Broca's declaration that the left hemisphere is
predominantly responsible for language-related behavior is only the
clearest and most dramatic example of an asymmetry of function in the human
brain. This functional asymmetry is related to hand preference and probably
to anatomical differences, although neither relationship is simple.
Evidence from a number of converging sources, notably the high incidence of
the language disturbance known as aphasia after left- but not
right-hemisphere damage, indicates that the left hemisphere is dominant for
the comprehension and expression of language in close to 99 percent of
right-handed people. At least 60 percent of left-handed and ambidextrous
people also have left-hemisphere language, but up to 30 percent have
predominantly right-hemisphere language. The remainder have language
represented to some degree in both hemispheres.
The posterior temporal region of the brain, which in the dominant hemisphere
is one of the regions responsible for language, is physically asymmetrical;
specifically, the area known as the planum temporale is larger in the left
hemisphere in most people. This asymmetry is more common in right-handers,
while left-handed individuals are likely to have more nearly symmetrical
brains. Reduced anatomical asymmetry has also been found in people with
right-hemisphere dominance for speech and in some developmental dyslexics
(people with reading disorders). These results point to some relationship
between handedness, cerebral dominance for language, anatomical asymmetry in
the temporal lobe, and some aspects of language competence. Certainly, there
is a tendency for right-handedness, left-hemisphere dominance for language,
and a larger left planum temporale to go together. However, there are
exceptions; for example, a few right-handers are right-hemisphere dominant
for speech, and some right-handers who have left-hemisphere speech do not
have a larger left planum temporale. In subjects who are atypical in one of
these respects—for example, by being left-handed—the relationship between
handedness, cerebral dominance, and anatomical asymmetry is much less
consistent. It follows, therefore, that language is not invariably located
in the hemisphere opposite the dominant hand or in the hemisphere with the
larger planum temporale.
Studies of patients in whom the corpus callosum (the bundle of nerve fibers
connecting the two halves of the brain) has been severed, allowing the two
hemispheres to function largely independently, have revealed that the right
hemisphere has more language competence than was hitherto supposed. These
patients show evidence of comprehension of words presented to the isolated
right hemisphere, although that hemisphere is not able to initiate speech.
The speech of patients with a lesion of the right hemisphere may lack normal
melodic quality, and they may have difficulty expressing and understanding
such things as emotional overtones. They may also have difficulty
appreciating some of the more subtle, connotative aspects of language, such
as puns, figures of speech, and jokes. Nevertheless, the dominance of the
left hemisphere for language, particularly the syntactic aspects of language
and language output, is the clearest example yet discovered of the
lateralization of higher cortical function.
The left hemisphere also appears to be more involved than the right in the
programming of complex sequences of movement and in some aspects of
awareness of one's own body. Thus, the disorders known as ideomotor and
ideational apraxia are more common after left-hemisphere damage. In these
disorders, the patient has difficulty carrying out actions involving several
movements or the manipulation of objects in an appropriate and skillful way.
The difficulty appears to be in programming the motor system to run off the
sequence of movements required to perform a complex action in the
appropriate order and with the appropriate timing.
A third category of deficits associated with left-hemisphere damage,
disorder of the body image, is much more difficult to define. It includes a
disorder called finger agnosia, in which the individual does not appear to
“know” which finger is which, being unable to indicate which one the
examiner touches without the aid of vision. Confusion of right and left is
also found after left-hemisphere damage, making it appear that the left
hemisphere is largely responsible for collating somatosensory information
into a special awareness of the body called the body image. The phenomenon
of the phantom limb, whereby patients “feel” sensations in amputated limbs,
indicates that the brain's internal representation of the body may persist
intact for some time after the loss of a body part. This internal
representation appears to be maintained chiefly by the left hemisphere.
