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

To navigate their environment, infants rely on intersensory facilitation when unisensory perceptual demand is high, a principle known as inverse effectiveness. Given that this principle was mainly documented in the context of audiovisual stimulations, here we aim to determine whether it applies to olfactory-to-visual facilitation. We build on previous evidence that the mother's body odor facilitates face categorization in the 4-month-old brain, and investigate whether this effect depends on visual demand. Scalp electroencephalogram (EEG) was recorded in two groups of 4-month-old infants while they watched 6-Hz streams of visual stimuli with faces displayed every 6th stimulus to tag a face-selective response at 1 Hz. We used variable natural stimuli in one group (Nat Group), while stimuli were simplified in the other group (Simp Group) to reduce perceptual categorization demand. During visual stimulation, infants were alternatively exposed to their mother's versus a baseline odor. For both groups, we found an occipito-temporal face-selective response, but with a larger amplitude for the simplified stimuli, reflecting less demanding visual categorization. Importantly, the mother's body odor enhances the response to natural, but not to simplified, face stimuli, indicating that maternal odor improves face categorization when it is most demanding for the 4-month-old brain. Overall, this study demonstrates that the inverse effectiveness of intersensory facilitation applies to the sense of smell during early perceptual development. RESEARCH HIGHLIGHTS: Intersensory facilitation is a function of unisensory perceptual demand in infants (inverse effectiveness). This inverse relation between multisensory and unisensory perception has been mainly documented using audiovisual stimulations. Here we show that olfactory-to-visual facilitation depends on visual demand in 4-month-old infants. The inverse effectiveness of intersensory facilitation during early perceptual development applies to the sense of smell.