Split Brain Research Flashcards
Split brain:
an operation used in some cases of servere epilepsy; the technical term for the operation is ‘commissurotomy’
Divided field:
technique devised by Sperry to present visual stimuli to either left or right hemisphere in his studies of split-brain patients; can also be used with neurotypical participants
Brain surgery and psychology reserach on epilepsy:
-patient HM -> studied after surgery caused profound amnesia
-split brain research -> examined effects of separating brain hemispheres
Epilepsy overview:
-a condition caused by uncontrolled electrical activity in the brain
-symptoms: convulsions and loss of consciousness (severe cases), milder cases often controlled with medication
-causes: sometimes unclear but can include: an imbalance in brain’s excitatory and inhibitory processes, scar tissue, brain damage, or previous surgeries (referred to as focus)
-surgical options: if a focus for epilepsy is identified, it can be possible to surgically remove it (as seen in the case of HM) if case is severe and disabling, however, if a focus cannot be identified, or it is in part of the brain that can’t be safely reached surgically, then another operation is possible
Epilepsy and split-brain:
-the two hemispheres of the brain are connected by the corpus callosum -> a bundle of 200-300 million fibres
-besides its normal functions, the corpus callosum also allows epilsepy discharges to travel down from one hemisphere to the other
-this means epilepsy can involve the whole brain
-in the 1940s, an operation was devised to cut the corpus callosum, preventing the epileptic discharges involving both hemispheres, and so reducing the severity of the attack
-this operation was called a commissurotomy as it was cutting a pathway connecting two hemispheres
-this operation was not very successful at reducing the symptoms of epielpsy, however, patients did not suffer ill effects from the operation, which was surprising as they had lost one of the major pathways in the brain
Sperry and the split brain:
-in the 1950s, Roger Sperry studied split-brain patients to explore brain hemispheres functions
-his earlier work with monkeys showed that both hemispheres have similar behavioural capacities, such as learning simple tasks
Human hemishphere functions:
-in humans, key functions like language are typically lateralised to the left hemisphere, which also controls the right hand
-the left hemisphere is considered dominant, while the right is seen as a minor partner
Split-brain patients and research challenges:
-Sperry saw split-brain patients (post-commissurotomy) as an opportunity to study hemisphere specialisation in humans
-a challenge arose because each eye sends information to both hemispheres, even when the hemispheres are separated
-Sperry had to design a method for projecting stimuli to each hemisphere separately
-based on earlier research into the visual systems of non-human animals,and he was able to apply his knowledge of visual systems to the problem of the split brain patient
-he devised an experimental procedure known as the divided field
Split brain information:
-divided field is based on the organisation of the human visual system, and the systematic arrangement of the visual pathways from each eye to the hemispheres
-note that the right side of the right eye and the right side of the left eye both connect to the right hemisphere; similarly to the left side of the left eye and the left side of the right eye both connect to the left hemisphere
-Sperry realised that with a split-brain patient focusing their eyes straight ahead, a stimulus presented out to their right (called right visual field or RVF) would be regstered only by the left side of each eye
-the left side of the eye projects to the left himpshere so the stimulus would be transmitted to the left hemisphere
-similarly, a sitmulus presented out to the left of the patient would be registered by the right side of each eye and projected to the right hemisphere
Split-brain procedure:
-in the intact human participant, a stimulus presented either to the right or the left hemisphere would immediatelt be conducted across the corpus callosum to the other hemisphere, so both hemispheres would be aware of the stimulus
-in split-brain patients, the corpus callosum has been cut, so a stimulus sent to the right hemisphere cannot be transmitted to the left hemisphere, and is effectively confined to the right hemisphere
-in this way, Sperry knew that he could present stimuli to each hemisphere separately by presenting them in either right or left visual field
-one limitation to the procedure was the natural tendency for the paticipant to move their eyes towards the stimulus, if the eyes move too much the stimulus is likely to be picked up by both hemispheres
-to prevent this happening, the stimulus could only be presented for a very brief period of time, for example around 200 miliseconds
-this meant in effect that Sperry could only present single words or pictures, even so, he was able to demonstrate some dramatic effecs in these early experimens
