The aim of this experiment was to investigate the effects of hemisphere
deconnection and to show that each hemisphere has different functions.
The participants were 11 ‘split-brain’
patients, that is, they were patients who had undergone disconnection of the
cerebral hemispheres. The participants had all undergone hemisphere
deconnection because they had a history of advanced epilepsy which could not
be controlled by medication.
Sperry used a number of ingenious tasks
in order to investigate lateralisation of brain function.
One of the tasks used to send information
to just one hemisphere involved asking participants to respond to visual
information. This involved blindfolding one of the participant’s eyes and
then asking them to fixate with the seeing eye on a point in the middle of a
screen. The researchers would then project a stimulus on either the left or
right hand side of the fixation point for less than 1/10 of a second. The
presentation time is so small to ensure that the participant does not have
time for eye movement as this would ‘spread’ the information across both
sides of the visual field and therefore across both sides of the brain.
As language is processed in the left
hemisphere, when a stimulus is presented to the left visual field of a
split-brain patient they should not be able to name the stimulus.
Below is a summary of some of the main
participants were presented with an image in one half of their visual field
and then presented with the same image in the other half of the visual field
they responded as if they had never seen the image before. If the same
image was presented in the original visual field the participants were able
to recognise the image as one they had seen before.
were not able to give a description of an image that was presented to the
left hand side of the visual field. The image was either not noticed or
just appeared as a flash. Although they could respond non-verbally by
pointing with their left hand to a matching picture or selecting an object
presented among a collection of other pictures and objects. This of course
only works with right-handed participants.
symbols were presented simultaneously, one on either side of the visual
field (e.g. a dollar sign on the left and a question mark on the right) and
the participant was required to draw with their left-hand (shielded from
their own view) what they had seen, they would draw the left visual field
symbol (a dollar sign). If they were required to say what they had just
drawn, the participant would say by name, the right visual field symbol (a
in the participants hand for identification by touch could be described or
named in speech or writing if they were in the right hand but if placed in
the left hand, the participant could either only make wild guesses or even
appeared to be unaware that anything at all was present. However, if the
object was taken from the left hand and placed in a ‘grab bag’, or was
scrambled among other test items, the participant was able to search out and
retrieve it with their left hand.
interesting example of lateralisation of function is when two different
objects were placed in each hand at the same time and then removed and
hidden for retrieval in a scrambled pile of test items. Each hand hunted
through the pile and searched out its own object. During the search each
hand was seen to explore, identify and reject the item for which the other
hand was searching. Although the performance of ‘normal’ participants would
be slowed down by the competing demands of the tasks, the people with
hemisphere deconnection could actually perform these double tasks in
parallel, as quickly as they could perform one of the tasks on its own. It
is worth noting though that even though Sperry showed that split-brain
patients were better at completing such highly unusual tasks that this would
have no advantage in the real world.
Sperry argued that his studies give
considerable support to his argument of lateralisation of function. That is,
that different areas of the brain specialise on different tasks, such as the
left hand side being responsible for language.
He also went on to argue that each
hemisphere has its own perceptions and memories and experiences.