Hemispheric Lateralisation & Split-brain Research Flashcards
Lateralisation/ Localisation
1) The brain is lateralised i.e. two hemispheres.
2) Some functions are localised & appear in both left & right hems (LH & RH).
E.g. auditory, visual, motor, somatosensory appear in both whereas broca’s & wernicke’s are only in LH.
Contralateral & Ipsilateral
Ipsilateral –> structures on same side of the body or brain
Contralateral –> structures on opposite sides of the body.
1) Left visual field (LVF) of both eyes is connected to the RH & right visual field (RVF) of both eyes is connected to the LH.
2) Enables the visual areas to compare slightly different perspective from each eye & aids depth perception.
3) Same arrangement for auditory areas.
4) RH controls the left side of the body & vice versa (cross-wired).
What is Split-brain Research?
1) ‘Split-brain’ –> 2 hems surgically separated by cutting the connections e.g. corpus callosum.
2) Used to treat symptoms of severe epilepsy.
Sperry (1968) Split-brain Research
METHOD:
1) Studied 11 split-brain pps by having a word/image presented to LVF (processed by RH), or to RVF (processed by LH) –> presenting to one H meant the info isn’t transferred from that H to the other as corpus callosum has been cut.
2) Range of visual and tactile tests:
- Visual tests - Images were flashed into their LVF/RVF and were asked to name, describe or draw the objects.
- Tactile test - Objects were placed in either their left or right hand behind a screen.
Sperry (1968) - Findings
1) Presented to RVF:
- Can describe images and objects (language centres in LH).
2) Presented to LVF:
- Cannot name/describe image/object (no lang centres in RH).
- Can select matching object behind screen using left hand.
–> Demonstrates how certain functions are lateralised in the brain, shows that LH is dominant in terms of speech & lang (Broca & Wernicke) & RH is dominant in visual-motor tasks.
Methodological Evaluation
1) Sperry’s research was well controlled/designed & had standardised procedures - Also designed a procedure which could be replicated.
2) Were only 11 pps in the original study - small samples mean that these findings may not be able to be generalised to everyone, especially those who do not have a disorder such as epilepsy.
–> We cannot be certain that lateralisation occurs for everyone in the same way.
Limitation - Split-brain Research
CONFOUNDING VARIABLES
1) In Sperry’s research the behaviour of the s-b pps was compared to a neurotypical control group - BUT none of control group had epilepsy, any differences between the groups may be due to epilepsy not the split-brain - Hard to establish cause & effect.
–> Means some of the unique features of the s-b pps’ cognitive abilities might have been due to their epilepsy.
Strength - Hem Lateralisation
EVIDENCE OF LATERALISED BRAIN FUNCTIONS IN ‘NORMAL’ BRAINS
1) In PET scans, when ‘normal’ pps are shown a whole image, the RH is more active// When required to focus on finer detail, LH tend to dominate.
–> Suggests hemispheric lateralisation is a feature of normal brain as well as the split-brain.
Limitation - Hem Lateralisation
IDEA OF DOMINANT HEMISPHERES MAY BE WRONG
1) There may be different functions in the RH & LH but research suggest ppl don’t have a dominant side, creating a diff personality.
2) Nielson et al. analysed 1000 brain scans, finding ppl used certain hems for certain tasks but no dominance.
–> Suggests notion of right/left-brained ppl is wrong.
Evaluation - Lateralisation vs Plasticity
1) Lateralisation is adaptive, enabling 2 simultaneous tasks with greater efficiency.
2) On the other hand, neural plasticity is also adaptive - After damage to brain, language function can ‘switch sides’ (Holland et al.)
—> Suggests lateralisation is first preference but ultimately plasticity is more important because it deals with loss of lateralisation.
Research Evaluation - Szaflarski et al.
LATERALISATION CHANGES WITH AGE
1) Found lang became more lateralised to the LH with increasing age in children & adolescents, but after age of 25, lateralisation decreased.
2) One possibility for why is that using the extra processing resources of the other hem may in some way compensate for age-related declines in function.
—> This implies that a lateralised brain is in fact only a feature of young adults and not true for everyone.
Research Evaluation - Gazzaniga
LANGUAGE MAY NOT BE RESTRICTED TO LH
1) Some early discoveries from split-brain research have been disconfirmed by more recent discoveries.
E.g. split brain research had suggested that the RH was unable to handle lang at all. Damage to the LH was found to be far more detrimental to lang function than was damage to the right.
2) BUT, case studies (on plasticity) have demonstrated that this was not necessarily the case.
E.g. A patient, known as J.W. who had damaged his LH, developed the capacity to speak out of the RH about info presented to the left or to the right brain (Turk et al).
