Human Physiology, 14th edition (2016)

The Central Nervous System 217

separately presented with sensory images and the patient is asked
to perform tasks (speech or writing or drawing with the contra-
lateral hand), it has been learned that each hemisphere is good at
certain categories of tasks and poor at others ( fig. 8.13 ).
In a typical experiment, the image of an object may be
presented to either the right or left hemisphere (by presenting
it to either the left or right visual field only; see chapter 10,
fig. 10.32) and the person may be asked to name the object.
Findings indicate that, in most people, the task can be performed
successfully by the left hemisphere but not by the right. Similar
experiments have shown that the left hemisphere is generally the
one in which most of the language and analytical abilities reside.
These findings have led to the concept of cerebral dominance,
which is analogous to the concept of handedness—people gener-
ally have greater motor competence with one hand than with the
other. Since most people are right-handed, and the right hand is
also controlled by the left hemisphere, the left hemisphere was

Olfaction Olfaction

Speech, writing

Left ear Right ear

Simple

language

comprehension

Spatial

concepts

Left visual

half

field

Right visual

half

field

Split

brain

Main language

center

Calculation

Clinical Investigation CLUES

Kevin suffered paralysis of his right arm, and his speech

was affected by the accident.

• What is the likely explanation for these two
  observations?


• If his left arm were paralyzed instead, would his
  speech likely have been affected?

naturally considered to be the dominant hemisphere in most peo-

ple. Further experiments have shown, however, that the right hemi-

sphere is specialized along different, less obvious lines—rather

than one hemisphere being dominant and the other subordinate,

the two hemispheres appear to have complementary functions. The

term cerebral lateralization, or specialization of function in one

hemisphere or the other, is thus now preferred to the term cerebral

dominance, although both terms are currently used.

Experiments have shown that the right hemisphere does

have limited verbal ability; more noteworthy is the observation

that the right hemisphere is most adept at visuospatial tasks.

The right hemisphere, for example, can recognize faces better

than the left, but it cannot describe facial appearances as well

as the left. The right hemisphere of split-brain patients, acting

through its control of the left hand, is better than the left (con-

trolling the right hand) at arranging blocks or drawing cubes.

Patients with damage to the right hemisphere, as might be pre-

dicted from the results of split-brain research, have difficulty

finding their way around a house and reading maps.

Perhaps as a result of the role of the right hemisphere in

the comprehension of patterns and part-whole relationships,

the ability to compose music, but not to critically understand it,

appears to depend on the right hemisphere. Interestingly, dam-

age to the left hemisphere may cause severe speech problems

while leaving the ability to sing unaffected.

The lateralization of functions just described—with the

left hemisphere specialized for language and analytical ability,

and the right hemisphere specialized for visuospatial ability—is

true for 97% of all people. It is true for all right-handers (who

account for 90% of all people) and for 70% of all left-handers.

The remaining left-handers are divided about equally into those

who have language-analytical ability in the right hemisphere

and those in whom this ability is present in both hemispheres.

It is interesting to speculate that the creative ability of a

person may be related to the interaction of information between

the right and left hemispheres. The finding of one study—that

the number of left-handers among college art students is dis-

proportionately higher than the number of left-handers in the

general population—suggests that this interaction may be

greater in left-handed people. The observation that Leonardo

da Vinci and Michelangelo were both left-handed is interest-

ing in this regard, but obviously does not constitute scientific

proof of this suggestion. Further research on the lateralization

of function of the cerebral hemispheres may reveal much more

about brain function and the creative process.

Figure 8.13 Different functions of the right and
left cerebral hemispheres. These differences were revealed
by experiments with people whose corpus callosum—the tract
connecting the two hemispheres—was surgically split.