FoundationalConceptsNeuroscience

appeared to have little impact on the animal’s behavior, leading neu-
rosurgeons to believe that the callosotomy procedure could be safely
performed on humans. And indeed, superficially it is hard to tell that
there is anything amiss with a split-brain patient. But if you look care-
fully, as Sperry did, some dramatic effects of the surgery are revealed.
Sperry’s experimental setup was very simple. The patient sat in
a chair and looked at the center of a small screen. Onto the screen a
projector would briefly flash pictures or words into either the patient’s
left or right visual field. If the picture is shown in the left visual field,
then that visual information goes to the right hemisphere of the brain
(Fig. 18.3). Ina normal person, the information associated with the
visual stimulus is rapidly relayed across the corpus callosum and is
available to both hemispheres after a very brief period of time. How-
ever, in a split-brain patient, the visual information is only in the
hemisphere contralateral to where the picture had been flashed and is
not shared between hemispheres.
Now consider the following experiment. A picture of a spoon is
flashed into the split-brain patient’s right visual field. The visual in-
formation goes to the person’s left cerebral hemisphere. The patient
is asked to report what he saw on the screen. He answers correctly,
saying he saw a spoon. However, if the picture of a spoon is flashed
into the patient’s left visual field and he is asked to report what he saw,
he can’t say. He may simply answer with “I don’t know.” This finding
is consistent with language expression lateralized to the left hemi-
sphere in most people. Even though the person may have known that
he saw a spoon, he can’t utter the word.