Lecture 11 and 12: Corpus Callosum and Split-brain Flashcards
What is the Corpus Callosum?
- Large white matter tract with over 200 million axons
- Function
- transfer and integration of information between the two hemispheres
- Interhemispheric communication
What are the other commisural fibres?
- Corpus callosum
- Anterior commissure
- Posterior commissure
- Hippocampal commissure
Here is an image of an structural MRI of the CC
Here is a diffusion MRI (dMRI) of the CC
How did researchers decided how to divide the Corpus Callosum?
- From experiments in monkeys and clinical work in humans
- The regions may be roughly associated with various cortical regions, although there is a considerable overlap
- no exact one-to-one correspondence
True or False
There is not really landmarks on the CC
True,
to divide the CC they really went with measurements and divided it according to portions of the CC
What is the Witelson parcellation?
It’s how the CC is divided
Name the division of the CC
- Rostrum
- Genu
- Rostral Body
- Anterior Mid-body
- Posterior Mid-body
- Isthmus
- Splenium
True or False
You can parcellate the CC as you want
True, there is not rule for how it is divided. You can divide it into 25 subregions as well for example:
True or False
The Microstructure of the CC, is the same across every division.
False.
- The microstructural organization of axons vary along the CC
- Fibre density
- axon diameter
True or False
It is impossible to measure how fast the information is transferred from one hemisphere to the other
False, it can be measured with the Poffenberger paradigm
What is the Poffenberger paradigm?
- 1912
- Measures the interhemispheric transfer time (IHTT)
- Procedure:
- Flash a figure on the left or right visual field and ask the patient to press a button with the left or right hand.
- Measure the time it takes to click the button and subtract the amount of time it takes for the uncrossed reaction (crossed uncrossed difference)
- amount of time it takes to cross the hemisphere
- Results:
- Takes longer when interhemispheric transfer of information is necessary
- Difference is subtle
- Takes longer when interhemispheric transfer of information is necessary
Where does the interhemispheric transfer occur in the Poffenberger paradigm?
- One way of studying this:
- manipulate the motor and visual aspects
- See which manipulation affects more the transfer time
- Another way is to study lesions and see if it take more time or not
What is the Horse Race Model?
- The model says that the transfer probably occurs at both locations at the same time but the motor transfer wins the race because the axons in CC are larger and more myelinated.
- transfers better
You are using the Poffenberger paradigm, what happens if you flash the stimuli in the left visual field and ask the patient to answer with their left hand?
The information stays within the right hemisphere. It doesn’t crosses the CC
You are using the Poffenberger Paradigm and you flash the stimuli in the left visual field and ask the patient to answer with their right hand. According to the Horse Race Model, what happens?
Because the information has to cross to the left hemisphere to move the right hand, both the motor system and visual system transmit the information. According to the Horse race model, the motor transfer is faster and arrives at the left hemisphere first since the axons in the CC are bigger and more myelinated.
True or False
The two hemispheres work together
True.
- Motor coordination
- Language
- affective prosody (intonation in language)
- Emotion processing
True or False
The CC is different depending on the sex of a person.
True,
Some CC subregions are larger in women
True or False
The CC is smaller in left handed people.
False
The CC is larger for left-handed people
What are some acquired abnormalities of the corpus callosum?
- Callosotomies
- Traumatic brain injuries
- Stroke
What are some developmental abnormalities of the corpus callosum?
- Corpus callosum Agenesis
- Variation in developmental disorders
- Autism
Who studied the Split-brain?
- Geschwind
- Sperry
- Bogen
- Gazzaniga
What has Sperry’s role when studying the split-brain?
- He studied the specialization of each hemisphere
- He performed surgery cut of the CC
- initially he didn’t notice any cognitive damage
- After creating the correct tests, he could find specific deficits
- Thus learn about the specialization of the hemispheres
What experiments with Sperry do?
- Mainly with cats and monkeys (with and without cut CC)
- Cut optic chiasm as well
- Procedure:
- Trained the animals to do some task while covering one of the eyes.
