Recording procedure
The neural responses to a click stimulus and to the consonant-vowel /da/
were recorded while the neonates were naturally sleeping in their
cradle. A vertical montage was used with the active electrode located
above nasion at the 10% of the distance from nasion to
inion59 (Fpz according to the International 10-20 EEG
system), the ground electrode placed between Fpz and nasion, and the
reference electrodes placed at the mastoids. According to previous
studies39,49,51-53, only the right mastoid
(ipsilateral reference) was used. Stimulus presentation, EEG recording
and data repository were carried out by the portable SmartEP equipment,
including the cABR and Advanced Hearing Research modules
(Intelligent Hearing Systems, Miami, Fl, USA). Firstly, to confirm
auditory pathway integrity, two blocks of 2000 ABRs to a click stimulus
(100 µs square; 60 dB SPL) were recorded. Then, the FFR was collected
following the standards published in our previous
studies39,53. Four blocks of 1000 artifact-free
responses were recorded to the consonant-vowel /da/. The speech stimulus
was created using a Klatt-based synthesizer60 and
modified in Praat61. The stimulus duration was 170 ms
(10 ms onset period, 47 ms for the consonant transition and 113 ms for
the vowel region) with a constant fundamental frequency
(F0) of 113 Hz. During the consonant transition, the
first and second formants (F1, F2)
varied from 553 to 688 Hz and 1438 to 1214 Hz, respectively, remaining
stable in the vowel region. The speech stimulus was presented at 60 dB
SPL every 100.27 ms in alternating polarities though ER3C shielded
earphones of 300 Ω (Etymotic Research Inc., Elk Grove Village, IL, USA).
The continuous EEG sampled at 13333 Hz was online bandpass filtered from
30 to 1500 Hz and epoched in sweeps of 270.27 ms (-40.95 ms to 229.32
ms). Any sweep containing activity > ±30 µV was rejected
during the recording. Less than 3% of sweeps were rejected in click or
/da/ stimuli blocks and impedances were kept <8 kΩ throughout
the entire recording. Responses were averaged for each stimulus and
neonate and the subsequent evoked potentials were offline bandpass
filtered from 80 to 1500 Hz.