Discuss research into hemispheric lateralisation. 16 marks
Hemispheric lateralisation refers to each hemisphere of the brain having a different function.
Research into hemispheric lateralisation is normally done on severe epilepsy patients who have had
there corpus callosum (bundle of nerve fibres joining the two hemispheres) severed to reduce their
seizures. Sperry’s research used epilepsy patients who had had this procedure as because of the
severed corpus callosum, the hemispheres of the brain can’t communicate and so we can see each
hemisphere working independently. Sperry’s participants were placed very close to a screen with a
focus point for them to look at in the middle of it. The words ‘key’ and ‘ring’ were displayed
simultaneously on the left and right sides of the screen, respectively, for 1/10 th of a second. The
participants were then asked to say what they had seen and pick up what they had seen with their
left hand. All participants said only ‘ring’, and only picked up the key. Sperry concluded this was
because language is lateralised to the left hemisphere, so they could only verbalise what they had
seen in their right visual field (as this is controlled by the left hemisphere), meaning they could only
verbalise ‘ring’. As spatial awareness and the movement of the left side of the body are both
controlled by the right hemisphere, they only picked up what they had seen in the left visual field
(controlled by the right hemisphere). Sperry concluded that because of the severed corpus callosum,
each hemisphere was working independently and couldn’t integrate information, so we can see that
language is lateralised to the left, and spatial awareness to the right.
A strength of Sperry’s research is that it used scientific methodology. It was conducted in a
laboratory environment, where extraneous variables were controlled for and standardised control
methods put in place, such as the words being flashed for only 1/10 th of a second and the focus point
on the screen so that the participants had no time to move their eyes, and couldn’t process the
information through both visual fields. Because of these standardised procedures, the experiment is
replicable, meaning reliability can be found in the findings. Furthermore, the control measures
meant the study had a high internal validity, which is a strength of this reseach’s methodology, and
useful when studying hemispheric lateralisation.
A limitation of research into hemispheric lateralisation however, is that it has low population
validity. The research, including sperry’s experiment, is only done on severe epileptic patients who
have had their corpus callosum severed, of which there are very few, often leading to small sample
sizes in the research, which lowers the generalisability. Furthermore, the extent to which the corpus
callosum has been severed differs from patient to patient, with some having it only partially cut and
others completely, greatly differing in the amount of inter hemispheric communication the
participants have. This could have a large effect on our findings, lowering the internal validity.
Finally, there has been evidence of epilepsy having an effect on brain functions, meaning that the
brains of epileptic patients could already be different to that of neurotypicals, further lowering the
generalisability of the studies. This is a limitation as it means sperry’s findings as well as others into
hemispheric lateralisation can’t be generalised to the wider population, giving us very low
population validity.
Contradictory research by Turk et al limits sperry’s findings into hemispheric lateralisation. Turk et
al’s case study on JW found that after damage to the left side of his brain, he was able to use the
right hemisphere of his brain to talk. This supports the idea of plasticity, as neuroreorganisation
occurs as the language centres in the left hemisphere had their roles taken up by the right
hemisphere, for example Broca’s area in the left frontal lobe taken up by the corresponding area in
the right hemisphere. This supports equipotentiality, the idea that AK areas of the brain have the
ability to complete the functions of other areas, and contradicts the concepts of hemispheric
lateralisation.
Hemispheric lateralisation refers to each hemisphere of the brain having a different function.
Research into hemispheric lateralisation is normally done on severe epilepsy patients who have had
there corpus callosum (bundle of nerve fibres joining the two hemispheres) severed to reduce their
seizures. Sperry’s research used epilepsy patients who had had this procedure as because of the
severed corpus callosum, the hemispheres of the brain can’t communicate and so we can see each
hemisphere working independently. Sperry’s participants were placed very close to a screen with a
focus point for them to look at in the middle of it. The words ‘key’ and ‘ring’ were displayed
simultaneously on the left and right sides of the screen, respectively, for 1/10 th of a second. The
participants were then asked to say what they had seen and pick up what they had seen with their
left hand. All participants said only ‘ring’, and only picked up the key. Sperry concluded this was
because language is lateralised to the left hemisphere, so they could only verbalise what they had
seen in their right visual field (as this is controlled by the left hemisphere), meaning they could only
verbalise ‘ring’. As spatial awareness and the movement of the left side of the body are both
controlled by the right hemisphere, they only picked up what they had seen in the left visual field
(controlled by the right hemisphere). Sperry concluded that because of the severed corpus callosum,
each hemisphere was working independently and couldn’t integrate information, so we can see that
language is lateralised to the left, and spatial awareness to the right.
A strength of Sperry’s research is that it used scientific methodology. It was conducted in a
laboratory environment, where extraneous variables were controlled for and standardised control
methods put in place, such as the words being flashed for only 1/10 th of a second and the focus point
on the screen so that the participants had no time to move their eyes, and couldn’t process the
information through both visual fields. Because of these standardised procedures, the experiment is
replicable, meaning reliability can be found in the findings. Furthermore, the control measures
meant the study had a high internal validity, which is a strength of this reseach’s methodology, and
useful when studying hemispheric lateralisation.
A limitation of research into hemispheric lateralisation however, is that it has low population
validity. The research, including sperry’s experiment, is only done on severe epileptic patients who
have had their corpus callosum severed, of which there are very few, often leading to small sample
sizes in the research, which lowers the generalisability. Furthermore, the extent to which the corpus
callosum has been severed differs from patient to patient, with some having it only partially cut and
others completely, greatly differing in the amount of inter hemispheric communication the
participants have. This could have a large effect on our findings, lowering the internal validity.
Finally, there has been evidence of epilepsy having an effect on brain functions, meaning that the
brains of epileptic patients could already be different to that of neurotypicals, further lowering the
generalisability of the studies. This is a limitation as it means sperry’s findings as well as others into
hemispheric lateralisation can’t be generalised to the wider population, giving us very low
population validity.
Contradictory research by Turk et al limits sperry’s findings into hemispheric lateralisation. Turk et
al’s case study on JW found that after damage to the left side of his brain, he was able to use the
right hemisphere of his brain to talk. This supports the idea of plasticity, as neuroreorganisation
occurs as the language centres in the left hemisphere had their roles taken up by the right
hemisphere, for example Broca’s area in the left frontal lobe taken up by the corresponding area in
the right hemisphere. This supports equipotentiality, the idea that AK areas of the brain have the
ability to complete the functions of other areas, and contradicts the concepts of hemispheric
lateralisation.
