Functional localisation and split brains Flashcards
(20 cards)
What is epilepsy?
A condition that effects 1% of the world’s population. Involves seizures caused by abnormal electrical activity in the brain, presents as sudden disturbances in movement, sensation or behaviour. Before a seizure will have relatively normal minimal activity, seizure involves what is effectively an electrical storm of over activity in the brain.
What are the two main forms of epilepsy?
Primary generalised seizures and partial (focal) seizures
What is another form of epilepsy?
Secondarily generalised seizure
- occurs when a partial seizure spreads to involve both cerebral hemispheres. Can be preceded by focal symptoms. Presence of localising neurological signs after seizures is a clue that there is a discrete cortical origin of the epilepsy
What are primary generalised seizures?
Seizures which diffusely involve both hemispheres at onset and consciousness is usually lost
What are partial seizures?
Focal seizures that have a discrete cortical origin and clinical features reflect the function of the affected area
First evidence for focal localistion of the brain
Partial seizures that originate in primary motor cortex cause convulsions in opposite half of body.
Seizures typically begin with involuntary movements in one part of the body as the wave of abnormal electrical activity spreads, clinic movements spread to consecutive body parts.
How can information be isolated to a single hemisphere e.g. due to epilepsy?
Corpus callosum surgery for epilepsy hope to minimise effects of callosal propagation of epileptic seizures over the whole brain.
Acute disconnections syndrome presents but rapidly normalises after corpus callosum surgery
In specialist studies found that patients could see occurrences but not vocalise them.
What are symptoms of Acute disconnection syndrome?
Mutism
Dyspraxia (lack of coordination, especially for left hand)
Contradictory ‘anarchic’ actions - one hand picks something up but the other hand changes this
How can information be processed by a single hemisphere?
Vision and touch could be processed solely in one hemisphere.
For example:
Whilst focussing on central point, lights flashed for a tenth of a second across both LVF and RVF. Patients would report just flashed in RVF. When only flashed on LVF would report not seeing anything but could point to where lights flashed.
Similarly, objects held in one hand could not be matched by others as processed in one hemisphere and not other. Able to name object when in R hand. WHen in left could not verbally identify but could nonverbally.
FIne motor control sometimes limited to within one hemisphere
How are functions lateralised in the human brain?
Language is laterilised to the left side of the brain. Although this had been suspected for a long time it had not been formally tested.
Visuospatial information is localised to the R hemisphere
What was the procedure of Ronald Myer’s seminal experiment?
Animals were taught to associat one pair of symbols with a food reward. Animals were taught only with one eye. This eye was trained while the other was blindfolded. See what would happen when the animal’s only used the untrained eye.
Animals who had the corpus callosum cut, even if just around the visual cortex, due to the corpus callosum being homotopic, visual information still could not cross. However, aside from very specific circumstances, commissurotomy did not have any side-effects.
Why was the procedure used in Ronald Myer’s seminal experiment?
In animals/humans without damage to the corpus callosum when a discrimination task occurs in one eye without the other it can be recalled using the other eye due to interocular transfer which is permitted by the optic chiasm. This procedure hopes to demonstrate the possibility of information transfer through the chiasm.
What experiments have been conducted around the corpus callosum?
Ronal Myer’s seminal experiment
Initial split brain procedure
Visuospatial 3D shape experiment
Gazzaniga et al (2000) finger position experiment
Juhn Wada
What was the initial split brain procedure?
- Original set up allowed the researchers to flash stimuli into the left and or right visual fields of the subjects
- Objects could be placed on the table behind the screen to be pointed at or handled
- Hands could be seen or hidden
Images were flashed very briefly to stop eye movements
What is the theory behind the original split brain experiment?
When a person fixates centrally, information that is to the right of fixation is projected to the visual cortex of the left cerebral hemisphere and information to the left of fixation is projected to the visual cortex of the right hemisphere
so
If an image was flashed to the RVF the patients could describe what they saw, same letter, image, scenes.
If an image was presented to the LVF the patients would draw a blank but if asked to pick up object with L hand it would match
As in a split brain images to the left visual field are processed in the right visual cortex and unable to transfer to the left hemisphere where Broca’s area is. But the R hemisphere can still control motor movements of the L hand.
Sometimes with partial split some information can cross allowing for communication of concept.
How were visuospatial abilities tested?
Asked to copy a 3D image presented to the LVF and RVF.
Images only had 3D form when were drawn by the L hand.
Confirmed in a pattern block task
Shows spatial awareness is localised to right hemisphere explaining why spatial neglect is was in R hemispheric damage.
Why test for visuospatial abilities this way (original split brain experiment?
Information only goes to one hemisphere and helps to determine where visuospatial information is specialised to.
How were split brains tested by Gazzaniga MS (2000)?
Outlines of hand and fingers held in a variety of postures flashed to contralteral hemisphere.
Readily mimic posture with opposite hand but can only replicate simple hand shapes with ipsilateral hand.
What did the findings of Gazzaniga MS (2000) show?
Disconnected hemisphere can control both arms however dextrous control can only be completed by the contralateral hemisphere.
What did Juhn Wada do to test lateralisation?
Injected anaesthetic into half of the brain to ‘put to sleep’
Then tested language and memory
Why did Juhn Wada test the brain in this way?
Argued that to test lateralisation tests had to be functional and could not be correlational ie through brain imaging studies.
