3. hemispheric lateralisation Flashcards
The brain is lateralised - there are two sides of the brain which are called
hemispheres.
For some functions, the localised areas appear in both hemispheres.
In the case of language, the two main centres are only in the LH (for most people) - Broca’s area is in the x and Wernicke’s area is in the y - language is lateralised (performed by one hemisphere rather than the other).
x = left frontal lobe
y = left temporal lobe
Many functions are not lateralised. For example, vision, motor and somatosensory areas appear in both hemispheres.
· But 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 eves is connected to the LH
This enables the visual areas to compare the slightly different perspective from each eye and aids depth perception.
AO3: strength of hemispheric laterlisaton
RESEARCH SUPPORT - Fink et al
One strength is research showing that even in connected brains the two hemispheres process information differently.
For example, Gereon Fink et.al. 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.
AO3: limitation of hemispheric laterlisaton
ONE BRAIN - Nielsen
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. Nielsen et al. 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.
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. 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.
SPERRY’S RESEARCH
Sperry devised a system to study how two separated hemispheres deal with speed and vision.
PROCEDURE:
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 RVF (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.
SPERRY CONCLUSIONS: 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.
FINDINGS: when a picture of an object was shown to a participant’s RVF (linked to LH), the participant could x. But they could not do this if the object was shown to the LF (RH) - they said there was ‘nothing there’.
This is because, y
Although participants could not give verbal labels to objects projected to the LVF, they could select a matching object out of sight using their z. The left hand was also able to select an object that was most closely associated with an object presented to the LVF (for instance, an ashtray was selected in response to a picture of a cigarette).
x = describe what was seen
y = 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.
z = left hand (linked to RH).
AO3: strength of split brain research
RESEARCH SUPPORT - Gazzaniga
One strength is support from more recent split- brain research.
Gazzaniga 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’.
This supports Sperry’s earlier findings that the ‘left brain’ and right brain’ are distinct.
AO3: limitation of split brain research
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.
