Outline and evaluate hemispheric lateralisation and split brain research. (16 marks)
Hemispheric lateralisation is the idea that each hemisphere is functionally different, and that
certain mental processes and behaviours are mainly controlled by one hemisphere. Information is
communicated between the two hemispheres via the corpus callosum, allowing us to perform tasks
requiring both hemispheres.
Hemispheric lateralisation is demonstrated by Sperry’s split brain research, using
participants who had had their corpus callosums severed in order to control severe epileptic
seizures. Participants would focus on a central fixation point, and an image would be displayed for
one tenth of a second to one side; images presented in the right visual field would be processed by
the left hemisphere, and vice versa. Participants could describe images on the right, but not those on
the left, demonstrating that language is lateralised and controlled by the left hemisphere only. Also,
images on the right could be drawn with the right hand, and images on the left could be drawn with
the left hand, demonstrating how movement of the left side of the body is controlled by the right
hemisphere and vice versa.
A limitation of split brain research is that it may lack internal validity, meaning causal
connections drawn may be inaccurate. Sperry’s control group had intact corpus callosums but also
had no history of epilepsy, meaning they differed in two ways from the experimental group. This
means we cannot be sure whether differences between the groups are due to the lack of
communication between hemispheres, or neural changes as a result of previous seizures or epilepsy
medications. Furthermore, the study only had 11 participants, and this small sample may not reflect
the general population, weakening our understanding as it cannot be generalised universally.
A strength of lateralisation is that it makes evolutionary sense. Lateralisation enhances our
ability to multitask, and this would have been adaptive for hunters during the EEA; this theory is
supported by Rogers, who found that lateralisation in chickens allows them to find food and watch
for predators simultaneously. This demonstrates how valuable lateralisation is for our neural
processing capacity, strengthening the concept.
Modern neuroscientists argue that hemispheric lateralisation is limited, as it is not as ‘fixed’
as was once predicted. Brain plasticity can allow behaviours performed by one hemisphere to be
effectively performed by the other; for example split-brain patient JW learned to describe stimuli
presented to his left side despite the language centres being in the left hemisphere. Furthermore,
recovery after hemispherectomy for Rasmussen’s syndrome shows how effective plasticity can be.
Hemispheric lateralisation to some extent explains differences in functioning within the brain,
however advancements in technology – such as PET scans – that allow research into plasticity
suggests that the concept is only a partial explanation, weakening its validity.
Word count: 178 + 272
Hemispheric lateralisation is the idea that each hemisphere is functionally different, and that
certain mental processes and behaviours are mainly controlled by one hemisphere. Information is
communicated between the two hemispheres via the corpus callosum, allowing us to perform tasks
requiring both hemispheres.
Hemispheric lateralisation is demonstrated by Sperry’s split brain research, using
participants who had had their corpus callosums severed in order to control severe epileptic
seizures. Participants would focus on a central fixation point, and an image would be displayed for
one tenth of a second to one side; images presented in the right visual field would be processed by
the left hemisphere, and vice versa. Participants could describe images on the right, but not those on
the left, demonstrating that language is lateralised and controlled by the left hemisphere only. Also,
images on the right could be drawn with the right hand, and images on the left could be drawn with
the left hand, demonstrating how movement of the left side of the body is controlled by the right
hemisphere and vice versa.
A limitation of split brain research is that it may lack internal validity, meaning causal
connections drawn may be inaccurate. Sperry’s control group had intact corpus callosums but also
had no history of epilepsy, meaning they differed in two ways from the experimental group. This
means we cannot be sure whether differences between the groups are due to the lack of
communication between hemispheres, or neural changes as a result of previous seizures or epilepsy
medications. Furthermore, the study only had 11 participants, and this small sample may not reflect
the general population, weakening our understanding as it cannot be generalised universally.
A strength of lateralisation is that it makes evolutionary sense. Lateralisation enhances our
ability to multitask, and this would have been adaptive for hunters during the EEA; this theory is
supported by Rogers, who found that lateralisation in chickens allows them to find food and watch
for predators simultaneously. This demonstrates how valuable lateralisation is for our neural
processing capacity, strengthening the concept.
Modern neuroscientists argue that hemispheric lateralisation is limited, as it is not as ‘fixed’
as was once predicted. Brain plasticity can allow behaviours performed by one hemisphere to be
effectively performed by the other; for example split-brain patient JW learned to describe stimuli
presented to his left side despite the language centres being in the left hemisphere. Furthermore,
recovery after hemispherectomy for Rasmussen’s syndrome shows how effective plasticity can be.
Hemispheric lateralisation to some extent explains differences in functioning within the brain,
however advancements in technology – such as PET scans – that allow research into plasticity
suggests that the concept is only a partial explanation, weakening its validity.
Word count: 178 + 272