Key: study Sperry
1965
-Sperry studied split brain patients (with severed corpus callosum) to see how the brain’s hemispheres function independtly
-he found the left hemisphere controls language and speech, while the right hemisphere handle spatial and visual tasks
-this proved the brain is lateralized, with each side having specialized functions
research findings with the split brain- visual processing:
-Sperry’ss study reinforced the idea that language was mainly a left hemisphere function
-what really changed the view on lateralisation of functions across the hemishperes were the findings were the findings of studies using non-bverbal stimuli
-Sperry and his collaborators (Gazzaniga, 2005) repeated the divided field study with split-brain patients, but this time using faces as the simuli rather than words
-in one such study, a different face was presented to each hemisphere at the same time, then the split brain patient was fiven a set of faces, including the ones presented, and asked to choose the one that they had seen earlier
-they would choose the one presented to the the right hemisphere
-in a series of similar studies, Sperry demonstrated that the right hemisphere was better at identifying faces than the left hemisphere
-he was also able to show that the right hemisphere was better at matching shapes, and in general the right hemisphere showed a superiority over the left hemisphere in what we call visualspatial tasks
-so, one result of the splitbrain studies was a change in the way we viewed the two hemispheres
-no longer did we have a dominant right hemisphere and a minor left hemisphere, but instead we have a verbal left hemisphere and a visuospatial right hemisphere
Key study: Turk et al.
2002
-Turk et al- studied a split-brain patient (JW) to explore the right hemisphere’s role in language
-unlike Sperry’s findings, JW’s right hemisphere could process written words and even produce speech, suggesting that language abilities are not always limited to the left hemisphere
-this challenged the idea of strict lateralization in language processing
evaluation of split-brain research:
-the word of Sperry and others on split-brain patients patients was groundbreaking and changed our views in hemispheric function
-however, there are considerable issues with split-brain research:
-there are very few of these patients, and only between 10 and 15 have been subjected to extensive systematic study, this is a very small sample size
-those studied are an extremly varied group, they differ in age and sometimes gender and handedness, age at which they develop epilepsy, age at which they had the sommissurotomy, and age at which they were tested
-their operations were not always comparable, besides the corpus callosum, there are smaller pathways connecting the two hemispheres, such as the anterior commissure
-in some cases this was cut along with the corpus callosum, but in other cases it was left intact, possibly allowing for some communication between hemispheres
2.0 evaluation of split brain research:
-given these issues, we cannot be confident in building a model of hemispheric lateralisation using only split-brain research
-there is confidence behind language lateralisation because of the extensive case studies on language impariment after brain damage
-additionally, since Sperry’s word in the 1960s and 1970s, his techniques have been modified for use with intact (neurotypical) patients
-using Sperry’s divided field, the neurotypical participants is presented with two stimuli at the same time, one to each hemisphere
-the stimulus in the RVF is transmitted to the left hemisphere, while the sitmulus in the LVF is transmitted to the right hemisphere
-with brief presentation, about 200 ms, the participant will usually report only one of the stimuli
-if the stimuli are words, the one presented in the RVF is the one most likely to be reprted; this is called a right visual field advantage for words
-if the stimuli are faces or drawings, then the one presented in the LVF is most likely to be reported; this is called a left visual field advantage for visuospatial stimuli
split brain research overview:
-stimulated by split-brain research, findings from a variety of studies have lef to a general model of hemispheric specialisation
-the left hemisphere is seen as verbal and the right hemisphere as visuospatial
-to process language we need to break down incoming speech into separate words spread out over a stime interval (e.g. listening as a friend talks to you)
-it is only at the end of a sentence that we put everything together to understand what was said
-therefore the left hemisphere is also seen as analytical, working best when stimuli need to be broken down into their components parts
-the right hemisphere is better at processing faces, faces are not taken in bit by bit, but usually as one whole stimulus. therefore the right hemisphere is seen as holistic or Gestalt processor