The special functions of the right hemisphere were recognized later than
those of the left hemisphere, although a case of “imperception” reported by
the English neurologist John Hughlings Jackson in 1876 foreshadowed later
findings. Jackson's patient, who had a lesion in the posterior part of the
right hemisphere, lost her way in familiar surroundings, failed to recognize
familiar places and people, and had difficulty in dressing herself—all of
which became well-recognized consequences of right-hemisphere damage. The
right hemisphere, then, appears to be specialized for some aspects of
higher-level visual perception, spatial orientation, and route finding
(sense of direction), and it probably plays a dominant role in the
recognition of objects and faces. The specialization of the right
hemisphere, however, is less absolute than that of the left hemisphere in
that these skills are less lateralized than language.
There has been considerable speculation as to why the human brain should be
functionally asymmetrical. Initially, both functional and anatomical
asymmetry were thought, like language, to be a uniquely human trait, but
less pronounced asymmetries have now been found in lower animals. One
suggestion is that it is necessary to have language represented in a single
hemisphere to avoid competition between the hemispheres for control of the
muscles involved in speech. Another suggestion is that it is efficient to
have the language system represented in a restricted area on one side of the
brain because information needs to be transferred over short distances and
fewer connections. A third suggestion is that the dominance of the left
hemisphere over the right hand and skilled movement preceded its dominance
over language. According to this view, language subsequently developed in
the same hemisphere because language implies speech, which requires precise
programming of sequences of movement in the articulatory musculature. All
these views have something to recommend them, but none has been conclusively
proved correct or has been generally accepted. Also, there remain some facts
that are difficult to explain by any theory. For example, all the above
theories would predict that bilateral and, in some cases, right-hemisphere
language representation would be disadvantageous, but this does not seem to
be generally true.
The language area of the brain surrounds the Sylvian
fissure in the dominant hemisphere. This area is divided into two major
components named after the pioneers Paul Broca and Carl Wernicke. Broca's
area lies in the third frontal convolution, just anterior to the face area
of the motor cortex and just above the Sylvian fissure. This is often
described as the motor, or expressive, speech area; damage to it results in
Broca's aphasia, a language disorder characterized by deliberate,
telegraphic speech with very simple grammatical structure though the speaker
may be quite clear as to what he wishes to say and may communicate
successfully. Wernicke's area is in the superior part of the posterior
temporal lobe; it is close to the auditory cortex and is classically
considered to be the receptive language, or language comprehension, centre.
A patient with Wernicke's aphasia has difficulty understanding language; his
own speech is typically fluent but is empty of content and characterized by
circumlocutions, a high incidence of vague words like “thing,” and sometimes
neologisms and senseless “word salad.” The entire posterior language area
extends into the parietal lobe and is connected to Broca's area by a fiber
tract called the arcuate fasciculus. Damage to this tract has been
implicated in conduction aphasia, a disorder in which the patient can
understand and speak but has difficulty in repeating what is said to him.
The suggestion is that, in this condition, language can be comprehended by
the posterior zone and spoken by the anterior zone, but it can not be easily
shuttled from one to the other.
It is important to note that aphasia is a disorder of language and not of
speech (although an apraxia of speech, in which the programming of motor
speech output is affected, may accompany aphasia). The writing and reading
of aphasic patients, therefore, usually commits the same type of error as
their speech, while the reverse is not the case. Isolated disorders of
writing (dysgraphia) or, more commonly, reading (dyslexia) may occur as
well, but these reflect a disruption of the additional processing required
for these activities over and above that required for language.
One particular form of dyslexia deserves mention, as it is a clear example
of a disconnection syndrome—a disorder resulting from the disconnection of
two areas of the brain rather than from damage to a “centre.” This is
dyslexia without dysgraphia, or letter-by-letter reading, so called because
it is not associated with writing disturbance and because the patients tend
to attempt to read by spelling words out loud letter by letter. It usually
results from a lesion in the posterior part of the left hemisphere that
disconnects the visual areas of the brain from the language areas. This
renders the language areas effectively blind, so that they cannot be brought
to bear on visible language such as the written word. Writing is unaffected
because the right hand is still connected to the left hemisphere, and, if
letters can be spoken out loud correctly (which is not always the case), the
patient will be able to hear himself say them and reintegrate them into
words. Disconnection syndromes are an important concept in understanding
behavioral disorders associated with brain damage. The possibility that
deficits are caused by disconnection must always be borne in mind.