- Test
- Tested the training while covering the other eye
- Results:
- Both hemispheres act independently
- What the animals with split-brain learned was not remembered by the other hemisphere. They had to learn it all over again.
- Control group: training transferred to the other hemisphere
When does the split-brain phenomenon occurs?
- when both the CC and anterior commissure (AC) are cut
What experiments were done with humans?
- Participants:
- People with plit CC
- Procedure:
- Visual stimulation
- fixed gaze on the central poin
- lights flashed on both the right and left hemifield
- Tactile stimulation
- Placed object on the right hand
- Placed object on left hand
- Motor control of the hand
- Dictate to the left hemisphere some control over left hand
- Dictate contradictory information to each hemisphere
- Visual stimulation
- Results:
- Visual
- R hemifield → Could say where they saw the light
- L hemifield → Couldn’t say but could point
- Tactile:
- R hand → could name the object
- L hand → not able to name or describe it, but could match it with same object in collection
- Motor
- Poor control (since not the left hemisphere responsible for moving the left hand)
- R hand moved differently from the left
- Visual
Why if you flash a light to the left hemifield, the patients claim that they didn’t see anything?
Because that information goes to the right hemisphere which is not the language dominant hemisphere. + the split brain phenomenon doesn’t let the information to go to the other side
If you have a split brain and. your left hand it tied down, you can only use your right hand. If you send to the left hemisphere the command “raise your hand” and to the right hemisphere the command “tap your head” at the same time, what would you do?
Raise the right hand since the hemisphere having full control of my right hand (L hemisphere) would overrule the other.
What is a Dichotic listening task?
- Present two different sounds simultaneously in each ear
- Ask what they heard:
- report what they heard in right ear (left hemisphere)
- Aks to focus on what is said on the left ear
- can do it
- They have selective auditory attention
Missing Lectrue 11 p.35-37
What is the Chimeric face test?
- Participants are shwon an image where haft of the face is the one of a man and the other one of a woman.
- Each hemisphere seems to complete the image (imagines the other half) so they both think that saw a full image
- They could SAY that they saw a man or woman (if shown in the right hemifield)
- They could POINT to the man or women (if shown in the left hemifield)
What is the helping hand phenomenon?
- If right hand knows the answer, it might try to correct the left hand
What is cross-cuing?
- Language abilities on the right hemisphere
- comprehension
- info from one hemisphere might try to help the other hemisphere
When can you see the coss-cuing?
When you leave the patients a sencond chance to guess.
If they are correct they will not change the answer if they are wrong, they will frown or shake their head and correct their anser
What is a positive symptom of a split brain?
Some tasks can be done faster as one hemisphere works independently of the other.
What are the lateralization functions of the right hemisphere?
- Spatial
- visuo-motor tasks
- music
- Left side of the body
Which hand is better at drawing in visuo-constructional tasks in split-brain patients?
- The left hand since it has access to the spatial information in the right hemisphere
Which hemisphere is better at generating emotional reactions? and how do we know that?
- Both
- If show funny image on the left visual field, the person with laugh but will not be able to tell you why
True or False
There is a hemisphere that is better at processing emotions.
True
The right one
What effects can you see when callosotomy is performed in younger individuals?
The disconnection syndromes observed in split-brain adults are not observed.
Some of the deficits might be seen at first but they improve with time (especially when younger than 13)
Why young callosotomy patients do not display the same deficits as adult patients?
Because of their brain plasticity, the brain is capable of reorganization.
True or False
Even though the disconnection syndromes in young callosotomy patients is not the same as in adults, there are some problems that persist.
True
- Light memory deficits (acquisition of new information)
- Pragmatic language deficits
- Reading difficulties
Describe the Alien hand syndrome.
- Feeling that one of your hands is not yours and acts in an independent way
True or False
Even though your hand doesn’t seem to be yours in the alien hand syndrome you can still fully control it.
False
- There is involuntary motor activity in one hand
- Patients can try to control the alien hand with the other hand without success
In the alien hand syndrome, is there involuntary movements? If yes, what explains this?
- Yes
- It happens because the homologous motor frontal regions are disconnected
- No proprioceptive feedback between the homologous parietal areas
What is the timeline of the development of the commissural fibres in the brain?
