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Chapter 15
Human Communication
Speech production and
comprehension
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Most of what we know about the physiology of language is from
studying individuals that have suffered cerebrovascular accidents (a
stroke; caused by blockage or rupture of a blood vessel in the brain);
the interruption of blood flow deprives a region of the brain of its
blood supply, which causes cells in that region to die
Also, information from patients with seizure disorders that require
brain surgery, and functional imaging studies
Aphasia – most important category of speech disorders; difficulty in
producing or comprehending speech not produced by deafness or a
simple motor deficit; caused by brain damage
Lateralization
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Verbal behavior is a lateralized function; that is, areas of the left
hemisphere are primarily responsible for speech production and
comprehension
The left hemisphere is specialized more for the analysis of sequences
of stimuli, occurring one after the other, which may explain why
speech centers are located there
However, although the circuits that are primarily involved in speech
comprehension and production are located in one hemisphere, the R
hemisphere also plays a minor part
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e.g. understanding semantics, prosody (i.e. normal rhythm and stress in
speech)
Speech production
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Damage to a certain region of the
inferior left frontal lobe (Broca’s
area) disrupts the ability to speak
Broca’s aphasia – speech disorder
characterized by slow, laborious, and
nonfluent speech
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Have difficulty saying function
words (e.g. the, in, some), but can
manage to say content words (e.g.
nouns, verbs, etc.)
However, can still comprehend
speech for the most part
Broca’s aphasia
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Broca’s area contains memories of the sequences of muscular movements
that are needed to articulate words
Often become frustrated by their inability to speak correctly; however,
comprehension is not perfect
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3 major speech deficits with Broca’s aphasia:
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Difficulty in comprehending meaning from word order (“The horse kicks the
cow” vs. “The cow kicks the horse”)
Agrammatism – difficulty in comprehending or properly employing
grammatical devices, such as verb endings and word order
Anomia – difficulty in finding (remembering) the appropriate word to describe
an object, action, or attribute
Difficulty with articulation – mispronounce words, often realizing it afterwards,
and trying to correct it
Damage to different areas in and around Broca’s area leads to different
symptoms of aphasia
Left insular cortex – controls speech articulation (damage can cause apraxia
of speech: impairment in the ability to program movements of the tongue,
lips, and throat required to produce the proper sequence of speech sounds
Speech comprehension
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Must not just recognize words, we must understand their meaning
Wernicke’s area contains neural circuits that accomplish this task
Wernicke’s aphasia – a form of aphasia characterized by poor speech
comprehension and fluent but meaningless speech
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Comprehension tested by directing movement toward objects asked
about by experimenter is also poor (e.g. “Point to the ink bottle” –
patient cannot point to ink bottle)
However, patients seem unaware of their deficit, unlike with Broca’s
aphasia
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They do not recognize that their speech is faulty, nor that they do not
comprehend other speech
Wernicke suggested that this area is a location where memories of the
sequences of sounds that constitute words are stored
Wernicke’s aphasia
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Recognition: Pure word deafness
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Recognition (perceptual task) vs. comprehension (involves retrieval of
additional info from memory)
Damage to the L temporal lobe can produce pure word deafness
(ability to hear, speak, and usually to read and write without being able
to comprehend the meaning of speech) without disrupting other
functions; e.g. can recognize non-speech sounds
Not just an inability to understand the meaning of words, since most
can read or write
Speech sounds activate the superior temporal cortex
The more crucial auditory component is timing, not pitch
Can be caused by either damage to Wernicke’s area or to auditory input
to Wernicke’s area
Wernicke’s aphasia
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Comprehension: Transcortical sensory aphasia
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Posterior language area – region that surrounds the posterior part of the
lateral fissure, near the junction of the temporal, occipital, and parietal
lobes
Damage to this area results in Transcortical sensory aphasia (difficulty
in comprehending speech and producing meaningful spontaneous
speech but can repeat speech
Can recognize words (e.g. can repeat phrases) but cannot comprehend
them
Can be seen as Wernicke’s aphasia without a repitition deficit
Wernicke’s aphasia
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What is meaning?