Memory is one of the most widely studied cognitive
functions, and a number of different aspects of memory are recognized. The
labels short-term memory, primary memory, and working memory refer to the
temporary storage of information that is necessary for the performance of
many cognitive tasks. In order to understand this sentence, for example, a
reader must maintain the first half of the sentence in working memory while
reading the second half. This working memory has been graphically described
as the memory one uses to hold a telephone number in mind after looking it
up in a directory and while dialing. The capacity of working memory is
limited, and it decays if not rehearsed. Long-term memory, secondary memory,
and reference memory refer to the storage of information for longer periods.
The capacity of long-term memory is very large—in practice unlimited—and it
can endure indefinitely. In addition, psychologists distinguish episodic
memory, a memory of specific events or episodes normally described by the
verb remember, from semantic memory, a knowledge of facts normally said to
be known rather than remembered.
Almost
certainly, memory is stored over wide areas of the brain rather than in any
single location. However, amnesia, a disorder of memory, can occur after
localized bilateral lesions in the limbic system—notably the hippocampus on
the medial side of the temporal lobe, some parts of the thalamus, and their
connections. This probably implies that these structures, rather than
actually constituting a memory store, are important in the laying down of
memories and in their recall when needed. Memory impairment resulting from
damage in these areas is a disorder of long-term episodic memory and is
predominantly an anterograde amnesia—that is, it typically affects the
memory of events occurring after the illness or accident causing the amnesia
more than it does memories of the past. Substantial retrograde amnesia (loss
of the memory of events occurring before the onset of amnesia) rarely if
ever occurs without significant anterograde amnesia as a result of brain
damage, although it may occur alone in psychiatric illness.
Although amnesia is a disorder of long-term episodic memory and leaves
short-term and semantic memory intact, both of the latter can be affected by
brain damage. Some parietal lobe lesions may affect short-term memory
without affecting long-term memory; this fact has contributed to a revision
of the old theory that there are distinct short- and long-term stores, the
latter being accessible only via the former. It has been suggested that
short-term memory impairment—at least for verbal material—can be further
subdivided into auditory and visual domains; however, these disorders
manifest themselves in difficulty in understanding spoken and written
language rather than in memory impairment (i.e., they appear more like
aphasia and dyslexia). Impairment of semantic memory, too, results in an
impairment that resembles a loss of concepts or a language deficit more than
it resembles what would usually be described as a memory impairment. Some
forms of visual agnosia have been interpreted as semantic memory impairment,
since the patients are unable to recognize objects such as chairs because
they no longer “know” what chairs are or what they look like (or can no
longer access that knowledge)
The frontal lobes are the part of the
brain most remote from
sensory input and whose functions are most difficult to capture. They can be
thought of as the executive that controls and directs the operation of the
brain systems dealing with cognitive function. Indeed, the deficits seen
after frontal lobe damage have been described as a “dysexecutive syndrome.”
Frontal lobe damage can affect people in any of several ways, and the
results are at once subtle and drastic. On the one hand, they may have
difficulty initiating behavior, in extreme cases being virtually unable to
move or speak but more often simply having difficulty in getting started on
a task. On the other, they may perseverate, being apparently unable to stop
a behavior once started. Rather than appearing apathetic and hypoactive,
they are uninhibited, rude, and boorish. Such people may also have
difficulty in planning and problem solving and may be incapable of creative
thinking. Mild cases of this deficit can be revealed by a difficulty in
solving mental arithmetic problems that are couched in words, even though it
can be shown that the patient is capable of remembering the question and
performing the required calculation. In such cases it appears that the
patient simply cannot work out what to do, a difficulty described as a
failure to select the appropriate cognitive strategy.
A unifying theme in these disorders is the notion of
inadequate control or organization of pieces of behavior that may in
themselves be well formed. Frontal lobe patients are easily distracted.
Although their deficits may be superficially less dramatic than those
associated with posterior lesions, they can have a drastic effect on
everyday function. Irritability and personality change are also frequently
seen after frontal lobe damage.
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