PSYC 301 - Split Brain Patients - M2

in a healthy brain (2)

• left and right hemispheres are connected by a few white matter tracts, commissures 

• the largest is the corpus callosum 

callosotomy

cutting of corpus callosum

commissurotomy 

cutting of any commisures 

in the 1940s (2)

• researchers show that epileptic discharge can spread from one hemisphere to another via corpus callosum in monkeys

• first limited callosotomy procedure in humans to control seizures in patients with intractable epilepsy, not successful

1950s

Roger Sperry and colleagues study split brain in rats, cats and monkeys to assess each hemisphere separately 

1960s (3)

• scientists return to the idea of callosotomy, serving of the corpus callosum in humans 

• new and more complete commissurotomy performed, successfully controls epilepsy 

• adaptation of animal techniques for use in human patients 

Roger Sperry and Michael Gazzaniga

small number of human subjects received surgery to sever their corpus callosum as treatment for intractable epilpesy - ethical bc it was the best available surgery at the time

how can each hemisphere of the cortex be studied separately (2)

• through clever experiment design

• if not under carefully controlled environments, patients will be seen as normal and independent

what did these experiment rely on

• the fact that each hemisphere sees only the contralateral visual field with some midline overlap

• e.g. right primary visual cortex only gets information from the left side of space and vice versa

what did this mean 

we can selectively “drop” visual information into each hemisphere of the split brain patient if they maintain fixation, eyes not moving ard 

what does the right hemisphere do (3)

• sees left visual filed

• does not speak

• controls left hand - when asked to say what is seen on left visual field, cant say but when asked to draw with left hand they can draw it

what does the left hemisphere do (3)

• sees right visual field

• can speak - speech production

• controls right hand

e.g. HOT + DOG (4)

• right hem sees: HOT

• left hem sees: DOG

• left hand draws: HOT

• voice says: DOG

using other sensory modalities - touch (2)

• each hemisphere feels touch input from the contralateral side of the body - sensory input crosses at brainstem

• e.g right hemisphere detects touch information from left hand and vice versa

what does this mean we can do 

• we can selectively “drop” touch information into each hemisphere of the split brain patient if they don’t see it and don’t use cross midline touch 

• speech can only tell us abt objects in the right hand 

anomia

inability to name objects in the left hand

what is important to know

that both hemisphere need to understand the task

eye movement 

eyes move tgt and either hemisphere can control saccadic eye movements via lower structures 

by looking at task performance, we can

make inferences abt cortical and subcortical systems

task performance (4)

• sense of self

• visual search 

• left hemisphere as “interpreter” 

• producing facial expressions 

what is preserved even without corpus callosum (2)

• responding to some types of stimuli, judging parallel lines and apparent motion

• probably subcortical involvement

sense of self (3)

• both hemisphere can make judgement on who is this person in face morph 

• when both hemisphere is presented with 100% JW, know that it is him but if its 10% JW, he knows it is not him 

• right hemisphere plays it more conservative, doesn’t switch to that is me until there is more weighting of himself in the face morph 

visual search (3)

• instruction: find the dark circle

• in normal subjects, each distractor linearly adds to the search time

• in split brain patients, adding distractors increases search time by half, as each hemisphere searches its array in parallel, simultaneously

• however, left hemisphere is more strategic than the right

the left hemisphere as “interpreter” (5)

• wanted to understand how the left hemisphere speaking wld account for the actions of the left hand

• shown: snow + chicken feet

• chooses from an array of pictures with each hand: shovel (RH) + chicken (LH) - asked why they chose the pictures

• they will now why they chose chicken but doesn’t know why the shovel was chosen so the left hemisphere will say a plausible answer

• left hemisphere in healthy individuals gives us a sense that our behaviour is coherent

producing facial expressions (2)

• if spoken casually, facial expressions look normal 

• if asked to produce expression intentionally, only the left hemisphere can generate voluntary facial expressions and the right side of face moves first 

agenesis of the corpus callosum (4)

• condition where corpus callosum doesn’t form in children

• occurs in 1:4000 live births with a variety of causes, genetic and non genetic

• often discovered as an incidental finding

• can either be complete (no corpus callosum at all) or partial (some fibre)

what remains normal in agenesis of the corpus callosum

language skills and IQ unless other abnormalities are present

comparing these children with split brain patient

they have minimal “disconnection syndrome”

why is that

plasticity in children allow alternative cross- hemispheric pathways to be reinforced

but what happens if the task requires very complex integration of information across hemisphere (e.g. comparing visually complex shapes across the midline and fast)

some impairment can be seen