Chapter 3 Biological Substrates of Speech Development: - PowerPoint Presentation

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

IntroductionSpeech is the finishing stage of transducing a thought into sound wavesSpeech sounds are generated by constricting the airstream out of (or into) the lungsMultiple structures and systems converge to generate the complex movement sequences of speechThese 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 saidThis chapter describes some general principles of human development and traces the developmental trajectories of relevant structures and systems 2

Developmental Trajectories3 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 structuresEndoderm: lining of digestive tract, lining of respiratory systemMesoderm: skeleton, muscles, cardiovascular system, reproductive systemEctoderm: skin, central and peripheral nervous system, lens of eye 4Less than 4 mm, heart starts beating830 mm, all major structures and organs are formed9 - birthStructures and organs grow and become more refined22Fetus can survive if born prematurely~ 35 - 38Birth Prenatal Development

Central and Peripheral Nervous Systems (CNS and PNS)Central nervous systemBrain and spinal cord (structures encased in bone plus the retina of the eye)Prenatal Week 4 and 5Neural tube, central canal Rostral end: three bulgesForebrainTelencephalon (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)4

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.

CNS Structure Function Cerebral cortex Sensory processing (visual, auditory , tactile), cognition, linguistic processing, motor events ThalamusRelay center for sensory informationHypothalamusAutonomic functionsEpithalamusRegulation of sleep/wake cycleCerebellumCoordinated and smooth motor functioningMidbrainVarious functions: conduct information, support motor control, auditory and visual perceptionPonsMediate between cerebellum and motor cortex, mediate between higher brain centers and spinal cordMedullaMonitoring of bloodstream for oxygenation and toxins; regulation of heart rate, relay stations for auditory and vestibular informationNeurons 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. 6

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 againAfter birth, there is an excess of neurons and synapsesThese are lost soon (pruning, apoptosis)Neurons generally do not reproduce except in areas important for generating new memories (caudate nucleus, hippocampus) 7

In general, the sensory and motor centers of one hemisphere are connected to the body regions on the opposite sideThe two cerebral hemispheres are not entirely symmetricalLeft insula is larger than rightLeft planum temporale is larger than rightIn most individuals, speech and language processing is mostly lateralized to the left hemisphere 8

Peripheral Nervous SystemPrenatal week 3: Small part of the ectoderm moves to position parallel to the neural tube and forms the neural crestPrenatal week 4: Spinal nerve fibers protrude from the spinal cord; spinal sensory neurons form ganglia; motor and sensory elongate and grow into the limbsPrenatal weeks 5 and 6: Cranial nerves begin to appear 9

Cranial NervesSomatic efferent, originate in brainstemIII O culomotor (eye movement, pupil constriction, proprioception of eye) IV Trochlear (eye movement) VI A bducens (eye movement) XII Hypoglossal (tongue movement) Pharyngeal arch nervesV 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 sensesI Olfactory (smell)II Optic(vision)VIII Vestibulocochlear (hearing, balance)10

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

Respiratory SystemPrenatal week 4: Tracheal budPrenatal week 5: Two bronchial buds that will keep subdividingPrenatal weeks 16 to 26: Lung tissue becomes more vascularized and develops terminal saccules By prenatal week 24: 17 orders of branchesPrenatal 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) L ung volume correlates with breathing frequency (decreases with age) and maximum phonation time (increases with age; decreases again with senescence) 12

LarynxFirst site of air constriction in egressive airstreamCartilagenous structureAdduction of the thyroarytenoid muscles (“vocal folds”) produces buzzing sound perceived as voicingLaryngeal 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 Hz133S1 Newborn crying

ArticulatorsLargely derived from the embryo’s pharyngeal arch apparatusAt birth, the positioning of the articulators differs from that in adultsEpiglottis sits high in the vocal tract, nearly touching the velumLarynx sits high in the vocal tract Lips are roundFirst primary teeth erupt around age 6 or 7 months; permanent teeth erupt around 6 or 7 years 14

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

Auditory SystemExternal ear (auricle, ear canal): funnels sound into the headMiddle ear (tympanic membrane, ossicles): transduces sound into mechanical vibrationsCochlea: transduces mechanical vibrations into neural impulses Vestibulocochlear nerve (CN VIII): carries neural impulses to the brainstemSeveral 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 16

Outer earPrenatal week 6: auricular hillocks begin to appear on the sides of the embryo’s neckBy prenatal week 10: hillocks move to their position at the sides of the headBy prenatal week 32: folded structure of auricle is completeMiddle earBy prenatal week 16: ossicles have formed as cartilageBy 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 form17

Prenatal exposure to soundFetuses 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. p rosodic elements of speech (e.g., lexical stress, intonation) are transmitted to the fetus At birth, newborns canDistinguish their mother’s voice from the voice of another womanDistinguish the prenatally ambient language from languages with a different prosodic patternDistinguish between many of the world’s speech sounds (this ability is reduced to phonemic contrasts in the ambient language by age 1 year)18

All Players in Concert: The Orchestration of SpeechSome typical characteristics of speech productionBefore 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 W ithin 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/)19

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) 20

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

ConnectionsChapter 7 provides a detailed overview of the development of prosody, which involves skilled use of respiratory and phonatory systemsChapter 8 addresses ways adults approach acquiring speech sounds in a second languageChapters 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 22

Concluding RemarksEven 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 saidOne reason is that the structures are not yet in an optimal spatial orientation for speechAnother 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 23