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Si us plau utilitzeu sempre aquest identificador per citar o enllaçar aquest document: https://hdl.handle.net/2445/221303
Early developmental trajectories in the encoding of speech sound features: the Influence of sex and bilingualism
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[eng] Early speech acquisition relies on the appropriate maturation of the auditory system and
the fine-tuning to a range of speech sound features that define a particular language. This
process initiates early in development as the auditory system becomes functional during the
third trimester of gestation. From that moment, fetuses are exposed to a variety of sounds, both
internal and external to the maternal womb, and the speech acoustic signal is first encountered.
Still, the auditory system and infant acoustic capacities are not fully developed at birth and
overcome a marked maturation during the first infant year, with key developmental processes
such as neuronal migration to the auditory cortex and myelination of the auditory brainstem
extended into this infant stage. Along this developmental journey, infant auditory capacities are
shaped by both genetic and environmental factors, which ultimately affect how speech sounds
are perceived. Biological factors like sex introduce early genetic differences, while
environmental experiences like bilingualism influence the auditory input infants receive. Despite
the clear impact of both sex and bilingualism on speech perception development, the underlying
changes in the neural encoding of speech sounds remain unexplored. The present thesis aimed
at investigating the neural maturation underlying speech sounds encoding during the first infant
year, and how sex and bilingual experiences shape this developmental trajectory.
Speech perception undergoes significant changes during the first year of life, when infants
begin to explore and adapt to the speech sounds in their environment. Milestones in language
development, such as the onset of consonant-vowel articulations at around six months and the
first spoken words by the end of the first year, rely on infants’ accurate phoneme perception
and native-like attunement to speech sounds. For instance, infants need to differentiate
between similar yet distinct sounds to discern phonemic categories relevant to their language.
The development of phonetic perception has been an area of growing interest, with auditory
evoked potentials like the Mismatch Negativity Response (MMN) frequently used to study this
process. However, research into the auditory maturation that supports these infant capacities
remains limited.
For phoneme discrimination, infants must accurately encode the acoustic features that
compose speech signals. The proper encoding of key speech-sound features –such as voice pitch
and formant structure– guides early speech acquisition by providing essential acoustic cues to
the infant. The frequency-following response (FFR) has recently emerged as a valuable auditory
evoked potential capable of capturing the neural encoding of these speech features, making it
ideal for studying their development during infancy. Yet, there is still a gap in the literature
examining how the neural encoding of speech sounds develops alongside early speech
acquisition. The first objective of the present thesis was to disentangle the neural developmental
underlying the encoding of key speech-sound features along the auditory pathway during the
first year of life.
By longitudinally recording FFR responses at birth, six months and twelve months of age,
this thesis investigates its first aim through two complementary studies. In the first study, FFRs
were recorded in response to a two-vowel stimulus (/oa/) from a cohort of 37 healthy-term
infants across these three developmental stages. This study focused on analyzing neural
transmission latency, as well as the neural encoding of voice pitch and formant structure. The
second study extended the sample to 73 infants, maintaining the same experimental procedure
and stimulus while incorporating additional variables, such as perinatal linguistic exposure and
sex. To illustrate developmental trajectories in the neural encoding of speech sound features,
linear mixed-effects models were employed for both voice pitch and formant structure.
Together, these studies provide a comprehensive picture of the typical neural maturation
involved in encoding voice pitch and formant structure of speech during the first postnatal year.
Crucially, they converge on identifying the first six months as a critical period for the maturation
of neural encoding mechanisms essential for speech acquisition. Particularly, they reveal rapid
advancements in the encoding of formant structure, which is crucial for discerning subtle
acoustic differences delimiting phonetic categories.
A sensitive period for the mapping of phonetic categories emerges during the first year of
life, with biological and environmental factors influencing speech perception development. For
example, sex-related biological factors introduce early genetic differences that extend to speech
perception. Additionally, speech acquisition is language-dependent, with early auditory
experiences significantly shaping infants' acoustic capacities from the onset of hearing in utero,
likely affecting their future speech abilities. The second objective of this thesis was to explore
the mediating effects of sex and bilingual exposure on the neural development of speech
encoding across the first infant year. This goal was addressed in the second study, which
uncovered distinct developmental trajectories associated with sex and bilingualism, highlighting
their role in shaping neural attunement to key speech-sound features. By revealing the positive
effect of bilingualism in neural speech encoding by the end of the first year, the findings suggest
that enriched linguistic environments during this critical period could positively influence
auditory processing and speech development.
Overall, the findings enclosed in the present thesis provide important insights into the
neural mechanisms underpinning early speech perception and acquisition, revealing biological
and environmental influences that shape individual variability. Future research examining how
these early neural patterns predict later language outcomes could extend the results from this
thesis, potentially contributing to early detection of language delays or disorders.
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PUERTOLLANO RODRÍGUEZ, Marta. Early developmental trajectories in the encoding of speech sound features: the Influence of sex and bilingualism. [consulta: 29 de novembre de 2025]. [Disponible a: https://hdl.handle.net/2445/221303]