Hemispheric lateralisation and split-brain research

Is language lateralised or localised?

Is language lateralised or localised?

Lateralised- as it is performed in the left hemisphere. Broca’s area is in the left frontal lobe and Wernicke’s area is in the left temporal lobe

The right hemisphere can only produce rudimentary words and phrases but contributes emotional context to what is being said. LH may be the analyser, but RH is the synthesiser.

Explain how the motor area works (wiring)

Right hemisphere controls the left side of the body and vice versa. It is cross wired (contralateral wiring).

Explain how vision works (wiring)

Wiring is both contralateral and ipsalateral (opposite and same-sided).

Each eye receives light from the LVF and the RVF. 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 is split-brain research?

A split-brain patient has had their two hemispheres are surgically separated (commissurotomy) by cutting the connections e.g. the corpus callosum.

Used to treat severe epilepsy to reduce the ‘electrical storm’ across hemispheres. Split-brain research (like Sperry) studies how the hemispheres function when they can’t communicate with each other.

Sperry procedure

11 split-brain participants were studied using a special setup in which participants were asked to look at a fixation point and an image could be projected to a participant’s RVF (processed by LH), and the same, or a different, image could be projected to the LVF (processed by RH).

Presenting the image to one hemisphere meant that the information could not be conveyed from that hemisphere to the other.

Sperry findings

Object shown to RVF

• Participant can describe what is seen, as their language centres are in their LH

Object shown to LVF

• Cannot name object, as their are no language centres in RH

• Can select matching object behind screen using their left land

• Can select object closely associated with picture (e.g. ashtray for photo of cigarette)

• When a pinup photo was shown to the LVF, the participant giggled (emotional reaction) but reported seeing nothing.

Sperry conclusions

This study demonstrates how certain functions are lateralised in the brain and support the view that the LH is verbal and the RH is ‘silent’ but emotional.

AO3: Strength: (Lateralisation): evidence of lateralised brain functions in ‘normal’ brains

PET scans show when ‘normal’ participants attend to global elements of an image, the RH is more active. When required to focus on finer details, the specific areas of the LH tend to dominate.

This suggests that hemispheric lateralisation is a feature of the normal brain as well as the split-brain

AO3: Limitation: (Lateralisation): Idea of analyser vs synthesiser brain may be wrong

There may be different functions in the RH and LH but research suggests people do not have a dominant side, creating a different personality. Nielsen analysed 1000 brain scans, finding people did use certain hemispheres for certain tasks but no dominance.

This suggests that the notion of right or left brained people is wrong (e.g. ‘artist’ brain)

AO3: Strength: (Sperry): Support from more recent split-brain studies

Luck showed that split-brain participants are better than normal controls (twice as fast at identifying the odd one out in an array of similar objects). In the normal brain, the LH’s superior processing abilities are ‘watered down’ by the inferior right hemisphere.

This supports Sperry’s earlier findings that the ‘left brain’ and ‘right brain’ are distinct in terms of functions and abilities.

AO3: Limitation: (Sperry): Causal relationships are hard to establish

In Sperry’s research, the behaviour of the split-brain participants was compared to a neurotypical control group. However, none of the control group had epilepsy. Any differences between the groups may be due to epilepsy not the split-brain (confounding variable).

This means that some of the unique features of the split-brain participants’ cognitive abilities might have been due to their epilepsy.