The dominant left hemisphere
- Asphasia - deficit in language comprehension of production due to brain damage, usually on the left.
Broca - reported results of post-mortem examination of two asphasic patients.
- Patients with deficits in the use of language that are not attributable to general sensory, motor, or intellectual dysfunction.
- Both had left hemisphere damage centered in the left pre-frontal lobe, just in front of the primary motor face area.
- Became known as Broca's area is associated with grammar and speech production.
Apraxia - difficulty performing movements with either side of the body when asked to do so, but not when performing them spontaneously.
Liepmann discovered that apraxia was associated with left-hemisphere damage.
Complex activities were performed by the left hemisphere; the left and right hemispheres thus became known as dominant and minor hemispheres, respectively.
Imaging studies reveal more activity in the left hemisphere than the right during language related activities.
Tests of cerebral lateralisation
- 95% of right-handed participants are left-hemisphere dominant for speech.
- 70% of left-handed participants are also left-hemisphere dominant for speech.
- However, left hemisphere damage can cause the right hemisphere to become dominant for speech and the left hand to be preferred.
The split-brain experiment
- In 1953, Myers and Sperry performed an experiment on cats. It provided a means of comparing the function of two hemipsheres and the corpus callosum.
- The corpus callosum transfers learned information from one hemisphere to the other. When cut, each hemisphere functions independently.
- There were 4 types of cat used: 1) corpus callosum severed, 2) Optic chiasms severed, 3) Both systems severed, and 4) intact controls.
- Phase 1: Cats learned a lever-press pattern discrimination task with a patch over one eye; all four groups readily learned this simple task.
- Phase 2: Patch was switched to the other eye.
- Groups 1,2, and 4 performance kept same.
- In contrast, group 3 (both systems severed), the group acted as if the task were completely new to them.
Conclusion of experiment
The cats' forebrain can act as 2 separate things, capable or independent learning, can store memory.
Test of split-brain patients
- First operations to severe corpus callosum performed in the 1940's - performed on patients with life-threatening cases of epilepsy to reduce the severity of convulsions by restricting epileptic discharge to half of the brain.
- Remarkably, operation is effective and appeared to have few obvious effects.
- After Myers and Sperry experiment, there was a clearer idea of corpus callosum function.
- The neuropsychological testing of these split-brain patients has revealed some amazing things about the human brain.
- Visual stimuli are flashed to the right or left of a fixation point on a screen.
- Also tactual information is presented to one hand under a ledge or in a bag.
- These tests confirmed the conclusion that the split brain patients have two independent streams of consciousness.
- Visual information from right visual field will be transferred to left - but not in split-brain patients.
Two independent streams of consciousness
- Object presented to the left hemipsphere either by touching something with the right hand or viewing something in the right visual field.
- The patient able to:
- Pick out the correct object with the right hand, but could not pick out the correct object with the left hand.
- Could name the correct object
- Object presented to the right hemisphere, either by touching something with the left hand, or viewing something in the left visual field. The patients could:
- Could pick out the correct object with the left hand, but could not pick out the correct object with the right hand.
- Claimed nothing had been presented.
- Represents communication between hemispheres via a non-neural route.
- For example, a red or green light is flashed in the left visual field; the split-brain patient was then asked to name the colour, red or green.
- Patients get 50% correct (i.e. by guessing or by chance) - they found that one patient performed almost perfectly.
- On the trials when this patient initially said the incorrect colour, his head shook and he changed his guess.
- Apparently, the right hemisphere heard the incorrect guess of the left-hemisphere that it was wrong by shaking the person's head, when only first guesses were counted, performance fell to 50%.
Learning two things at once.
- Split brain patients are capable of learning two things at once.
- If a split-brain patient is visually presented two objects at the same time (e.g. a pencil in the LVF and apple in the RVF, s/he can reach into two different bags as the same time, one with each hand, and pull out the two objects - a pencil in the left hand and an apple in the right.
- Occurs when the two hemispheres are presented with different information about the correct choice and then are asked to reach out and pick up the correct object.
- Usually the right hand will reach out to pick out what the left hemisphere saw, but the right hemisphere seeing what it thinks is an error makes the left hand grab the right hand and pull it over to the correct object.
Differences in the left and right hemipsheres
- Language is the most lateralised of all abilities; the left-hemisphere is better than the right at most language-related tasks.
- However, the right hemisphere is able to understand single written and spoken words.
- The right hemisphere better at tasks involving spatial ability, emotional stimuli and musical tasks.
The two hemispheres seem to engage different types of memory processing:
- LH attempts to place its experience in a larger context
- The RH attends strictly to the Gestalt perceptual characteristics of the stimulus.
- This is usally termed analytical (LH) versus holistic (RH).
- Thus, the RH should not be regarded as the minor hemisphere, it has different abilities, not less important ones.
Differences in the left and right hemispheres
- There are also anatomical asymmetries in the human brain; for example the planum temporale and frontal operculum (language related areas) are larger in LH.
- However, Hesci's gyrus is larger in RH.
- The anatomical asymmetry detected in musicians by MRI.
- In most people, the planum temporale is larger in the left hemisphere
- than in the right; this difference was found to be greater in musicians with perfect pitch than in either musicians without perfect pitch or controls.
- Suggests that there are two fundamentally different modes of thinking, an analytic mode (LH) and synthetic mode (RH), and that the neural circuitry for each is fundamentally different.
- LH operates in logical, sequential, analytic fashion (pieces of the whole).
- RH (the whole) makes immediate, overall synthetic judgements.
Suggests that LH is specialized for fine motor movement of which speech in but one example.Two lines of evidence:
- Lesions of the LH disrupt facial movements more than RH lesions do, even when they are not related to speech.
- Degree of disruption of non-verbal facial movements is positively correlated with the degree of asphasia.
- The primary function of the LH is language; based on studies of deaf people who communicate using SL; this ability is lost if these people suffer damage to the LH, even when they are able to make the movements required.
Inferior left pre-frontal lobe in left hemisphere. Damage leads to deficits primarily, but also comprehensions of speech production.
Left temporal lobe, just posterior to the primary auditory cortex. Damage leads to deficits in semantic language comprehension and speech is incomprehensable, despite having correct grammar, rhythm, and intonation.
Damage to pathway connecting Broca's and Wernicke's area. Comprehension and spontaneous speech are intact but patient not able to repeat words they have just heard.
Damage to the left angular gyrus (area of left temporal and parietal cortex posterior to Wernicke's). Inability to read despite intact language comprehension and production caused thorough damage.
Also due to damage to left angular gyrus. Inability to write despite intact language comprehension and production. Involvement of left angular gyrus in both alexia and agraphia - responsible for language related visual input.