Early Vocal Development in Tuberous Sclerosis Complex

Abstract

Background

Our goal was to assess for the first time early vocalizations as precursors to speech in audio-video recordings of infants with tuberous sclerosis complex (TSC).

Methods

We randomly selected 40 infants with TSC from the TSC Autism Center of Excellence Research Network dataset. Using human observers, we analyzed 74 audio-video recordings within a flexible software-based coding environment. During the recordings, infants were engaged in developmental testing. We determined syllables per minute (volubility), the number of consonant-vowel combinations, such as ‘ba’ (canonical babbling), and the canonical babbling ratio (canonical syllables/total syllables) and compared the data with two groups of typically developing (TD) infants. One comparison group's data had come from a laboratory setting, while the other's had come from all-day Language Environment Analysis recordings at home.

Results

Compared with TD infants in laboratory and all-day Language Environment Analysis recordings, entry into the canonical babbling stage was delayed in the majority of infants with TSC, and the canonical babbling ratio was low (TD mean = 0.346, SE = 0.19; TSC mean = 0.117, SE = 0.023). Volubility level in infants with TSC was less than half that of TD infants (TD mean = 9.82, SE = 5.78; TSC mean = 3.99, SE = 2.16).

Conclusions

Entry into the canonical stage and other precursors of speech development were delayed in infants with TSC and may signal poor language and developmental outcomes. Future studies are planned to assess prediction of language and developmental outcomes using these measures in a larger sample and in more precisely comparable recording circumstances.

Introduction

Tuberous sclerosis complex (TSC) is associated with a variety of neurodevelopmental issues collectively known as TSC-associated neuropsychiatric disorders (TAND),¹ including significant problems in communication and language.2, 3 In fact, only 28% of individuals living with TSC have normal language.⁴ These communication and language concerns may be in part attributable to the fact that TSC is a leading cause of autism spectrum disorder (ASD), impacting up to 50% of individuals with TSC.4, 5, 6, 7

However, detailed studies of language in TSC are rare.⁴ The goal of this preliminary study was to examine the origin of language in infants with TSC using audio-video recordings from the TSC Autism Centers of Excellence Research Network (TACERN). Such vocal development research has never been conducted in TSC, and consequently there has been little basis upon which to evaluate possible early vocal predictors of the language difficulties attendant to TSC. We aimed to analyze precursors to vocal development in infants with TSC. We hypothesized that these infants would display delays in these precursors. Our long-term goal is to develop a body of knowledge in TSC about a variety of precursors to speech and language, both vocal and nonvocal, precursors that may help to illuminate the development of language difficulties and other neurodevelopmental disorders in infants with TSC. We use methods not previously used in TSC research and introduce necessary terminology from the literature on early language development (see key terminology box).

Key Terminology (see the Supplementary Material for additional details)

Protophones: Vocalizations that are precursors to speech, occurring from the first day of life and throughout the first year, excluding cries and laughs (see [8] for recordings of all the types).

Precanonical protophones: Vocalizations that predominate in the first half year, differentiated primarily by phonatory (vocal fold vibration) characteristics, and categorized often as vocants (vowel-like sounds), squeals, and growls.

Canonical babbling (CB): Protophones that first appear in the second half year of life, showing well-formed (i.e., canonical) syllable structure as is required for words in speech, typified by consonant-vowel combinations such as ‘ba,’ ‘da,’ and ‘na,’ or in reduplicated babbling such as ‘bababa’ or ‘nanana.’

Volubility: The total number of protophone syllables, both canonical and precanonical, per minute.

Canonical babbling ratio (CBR): The number of canonical syllables in a recorded sample divided by the total number of syllables (the volubility).

Canonical babbling stage: In the traditional literature, infants have been categorized as in the canonical stage when CBR ≥0.15. Canonical babbling stage is required for children to acquire vocabulary, and late-onset or low CBR are predictive of communication disorders and delays.

LENA (Language Environment Analysis): The LENA system uses a small wearable device worn by infants in their natural environment. The device records all sounds from the infants and individuals in the infant's environment.

TACERN (TSC autism centers of excellence): An NIH-funded initiative designed to optimize multisite research and share data, which includes Boston Children's Hospital, Cincinnati Children's Hospital Medical Center, the University of Alabama at Birmingham, University of California at Los Angeles, and the University of Texas at Houston.