Hemispheric lateralisation and split brain research

Localisation and lateralisation

The brain is lateralised. (There are two sides which are called hemispheres ‘hemispheric lateralisation’)

 

• The ideas that the 2 halves of the brain are functionally different.

• Langaueg is lateralised to the left hemisphere, facial recognition is lateralised to the right hemisphere.

 

For some functions the localised areas appear in both hemispheres.

 

For example, the visual centre is in the left and right occipital love in the left hemisphere (LH) and thr right hemisphere (RH) respectively.

Left and right hemispheres

In the case of langauge, the two main centres are only in the LH (for most people) - Broca’s area is in the left frontal lobe and Wernickes 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 redimntaey words and phrases but contributes emotional content to what is being said. (This has led to the suggestion that the LH is the analyser whilst the RH is the synthesiser.)

Both hemispheres

Many functions are not lateralised - for example vision, motor and somatosensory centres appear in both hemispheres.

 

But there is a further twist in the case of the motor centre in 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.

Vision

In the case of vision the situation is even more complex - it is both contralateral and ipsilateral (opposite and same-sided). Each side recovers 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 centres to compare the slightly different perspective from each eye and aids depth perception.

 

There is a similar arrangement for auditory input to the auditory centre, the disparity from the two inputs helps us locate the source of sounds.

Hemispheric lateralisation - STRENGTH

Lateralisation in the connected brain

 

One strength is research support showing that even  in connected brains the two hemispheres process information differently.

 

For example, Fink (1996) used PET scans to identify which brain areas were active during a visual processing task.

 

When participants with connected brains where asked to attend to global elements of an image (such as looking at a pictyre of a whole forest), regions of the RH where 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.

Hemispheric lateralisation - WEAKNESS

Age and lateralisation

 

One limitation of lateralisation is that it may decline as we get older.

 

Szaflarski and colleuges (2006) used fMRI to asses brain activity during a verb - generation task in 170 healthy right haded participants aged 5 to 67 years.

 

The findings revealed that left hemisphere dominance for language increases from ages 5 to 20, stablises between 20 and 25 and gradually decreases from 25 onwards.

 

This suggests, at least for language, that there is a shift towards bilateral representation, possibly to compensate for age-related cognitive changes

Split brain research

• Commisurotomy (cutting the corpus callosum)

 

A ‘splint brain’ operation involves severing the nerve (the corpus callosum) that connect the RH and LH. This sugfrical procedure is a treatment for epilepsy because it reduces the characteristic ‘electrical storm’ across hemispheres.

 

Split - brain research studies how the hemispheres function when they can’t communicate with each other.

 

Sperry’s research - Nobel prize winner 1981

 

Roger Sperry (1968) devised a system to study how two separated hemispheres deal with, for example, speech and vision.

SB procedure

Procedure: eleven people who has a split brain operation were studied using a special set up in which an image could be projected to a participants RVF (processed by the LH) and the same or different image could be projected to the LVF (processed by the RH)

 

In the neurotypical brain, the corpus callosum would share the information between both hemispheres. However presenting the image to one hemisphere of a split brain participant meant that the information cannot be converted from that hemisphere to the other.

SB findings

when an image (word or picture) was shown to a participants RVF (linked to the LH), the participants could describe what was seen. But they  could not do this if the image 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 langauge centres in the LH, but this is not possible in the split brain.

 

Although participants could not give verbal labels to images projected to the LVF, they could select a matching object out of sight using their left hand (linked to RH).

 

The left hadn was also able to select an object that was most closely associated with an image presented to the LVF (for instance, an ashtray was selected in response to a a picture of a cigarette, or the word ‘cigarette’.

 

If a pin up picture was shown to the LVF there was an emotional reaction ( a giggle) but the participants usually reported seeing nothing or just a flash of light.

Feilds

• RIGHT VISUAL FIELD: when an image was projected to the right half of the visual field (and therefore the left hemisphere), it could be described normally.

• LEFT VISUAL FIELD: when it was projected to the left half of the visual field, they insisted they did not see anything and that there was only a flash of light on the left side.

 

BUT if asked to find what they just saw with their left hand, they immoderately found the object they just insisted they had not seen.

 

Conclusions: these observations show how certain functions are laterlisated in the brain and support the view that the LH is verbal and the RH is ‘silent’ but emotional.

Split brain a03 - strengths

Research support

 

One strength is research support fron more recent split-brain research.

 

Steven Luck and colleuges (1989) showed that split- brain participants usuallu perform better than connected controls on certai tasks.

 

For example, they were faster than controls at identifying the odd one out in an array of similar objects.

 

In the connected 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 the ‘right brain’ are distinct.

 

• High control and standardised (prevents P’s from taking in information into both hemispheres)

Split brain a03 - weaknesses

Generalisation issues

 

One limitation of Sperry’s split brain participants was compared to a neurotypical control group.

 

However, none of the participants in the control group had epilepsy, which is a major extraneous variable.

 

Any differences that were observed between th two groups may be the result of the eplisepsy rather than the split brain.

 

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