Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 19

Learning, Memory, Language, & Speech 295

lose it” phenomenon is as yet unknown, but it certainly sug-
gests that the hippocampus and its connections have plasticity
like other parts of the brain and skeletal and cardiac muscles.

LANGUAGE & SPEECH

Memory and learning are functions of large parts of the brain,
but the centers controlling some of the other “higher functions
of the nervous system,” particularly the mechanisms related to
language, are more or less localized to the neocortex. Speech
and other intellectual functions are especially well developed
in humans—the animal species in which the neocortical man-
tle is most highly developed.

COMPLEMENTARY SPECIALIZATION

OF THE HEMISPHERES VERSUS

“CEREBRAL DOMINANCE”

One group of functions more or less localized to the neocortex
in humans consists of those related to language, that is, under-
standing the spoken and printed word and expressing ideas in
speech and writing. It is a well-established fact that human lan-
guage functions depend more on one cerebral hemisphere than
on the other. This hemisphere is concerned with categorization
and symbolization and has often been called the
dominant
hemisphere.
However, it is clear that the other hemisphere is
not simply less developed or “nondominant;” instead, it is spe-
cialized in the area of spatiotemporal relations. It is this hemi-

sphere that is concerned, for example, with the identification of

objects by their form and the recognition of musical themes. It

also plays a primary role in the recognition of faces. Conse-

quently, the concept of “cerebral dominance” and a dominant

and nondominant hemisphere has been replaced by a concept

of complementary specialization of the hemispheres, one for se-

quential-analytic processes (the

categorical hemisphere

) and

one for visuospatial relations (the

representational hemi-

sphere

). The categorical hemisphere is concerned with lan-

guage functions, but hemispheric specialization is also present

in monkeys, so it antedates the evolution of language. Clinical

Box 19–2 describes deficits that occur in subjects with represen-

tational or categorical hemisphere lesions.

Hemispheric specialization is related to handedness. Hand-

edness appears to be genetically determined. In 96% of right-

handed individuals, who constitute 91% of the human popu-

lation, the left hemisphere is the dominant or categorical

hemisphere, and in the remaining 4%, the right hemisphere is

dominant. In approximately 15% of left-handed individuals,

the right hemisphere is the categorical hemisphere and in

15%, there is no clear lateralization. However, in the remain-

ing 70% of left-handers, the left hemisphere is the categorical

hemisphere. It is interesting that learning disabilities such as

dyslexia

(see Clinical Box 19–3), an impaired ability to learn

to read, are 12 times as common in left-handers as they are in

right-handers, possibly because some fundamental abnormal-

ity in the left hemisphere led to a switch in handedness early

in development. However, the spatial talents of left-handers

may be well above average; a disproportionately large number

FIGURE 19–6
Comparison of a normal neuron and one with abnormalities associated with Alzheimer disease.
(From Kandel ER,
Schwartz JH, Jessell TM [editors]:
Principles of Neural Science,
4th ed
.
McGraw-Hill, 2000.)

A Normal

Neuropil

threads

Neurofibrillary

tangles

B Alzheimer disease

Neurites

Senile plaque

Abnormal

membranous

organelles

Aβ

(fibrillar)

Paired helical

filaments

Nerve

terminals