Download INTRODUCTION: LANGUAGE DISORDERS IN ADULTS

INTRODUCTION:
LANGUAGE DISORDERS IN
ADULTS
1
Two Alternative Views Have
Been Advanced on the
Relationship Between Brain
and Behavior
2
Current views of nerve cells, the
brain, and behavior have emerged
relatively recently from a fusion,
at the end of the nineteenth
century, of four experimental
traditions: neuroanatomy,
physiology, biochemical
pharmacology, and behavior.
3
The anatomical complexity of
nervous tissue was not
appreciated before the invention
of the compound microscope.
4
Until the eighteenth century
anatomists thought nervous tissue
to be glandular in function. They
considered nerves to be ducts
conveying the fluid secreted by
the brain and spinal marrow to
the periphery.
5
Histology of the nervous system
became a modern science during
the nineteenth century,
culminating in the investigations
of Camille Golgi and Santiago
Ramon y Cajal, who shared the
sixth Nobel Prize for Medicine in
1906.
6
Golgi developed the histological silver
impregnation methods that allowed
visualization of the whole neuron with
all its processes: the cell body, the
dendrites, and the axon. Cajal developed
some of the key conceptual insights and
much of the empirical support for the
neuron doctrine--the principle that the
nervous system is made up of discrete
signaling elements, the neurons.
7
Neurophysiology, the second
scientific discipline fundamental to
the modern view of nervous
function, also began in the
eighteenth century with the
discovery by Luigi Galvani of the
importance of electricity to animal
physiology.
8
Again, it was during the
nineteenth century that the
foundations of electrophysiology
were laid by Emil DuBoisReymond and Hermann von
Helm- Holtz.
9
Biochemical pharmacology started
with Claude Bernard, Paul Ehrlich,
and I. N. Langley, each of whom
realized that drugs interact with
specific receptor molecules on the
surface of cells, an insight that
became the basis of the modem
study of chemical synaptic
transmission.
10
The fourth discipline
important for determining the
relationship between brain and
behavior has the longest history.
It is difficult to trace the history
of psychology briefly.
11
In the West, ideas about mind and soul
are derived from antiquity; behavior, the
manifestation of mind in the physical
world, was not approached
systematically until the nineteenth
century, when the work of Charles
Darwin on the evolution of behavior
allowed psychology to develop as a
discipline independent of philosophy and
to become experimental.
12
Aspects of the merger of anatomy,
physiology, and behavior can be traced
to a series of experiments by Pierre
Flourens, a French neurologist working
in the nineteenth century who produced
lesions of various parts of the nervous
system of animals in order to examine
how their behavioral capability was
altered by the removal of that portion of
the brain.
13
This approach led Flourens to conclude
that the various sensory and motor
functions are not localized to specific
regions in the cerebral cortex. Thus, by
the middle of the nineteenth century, it
was generally believed that the cortex
acted as a whole for each of its mental
functions, and that any of its parts was
able to perform all of its functions.
14
Injury to a specific area of the
cortex would therefore affect all
higher functions equally. The
acceptance of this belief
(subsequently called the
aggregate field view of the brain)
was based only partially on
Flourens' experimental work.
15
It also represented a philosophical
reaction against phrenologists who had
argued that highly elaborate and abstract
mental functions--including generosity,
mother love, and secretiveness--were
localized discretely in mosaic fashion to
specific domains of the brain, domains
that gave rise to identifiable bumps on
the overlying skull.
16
At the end of the nineteenth
century J. Hughlings Jackson, a
British neurologist, broke with this
aggregate field view. Jackson's
clinical studies of focal epilepsy
(convulsions beginning on one side
of the body) showed that different
motor and sensory activities are
localized to different parts of the
brain.
17
These studies were later
elaborated systematically by the
German neurologist Karl
Wernicke and by Ramon y Cajal
into an alternative view of brain
function called cellular
connectionism.
18
Cajal provided the histological basis for
considering the neuron to be the
signaling unit of the brain. He also
showed that neurons connect to one
another in a highly precise fashion.
Wernicke showed that behavior is
mediated by specific regions and through
localizable pathways connecting sensory
and motor structures.
19
The history of the dispute
between the aggregate field and
the cellular connection views of
cortical function can best be
illustrated in the analysis of
language, the highest and most
characteristic human function.
20
Before we consider the relevant
clinical and anatomical studies
concerned with the localization
of language, it is useful to survey
the structure of the brain.
21
Introduction to the Anatomy of
the Central Nervous System
22
The central nervous system is a
bilateral and essentially
symmetrical group of structures,
consisting of six main parts:
23
(1) The spinal cord receives
information from the skin and
muscle and sends out motor
commands for movement.
24
(2) The brain stem, the rostral extension of the
spinal cord, is subdivided into three regions:
the midbrain, the pons, and the medulla. The
brain stem receives information from the skin
and muscles of the head and neck and in turn
controls those muscles. The brain stem also
contains collections of the cell bodies of most
of the cranial nerves such as the auditory and
vestibular nerves and is essential for
processing the special senses.
