5. Hemispheric lateralisation and split-brain research

Motor areas are contralateral

Contralateral= cross-wired

-LH controls the right side of the body

-RH controls the left side of the body

Visual fields are also contralateral

Info from the left side of both eyes travels to the RH.—> LVF is processed by the RH.

Info from the right side of both eyes travels to the LH.—> RVF is processed by the LH

Why can’t we say that visual fields are lateralised?

We can’t say “motor areas are lateralised”, as both hemispheres have motor cortices. However, we can say “processing of the LVF is lateralised to the RH, as only the RH processes info from the LVF.

What is Corpus Callosum?

A thin piece of tissue connecting the two hemispheres. (Function: allows L and R hemispheres to communicate)

What is Split brain patients?

A person whose corpus callosum has been cut, so two hemispheres can’t communicate with each other. Done to reduce epileptic fits.

Sperry Split-Brain Research: Speech

Procedure: 11 split brain individuals were studied. Image was projected to the LVF (processed by RH), patients were asked to verbalise what they saw. Procedure repeated for the RVF.

Findings: When object was shown to their LVF (processed by RH), they couldn’t name object.

When object was shown to their RVF (processed by the LH), they could verbally describe object

Explanation: because info in LVF is processed by RH, where there is no language centres. And as corpus callosum is cut, info can’t reach LH language centres, so patients can’t verbalise what they saw. And info in RVF is processed by the LH where the language centres are, so they can verbalise what they saw.

Sperry split-brain research: Tactile/touch/draw

Procedure: objects show to the LVF (processed by the RH), pp were asked to verbalise what objects were shown and also asked to select a matching object using left hand or draw the object with left hand.

Findings: pp could name them (as processed by RH which doesn’t have language centres), but could select a matching object using left hand/could draw object using left hand.

Explanation: cause info shown in LVF is going to RH which controls left side of the body, RH can “understand” what the object was and select/ draw it with left hand. (RH just can’t verbalise it)

Evaluations ( split brain reader h into hemispheric lateralisation)

Research support

Ev: research carried out further investigation with split-brain patients and found that the two hemispheres have different but complementary functions. LH was better at language processing, whereas the RH was better at visual-spatial tasks. Ex: findings replicate the Sperrys original research, demonstrating consistency across studies. L: high reliability, as similar findings have been repeatedly obtained.

Confounding variables

Ev: Sperrys split-brain patients were compared to a neurotypical control group, but all of the patients had severe epilepsy and had undergone surgery to sever the corpus callosum. Ex: makes it difficult to determine whether the differences found were caused by severing the corpus callosum or by the long-term effects of epilepsy itself. L: cause and effect relationships cannot be established, reducing the internal validity.

Lacks mundane realism

Ev: pp were required to stare at a fixed point while images were flashed briefly to one visual field, which is not how people normally process info in everyday life. Ex: the tasks were highly artificial and don’t reflect how the hemispheres operate during normal behaviour. L: findings may not generalise to real-world situations, reducing the external validity.