Fig. 1.

(A) Estimated projection of the optodes on the brain of a 30 w GA preterm infant. The geometric layout of the optical probe and its projection on the brain mesh of an individual 30 wGA preterm infant (courtesy of P. Hüppi and J. Dubois). Numbers 1–16 correspond to the light sources and letters A, B, C and D correspond to the detectors. (B) Auditory stimulation paradigm. Five series of four syllables were presented (SOA = 600 ms) in a block (duration 20 s), followed by 40 s of silence. Three types of blocks were randomly presented for a total duration of 108 min. In standard blocks, the same syllable (/ba/or/ga/) was presented, whereas, in deviant blocks, a change of voice (DV) or a change of phoneme (DP) was presented three times. (C) HB-ERP and HD-fOI system. Left: Preterm neonate with high-density EEG cap. The infants were tested while sleeping at night to avoid daylight and the intense daytime activity of a neonatal care unit. They were placed in the supine position on a comfortable pad in a dark and quiet incubator. The incubator was further protected from ambient light by dark sheets. Stimuli were binaurally presented at a comfortable hearing level (≈70 dB) via speakers placed at the infant’s feet, at a distance of 30 cm from the subject’s head. Right: A special probe made from soft, flexible foam (thickness = 5 mm) was designed to comfortably maintain the source and detector fibers on the infant’s head. A patch comprising two detectors and eight emitters (optodes) perpendicular to the head surface was placed on each side of the head. Each of the eight optodes contained two wavelength emitter glass fibers (690 and 830 nm). The probe was smoothly secured to the infant’s head to cover the perisylvian areas on each side with straps and foam padding. The probe was designed as a round-grid layout, maintaining a distance of 15 mm between the optodes (Fig. 1A). This layout allowed 10 measuring points (channels) that were simultaneously sampled on each hemisphere.