Lecture 5 - Laterality #2 Flashcards
Name 4 areas and 2 comissures of the corpus callosum
Four main areas of the corpus callosum: splenium, mass intermedia, hippocampal commissure, rostrum.
There are also commissures connecting the hemispheres: anterior commissure and the posterior commissure.
What is Banich’s Model of Interhemispheric processing?
Banich(1995) model: explains interhemispheric control in terms of metacontroland proposes that:
1) The transfer of info occurs via channels provided by the corpus callosum and subcortical commissures.
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2) The hemispheres interact but only one assumes a dominant role in the interaction.
3) Callosalchannels allow for a more efficient processing of complex information than would one hemisphere alone.
Inhibition Model of Interhemispheric processing
For inhibition vs excitation models of interhemispheric processing, the way I think of it is that the inhibition model is all about one hemisphere inhibiting the other (and that best processing occurs when one hemisphere dominates the other), while the excitation model is all about both hemispheres working together (and that best processing occurs when both hemispheres are involved).
- Callosal inhibition is thought to allow for intrahemispheric processing to enable more efficient during simple tasks. When this inhibition becomes mutually exclusive it allows a single hemisphere to take control and dominate processing.
- Hemisphere assumed to be superior for a specific function is not always the hemisphere that undertakes that function eg. the hemisphere that takes control is not always the one with the greatest ability.
What mechanism underlies inhibition?
Metacontrol:
•The neural mechanisms that distinguish between hemispheric ability and hemispheric dominance and determine which hemisphere will attempt to control cognitive operations.
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•Based on the idea that transferring and integrating information between both hemispheres requires time and energy, and it can therefore be more efficient to use one hemisphere and inhibit the other in simple tasks.
The theory behind Excitation Model of Interhemispheric processing,
Supporting evidence
For inhibition vs excitation models of interhemispheric processing, the way I think of it is that the inhibition model is all about one hemisphere inhibiting the other (and that best processing occurs when one hemisphere dominates the other), while the excitation model is all about both hemispheres working together (and that best processing occurs when both hemispheres are involved).
- The main theory behind the excitatory model is the reinforcement of information transfer and integration between hemispheres, activating the unstimulated hemisphere.
- Supporting evidence comes from callosotomy patients for epilepsy: sectioning the corpus callosum stops the spread of discharge to the other hemisphere, blocking the signal which activates the other hemisphere.
- Resulting disconnection syndromes show that these patients are unable to integrate information from each hemisphere, thereby indicating that communication between hemispheres is necessary for normal behaviour.
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What is a Callosotomy?
•When the corpus callosum is severed, it is called a callosotomy.
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•When other commissural fibers (eg. the anterior commissure, dorsal and ventral hippocampal commissures) are severed, this is called a commissurotomy.
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Characteristics of CallosotomyPatients
- Have normal post-surgery intelligence, personality, and general behaviour.
- In first few days after surgery, seem to have difficulty following complex multiple commands, even though individual commands are understood.
- Patients are often mute and have difficulty using the left hand/arm.
- After a few months: perform at a normal social and intellectual level but aspects of their behaviour are irregular.
How Do Other CallosalSyndromes Occur?
- Through disruptions to the development of the corpus callosum occur during the 5th to 16th week of pregnancy.
- Prenatal infections or viruses (eg. rubella).
- Chromosomal (genetic) abnormalities (eg. trisomy 8 and 18, Andermannsyndrome, and Aicardi syndrome).
- Toxic metabolic conditions (eg. Fetal Alcohol Syndrome).
- Blockage of the growth of the corpus callosum (eg. cysts).
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Prevalence of CallosalDisorders
•Estimates of the frequency of corpus callosum disorders vary greatly.
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•Current research suggests that as many as 1 person in 3,000 is affected by some disorder of the corpus callosum.
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Development of Corpus Collossal
•Typical infant brain: corpus callosum develops between 12 to 16 weeks after conception (near the end of the first trimester).
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•While the entire structure developsprior to birth, the fibers of the corpus callosum continue to become more and more effective and efficient on into adolescence.
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•By the time a child is approximately 12 years of age, the corpus callosum functions essentially as it will in adulthood, allowing rapid interaction between the two sides of the brain.
How does the Corpus Callosum change throughout lifespan?
•Recent studies have shown a decrease in lateralization with age.
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•Tasks strongly lateralized for young adults can become bilateral in older brains.
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•A possible explanation could be that the neuronal processing in one hemisphere is diminished, requiring the two hemispheres to work together in order to solve the task.
CallosalSyndrome Diagnoses : Complete Agenesis of the Corpus Callosum
Partial Agenesis
Dysgenesis
CompleteAgenesis of the Corpus Callosum:
- If the nerve fibers don’t cross between the hemispheres of the brain during the critical prenatal time for development of the corpus callosum, then they never will.
- Some smaller connections between the hemispheres develop in most individuals with ACC.
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PartialAgenesis of the Corpus Callosum:
- Corpus callosum begins to develop, but something stopsit from continuing.
- Since the corpus callosum generally develops from front to back, the part of the corpus callosum that is present in usuallywill be toward the front of the brain, with the back portion missing.
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Dysgenesisof the Corpus Callosum:
• The corpus callosum developed, but in some incomplete or malformed way.
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What are the Characteristics of Callosal Syndrome Patients
- Delays in attaining developmental milestones.
- Clumsiness and poor motor coordination.
- Atypical sensitivity to particular sensory cues.
- Difficulties on multidimensional tasks.
- Challenges with social interactions.
- Mental and social processing problems become more apparent with age.
- Limited insight into their own behavior, social problems, and mental challenges.
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Why language and communication abilities can be reduced in people with callosal syndromes.
•Since the left visual field is processed in the right hemisphere, a person with a callosotomyis unable to describe objects to the left, because the “talking” hemisphere has not seen anything.
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- It can be demonstrated that stimuli to the right hemisphere for example give emotional response, but because of the severed corpus callosum it cannot be verbalized.
- Especially lateralized processes that require interhemispheric cooperation, eg. combining tactile information entering the right hemisphere with the speech process present in the left hemisphere became impossible by the complete removal of the corpus callosum.
What happens during dichotic listening for those with split brain (collosotomy patients)?
•When split brain patients undertake a dichotic listening task, there is almost complete right ear advantage.
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One theory behind this:
•Because there is no corpus callosum, the left ear does not transfer info to the RH. Therefore, right ear becomes dominant due to RH specialization for attention.
