Ganong's Review of Medical Physiology, 23rd Edition

298 SECTION III Central & Peripheral Neurophysiology
appears that it is due to lesions in and around the auditory
cortex (areas 40, 41, and 42).
When a lesion damages the angular gyrus in the categorical
hemisphere without affecting Wernicke’s or Broca’s areas,
there is no difficulty with speech or the understanding of
auditory information; instead there is trouble understanding
written language or pictures, because visual information is not
processed and transmitted to Wernicke’s area. The result is a
condition called anomic aphasia.
The isolated lesions that cause the selective defects
described above occur in some patients, but brain destruction
is often more general. Consequently, more than one form of
aphasia is often present. Frequently, the aphasia is general
(global), involving both receptive and expressive functions.
In this situation, speech is scant as well as nonfluent. Writing
is abnormal in all aphasias in which speech is abnormal, but
the neural circuits involved are unknown. In addition, deaf
subjects who develop a lesion in the categorical hemisphere
lose their ability to communicate in sign language.
Stuttering has been found to be associated with right cerebral
dominance and widespread overactivity in the cerebral
cortex and cerebellum. This includes increased activity of the
supplementary motor area. Stimulation of part of this area has
been reported to produce laughter, with the duration and
intensity of the laughter proportionate to the intensity of the
stimulus.
RECOGNITION OF FACES
An important part of the visual input goes to the inferior temporal
lobe, where representations of objects, particularly faces,
are stored (Figure 19–9). Faces are particularly important in
distinguishing friends from foes and the emotional state of
those seen. In humans, storage and recognition of faces is
more strongly represented in the right inferior temporal lobe
in right-handed individuals, though the left lobe is also active.
Lesions in this area cause prosopagnosia, the inability to recognize
faces. Patients with this abnormality can recognize
forms and reproduce them. They can recognize people by
Stores
biographical
information
Connects facial features
to biographical information
Extracts
facial
features
FIGURE 19–9 Areas in the right cerebral hemisphere, in
right-handed individuals, that are concerned with recognition of
faces. (Modified from Szpir M: Accustomed to your face. Am Sci 1992;80:539.)
their voices, and many of them show autonomic responses
when they see familiar as opposed to unfamiliar faces. However,
they cannot identify the familiar faces they see. The left
hemisphere is also involved, but the role of the right hemisphere
is primary. The presence of an autonomic response to
a familiar face in the absence of recognition has been explained
by postulating the existence of a separate dorsal pathway
for processing information about faces that leads to
recognition at only a subconscious level.
LOCALIZATION OF OTHER FUNCTIONS
Use of fMRI and PET scanning combined with study of patients
with strokes and head injuries has provided further insights—or
at least glimpses—into the ways serial processing of
sensory information produce cognition, reasoning, comprehension,
and language. Analysis of the brain regions involved
in arithmetic calculations has highlighted two areas. In the inferior
portion of the left frontal lobe is an area concerned with
number facts and exact calculations. Frontal lobe lesions can
cause acalculia, a selective impairment of mathematical ability.
There are areas around the intraparietal sulci of both parietal
lobes that are concerned with visuospatial representations
of numbers and, presumably, finger counting.
Two right-sided subcortical structures play a role in accurate
navigation in humans. One is the right hippocampus, which is
concerned with learning where places are located, and the other
is the right caudate nucleus, which facilitates movement to the
places. Men have larger brains than women and are said to have
superior spatial skills and ability to navigate.
Other defects seen in patients with localized cortical lesions
include, for example, the inability to name animals, though the
ability to name other living things and objects is intact. One
patient with a left parietal lesion had difficulty with the second
half but not the first half of words. Some patients with parietooccipital
lesions write only with consonants and omit vowels.
The pattern that emerges from studies of this type is one of
precise sequential processing of information in localized brain
areas. Additional research of this type should greatly expand
our understanding of the functions of the neocortex.
CHAPTER SUMMARY
■ Long-term memory is divided into explicit (declarative) and
implicit (nondeclarative). Explicit is further subdivided into
semantic and episodic. Implicit is further subdivided into
priming, procedural, associative learning, and nonassociative
learning.
■ Synaptic plasticity is the ability of neural tissue to change as reflected
by LTP (an increased effectiveness of synaptic activity) or
LTD (a reduced effectiveness of synaptic activity) after continued
use.
■ Hippocampal and other temporal lobe structures and association
cortex are involved in declarative memory.
■ Alzheimer disease is characterized by progressive loss of shortterm
memory followed by general loss of cognitive function.