hemispheric lateralisation Flashcards
localisation and lateralisation
On the previous spread you learned about localisation of brain areas. This refers to the fact that some functions, such as vision and language, are governed by very specific areas in the brain. You also learned that the brain is lateralised i.e. there are two sides which are called hemispheres (thus hemispheric 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.
left and right hemispheres
In the case of language, the two main centres are only in the LH (for most people) - Brocas area is in the left frontal lobe and Wernick’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. 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.
Many functions are not lateralised. For example 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 LH controls movement on the right.
In the case of vision the situation is even more complex - 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 (see diagram on right).
This enables the visual areas to compare the slightly different perspective from each eye and aids depth perception. There is a similar arrangement for auditory input to the auditory area and the disparity from the two inputs helps us locate the source of sounds.
split brain research
A ‘plit-brain operation involves severing the connections between the RH and LH, mainly the corpus callosum. This is a surgical procedure used to reduce epilepsy. During an epileptic seizure the brain experiences excessive electrical activity which travels from one hemisphere to the other. To reduce fits these connections are cut, ‘splitting’ the brain in two halves. Split-brain research studies how the hemispheres function when they can’t communicate with each other.
sperrys research
Roger Sperry (1968) devised a system ta study how two separated hemispheres deal with, for example, speech and vision.
Procedure Eleven people who had a split-brain operation were studied using a special set up (see left) 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 L (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.
Findings 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. This is 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.
Although participants could not give verbal labels to objects projected to the LVF, they could select a matching object out of sight (see picture) using their left hand (linked to RH). The left hand was also able to select an object that was most closely associated with an object presented to the LV for instance, an ashtray was selected in response to a picture of a cigarette).
If a pinup picture was shown to the LVF there was an emotional reaction (e.g. a giggle) but the ppts reported seeing nothing
Conclusions These observations show how certain functions are lateralised in the brain and support the view that the LH is verbal and the RH is ‘silent’ but emotional.
Strength-lateralisation in connected brain
One strength is research showing that even in connected brains the two hemispheres process information differently.
For example, 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.
limitation- one brain
One limitation is 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, i.e. not artist’s brain or mathematician’s brain.
This suggests that the notion of right- or left-brained people is wrong.
strength-Sperry-research support
One strength is 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.
l-sperry-generalisation issues
One limitation of Sperry’s research is that causal relationships are hard to establish.
The behaviour of Sperry’s split-brain participants was compared to a neurotypical control group.
An issue though is that none of the participants in the control group had epilepsy. This is a major confounding variable. Any differences that were observed between the two groups may be the result of the epilepsy rather than the split brain.
This means that some of the unique features of the split-brain participants’ cognitive abilities might have been due to their epilepsy (though Fink’s research, above, supports Sperry’s conclusions).
