Biological Substrates of Speech Development

Ray D KentUniversity of Wisconsin-Madison

kent@waisman.wisc.edu

3 Major Themes

• Performance Anatomy – Structure is shaped partly by function

• Developmental Motor Control– Early distinction between motor control for

speech vs. motor control for nonspeech acts• Action-Perception Linkages

– Actions and the perceptions of those actions are fused in cortical representations that are present in neonates

Performance Anatomy

Developmental Motor Control

Action-perception Linkages

Babbling andearly words

Who Babbles?

Setting the Stage

• How is babbling affected by the ambient language (babbling drift)?

• How does babbling relate to early words?• How is babbling influenced by clinical

conditions?• Does babbling have clinical predictive

value?

Effect of Ambient Language• An effect of ambient language on infant sound

production has been observed by • 2 months (Ruzza, Rocca, Boero, & Lenti, 2003), • 6 months (Boysson-Bardies, Sagart, & Durand, 1984), • 9 months (Boysson-Bardies, Vihman, Roug-Hellichjius,

Durand, Landberg, & Arao, 1992), • 10 months (Boysson-Bardies, Halle, Sagart, & Durand, 1989;

Boysson-Bardies, Sagart, Halle, & Durand, 1986) • 12 months (Chen & Kent, 2005; Grenon, Benner, & Esling,

2007; Koponen, 2002; Levitt & Utman, 1992; Whalen, Levitt, & Wang, 1991).

Hearing Loss in Infancy

• Research on infants with hearing loss shows that their vocalizations differ from those of normal-hearing infants by the age of 8 to12 months of life.

• Specifically, delays in the onset of canonical babbling, along with reduced phonetic variation, have been reported for infants with hearing loss.

[Kent, Netsell, Osberger, & Hustedde, 1987; Koopmans-van Beinum, Clement, & van den Dikkenberg-Pot, 2001b; McGowan, Nittrouer, & Chenausky, 2008; Oller & Eilers, 1988; Scheiner, Hammerschmidt, Jurgens, & Zwirner, 2006; Stoel-Gammon & Otomo, 1986]

Tracheostomized Infants• Studies of infants tracheostomized during all or

part of the period when babbling is expected [Bleile, Stark, & McGowan, 1993; Kamen & Watson, 1991; Kertoy,

Guest, & Quart, 1999; Kraemer, Plante, & Green, 2005; Locke & Pearson, 1990].

• As a consequence of the medical intervention, the infants in these studies had limited opportunity to produce speech-like sounds associated with normal phonation and other laryngeal function.

• The general conclusion was that these children experienced difficulties with speech and language that persisted well beyond the time of decannulation

Babbling as a Predictor of Communication Outcome

• Babbling, especially with regard to its CV and consonantal composition, has been demonstrated to have predictive value for subsequent speech and language outcomes in children with a variety of disorders, including – orofacial clefting (Chapman, Hardin-Jones, & Halter, 2003;

Lohmander-Agerskov, Soderpalm, Friede, & Lilja, 1998; Scherer, Williams, & Proctor-Williams, 2008),

– otitis media (Rvachew, Slawinski, Williams, & Green, 1999), – expressive language delay (Fasolo, Majorano, & D’Odorico,

2008; Whitehurst, Smith, Fischel, & Arnold, 1991),– infants considered at high risk (Oller, Eilers, Neal, & Cobo-

Lewis, 1998).

The Anatomic Basis of Speech

• The present focus is on the craniofacial system in which the vocal tract resides, but the laryngeal and respiratory systems cannot be neglected

• The human craniofacial anatomy is unique in both its macro-anatomy and micro-anatomy

• This anatomy is molded by genetics and by function (use)

Chimpanzee vs Adult Human Vocal Tracts

The Head, Craniofacial System, and Vocal Tract

• Craniofacial evolution is fundamental to the origin of vertebrates (Trainor, 2005)

• “…there is no theory of segmentation that can account for all cephalic iterative structures” (Northcutt, 2008)

• “…no structural component has autonomy of form” (Kean & Houghton, 1987)

Rationale for Research

• Craniofacial malformations are involved in three fourths of all congenital birth defects in humans (Chai & Maxson, Dev Dys, 2006)

• Models of voice and speech production are based largely on the anatomy and physiology of adult males and do not take account of sex and age differences

• We lack a comprehensive theory of speech development that exploits available information on developmental biology

Vocal Tract Length

How Does the Craniofacial System Grow?

• The human head is a complex anatomical system consisting of uniquely shaped elements and a variety of tissue types.

