PSYC 311 Lecture 7: The Corpus Callosum and Disconnection Syndromes

What is the Corpus Callosum? What is the function of the Corpus Callosum?

The corpus callosum is a very larger white matter tract that contains over 200 million axons. The corpus callosum is responsible for transferring and integrating information between the two hemispheres.

Do the fibres of the corpus callosum cross?

NO! The corpus callosum is between homologous brain regions as the fibres in the corpus callosum don't cross.

What type of fibre is the corpus callosum? What are the names of other fibres in this group?

The corpus callosum is a commissural fibre. There are other commissural fibres such as the anterior commissure, the hippocampal commissure, and the posterior commissure.

What work does the corpus callsoum divisions come from? What is it roughly associated with?

Corpus callosum divisions come from experimental work with monkeys (ablation and tracing) and clinical worth with humans.
These callosal regions may be associated roughly with various cortical regions, although there is considerable overlap. Not a completely 1:1 relationship with the cortical region and function.

How is the corpus callosum parcellated? How many sub regions are there?

The corpus callosum can be parcellated however you want. The corpus callosum was divided into 25 sub regions and mapped onto the cortex.

Does the microstructure of the corpus callosum varry? What affects the conduction of information? What is possible to measure?

Microstructural organizations of axons vary along the CC. Properties such as fibre density and axon diameter affect the conduction of information. For example, information will travel faster in longer axons.
It is possible to measure the speed of information transfer by the CC to two homologous regions.

What does the poffenberger paradigm measure? Where is interhemispheric transfer the fastest: in the motor or visual region?

The poffenberger paradigm wanted to measure the interhemispheric transfer time (IHTT). Transfer is faster at the motor region because lesions in the visual corpus callosum affect transfer time less than lesions in the motor corpus callosum

What functions rely on interhemispheric communication?

Interhemispheric communication is essential for motor coordination as the two motor and visual areas need to be working together. Language requires interhemispheric communication even if it is lateralized in the left hemisphere. Specifically,
for pragmatics and for understanding non-literal expressions, affective prosody. Interhemispheric communication is also required for emotion processing. Specifically, for facial expression, regulation and managing emotions, and verbal expression

What are the 3 corpus callosum differences/abnormalities discussed?

1) Normal variations in the population
- Sex: some sub regions of the corpus callosum are larger in women.
- Manual preference: corpus callosum is larger in left handed individuals
- A larger CC indicates that there is potentially a greater need for interhemispheric communication and that we have less lateralized functions
2) Acquired abnormalities
- Colostomies (cutting the corpus callosum)
- Traumatic brain injuries
- Stroke
3) Developmental Abnormalities
- Corpus callosum agenesis
- Variations in development disorders like autism.

When and who were aware of that interhemispheric communication was essential for high order cerebral functions?

As early as 19th century. Werknicke, Dejerine, and others.

What did Geschwind study in 1965?

Disconnection syndrome in animals and man based on lesion studies

What did Sperry, Bogen, and Gazzangia study in 1969?

Interhemispheric relationships. Syndromes of hemisphere disconnection

What did Roger Sperry study? What surgeries did he conduct? How does his first observations compare to his later ones?

Sperry studied the functional specialization of each hemisphere. Conducted surgeries to cut the corpus callosum in some epileptic patients. His first observation was that there was no apparent cognitive damage but when designing the right experiments, were able to find specific deficits and learn about hemispheric specialization.

What was Roger Sperry's main discovery?

When you cut the connections between the two brain hemispheres: each hemisphere functions independently as if each was a complete brain.If you train one side of the brain, the other side of the brain doesn't know anything about it.

split brain phenomenon only occurs under what conditions?

If section the corpus callosum and anterior commissure.

When a patient has a split brain, how do they perform on tactile stimulation/discrimination tests?

Object in the right hand: able to name and describe the object because the somatosensory information form the right hand goes to the left parietal lobe and can be transferred to the language areas in the left hemisphere. Object in left hand:
·Not able to name or describe and able to match it to the same object in a collection of object presented visually

When a patient has a split brain, how do they perform on visual stimulation tests?

Can see the flashing light in the right hemifield but not in the left hemified. Can only point to where the light was in the left hemified. This is because speech is in the left hemisphere, thus, if you flash the light in the left hemifield the information will go to the right hemisphere but it cannot cross to the language area so they cannot produce an answer

What is the Dichotic Listening task? What results do we see with patient that have sectioned Corpus Callosum?

The Dichotic Listening Task is another task used to test the phenomenon of split-brain. Essentially subjects are presented two different sound simultaneously in each ear. When you ask the subject with split brain to say what they heard most frequently they report what they heard in the right ear. Dominance in what is heard in the right ear as it goes to the left hemisphere to be verbalized. 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 as they have selective auditory attention. Essentially, patients with sectioned corpus callosum show deficits to name what is presented to the left ear.

What is the Chimeric Face Test?

Subjects presented with a picture that has half the face of a woman and a man. If the patient with split brain has to SAY if they saw a man or a woman, they will say that they saw a woman. If you ask the patient to point (right hand) at which face they saw, they will point to the man. The one hemisphere completes a symmetrical face so each hemispheres think they saw a full face.

What is the helping hand phenomenon?

The helping-hand phenomenon is when the right hand that 'knows' the answer may try to correct the left hand (for example, by bringing it to the right object).

What is cross-cuing?

Cross-cuing occurs in some participants as some language abilities are observed in the right hemisphere. So they have some language comprehension.

What is a positive symptom that may be observed in some split brain patients?

