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Chapter 16 – Lateralisation, language, and the split brainThe left brain and right brain are entirely separate, apart from the commissures which can be seen in
the midsagittal brain section.
Discovery of the specific contributions of the left-hemisphere damage to aphasia and apraxia.
Aphasia occurs when the brain is damaged and there is a deficit in language production or
comprehension. Broca noted his patients had left hemispheric lesions in the frontal lobe. This area
became known as Broca’s area. Apraxia was discovered a few years after this by Liepmann, who
realised that people with left hemisphere damage found it hard to conduct limb movements on
command. They could move automatically, but when told to, could not process the information.
Techniques to assess cerebral lateralisation:
- Sodium amytal test: It is given before major neurosurgery to plan what to tackle. Sodium
amytal is injected into the neck’s carotid artery on one side. It anaesthetises one brain side,
helping to assess the other aside’s functions. When the left brain is numb, the patient is
usually mute and has issues understanding given tasks.
- Dichotic listening test: Non-invasive. Playing the sound of three numbers, different numbers,
in each ear to see which side of the brain picks up the sounds better. Right ear sounds were
reported better, and since the left brain controls the right side of the body, this indicates
that the left brain is responsible for language.
- Functional brain imaging: PET scans and fMRIs are conducted when a patient is reading. This
reveals greater activity in the left-brain
Dextrals are right-handed and sinestrials are left-handed. In dextrals, 60% of left-brain lesions and
2% of right brain lesions cause one to be diagnosed with aphasia. But for sinestrals, 30% of people
with left-brain damage lesions and 24% of people with right brain lesions are said to be aphasic,
suggesting that language capabilities may be in different regions depending on handedness. Left-
handed folks have a more varied and unpredictable lateralisation understanding. 69% of speech
characteristics are found to be in those without left-brain damage in sinestrals, but 92% in dextrals.
Sex differences: When males suffer unilateral strokes, they are more likely to suffer aphasia,
suggesting they are more likely than women to have lateralised brain functions. This has not been
well researched, however.
The split brain – patients whose brain has been split by commissurotomy
The corpus callosum is the largest cerebral commissure. There are 200 million axons in the CC.
Myers and Sperry cut the CC of cats. They found that the CC transfers information between both
sides and that sometimes, even if the region is lesioned, the brain can operate independently. This
suggests that there are language abilities in the right brain; they may not be as strong as the left-
brain, but they are still present. When ONE of the optic chiasm or corpus collosum were transected,
then the cat still knew what lever to press when one other eye was blindfolded. They could do this
without fail. When both were lesioned, they could not figure out what was what, suggesting that
even with a cut optic chiasm on one side, the CC could transfer information to the other side and the
cat knew what to do. This happened despite the scotoma in the other side of the cat’s vision.
, If contralateral brain regions do not function, then information passing through from one side
cannot be understood on the other side. This is what happens when split brain monkeys cannot
move one arm in a certain way, as their somatosensory cortex and motor cortex are not working in
legion.
Commissurotomy in humans with epilepsy
If cutting the CC reduced seizures, it would be due to the source of electrical outburst being limited
to this region of origin, as opposed to taking over the other side of the brain after the CC relayed this
information. These are better than lobotomies, as the brain can still function somewhat without this
relay of information.
But with a lobotomy. Certain pathways specific to brain regions are cut# off, and the key regions are
severed, like the hippocampus, which reduces cognitive capacity. If the brin can operate
independently in some cases, then cutting the centre piece should cause minimal damage and
sluggishness, unlike lobotomies, which rid someone of a function entirely even if it helped cure
localised epilepsy. The nucleus accumbens is in the prefrontal cortex, meaning the pathway for
pleasure is transected. The amygdala is linked to the pathway where the nucleus accumbens is,
although it is not directly in the prefrontal cortex. This lesion can make it hard to process pleasure
and happiness.
Left and right brain study: An apple being placed in the right hand was easy for the left
hemisphere to vocalise and name. This is because the right hand picked up the information of the
stimuli and this information was quickly coded in the language specialist left-brain. However, even
though the right-brain is good at perceiving that there is an apple in the left hand, it is hard to
vocalise and say that it was an apple. Even when the word is SEEN on the screen by the left eye, it is
difficult to process what exactly was seen on the screen as it was a word, not something in space.
This made it hard for the right brain to know what was said, but volunteers still managed to
effectively pick up the apple that they recognised but could not identify vocally. (Gazzaniga and
Sperry).
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