MRI High-speed CT

Craniofacial anatomy shaped by biomechanical forces

Moss’s Functional Matrix theory

Bosma’s theory ofPerformance Anatomy

Developmental Performance Anatomy based on advances in biology

GeneticsMolecular biologyEmbryology

Scammon’s Morphogenetic Schedules

1800s

1930s

1970s1960s

Today

Original status

Compression

Tension

Original statusShear Bending

Original status Torsion

Tissue Growth Types, based on Scammon

Neural

Lymphatic

Somatic

Lingual

F ig u re 5 6 . G ro w th c u rv e s a d a p te d fro m S c a m m o n ( 1 9 3 0 )

Lingual(Vorperian & Kent)

Moss’ Functional Matrix

• “The functional matrix is primary and the presence, size, shape, spatial position, and growth of any skeletal unit is secondary, compensatory, and mechanically obligated to changes in the size, shape, spatial position of its related functional matrix” (Moss, 1968).

• The functional matrix incorporates relevant soft tissues, including muscles, glands, nerves, and the spaces.

Bosma’s Functional Anatomy

• Bosma (1975, 1976) theorized that the vocal tract has a “performance anatomy,” meaning that its structure is determined by how the system is used.

• He further suggested that different models of speech production would be required to account for different ages of development

Long-face Syndrome aka “adenoid facies”

Increased vertical height in lower third of faceExcessive dento-alveolar height“Gummy” smileHigh arched palateSteep mandibular plane

Cause: Nasal obstruction

Source: Dr. Christel Hummert

FMFemale13y 6m

Mouth breather; Enlarged pharyngeal tonsil (adenoid)

*

Recent Clinical Evidence• (1) Individuals with large volumes of the masseter and medial pterygoid muscles

have relatively flat mandibular and occlusal planes, along with small gonial angles.• (2) Congenital Fiber-Type Disproportion myopathy is associated with a narrow

maxillary arch, labial incompetence, severe skeletal open bite, and weakness of the masticatory muscles.

• (3) Children with obstructive sleep apnea have increased overjet, reduced overbite, and narrower upper and shorter lower dental arches.

• 4) Compared to a control group, children who received activator-headgear Class II treatment for at least 9 months had a greater reduction in ANB angle, a greater increase in pharyngeal area, pharyngeal length, and the smallest distance between the tongue and posterior pharyngeal wall.

• (5) Children with otitis media with effusion have an altered facial morphology, as reflected in measures of anterior cranial base length, upper facial height, size of the hard palate, facial depth, facial axis, mandibular length, and inferior pharyngeal airway.

• (6) Individuals with Duchenne muscular dystrophy have an altered craniofacial morphology that appears to result from an imbalance of strength in the orofacial muscles.

Lamina Propria of Vocal Folds

• A recent study of unphonated vocal folds in three young adults evinced abnormalities in vocal fold mucosa presumably due to the lack of mechanical stimulation normally provided by phonation

• The vocal fold mucosae were hypoplastic and rudimentary, lacking a vocal ligament, Reinke's space, and layered structure. – (Sato, Nakashima, Nonaka, & Harabuchi, 2008)

Developmental Performance Anatomy

• Endogenous and exogenous factors combine to influence postnatal craniofacial development.

• It is likely that the craniofacial and extraocular muscles have distinct patterns of gene expression.

• Interaction between genetics and extrinsic factors begins in embryology, where morphogenesis depends on the reactions of cells to the conditions created by their own growth and the growth of proximal cells.

Palatal Shapes

Typically developing Down syndrome

3-D modeling Based on Imaging Data

Yellow -- mandible

Blue -- vocal tract

Red -- palate

Green -- hyoid bone

Performance anatomy

Speech motor control

Action-perception linkage

Emergence of Speech Motor Control

• A popular conception is that motor control for speech builds on pre-existing motor control for nonspeech behaviors (e.g., feeding)

• This idea is a core assumption to MacNeilage and Davis’ Frame-Content Theory

• Recent evidence prompts a reconsideration of this idea

Speech and Nonspeech Motor Development

• The central conclusion of several studies is that, early in infancy, speech-like movements are distinct from movements for nonspeech behaviors.

• Accordingly, speech motor control appears to develop in parallel with nonspeech motor functions, rather than being derived from them.

[Moore & Ruark, 1996; Ruark & Moore, 1997; Steeve, Moore, Green, Reilly, & McMurtrey, 2008; Wilson, Green, Yunusova, & Moore, 2008)]

Mammalian Muscle Fibers

• There are at least nine different mammalian MyHC isoforms. – Embryonic and neonatal are developmental isoforms

– Cardiac alpha and beta are "slow" forms expressed in the heart. The cardiac beta is also found in slow skeletal muscle fibers (in which case it is called type I).

