Hemispheric Specialization
Language
INTRODUCTION
A. Association Areas
1. Association areas are regions of the
neocortex separate from those
devoted to specific
sensory and motor
functions
2. Three regions
a. Prefrontal
b. Limbic
c. Parietal-TemporalOccipital
3. Input from all sensory
systems and other
cortical areas
4. Output to other cortical areas,
especially other association areas
5. Functions: participate in
a. sensory perception (e.g. "wet")
b. voluntary movement (e.g. decision to initiate movement)
c. cognition
d. memory
e. emotional responses
f. language
6. Often characterized by Lateralization:
specialized functions located on either the left
or right association cortex (lack of symmetry)
B.. Lateralization (Lateral Specialization)
1. Description: although most functions are bilaterally
represented in the cerebral cortex, as in the
remainder of the central nervous system, for certain
functions, one hemisphere is the more important or
is the only important site ("dominant hemisphere")
2. Evidence
a. “split brain” experiments: section corpus
callosum, present stimulus to one hemisphere
only, test response: two independent
hemispheres
b. loss of function with unilateral cortical lesions
c. in vivo imaging for
localizing neuronal activity
d. gross anatomical
asymmetry
e. right-left handedness and
similar phenomena
C. Left and Right Association Cortex
Note: Not all individuals show the same pattern or degree of
hemispheric specialization. The following description applies
to the large majority of the population.
1. Left hemisphere specialization (dominant or
categorical hemisphere)
a. understanding and manipulating language:
recognition, use, and understanding of words
and symbols
b. speech
c. writing
d. identification of objects by name
e. mathematics, logic, analysis
2. Right hemisphere specialization (nondominant or representational hemisphere)
a. spatial-temporal relationships
b. identification of objects by shape or form
(e.g. stereognosis, face recognition)
c. attention based on form recognition
d. recognition of emotion
e. recognition of tunes, rhythms
f. synthesis, holistic problem solving
3. Relation to handedness
4. Assumption of function after injury to a.
handed individuals (90% of population)
1) left hemisphere dominant:
2) right hemisphere dominant:
right96%
4%
b. left-handed individuals (10% of population)
1) left hemisphere dominant:
70%
2) right hemisphere dominant:
15%
3) neither hemisphere dominant;
15%
4. Assumption of function after injury to dominant
hemisphere
a. in early childhood, the opposite hemisphere can
assume much of the function of the injured
hemisphere
b. in adults, injury to a hemisphere leads to moreor-less permanent disability
c. even in adults, the opposite hemisphere is able
to assume some function after injury
D. Cognition
1. Define: The ability to attend to external
stimuli or internal motivation, to
determine the significance of such
stimuli, and to plan appropriate
responses (analysis of input, memory,
strategy of action)
2. Evaluation: Cognitive screening
tests; e.g. Mini-Mental State Exam
(MMSE)
INTRODUCTION (continued)
E. Pathology
1. Apraxia (G., lack of action): condition of
being unable to perform a skilled
movement despite intact neuromuscular
function and desire to do the movement
2. Agnosia (G., lack of knowledge): unable
to recognize an object via a specific
sensory modality even though basic
sensation using that modality is normal
3. Aphasia (G., lack of speech):
impairment of any language ability; may
include difficulty in producing or
comprehending spoken or written
language
4. Neglect: condition of being unable to
direct attention
5. Dementia: loss of cognitive ability (more
loss than typical of aging) in a previously
normal person
CORTICAL REPRESENTATION OF SPEECH
AND LANGUAGE
A. Speech Representation
1. Broca’s area
a. location: frontal lobe, adjacent to area of primary
motor cortex controlling muscles of vocalization
b. output: primary motor cortex vocalization muscles
c. input: Wernicke’s area
d. role: in dominant hemisphere, processes information
into detailed pattern necessary for vocalization
e. pathophysiology: motor aphasia
2. Wernicke’s area
a.
b.
c.
d.
e.
location: posterior region of the superior temporal gyrus
input: visual, auditory, and somatosensory interpretive areas
output: Broca's area
role: language, thought, “intelligence”
pathophysiology: inability to recognize or form coherent
thought; sensory aphasia
LANGUAGE PATHOPHYSIOLOGY
A. Aphasias
1. Wernicke's aphasia (Receptive aphasia, Fluent aphasia)
a. Lesion - usually left posterior part of temporal lobe,
although extends to ventral part of parietal lobe
b. Visual and auditory language input can be impaired,
but speech is fluent
c. Patients use the wrong word or combination of
words, add additional syllables to words or words to
phrases, or tend to make up words
d. Speech contains little meaning because of the
abundance of words; e.g., when asked where he
lived, the patient said "I came here before here and
returned there" (i.e., called empty speech)
e. Patients are usually unaware of their errors
2. Broca's aphasia (Expressive aphasia, Nonfluent aphasia)
a. Lesion: area 44 and 45 on the dominant (usually,
left) side
b. Characterized from complete muteness to slowed,
deliberate speech constructed from simple
grammatical structures (e.g., the phrase "the large gray
cat" may become "cat")
c. Comprehension of spoken and written language is
seldom disturbed (but expression by writing often
exhibits the same problems as when speaking)
d. Patients are generally aware of their errors
e. Patients can sometimes express themselves by singing
3. Conduction aphasia
a. Lesion of the pathway (arcuate fasciculus) between
Wernicke's area and Broca's area
b. Characterized by incorrect words, inability to repeat,
deficits in naming
4. Speaking in sign language
a. Deaf people can become aphasic for sign language
following lesions in the left hemisphere. The deficits
can involve deficits in expression with good
comprehension (Broca’s), or signing fluently but with
many mistakes while having difficulty comprehending
the signing of others (Wernicke’s). In all cases, the
ability to move the hands is not impaired.
b. Thus, signing and language use similar structures of
the left hemisphere
B. Other Language Disorders
1. Disorders of reading and writing
a. Pure word blindness - unable to read but can
understand when words are spelled and is able
to spell words correctly (Alexia)
b. Cannot comprehend written words but can copy
them (If unable to write words, termed agraphia)
c. Typically involve damage to the part of the
corpus callosum which connects the visual
associational cortex
2. Dyslexia as a developmental disorder
of reading
“Dyslexia is an inability to read effortlessly or with
complete understanding, even though the cognitive
and intellectual capacities of the children are usually
normal or even superior.”
a. Impairment of the phonemic (phoneme = unit of
language expression) processing
b. Problems with a sequence of characters
c. Tendency to read from right to left (particularly
difficult for some words such as "was" and "saw"
d. Difficulty distinguishing between letters that have
the same configuration but different orientation
(e.g., p and q, b and d, m and w)
e. Disproportionate percentage of left-handers are
dyslexic
f. Postulated anatomical basis: an abnormality in
the migration of neurons to the left cortex during
development.