- Anterior commissure: develops around the 10th week of gestation
- Corpus Callosum: develops in-utero starting around week 12 until 6th or 7th month
How does the corpus callosum develop?
The development starts with the genu and it makes it way to the back.
This is called rostro-caudal development
In the development of the commissural fibres, what aspect of the development continues after birth?
The myelinisation process
Defect in neuronal migration
- before week 10:
- no AC
- no CC
- between week 10 and 12:
- no CC
- Agenesis
- between week 12 and 20:
- parietal agenesis of the CC
What is agenesis of the corpus callosum?
It is when you are born without a corpus callosum
What happens when someone has agenesis of the corpus callosum?
- This means that they don’t have a corpus callosum
- Sometimes they don’t show split-brain syndrome
- Precocious plasticity mechanisms
- Normal IQ in ⅔ of cases
NOTE:
Agenesis of CC has different outcomes when it is isolated versus associated with other malformations
Agenesis of CC has different outcomes when it is isolated versus associated with other malformations
What are some of the deficits that come with agenesis of CC?
- motor coordination
- emotional
- social
- pragmatic language
What are some possible adaptation mechanisms that people with agenesis of the CC can have?
- Development of bilateral language functions
- Amytal sodium studies say not the case
- Residual commissures
- Anterior and sub-cortical commissure may be enhanced to transmit information
-
Ipsilateral tactile information
- normally eliminated in normal development
- each hemisphere has a bilateral representation
What is Ipsilateral tactile information?
It is when each hemisphere has a bilateral representation.
This is usually eliminated in normal development
Describe the study by Mancuso et al. 2019 where they explored the functional connectivity of agenesis or early section of the CC
- Patients:
- Patients with agenesis or early section of the CC
- Procedure:
- Resting-state fMRI:
- Results:
- Reductions in interhemispheric connectivity
- Surprising cases where intact connectivity between some brain areas
- Conclusion:
- Plasticity mechanisms and reorganization allow to develop sub-cortical networks
- white matter reorganization for interhemispheric communication
- Plasticity mechanisms and reorganization allow to develop sub-cortical networks
What is the case of Kim Peek?
- Complex case:
- Autism?
- Low IQ
- Agenesis of corpus callosum and other brain abnormalities
- Incredible memory abilities
- Was able to read the left and right page of a book simultaneously
What is the link between agenesis of the CC and autism?
- Motor, language and social deficits are similar between the two
- Neuroanatomical characteristic observed in autism:
- Volumetric and microstructural reductions of the CC
- 10% to 30% of the cases of Agenesis of the CC have autism spectrum disorder diagnostic or marked autistic traits
How does is the corpus callosum different in an individual in the autism spectrum disorder?
- CC:
- reduction in volum
- microstructural alterations
- White matter and function:
- processing speed of the brain
- motor deficits
- sensorimotor integration
Altered interhemispheric communication
Explain the study named:
A greater involvement of posterior brain areas in interhemispheric transfer in autism:
fMRI, DWI and behavioural evidences
- Process:
- Measure interhemispheric transfer time
- visuo-motor task
- Measure bimanual coordination
- Purdue pegboard test
- Measure interhemispheric transfer time
- Results:
- Size AUT < TYP frontal and parietal
- No difference in terms of interhemispheric communication
- IHTT
- bimanual coordination
- Conclusion:
- Intact information transfer
- Transfer at the visual level rather than at the motor level
How can you divide the CC when you are studying specific regions?
- 25 areas
What does IHTT stand for?
Inter Hemispheric Transfer Time test
Structure-function relationship
IHTT and bimanual coordination are associated with different parts of the CC in each group
(See image)
What is the nature of the motor deficits of individuals with autism spectrum disorder?
- Slower unimanual condition:
- execution, anticipation and preparation of movement timing/speed
In individuals with ASD, why would their movements be impaired?
- Deficits in:
- spatial representation
- integration of visual information to plan and execute movements
What does the visual integration in motor deficits show?
- Agrees with reduced connectivity or atypical connectivity between frontal and parietal areas