25
(3) The cerebellum is important
for modulating motor movement
together with…
26
(4) the basal ganglia (the caudate
nucleus, the putamen, and the
globus pallidus).
27
(5) diencephalon (the thalamus,
hypothalamus, sub thalamus, and
epithalamus) is a key relay zone for
transmitting information about
sensation and movement and also
contains (in the hypothalamus)
important control regions for
homeostatic (autonomic) integration.
28
(6) The cerebral hemispheres,
capped by the cerebral cortex, are
concerned with higher
perceptual, cognitive, and motor
functions.
29
To understand the localization of
language, we are concerned primarily
with the cerebral cortex. The cortex of
each hemisphere is divided into four
anatomically distinct regions called lobes
the frontal, parietal, occipital, and
temporal. Each lobe has a number of
characteristic convolutions or infoldings
(an old biological trick for increasing
surface area).
30
The crests of the convolutions are called
gyri. The intervening grooves are called
sulci or (when deep and prominent)
fissures. The more prominent gyri and
the sulci are similar from one individual
to another and have specific names with
respect to each other (for example,
precentral gyms, central sulcus, and
postcentral gyrus).
31
The organization of the cerebral
cortex is characterized by two
important features. First each
hemisphere is concerned
primarily with sensory and
motor processes of the
contralateral side of the body.
32
Sensory information that enters
the spinal cord from the left side
of the body crosses over to the
right side of the nervous system
(either at the level of the spinal
cord or subsequently at the level
of the brain stem) before being
conveyed to the cerebral cortex.
33
In a similar fashion, the motor
areas in one hemisphere exert
control over the movements of
the opposite half of the body.
34
Second, although largely
symmetrical in structure, the
hemispheres are not completely
symmetrical and the two
hemispheres are not equivalent
in function.
35
Cognitive Function Can Be
Localized Within the Cerebral
Cortex
36
Much of what we know about the
localization of normal language
has come from the study of
aphasia, a disorder of language
that most commonly is found in
patients who have suffered from
stroke, an occlusion of a blood
vessel supplying a portion of the
cerebral cortex.
37
Many of the really important
discoveries in the study of
aphasia occurred in rapid
succession during the last half of
the nineteenth century and
formed one of the most exciting
chapters in the intellectual history
of human psychology.
38
The initial advance occurred in
1861 with the publication of a
paper by the French neurologist
Pierre Paul Broca.
39
Broca described the case of a
patient who could understand
language but who had lost the
ability to speak. Postmortem
examination of the brain showed
a lesion in the posterior portion
of the frontal lobe (an area now
called Broca's area).
40
Broca next collected eight cases, all of
which showed a lesion at this site. In
seven of the eight cases, the lesion
existed in' the left half of the brain. This
discovery led Broca to announce, in
1864, one of the most famous principles
of brain function: “ Nous parlons avec
l’hemisphere gauche!”(“We speak with
the left hemisphere!”).
41
Broca also noted that rare
exceptions to left hemispheric
localization of speech occurred,
and all were in left-handed
patients. This observation in turn
led to the generalization that
there is a crossed relationship
between hemispheric dominance
and hand preference.
42
Broca's work stimulated a
wider search for the cortical loci
of behavioral function--a search
that was soon rewarded.
43
In 1870, nine years after Broca's
initial discovery, Gustav Theodor
Fritsch and Eduard Hitzig
galvanized the scientific community
with their discovery that
characteristic movements of the
limbs can be produced in dogs by
electrically stimulating the precentral
gyrus in front of the central sulcus.
44
Moreover Fritsch and Hitzig
found that there was a cortical
representation for the individual
muscle groups and that the region
of the cortex devoted to each
group was small and discrete.
45
A further step was taken in 1876
by Karl Wernicke. At the age of 26
(having been out of medical school
for only 4 years) Wernicke published
a now classic paper entitled “The
symptom Complex of Aphasia: A
Psychological Study on an
Anatomical Basis.”
46
In this paper, Wernicke described a new type
of aphasia--an impairment of comprehension,
a sensory as opposed to a motor malfunction.
Whereas Broca's patient could understand but
could not speak, Wernicke's patient could
speak but could not fully comprehend.
Wernicke's new type of aphasia also had a
different locus from that described by Broca:
it was located in the posterior part of the
temporal lobe.
47
In addition to this discovery,
Wernicke formulated a theory of
aphasia that attempted to
reconcile and extend the two
existing theories of brain
function.
48
The phrenologists had argued
that the cortex was a mosaic of
specific functions; even abstract
mental attributes were localized
to single, highly specific cortical
areas.
49
The opposing aggregate field
school argued that mental
functions are not at all
represented topographically.
Wernicke used his findings and
those of Broca, Fritsch, and
Hitzig to argue that fundamental
mental functions are discretely
localized.
50
These functions are concerned
with simple perceptual and motor
activities. The elementary areas
for these simple functions are
interconnected in various ways.
