Lecture 16: Corpus Callosum and Split Brain

corpus callosum

- large white matter bundle, over 200 million axons->commissural fiber

corpus callosum function

> transfer and integration of info between 2 hemisphres

-> between homologous brain regions

> intrahemispheric communication

other commissural fibers

• anterior commissure

• hippocampal commissure

• posterior commissure

diffusion imaging of corpus callosum

- CC is white matter and made up of axons

- can study with diffusion imaging

- only left to right fibers so it is bright red

- can study microstructure and integrity of communication- if axons large or well myelinated in corpus callosum

parcellation of corpus callosum

- different subregions of CC have different functions

> transfer information from different barin areas

> not the same information

- usually parcellated by Whittles Parcellation

> study what different areas of corpus callosum do

normal variations in population

sex: some subregions of CC larger in woman than men

manual preference: CC larger in left handed

acquired abnormalities

callosotomies

traumatic brain injuries

storkes

developmental abnormalities

corpus callous agenesis

variations in developmental disorders

split brain background early

as early as 19th century, Wernicke, Dejerine and other aware that

inter-hemispheric communication essential for high order cerebral functions

split brain geschwind

Disconnection syndrome in animals and man based on lesion studies

Sperry, Bogen and Gazzaniga

interhemispheric relationships. Syndromes of hemisphere disconnections

Roger speedy split brain patient

> main discovery-> when you cut connections between 2 brain hemispheres: each hemisphere functions independently as if each was complete brain

> train one side of brain: other side of brain doesn’t know anything about it

> divide optic chiasm so visual information presented to one hemisphere

Sperry Human cases visual stimulation

patient fixes gaze onc ross in middle of screen

light flashed in L and R visual hemifield

light flash in only left hemifield

• patient does not see any light

• ask them to point to where light was flashing and point correctly

speech is in L hemisphere

Sperry Human cases tactile discrimination

object in R hand

• patient can describe and name object

object in L hand

• can’t name or describe it

• can match object to same one in pile of objects presented visually

Roger sperry

> studied functional specialization of each hemisphere

> surgery to cut corpus callosum in some epileptic patients

> first observation: no apparent cognitive damage

> when designing right experiments, able to find specific deficits and learn about hemispheric specialization

Sperry Human cases picture

picture or written information flashed in one hemifield

object placed out of view in one patient’s hand

if presented to left hemisphere (right visual hemifield)

• able to name and describe

presented to left hand or left visual hemifield and asked to name it

• wrong guess

• unable to name it

• give non-verbal answers

sperry and gazzangia

- left hemisphere can verbally say what is in right visual field :”apple”

- or show with right hand (not left hand)

- if you present only word in left visual hemifield and ask what word is they don’t know

> left hand can point or select spoon not right hand

dichotic listening tasks normal controls

• 2 different sounds same time in each ear

• patient say what they heard more dominantly in R ears because L brain dominant for language

• can attend specifically to what is being presented in L ear because selective auditory attention

• split Brian patients have difficulty in repeating what is presented to L ear

dichotic listening tasks word placed in center field

patient able to repeat ART but won’t say heart

2 cards on table with HE and ART

patient can point to and pick up HE with L hand but can only do same with ART with R hand

chimeric face test

- if patient has to say if they saw man or woman, they will say woman

- ask patient to point (left hand) at which face they will point man

- one hemisphere completes symmetrical face so both hemispheres “think” they saw full face

left hemisphere interpreter gazzaniga experiment

> show 2 pictures, one in each hemifield

> patient have to point with 2 hands at pictures of 2 objects corresponding

> left hand pointing at card with picture of snow shovel

> right hand pointing at card with picture of chicken

> ask why his left hand pointing at shovel: use shovel to clean out chicken shed

generating emotional reactions

> present funny picture to right hemisphere:

-> patient said she saw nothing when asked

-> but they did smile and chuckle

> when asked why you are laughing: “I don’t know, nothing..."

-> right hemisphere cannot verbally describe what was seen tu emotional reaction is there

-> left hemisphere did not see picture

positive symptoms

- some split-brain patients can:

> draw different pictures with each hand simultaneously

> do visual search asks faster than controls

-> experiments indicated that separated hemispheres were able to scan their respective hemifields independently

helping hand phenomenon

right hand that ‘knows’ answer may try to correct left hand

cross cuing

some language abilities in right hemisphere, some basic language comprehension

-> cross-cuing from one hemisphere to other may also happen

cross cuing experiment

simple: present green or red flash to right hemisphere

> patient answers at chance level at first but improves when second guess is allowed

>Why?

-> if answer guessed by chance (by left speaking hemisphere) is good answer, patient sticked to answer

-> if answer is wrong: right hemisphere hears left hemisphere’s guess and cues the left hemisphere that it’s wrong by frowning or by she of head

-> when answer is said out loud, right hemisphere (that saw light) hears answer and then able to correct what left hemisphere said

working simultaneously experiment

- experiments in monkeys

> present something separately to each hemisphere at same time

> each hemisphere memorizes different scenario

-> left eye: learns that if press button with cross-> food

-> right eye: learns that if press button with circle-> food

-> learned those two associations in same time it takes normal monkey to learn one

-> when CC sectioned: evidence that each hemisphere acts as only brain

lateralization of functions

contralateral motor control but left hemisphere is dominant for both planning and execution of movements-> right-handed individuals are better with their right hand

- left hemisphere; language and speech

- right hemisphere:

> music

> emotional processing

> visuospatial processing

facial expressions

- in normal individuals

> ask which face is most expressive;

-> in general people tend to select picture where left side of face is

more rexpressive

-> right hemisphere better at processing emotions

right hemipshere

- visual-constructional tasks: right hemisphere is better

- split-brain patients;

> when asked to draw example, even if split-brain patient s right-handed, better with left hand

- dominance for spatial awareness

block design task

> split-brain patients:

-> better performance with left hand