Some split brain patients can do visual search tasks faster than controls as they can search both sides of the screen simultaneously

In an experiment for cross-cuing, split brain patients are presented with a green or red flash to the right hemisphere. They answer at chance level at first but improves when a second guess is allowed. Why do they improve with a second guess?

If the answer guessed by chance (by left hemisphere) is the good answer, the patient stuck to the answer. If the answer is wrong, the right hemisphere hears the left hemisphere guess and cues the left hemisphere that it's wrong by frowning and shake of the head.
Essentially, when the answer is said out loud, the right hemisphere (that saw the light) hears the answer and then is able to correct what the left hemisphere said

What functions are lateralized in the left hemisphere and what functions are lateralized in the right hemisphere?

Left hemisphere = Language and speech
Right hemisphere =
Spatial, visuo-motor tasks, and music

In normal individuals, when you ask which face is most expressive what picture do they generally select? Why?

Generally, people tend to select the picture where the left side of the face is more expressive as the right hemisphere is better at processing emotions.

What function are both hemispheres equally good at?

Generating emotional reactions!

How does age effect the deficits we see in split brain patients? What does this show?

When colostomies are done in younger individuals, disconnection syndromes observed in split-brain adults not observed. Some deficits at first but they improve with time (especially when younger than 13). This shows that the immature brain has greater potential of adaptation after surgeries. Plasticity leading to a cerebral reorganization.

What deficits persist in split brain patients regardless of age?

1) Light memory deficits: acquisition of new information
2) Some pragmatic language deficits
3) Some reading difficulties.

What is Alien Hand Syndrome?

Alien Hand Syndrome is the feeling that one of your hand is not yours and acts in an independent manner (on its own). It's a very rare symptom of split-brain. Involuntary motor activity of one hand, patients can try to control the alien hand with the other hand without success.

Why does Alien Hand Syndrome happen?

Alien Hand Syndrome happens because the homologous motor frontal regions are disconnected. Essentially, there is no proprioceptive feedback between the homologous parietal areas.

When does the anterior commissure develop in utero?

10th week of gestation

When does the corpus callosum develop in utero? What part develops first?

starts around week 12 and develops until the 6th-7th month. The anterior portion of the CC (the genu) develops first.

What process continues after birth for increased interhemispheric communication after birth?

The Myelinisation process.

A defect in neuronal migration will have what consequence on the fetal brain if it occurs before week 10 of gestation?

no AC or CC

A defect in neuronal migration will have what consequence on the fetal brain if it occurs between week 10 and 12 of gestation?

no CC (agenesis)

A defect in neuronal migration will have what consequence on the fetal brain if it occurs before week 12 and 20 of gestation?

Partial agenesis of the CC (anterior parts would be intact but posterior parts that develop later may be absent).

What happens when you are born without a corpus callosum? How does isolation versus the association with other brain malformations affect the outcome?

Sometimes nothing!
Split brain syndrome not observed due to precocious plasticity mechanism (the brain adapts and uses alternative methods). Normal IQ in 2/3 of cases. Outcome may not be as good if not isolated and infact related to other brain malformations.

What are some deficits that can occur with agenesis of the corpus callosum?

1) Motor coordination (reaction time is slower which more complex tasks)
2) Emotional
3) Social
4) Pragmatic language.

List and discuss the three theories that could explain the possible brain adaptation mechanisms that could explain why we don't see many deficits.

1) Bilateral language function (each hemisphere is able) but some Amytal sodium say this is not the case.
2) Residual commissures.:
anterior commissure and subcortical commissures compensate especially to transfer visual information (as it connect temporal areas). So, maybe these fibres are enhanced in agenesis children.
3) Ipsilateral tactile information:
normally eliminated in normal development but not eliminated as much in agenesis children. Thus, each hemisphere has a bilateral representation of the somatosensory aspects.

What results did we see from the resting-state fMRIs of subjects with agenesis or early section of the Corpus Callosum?

Some important reductions in interhemispheric connectivity (or synchronization of brain activation) but also surprising cases of intact connectivity between some brain areas.

How did the authors explain the results seen in the resting-state fMRIs of subjects with agenesis or early section of the Corpus Callosum?

The authors explain this by plasticity mechanisms and reorganization allowing to develop sub-cortical networks (and anterior commissure connections). Some white matter reorganization allowing some interhemispheric communication but not be enough for more complex function. Essentially,
why we mostly observe deficits in higher-level functions

What characteristics are similar to both individuals of corpus callosum agensis and individuals on the autistic spectrum?

Motor, language, emotional, and social deficits.

What is the most consenual neuroanatomical characteristic observed in individuals on the autism spectrum?

Volumetric and microstructural reductions of the CC

What is the link between these white matter alterations noted in the brains of individuals on the autism spectrum and function?

Processing speed of the brain is slower, motor deficits, and sensori-motor integration. Also, evidence of altered interhemispheric communication which goes along with the reductions we see in the corpus callosum.

Despite structural reductions in the corpus callosum, why does information transfer (behavioral measures) remain intact in individuals on the autism spectrum?

In the brains of autistic patients, this information transfer occurs at the visual level rather than at the motor level. Essentially, they have an over activation of the occipital (visual lobe)

However, if we look more closely at motor tasks, what tasks are individuals on the autism spectrum slower at?

Slower for unimanual conditions. Reaction time slowed for execution speed, anticipation, preparation of movement. Essentially, simple reaction exercises can be slower

Atypical visual input and integration during motor action has been related with what?

atypical connectivity between parietal and motor areas. This shows the importance of visual integration in motor deficits.

What parts of the brain are activated during the performance of behavioral tasks in the typical group versus the autistic group?

Typical group shows more activation in the frontal and parietal lobe whereas the autistic group shows more activation in the occipital lobe.