Relationships between click auditory brainstem response and speech frequency following response with development in infants born preterm.

The speech evoked frequency following response (sFFR) is used to study relationships between neural processing and functional aspects of speech and language that are not captured by click or toneburst evoked auditory brainstem responses (ABR). The sFFR is delayed, deviant, or weak in school age children having a variety of disorders, including autism, dyslexia, reading and language disorders, in relation to their typically developing peers. Much less is known about the developmental characteristics of sFFR, especially in preterm infants, who are at risk of having language delays. In term neonates, phase locking and spectral representation of the fundamental frequency is developed in the early days of life. Spectral representation of higher harmonics and latencies associated with transient portions of the stimulus are still developing in term infants through at least 10 months of age. The goal of this research was to determine whether sFFR could be measured in preterm infants and to characterize its developmental trajectory in the time and frequency domain. Click ABR and sFFR were measured in 28 preterm infants at ages 33 to 64 weeks gestational age. The sFFR could be measured in the majority of infants at 33 weeks gestational age, and the detectability of all sFFR waves was 100% by 64 weeks gestational age. The latency of all waves associated with the transient portion of the response (waves V, A, and O), and most waves (waves D and E) associated with the quasi-steady state decreased with increasing age. The interpeak wave A-O latency did not change with age, indicating that these waves share a neural generator, or the neural generators are developing at the same rate. The spectral amplitude of F and the lower frequencies of the first formant increased with age, but that for higher frequencies of the first formant and higher harmonics did not. The results suggest that the sFFR can be reliably recorded in preterm infants, including those cared for in the neonatal intensive care unit. These findings support that in preterm infants, F amplitude continues to develop within the first 6 months of life and develops before efficient representation of higher frequency harmonics. Further research is needed to determine if the sFFR in preterm infants is predictive of long-term language or learning disorders.