Olfactory-to-visual facilitation in the infant brain declines gradually from 4 to 12 months

Abstract

During infant development, intersensory facilitation declines gradually as unisensory perception improves. However, this developmental trade-off has been mainly investigated using audiovisual stimulations. Here, fifty 4- to 12-month-old infants were tested to determine whether the facilitating effect of their mother’s body odor on neural face categorization, as previously observed at 4 months, decreases with age. In a baseline odor context, results revealed a face-selective electroencephalographic (EEG) response that increases and changes qualitatively between 4 and 12 months, marking improved face categorization. At the same time, the benefit of adding maternal odor fades gradually with age, indicating an inverse relation with the strength of the sole visual response, and generalizing to olfactory-visual interactions previous evidence from the audiovisual domain.

Olfactory facilitation of visual categorization in the 4-month-old brain depends on visual demand

“…As in previous studies showing a maternal odor effect on rapid face categorization (Leleu et al, 2020;Rekow et al, 2023Rekow et al, , 2021Rekow et al, , 2020, the first set (Nat Group) consisted in natural color images in which the item appears at variable locations, under variable lighting conditions, and from variable sizes and viewpoints, together with different facial expressions and external features (e.g., haircut) for faces. Items were embedded in their original background, implying figure-ground segregation.…”

“…Infants of both groups were exposed to two odor contexts: the mother's body odor and a baseline odor, which were delivered using white cotton t-shirts as in previous studies (Durand et al, 2013;Leleu et al, 2020;Rekow et al, 2023Rekow et al, , 2021Rekow et al, , 2020. T-shirts were laundered beforehand in the lab with a scentless hypoallergenic powder detergent (Persavon, France) and then stored in a hermetic zip-locked plastic bag.…”

“…For both groups, the procedure was identical to that of previous frequency-tagging EEG studies showing a maternal odor effect on rapid face categorization (Leleu et al, 2020;Rekow et al, 2023Rekow et al, , 2021Rekow et al, , 2020. After placement of the electrode cap, infants were installed in a baby car seat in front of the stimulation screen (Figure 1B) in a light-and sound-attenuated room equipped with an air-extractor located approximately 2 m above the seat and which continuously and silently renewed the air.…”

“…To increase signal-to-noise ratio, invalid segments were rejected using a data-driven criterion which consisted in identifying segments with no general visual response at first (6 Hz) and second (12 Hz) harmonics (used as a marker of the infant's attention to the stimulation sequence; Leleu et al, 2020;Rekow et al, 2023Rekow et al, , 2021Rekow et al, , 2020. Baseline-corrected amplitude spectra were extracted for each segment and Z-scores were calculated (see section 2.6) to estimate significance at 4 medial occipital electrodes (POz, Oz, O1, O2) that exhibit the largest response to a 6-Hz stream of complex visual stimuli in the infant brain (e.g., Leleu et al, 2020).…”

“…Although the following analysis pipeline was not pre-registered, as it was not a common practice in our lab at the time we designed the study, analyses were mostly planned before data collection based on previous studies on maternal odor influence on rapid face categorization (Leleu et al, 2020;Rekow et al, 2023Rekow et al, , 2021Rekow et al, , 2020. First, a fast Fourier transform was applied to the two segments (one per odor context) of each infant and raw amplitude spectra were extracted for all electrodes with a frequency resolution of 1/32 s = 0.03125 Hz.…”