Chapter 3
Biological Substrates of Speech
Development:
A Brief Synopsis of the Developing
Neuromuscular System
Beate Peter
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Introduction
• Speech is the finishing stage of transducing a thought into sound
waves
• Speech sounds are generated by constricting the airstream out of
(or into) the lungs
• Multiple structures and systems converge to generate the complex
movement sequences of speech
• These are essentially fully formed at birth, although not yet in
adult-like size and orientation
• It typically takes several years before a child can use them in such a
way that even unfamiliar listeners can understand what was said
• This chapter describes some general principles of human
development and traces the developmental trajectories of relevant
structures and systems
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Developmental Trajectories
Prenatal Development
Week
Event
1
Fertilized egg divides multiple times, forming blastocyst
2
Blastocyst attaches to uterine wall
3
Oblong disc, 1.5 mm. Three layers that will give rise to specific structures
Endoderm: lining of digestive tract, lining of respiratory system
Mesoderm: skeleton, muscles, cardiovascular system, reproductive
system
Ectoderm: skin, central and peripheral nervous system, lens of eye
4
Less than 4 mm, heart starts beating
8
30 mm, all major structures and organs are formed
9 - birth
22
~ 35 - 38
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Structures and organs grow and become more refined
Fetus can survive if born prematurely
Birth
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Central and Peripheral Nervous
Systems (CNS and PNS)
• Central nervous system
– Brain and spinal cord (structures encased in bone plus
the retina of the eye)
– Prenatal Week 4 and 5
• Neural tube, central canal
• Rostral end: three bulges
– Forebrain
» Telencephalon (to differentiate into cerebral hemispheres)
» Diencephalon (to differentiate into thalamus, hypothalamus,
epithalamus)
– Midbrain
– Hindbrain
» Metencephalon (to differentiate into pons and cerebellum)
» Myelencephalon (to differentiate into medulla)
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Figure 3.1 Differentiation of the neural tube into the structures of the adult brain (not
drawn to scale). a. Neural tube, 2 weeks post fertilization. b. Primary brain vesicles, 3
weeks post fertilization. c. Secondary brain vesicles, 4 weeks post fertilization. d. Adult
brain structures. D = diencephalon, Mes = mesencephalon, Met = metencephalon,
Myel = myelencephalon, P = prosencephalon, T = telencephalon.
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CNS Structure
Function
Cerebral cortex
Sensory processing (visual, auditory, tactile), cognition, linguistic
processing, motor events
Thalamus
Relay center for sensory information
Hypothalamus
Autonomic functions
Epithalamus
Regulation of sleep/wake cycle
Cerebellum
Coordinated and smooth motor functioning
Midbrain
Various functions: conduct information, support motor control,
auditory and visual perception
Pons
Mediate between cerebellum and motor cortex, mediate between
higher brain centers and spinal cord
Medulla
Monitoring of bloodstream for oxygenation and toxins; regulation
of heart rate, relay stations for auditory and vestibular information
Neurons
Form gray matter. Receives and integrates chemical and electrical
stimuli; if threshold is exceeded, generates an electrochemical
response that stimulates other neurons
Glial cells
Support of neurons (e.g., insulation, structural scaffolding, remove
debris). The insulating glial cells form white matter.
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• At birth, the brain structures, cortical layers, and
surface convolutions of the brain are formed.
• White matter continues to form until a peak of
volume is reached at age 39, then volume
declines again
• After birth, there is an excess of neurons and
synapses
– These are lost soon (pruning, apoptosis)
• Neurons generally do not reproduce except in
areas important for generating new memories
(caudate nucleus, hippocampus)
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• In general, the sensory and motor centers of one
hemisphere are connected to the body regions
on the opposite side
• The two cerebral hemispheres are not entirely
symmetrical
– Left insula is larger than right
– Left planum temporale is larger than right
• In most individuals, speech and language
processing is mostly lateralized to the left
hemisphere
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Peripheral Nervous System
• Prenatal week 3: Small part of the ectoderm
moves to position parallel to the neural tube
and forms the neural crest
• Prenatal week 4: Spinal nerve fibers protrude
from the spinal cord; spinal sensory neurons
form ganglia; motor and sensory elongate and
grow into the limbs
• Prenatal weeks 5 and 6: Cranial nerves begin
to appear
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Cranial Nerves
• Somatic efferent, originate in brainstem
–
–
–
–
III Oculomotor (eye movement, pupil constriction, proprioception of eye)
IV Trochlear (eye movement)
VI Abducens (eye movement)
XII Hypoglossal (tongue movement)
• Pharyngeal arch nerves
– V Trigeminal (sensory information from face, anterior tongue)
– VII Facial (facial movements, taste from anterior tongue)
– IX Glossopharyngeal (motor and sensory information including taste to and
from posterior tongue and throat)
– X Vagus (many functions including motor commands in pharynx, larynx, soft
palate)
– XI Accessory (motor control of larynx, pharynx, and soft palate; motor control
of neck muscles)
• Special senses
– I Olfactory (smell)
– II Optic(vision)
– VIII Vestibulocochlear (hearing, balance)
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Figure 3.2 Cranial nerves
V (Trigeminal),
VII (Facial),
IX (Glossopharyngeal),
X (Vagus), and
XII (Hypoglossal)
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Respiratory System
• Prenatal week 4: Tracheal bud
• Prenatal week 5: Two bronchial buds that will keep
subdividing
• Prenatal weeks 16 to 26: Lung tissue becomes more
vascularized and develops terminal saccules
• By prenatal week 24: 17 orders of branches
• Prenatal week 26 to birth: Lining of saccules thins and
becomes covered with surfactant (keeps walls of saccules
from sticking together)
• Prenatal week 32 through age 8 years: alveoli (exchange of
oxygen and carbon dioxide)
• Lung volume correlates with breathing frequency
