Human infants are able to acquire quickly a language by “simple” exposure to speech. This remarkable capacity is rather surprising knowing that the auditory cortices are still developing until the end of adolescence. Many psychoacoustic studies have explored how the adult auditory system processes the acoustic components of speech, showing that slow variations in sound amplitude over time (AM) are of crucial importance for speech perception. In the present study, we investigated the processing of AM using electroencephalography (EEG) in 3- and 10-month-old awake infants, as well as in young adults. We measured the auditory brain activity following the modulation frequency of sinusoidally amplitude modulated tones at 8 and 40 Hz. Results showed that the Amplitude-Modulation Following Response (AMFR) was observable at 8 Hz at all ages, but that only adults showed reliable responses at 40 Hz. In light of the previous literature, we propose that the neural processing sources of faster AM rates may be located preponderantly in subcortical regions at younger ages, while, in adults, both cortical and subcortical sources are activated. Such neuro-developmental trajectories might underlie the processing of fine acoustic cues that are necessary for speech and language development.

uditory detection of the Amplitude Modulation (AM) of sounds, crucial for speech perception, improves until 10 years of age. This protracted development may not be explained only by sensory maturation, but also by improvements in processing efficiency: the ability to make efficient use of available sensory information. This hypothesis was tested behaviorally on 86 6-to-9-year-olds and 15 adults using AM-detection tasks assessing absolute sensitivity, masking and response consistency in the AM domain. Absolute sensitivity was estimated by the detection thresholds of a sinusoidal AM applied to a pure-tone carrier; AM masking was estimated as the elevation of AM- detection thresholds produced when replacing the pure-tone carrier by a narrowband noise; response consistency was estimated using a double-pass paradigm where the same set of stimuli was presented twice. Results showed that AM sensitivity improved from childhood to adulthood, but not between 6 and 9 years. AM masking did not change with age, indicating that the selectivity of perceptual AM filters was adult-like by 6 years. However, response consistency increased developmentally, supporting the hypothesis of reduced processing efficiency in childhood. At the group level, double-pass data of children and adults were well simulated by a model of the human auditory system assuming a higher level of internal noise for children. At the individual level, double-pass data were better simulated when assuming a sub-optimal decision strategy in additionto differences in internal noise. Processing efficiency for AM detection is reduced in childhood, and this is explained by both systematic and stochastic inefficiencies.