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Words refer to particular objects, actions or relationships in the world
The meaning of a word then is defines by particular memories
associated with it, primarily stored in association cortex
First, we must recognize the sounds that constitute a word (Wernicke’s
area)
Next, the memories that constitute the meaning of that word must be
activated (through PLA) to the neural circuits that contain these
memories
Hebb rule: when we hear a word several times, a particular set of
neurons in the superior temporal lobe become active, and their
interconnections eventually become strengthened
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Cell assembly – an assembly of interconnected neurons
Wernicke’s aphasia
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What is meaning? (con’t)
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Damage to particular regions of the sensory association cortex can
damage particular kinds of info and thus abolish particular kinds of
meanings
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Damage to association cortex of L parietal lobe can produce
autotopagnosia (inability to name body parts or to identify body parts
that another person names)
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e.g. Patient cannot understand directional words (“The sky is down”) but
can use those same words in a non-directional meaning (“After exchanging
pleasantries, they got down to business”)
Have no difficulty understanding the meaning of other words
Studies of brain damaged patients suggest that comprehension of more
subtle, figurative aspects of speech (abstract concepts, moral dilemmas,
etc.) involves the R hemisphere
Wernicke’s aphasia
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Repetition: Conduction aphasia
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Arcuate fasciculus – a bundle of axons that connects Wernicke’s area
with Broca’s area
Damage to this pathway causes conduction aphasia (characterized by
inability to repeat words that are heard but the ability to speak normally
and comprehend the speech of others)
The symptoms that are seen in transcortical sensory aphasia and
conduction aphasia suggest that there are pathways connection the
speech mechanisms of the temporal lobe (Wernicke’s area) with those
of the frontal lobe (Broca’s area)
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The arcuate fasciculus simply conveys speech sounds from Wernicke’s area
to Broca’s area, and appears to play a role in STM of words and speech
sounds that have just been heard
Also a 2nd pathway, connecting the posterior language area and Broca’s area
is indirect and based on semantics
The arcuate fasciculus connecting both speech areas constitute the
phonological loop
Anomic aphasia
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Speech of patients with anomic aphasia (inability to remember the
appropriate word for something) is fluent and grammatical, and
comprehension is fine, but they cannot find the appropriate words
These patients often employ circumlocutions (strategy by which
people with anomia find alternative ways to say something when they
are unable to think of the most appropriate words)
Partial amnesia for words
Fluent anomia is caused by damage to the L temporal or parietal lobe,
sparing Wernicke’s area
More trouble finding nouns than other words
Damage to frontal lobe may lead to averbia (difficulty finding verbs
instead of nouns)
Aphasia in deaf people
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Some researchers believe that sign language preceded spoken
language with our ancestors
Broca’s area is activated when people observe and imitate finger
movements
The grammar of ASL (American Sign Language) is spatial, and thus
cannot be translated word for word to a spoken language
Aphasic disorders in deaf people may be caused by lesions to the R
hemisphere, which is primarily involved in spatial perception and
memory; However, all studies of deaf patients with aphasia for signs
reported lesions of the L hemisphere
The bilingual brain
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Evidence suggests that first and second languages share the same
brain regions
No evidence that brain damage is more likely to affect one language
more than the other
While neural representations of words for both languages seem to be
intermingled with each other, cell assemblies required for storage of
different grammatical rule appear to be separate (since languages can
have different grammatical structures)
Prosody: Rhythm, tone, and
emphasis in speech
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Prosody – the use of changes in intonation and emphasis to convey
meaning in speech besides that specified by the particular words; an
important means of communicating emotion
Patients with fluent aphasia and Wernicke’s aphasia is normal;
however, it is disturbed in patients with Broca’s aphasia
Prosody is a special function of the R hemisphere
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related to the more general role of this hemisphere in musical skills and
the expression and recognition of emotions
Patients with R hemisphere damage show a deficit in prosodic
comprehension
Speech production and
comprehension
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Go over table 15.1 in book, page 501 for
summary of all aphasias
Disorders of reading and writing
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Relation to aphasia
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The reading and writing skills of patients with aphasia almost always
resemble their speaking and comprehending abilities
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Patients with Wernicke’s aphasia have as much difficulty reading and
writing as they do speaking and understanding speech
Patients with Broca’s aphasia comprehend what they read, but their reading
aloud is poor
Patients with conduction aphasia have some difficulty reading, and when
they read aloud, will make semantic paraphasias (saying synonyms for
some of the words they read)
However, a few exceptions:
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Patients with severe fluent aphasia could not understand speech of others,
but can read and understand what they read
Disorders of reading and writing
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Pure alexia
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Loss of the ability to read without loss of the ability to write
Can recognize words that are spelled aloud
A perceptual disorder
Caused by lesions that prevent visual info from reaching the
extrastriate cortex of the L hemisphere
Disorders of reading and writing
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Toward an understanding of reading
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Reading involves at least 2 different processes:
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Direct recognition of the word as a whole
Sounding it out letter by letter
When we see a familiar word, we normally recognize it by its shape
and pronounce it (whole-word reading)
unfamiliar words require the recognition of individual letters and
knowledge of the sounds they make (phonetic reading)
Dyslexias – faulty reading; acquired type caused by brain damage,
developmental type becomes apparent when children learn to read
Disorders of reading and writing
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Dyslexias
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Surface dyslexia – deficit in whole-word reading; caused by lesion to
the L lateral temporal lobe
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Phonological dyslexia – can read whole-words but not sound words
out; difficulty with new words
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must sound all words out, easier with words like hand or table, more
difficult with words like yacht or sew
Must listen to own pronunciation to understand what they are reading
Caused by damage to L frontal lobe
Word-form (or spelling) dyslexia – can read a word only after spelling
out individual letters
Direct dyslexia – resembles transcortical sensory aphasia; able to read
aloud but not understand them; caused by damage to L frontal and
temporal lobes
Disorders of reading and writing
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Writing
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Depends on knowledge of words that are to be used, along with proper
grammatical structure of the sentences they are to form
One type of writing disorder involves difficulties in motor control
Another type involves problems in ability to spell words
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Involves more than one method
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Audition – must hear words to be able to spell them, especially longer, more
complex words
Transcribing an image of what a particular word looks like
Memorization of letter sequences
Motor memories
Brain damage can affect phonetic writing (phonological dysgraphia) –
cannot sound words out and writ them phonetically
Orthographic dysgraphia – can spell regularly spelled words (care or
tree) but not irregularly spelled words (half or busy)
Disorders of reading and writing
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Developmental dyslexias
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A reading difficulty in a person of normal intelligence and perceptual
ability
Of genetic origin or caused by prenatal or perinatal factors
Abnormal development of certain brain regions may cause this
Could be caused by magnocellular region of LGN – however not much
evidence to support this
Reading skill positively correlated with activation of L
occipitotemporal cortex