To make sense of their environment, human infants rely on odor cues along with cues from the other senses. Earlier studies have indeed found that infant visual perception can be facilitated by the reception of odor cues, the mother’s body scent boosting neural face categorization at 4 months of age. Interestingly, however, previous infant studies in the audiovisual domain found that such intersensory facilitation is noted only when unisensory perception is not fully effective, a principle known as ‘inverse effectiveness’. Therefore, we aim to evaluate here whether this principle also applies to olfactory-visual interactions by manipulating the demand of the visual task. We recorded scalp EEG in 2 groups of 4-month-old infants while they watched rapid streams of pictures (6 pictures/sec = 6 Hz) with human faces inserted every 6th picture to tag a face-selective response at 1 Hz in the EEG spectrum. Group 1 was exposed to variable natural images (high visual demand), while images in group 2 were simplified to make face categorization less demanding. During visual stimulation, infants were alternatively exposed to their mother’s body odor or to a baseline odor. We expected to replicate a maternal odor effect on face-selective neural activity in group 1 while no odor effect should be found in group 2. For both groups, we found an occipito-temporal face-selective neural response, but with a significantly larger amplitude for the simplified images, reflecting less demanding categorization. Importantly, maternal odor enhances the response to the natural, but not to the simplified, face images, supporting our hypothesis that maternal odor improves face categorization only when the visual task is demanding for the 4-month-old brain. Overall, this study reveals that the principle of inverse effectiveness can apply to olfactory-visual interactions in the developing brain.
This work was financially supported by the French National Research Agency (contract ANR-19-CE28-0009)