Beate Peter

1. Chapter 3Biological Substrates of Speech Development: A Brief Synopsis of the Developing Neuromuscular System Beate Peter

2. 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

3. Developmental Trajectories Prenatal Development

4. 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)

5. 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.

7. 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)

8. 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 planumtemporale is larger than right • In most individuals, speech and language processing is mostly lateralized to the left hemisphere

9. 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

10. 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)

11. Figure 3.2 Cranial nerves V (Trigeminal), VII (Facial), IX (Glossopharyngeal), X (Vagus), and XII (Hypoglossal)

12. 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)

13. Larynx 3S1 Newborn crying • 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

14. 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

15. Figure 3.3 Relative positions of craniofacial structures at birth and in adults (not drawn to scale)

16. 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

17. 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

18. 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 • 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)

19. 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 non-native 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/)

20. 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)

21. Figure 3.4 Meta-analysis of syllable durations (msec) in child productions of diadochokinetic tasks as a function of age (Fletcher, 1972)

22. 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

23. 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