Split brain Flashcards
Roger Sperry?
- Animal studies (cat, monkey; Myers & Sperry, 1958) demonstrated that sectioning the corpus callous & the anterior commissure profoundly limited the exchange of information between hemispheres (i.e. split-brain).
- The untrained hemisphere is ignorant of the task learned by the other hemisphere. Two mental systems co-habitating in one head?
Patient JW and experiment
51-year-old, right-handed Caucasian male, WWII veteran with seizures subsequent to war injuries (shot during a parachute jump behind enemy lines)
- He underwent a corpus callosotomy (transection of the corpus callosum connecting the hemispheres as treatment of intractable epilepsy).
- Experiment in which each hemisphere sees the object in the opposite visual field, and can make a response. Object (ie spoon) to the right goes to left hemisphere, vice versa.
- Short presentation ~150ms so can’t move eyes towards target voluntarily.
- The right hemisphere (which sees the object) cannot communicate with the left hemisphere (which can speak). So when you ask them to name the object they cannot. Part of how they discovered left is primarily language centre? look up
• Each hemisphere has its own perceptions, actions, concepts etc > each hemisphere responds independently > competition (usually one overrides the other).
So if present two different images to each hemisphere and ask to pick up the object seen with both hands, the right/left hands disagree, may pick up different objects or right hand may prevent left from performing the task.
Social ordinariness in split brain patients?
- In ordinary social situations the patients are indistinguishable from normal, except for certain memory problems?
- This is due to the eye movements compensating, allows input into both hemispheres.
what is the Corpus callosum
- Connected to prefrontal, premotor/motor, sensory, parietal, occipital, temporal areas.
- Genu, truncus, splenium (posterior) areas.
Cutting corpus callosum fibres
Gazzaniga, 2000
- If cut all white matter fibres except splenium, can still transfer visual information, but not tactile/motor etc.
- Posterior callosotomy (cut fibres connecting visual cortices and a bit more): still able to transfer sensorimotor information from R->L but not the reverse, transfer of attentional rescources and higher cognitive information.
- Full callosotomy: no sensorimotor transfer (but not totally true as anterior/post commisure) ie JW - presented SCRAPER to right hem and SKY to left, only drew sky.
- Spare fibres on each end: some transfer of information if words presented visually. ie. patients VP - presented words HEAD to right hem and STONE to left, could draw headstone.
- Partial callosotomy can still pass some info, ie knight presented to right hem, can give vague description and maybe work it out.
what is the anterior/posterior commissure?
•Mediates multisensory, inter-hemispheric messages of a complex nature. Even with full callosotomy may still have some integration processes preserved.
The anterior commissure (also known as the precommissure) is a bundle of nerve fibers (white matter), connecting the two temporal lobes of the cerebral hemispheres across the midline, and placed in front of the columns of the fornix. The great majority of fibers connecting the two hemispheres travel through the corpus callosum, which is over 10 times larger than the anterior commissure, and other routes of communication pass through the hippocampal commissure or, indirectly, via subcortical connections. Nevertheless, the anterior commissure is a significant pathway that can be clearly distinguished in the brains of all mammals.
What is controlled by left/right side of the brain?
- Left: Language, speech, writing, movement sequencing, math calculations, vision details, face processing, self recognition.
- Right: Visuospatial processing, overall configuration, attention, face recog of familiar faces, intentions of others.
describe visuospatial construction task
right hemisphere supiority:
- Asked to arrange blocks so they duplicate the pattern shown on a card.
- Split brain cannot duplicate with right hand, but can with left.
- therefore right hemisphere is superior at visuospacial processing.
Attention task
right hemisphere superiority:
- Visual search, look for items presented either on one side [left] (guided search) of screen or all over (standard search).
- Measured response time.
- Faster for a guided search in controls, but no difference for split brain.
- Therefore right hemisphere superior.
- Relationship to neglect and dmg to right hemisphere?
Face recognition
LH self, RH familiar faces:
- Prosoprognosia - inability to perceive faces.
- Presented faces of split brain patient JW and experimenter MG, morphed on a scale and presented to either LH/RH.
- Both hemispheres are capable of face recognition
- But LH shows recognition bias for self (able when 30% JW in LH, not until 70% JW for RH)
- RH shows bias for familiar faces ie MG (RH able 20% of MG, LH not until 70%).
- percentages not really important, just note that LH recognise much earlier in morph % than RH for self, opposite for familiar.
- Therefore LH dmg can’t recognise self in mirror, RH dmg can’t recognise others.
Racial bias
RH ORB:
- The own-race bias (ORB) in facial recognition is characterized by increased accuracy in recognition of individuals from one’s own racial group, relative to individuals from other racial groups.
- Delayed match-to-sample task for faces that varied both the race of the facial features (Caucasian/Japanese) and the cerebral hemisphere performing the task.
- Fixate on spot 500ms, presented with a face 1s, delay interval 5s, then had to identify which of two faces was seen, both presented on either R/L.
- JW: LH showed no effect of race on facial recognition, but RH showed significant advantage for Caucasian faces.
- Japanese 55-60% correct for L/R, a bit above chance.
- Caucasian 60% LH, 80% correct RH.
Understanding intentions of others
RH dominance:
- Mirror neurons, activated by goal of action.
- Key aspect of social interaction/cognition.
- 55yo woman was complete resection of corpus callosum due to epilepsy.
- Means inference task MIT (asking whether grasp was correct?) and Intention inference task IIT (if outcome is correct, ie pouring water into glass?).
- Present glass and bottle of water with an action performed on it?
- RH faster in IIT, inferring the intentions of the task, why it was done.
- LH faster in MIT, understanding how actions are performed.
Motor control patient DR
- Asked to make gestures, case DR.
- When asked to use right hand first, the left, was fine.
- When asked to use left first, then right, couldn’t do it.
- RH could only execute command when visually saw the model of requested posture.
Conscious/unconscious experience:
- Right side picture of leg of chicken one side, left side snow.
- Identify what they are and match to related pictures.
- When asked why, couldn’t verbally respond correctly and made up reason “the chicken claw goes with the chicken and you need a shovel to clean out the chicken shed”
- Therefore LH appears to have a special module that makes up an explanation as to why things happen as reported.
- RH can influence a LH specific response, even though LH is completely unaware it processes information?
Disconnected syndrome:
- Disconnection refers to the interruption of critical pathways between functional brain areas, resulting in specific behavioural syndromes.
- Disconnection syndromes are not due to damage to the brain area, but from damage to fibres between hemispheres/callosal fibres, fibres within hemispheres.
- Major disconnection syndromes: split-brain, conduction aphasia (poor speech repetition), ideomotor apraxia (no imitation of hand gestures & tool use), alien hand syndrome, neglect?
