Corpus Callosum & Split Brain Flashcards
Corpus callosum
-Large white matter tract, over 200 millions axons
Function:
- Transfer and integration of information between the two
hemispheres.
→ between homologous brain regions.
→ Interhemispheric communication
Microstructure
- Microstructural organisation of axons vary along the CC
- Properties such as fiber density and axon diameter affect the conduction of information
- It is possible to measure the speed of information transfer by the CC
Poffenberger paradigm
- measured Interhemispheric transfer time (IHTT)
- faster at motor level than visual level
- (left hand and left visual field faster than left hemisphere and left visual field)
Corpus callosum differences and abnormalities
1) Normal variations in the population
• Sex: some subregions of the CC are larger in women
• Manual preference: CC larger in left handed
2) Acquired abnormalities
• Callosotomies
• Traumatic brain injuries
• Stroke…
3) Developmental abnormalities
• Corpus callosum Agenesis
• Variations in developmental disorders: the example of autism
Split-brain
• Studied the functional specialisation of each hemisphere
• Surgery to cut the corpus callosum in some epileptic patients
- First observation: no apparent cognitive damage.
- But… when designing the right experiments, were able find specific deficits and learn about hemispheric specialisation
Split-brain visual stimulation
• The patient fixed his gaze on a central point on a board
• Lights are flashed in both the left and right visual hemi field
- Ask the patient: what did you see?
- Answer: ‘’lights flashing in the right hemi field’’
• Flash lights in only the left hemifield
- Ask what did you see: they deny having seen any lights
- If you ask them to point where the lights were flashed instead of asking for a verbal answer, they point to the right side
Split-brain Tactile stimulation/discrimination
• Object in the right hand: able to name and describe the object
• Object in the left hand:
- not able to name or describe
- able to match it to the same object in a collection of
object presented visually.
Split-Brain Motor control
-Dictate to the left hemisphere some control over the left hand: poor
-Dictate contradictory information to each hemisphere: a different movement to the right hand
→ the hemisphere having full control of the hand (the left one here) overrules the other
Dichotic listening task
Normal controls
• Present two different sound simultaneously in
each ear
• When you ask the subject to say what they heard,
most frequently they report what they heard in
the right ear → dominance
If asked to attend specifically to what is presented
to the left ear (or present a sound only in the left
ear), can do it
→ Selective auditory attention
Patients with section of the Corpus callosum
→ Deficits to name what is presented to the left ear
Split-brain: A picture of written information is flashed in one hemifield OR An object is placed out of view in one of the patient’s hand
1) If presented to the left hemisphere (right visual hemi-field or right hand):
• Able to name and describe (verbally and in writing)
2) Presented in left hand or left hemifield and asked to name it:
→ wrong guess
• Even when they have the object in their hand, still unable to name it (knowledge remains in the right hemisphere)
• But able to give a non-verbal answer (find the object with the left hand, identify the pencil by touch, or point)
Chimeric face test
left half is man right half is woman
• If the patient has to SAY if they saw a man or a
woman, they will say woman
• Ask the patient to point (left hand) at which face
they saw they will point the man,
• The one hemispheres completes a symmetrical face
so each hemispheres think they saw a full face
Helping-hand phenomenon
the right hand that ‘knows’ the answer may try to correct the left hand
Cross-cuing
In some participants, some language abilities is observed in the right hemisphere. Some language comprehension
→Cross-cuing from one hemisphere to the other may also happen
–> guess but if wrong will correct themselves through movement
Lateralisation of functions
• Language an speech: left hemisphere
• Right hemisphere:
- spatial
- visuo-motor tasks
- music
- better at processing emotions
• both can generate emotional reactionsAlien hand syndrome
• The feeling that one of your hand is not yours and acts
in an independent manner (on its own)
• Involuntary motor activity of one hand, patients can
try to control the alien hand with the other hand
without success
→ because the homologous motor frontal regions are
disconnected
- No proprioceptive feed back between the
homologous parietal areas