Mammalian Muscle Fibers, cont.– The remaining forms are found in fast skeletal muscle:

– Type IIA is found in most fast oxidative-glycolytic (FOG) fibers

– Type IIB and type IIX in fast glycolytic (FG) fibers. These are relatively rare and appear to be expressed primarily in the extraocular, laryngeal, masticatory, and lingual muscles.

– Type IIM and extraocular

Muscle Fiber Types

• Isoforms isted in order of contraction speed, from slow to fast:

I - IC - IIC - IIAC - IIA - IIAB - IIB – IIX• In addition, hybrid muscle fibers co-

express two or more isoforms, and these have special relevance to the craniofacial muscles where they are found in unusual proportions.

Vocal Folds

Soft palateLips

Mandible Tongue

Pharynx

0%10%20%30%40%50%60%70%80%90%

100%

Anterior Mid Ant Mid Post Posterior

IIC/IMIM/IICIIABIIBIIAI

Percentage of muscle area formed by different fiber types

Muscles of the tongue

Lingual Muscles

• Stal et al. noted that the muscle fiber composition of the tongue differs from that in the limb, orofacial, and masticatory muscles.

• The predominance of type II fibers and regional heterogeneity were interpreted as a means for fast and flexible actions in positioning and shaping the tongue. The combination of type I, IIA, and IM/IIC fibers may contribute to lingual bending.

Masticatory Muscles

TemporalisMasseterPterygoid

Large number of hybrid fibers

MylohyoidGeniohyoidDigastric

Fewer hybrid fibers and fewerfibers expressing MyHC-I, MyHC-fetal, & MyHC-cardiac alphaMore fibers expressing MyHC-IIA

Korfage, Brugman, and Van Eijden (2000)

Masticatory Muscles

• Koolstra (2002) notes that the human masticatory system seems to have more muscles than are needed for its purposes.

• The apparent surfeit of muscles is understandable when it is recognized that the masticatory system meets both mechanical and spatial requirements.

Masticatory Muscles – Distinctive Properties

• Contain at least four different isoforms of myosin heavy chain

• Have a continuous range of contraction speeds

• Have a high oxidative capacity and are therefore very fatigue resistant

(Weijs, 1997)

0%10%20%30%40%50%60%70%80%90%

100%

Pal-phar Uvular LVP TVP

IIC/IMIIABIIBIIAI

Fast Movements Slower, morecontinuous movements

Stal & Lindman, J. Anat., 2000

Palatal muscles

A New Pharyngeal Muscle

• Mu and Sanders (2008) describe a a newly discovered muscle, the cricothyropharyngeus

• This muscle has unusual MyHC isoforms including slow-tonic, alpha-cardiac, neonatal, and embryonic.

• They believed that this muscle may have a specialized function in speech, which may explain its uniqueness to humans.

Muscle Properties

Speech muscles have properties that seem highly suited to their specialized roles in phonation and articulation:

• Fatigue resistance• Rapid shortening • Very slow shortening• Functional variation within and across muscles

Performance anatomy

Speech motor control

Action-perception linkage

Looking to the Future --Neuroscience

“As for the future of the field, I thinklanguage development will be covered at different levels in several disciplines. There is very exciting brain research going on right now—for instance the discovery of mirror neurons provides a new way of interpreting early imitative behaviour.” IASCL - Child Language Bulletin - Vol 26, July 2006

Jean Berko Gleason

“Monkey see”

“Monkey do”

Dalai Llama Neurons

Mirror Neurons (aka Dalai Llama neurons)

• Discovered by Iaccomo Rizzolati of the University of Parma in 1995.

• V.S. Ramachandran predicted that mirror neurons would do for psychology what DNA did for biology by providing a unifying framework and help explain a host of mental abilities that have hitherto remained mysterious and inaccessible to experiments.

Mirror Neurons and Autism

Action-Perception Networks

• Can explain seemingly precocious imitative behaviors, such as neonates imitating adult facial gestures

• Can account for aspects of vocal imitation in infancy

• Provide a basis for the efficient learning of behaviors

• May be a neural foundation for language development

Developmental Profile Based on Fagan

• 7 months – onset of canonical babbling • 9 months - maximum frequency of repetitions

per utterance, after which frequency of repetitions declined

• 8.4 months - the mean age of onset of word comprehension

• 11.8 months - first word production

Babbling

• Babbling is a behavior based on a developmental anatomy that is shaped in part by its uses. Babble helps to create the anatomy for adult speech.

• Babbling draws on action-perception linkages present to some degree at birth but are refined with experience to create internal models that guide speech production.