and once using his right hand. The drawing to be copied is presented
on a piece of paper, and the patient can look at it for as long as he
wants and continue looking while doing the copying. He is moving
his eyes and head around while looking at the drawing, so the vis-
ual information is going to both sides of his brain, to both cerebral
hemispheres. Even though the patient is right-handed, note that the
drawing made with his left hand more accurately represents the over-
all shape of the original. What’s going on?
Motor control, like sensory perception, is contralateral—the right
hand is controlled by the left hemisphere, and the left hand is con-
trolled by the right hemisphere. These kinds of results suggest that
the right hemisphere is superior to the left in dealing with global
spatial analysis, in this case, the judgment and rendering of the three-
dimensional perspectival aspect of this simple drawing.
From the collective findings of many experiments of this type, a
picture has emerged regarding lateralization of particular functions
in the human brain. In general, and especially in right-handed in-
dividuals, the left hemisphere is superior to the right for language
comprehension and expression, numeric reasoning and arithmetic
calculation, and visual detail. Conversely, the right hemisphere is
generally superior to the left for nonverbal aspects of communication,
such as linguistic prosody (rhythm, intonation), tone of voice, and
body language, and for visual perspective and larger-scale spatial pat-
terns and relations, what might be called visual gestalt. The functions
between the two hemispheres are not completely separated; it’s just
that one hemisphere is more involved than the other for certain tasks.
The separate hemispheres of a split-brain patient have clearly been
demonstrated to each competently carry out tasks. Are the two cere-
bral hemispheres in a split-brain patient each separately conscious?