51
More complex intellectual
functions (with which the
phrenologists concerned themselves) arise out of the neural
interactions among the simple
perceptual and motor areas and
are mediated by the pathways
that interconnect them.
52
By extending the mosaic view of the
brain into connectionist framework,
Wernicke emphasized that the same
function is processed in parallel in
different regions of the brain (specific
aspects of the function being processed
at particular loci). Wernicke thereby
initiated the notions of parallel and
distributed processing that are so
prominent in current thinking.
53
Wernicke applied this theory to
his own work and that of Broca
by analyzing the motor and
sensory components of speech
and their interactions.
54
Fritsch and Hitzig had found that
stimulating the lower end of the
precentral gyrus led to bilateral
movements of the mouth, tongue, and
palate. Wernicke therefore argued that
Broca's area--which lies immediately in
front of this motor area--coordinates the
muscles of the mouth, tongue, palate,
and vocal cords into coherent speech.
55
Next Wernicke considered the
sensory component. The
Viennese psychiatrist Theodor
Meynert (the teacher of both
Wernicke and Sigmund Freud)
had found that the auditory
pathway projected to Heschl’s
gyrus in the temporal lobes.
56
Wernicke now argued that, as his
patient's brain showed, the
capacity for word selection is
found near this zone; lesions in
this area next to Heschl's gyrus
cause aphasia with loss of
comprehension.
57
Finally, Wernicke predicted a
third type of aphasia (later
discovered clinically) produced
by a very different type of lesion
from that in Broca's and
Wernicke's aphasias.
58
This additional type of aphasia
spared the receptive and motor
speech zones, but destroyed the
pathways connecting them by
interrupting the arcuate fasciculus
of the lower parietal region.
59
This syndrome, later called conduction
aphasia, is characterized by incorrect
word usage (paraphasia). Patients with
paraphasia omit parts of words,
substitute incorrect sounds in the word or
use words incorrectly. They cannot
repeat simple phrases although they
understand words that are heard and
seen and they can speak fluently.
60
Thus, at the,beginning of the
twentieth century, there was
compelling evidence that discrete
areas of the cortex are involved
in specific behaviors. However,
surprisingly, the dominant view
of the brain was not the cellular
connection but the aggregate
field view.
61
During the first half of this century a
number of major neural scientists,
including the British neurologist
Henry Head, the German
neuropsychologist Karl Goldstein,
and the American psychologist Karl
Lashley, continued to argue strongly
for an aggregate field view.
62
The most influential of these
proponents was Karl Lashley,
Professor of Psychology at
Harvard. Lashley attempted to
find the locus of learning in the
rat by studying the effects of
various brain lesions on the
complex task of learning to
master a maze.
63
Lashley could not find any
specific learning center; rather
the severity of the learning defect
produced by damage to the brain
depended upon the extent of the
damage and not on its precise
location.
64
This discovery led Lashley--and,
after him, many other
psychologists--to conclude that
learning did not have a special
locus and therefore could not be
related to specific neurons.
65
On the basis of these conclusions,
Lashley formulated a theory of
brain function called mass action,
which minimized the importance
of individual neurons and of
specific neuronal connections.
66
What was important according to
this mass action or aggregate
field view was brain mass, not
neuronal architecture.
67
Applying this logic to aphasia, Head
and Goldstein argued that disorders of
language cannot be attributed to specific
lesions, but result from alterations in
almost any cortical area. As a result of
cortical damage, regardless of site, the
patient regresses from a higher symbolic
language to a simple, automatic verbal
knowledge--from an abstract to a
concrete language characteristic of
aphasia.
68
Recently the work of Lashley
and of Head has been
reinterpreted. A variety of studies
have demonstrated that maze
learning, the task used by
Lashley, is unsuitable for
studying localization of function
because it involves complex
motor and sensory capabilities.
69
Deprived of one capability, an
animal can still learn with
another. In addition, a series of
important clinical and
experimental advances greatly
strengthened the evidence for
localization.
70
In the late 1950s Windel Penfield
stimulated the cortex of conscious
patients during brain surgery for
epilepsy carried out with local
anesthesia. As a necessary part of the
surgical procedure, Penfield
searched the cortex for areas that
produced disorders of language upon
stimulation.
71
His findings, based upon the verbal
report of conscious subjects,
dramatically confirmed the localization
indicated by Wernicke's studies.
Moreover, Penfield extended the studies
of Fritsch and Hitzig to humans; he
showed that the muscles of the body
were represented in great topographical
detail and the resulting map formed a
motor homunculus.
72
Recently, these clinical studies
have been synthesized and
extended by Norman Geschwind
at Harvard, who has pioneered in
the modem study of
asymmetrical representation of
function in the human cerebral
cortex.
73
Experimental results from applying
cellular techniques to the central nervous
system have led to similar conclusions.
For example, developmental and
physiological studies have indicated that
individual nerve cells connect to one
another in a precise way. As a result,
individual cells respond only to specific
sensory stimuli and not to others.
74