hemispheric lateralisation & split-brain research Flashcards
what is hemispheric lateralisation?
The idea that the two halves (hemispheres) of the brain are functionally different and that certain mental processes and behaviours are mainly controlled by one hemisphere rather than the other, as in the example of language (which is localised as well as lateralised).
what is split-brain research?
A series of studies which began in the 1960s (and are still ongoing) involving people with epilepsy who had experienced a surgical separation of the hemispheres of their brain to reduce the severity of their epilepsy. This enabled researchers to test lateral functions of the brain in isolation.
localisation & lateralisation
For some functions the localised areas appear in both hemispheres. For example in the case of vision, the visual area is in the left and right occipital lobe - located in the left hemisphere (LH) and right hemisphere (RH) respectively.
is language lateralised?
In the case of language, the two main centres are only in the LH (for most people) - Broca’s area is in the left frontal lobe and Wernicke’s area is in the left temporal lobe. So we say that language is lateralised - that is, performed by one hemisphere rather than the other.
what can the right hemisphere produce?
The RH can only produce rudimentary words and phrases but contributes emotional context to what is being said. This has led to the suggestion that the LH is the analyser whilst the RH is the synthesiser.
are all functions lateralised?
no as vision, motor and somatosensory areas appear in both hemispheres. But there is a further twist, in the case of the motor area the brain is cross-wired (contralateral wiring) - the RH controls movement on the left side of the body whilst the L controls movement on the right.
what happens with vision?
it is both contralateral and ipsilateral (opposite and same-sided). Each eye receives light from the left visual field (LVF) and the right visual field (RVH). The LVF of both eyes is connected to the RH and the RVF of both eyes is connected to the LH. This enables the visual areas to compare the slightly different perspective from each eye and aids depth perception.
what happens with auditory?
similar to vision, for auditory input to the auditory area and the disparity from the two inputs helps us locate the source of sounds.
what is the lateralisation in the connected brain strength?
-research shows that even in connected brains the two hemispheres process information differently.
-For e.g, Gereon Fink et al. (1996) used PET scans to identify which brain areas were active during a visual processing task. -When participants with connected brains were asked to attend to global elements of an image (such as looking at a picture of a whole forest) regions of the RH were much more active. When required to focus in on the finer detail (such as individual trees) the specific areas of the LH tended to dominate.
-This suggests that, at least as far as visual processing is concerned, hemispheric lateralisation is a feature of the connected brain as well as the split-brain.
what is the one brain limitation?
-the idea that the LH as analyser and RH as synthesiser may be wrong.
-There may be different functions in the RH and LH, but research suggests people do not have a dominant side of their brain which creates a different personality. Jared -Nielsen et al. (2013) analysed brain scans from over 1000 people aged 7 to 29 years and did find that people used certain hemispheres for certain tasks (evidence for lateralisation). But there was no evidence of a dominant side.
-This suggests that the notion of right- or left-brained people is wrong.
what is the lateralisation vs plasticity strength?
-strength as lateralisation is adaptive.
-Lateralisation is adaptive as it enables two tasks to be performed simultaneously with greater efficiency.
-Lesley Rogers et al. (2004) showed that lateralised chickens could find food while watching for predators but ‘normal chickens couldn’t.
-On the other hand, neural plasticity could also be seen as adaptive.
-Following damage through illness or trauma, some functions can be taken over by non-specialised areas in the opposite hemisphere.
-For instance, language function can literally ‘switch sides’ (Holland et al. 1996).
what is a ‘split-brain’ operation?
A ‘split-brain’ operation involves severing the connections between the RH and LH, mainly the corpus callosum. This is a surgical procedure used to reduce epilepsy. To reduce fits these connections are cut, ‘splitting the brain in two halves.
what was Sperry’s 1968 research?
11 people who had a split-brain operation were studied using a special set up in which an image could be projected to a participant’s RV (processed by the LH) and the same, or different, image could be projected to the LVF (processed by the RH). In the ‘normal’ brain, the corpus callosum would immediately share the information between both hemispheres giving a complete picture of the visual world. However, presenting the image to one hemisphere of a split-brain participant meant that the information cannot be conveyed from that hemisphere to the other.
what was the findings from Sperry’s research?
-When a picture of an object was shown to a participant’s RVF (linked to LH), the participant could describe what was seen. But they could not do this if the object was shown to the LVF (RH) - they said there was ‘nothing there’. because, in the connected brain, messages from the RH are relayed to the language centres in the LH, but this is not possible in the split-brain.
-could not give verbal answers projected to LVF they could select a matching object out of sight using their left hand (linked to RH).
-left hand also able to select an object that was most closely associated with an object presented to the LVF.
If a pinup picture was shown to the LVF there was an emotional reaction (giggle) but the participants usually reported seeing nothing or just a flash of light.
what is the research support strength?
-support from more recent split-brain research.
-Michael Gazzaniga (Luck et al. 1989) showed that split-brain participants actually perform better than connected controls on certain tasks.
-For example, they were faster at identifying the odd one out in an array of similar objects than normal controls. In the normal brain, the LH’s better cognitive strategies are ‘watered down’ by the inferior RH (Kingstone et al. 1995).
-This supports Sperry’s earlier findings that the ‘left brain’ and ‘right brain’ are distinct.
