patterns
broad characterization of processing in 2 hemispheres
LH - language + detailed, analytical sequential + visual [high freq] + tactile [fine grained]
RH - visuo=spatial patterns, music + global holistic + visual [low freq] + tactile [coarse grained]