(decreases with age) and maximum phonation time
(increases with age; decreases again with senescence)
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Larynx
•
•
•
•
•
•
•
First site of air constriction in egressive airstream
Cartilagenous structure
Adduction of the thyroarytenoid muscles (“vocal folds”) produces buzzing sound
perceived as voicing
Laryngeal opening and epiglottis visible at prenatal week 6
A newborn baby’s vocal folds are < 4 mm (high fundamental frequency)
Note the high fundamental frequency and rapid breath cycles in sound file 3S1
Laryngeal growth patterns diverge for males and females, resulting in different
fundamental frequencies
–
–
–
–
Age 1 year: 400 Hz to 500 Hz
Age 3 to 5 years: 300 Hz
Young adult men: 110 Hz
Young adult females: 200 Hz
3S1 Newborn crying
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Articulators
• Largely derived from the embryo’s pharyngeal
arch apparatus
• At birth, the positioning of the articulators differs
from that in adults
– Epiglottis sits high in the vocal tract, nearly touching
the velum
– Larynx sits high in the vocal tract
– Lips are round
– First primary teeth erupt around age 6 or 7 months;
permanent teeth erupt around 6 or 7 years
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Figure 3.3 Relative positions of craniofacial
structures at birth and in adults (not drawn to scale)
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Auditory System
• External ear (auricle, ear canal): funnels sound into the
head
• Middle ear (tympanic membrane, ossicles): transduces
sound into mechanical vibrations
• Cochlea: transduces mechanical vibrations into neural
impulses
• Vestibulocochlear nerve (CN VIII): carries neural impulses to
the brainstem
• Several relay stations: process and organize the neural
signal
• Auditory cortex: processes and integrates the neural signal
• Human sensitivity range: 20 Hz to over 20 kHz
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• Outer ear
– Prenatal week 6: auricular hillocks begin to appear on the sides of the
embryo’s neck
– By prenatal week 10: hillocks move to their position at the sides of the
head
– By prenatal week 32: folded structure of auricle is complete
• Middle ear
– By prenatal week 16: ossicles have formed as cartilage
– By prenatal week 24: ossification of ossicles is complete
• Inner ear
– Prenatal week 4: precursor of cochlea appears on the surface of the
myelencephalon, deepens into a pit, becomes detached from the
surface
– By prenatal week 22: cochlea reaches its adult size and form
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• Prenatal exposure to sound
• Fetuses have shown responses to sound stimuli as early as prenatal
week 17 (Hepper & Shahidullah, 1994)
• Sound environment includes mother’s voice, maternal organ sounds,
external sounds
• In body tissue and fluid, high frequencies are attenuated
• Fetuses are mostly exposed to sounds < 500 Hz
–
–
In speech signals, these frequencies represent vowels and sonorant
components of consonants
As a result. prosodic elements of speech (e.g., lexical stress, intonation) are
transmitted to the fetus
• At birth, newborns can
–
–
–
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Distinguish their mother’s voice from the voice of another woman
Distinguish the prenatally ambient language from languages with a different
prosodic pattern
Distinguish between many of the world’s speech sounds (this ability is
reduced to phonemic contrasts in the ambient language by age 1 year)
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All Players in Concert: The
Orchestration of Speech
• Some typical characteristics of speech production
– Before adults begin an utterance, they inhale an amount of air that
correlates with the length of the planned utterance (unknown
whether children do this as well)
– Adults and inhale more air when they plan to speak loudly (Hixon,
1973; Stathopoulos & Sapienza, 1997)
– Speakers take auditory and kinesthetic feedback into account while
speaking, detecting and repairing speech errors
– When speaking in a language that was acquired after the first
language, some speakers substitute native sounds for difficult nonnative ones (consult Chapter 8 for more on that topic)
– Coarticulation can occur
• Within words (lip spreading during the [fr] segments in the word “free” in
anticipation of the vowel /i/)
• Across words (lip rounding during the [fj] segments in the words “if you” in
anticipation of the vowel /u/)
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• Motor sequencing ability can be measured with
diadochokinetic tasks (rapid repetition of monosyllables,
e.g. [papapapa …], or multisyllables, e.g., [patapatapata …]
• Monosyllabic and multisyllabic repetition rates increase in
children as a function of age
• Multisyllabic rates outpace monosyllabic rates at age 11
years
• In some families with familial speech sound disorder,
children and adults with a history of speech difficulties had
slower multisyllabic rates, compared to monosyllabic rates
and the same relative deficit was seen in a hand motor
task(Peter & Raskind, 2011; Peter, Matsushita & Raskind,
2012)
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280
260
240
msec
220
200
/pa/
/ta/
180
/pata/
160
140
120
100
Age
6
7
8
9
10
11
12
Figure 3.4 Meta-analysis of syllable durations
(msec) in child productions of diadochokinetic
tasks as a function of age (Fletcher, 1972)
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Connections
• Chapter 7 provides a detailed overview of the
development of prosody, which involves skilled
use of respiratory and phonatory systems
• Chapter 8 addresses ways adults approach
acquiring speech sounds in a second language
• Chapters 10, 11, and 12 discuss how speech
sounds are acquired in languages other than
English
• Chapters 17, 18, and 19 address speech
development in children with structural or
functional differences
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Concluding Remarks
• Even though newborns have almost all the structures
necessary for speech production, it may take up to four
years to learn to speak in such a way that an unfamiliar
listener can completely understand what was said
• One reason is that the structures are not yet in an
optimal spatial orientation for speech
• Another reason is that speech production is an
exquisitely complex process
• Given these complexities, it is astonishing to think that
most children acquire speech without explicit
instruction
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