Characterizing olfactory brain responses in young infants

Odor perception plays a critical role in early human development, but the underlying neural mechanisms are not fully understood. To investigate these, we presented appetitive and aversive odors to infants of both sexes at one month of age while recording functional magnetic resonance imaging (fMRI) and nasal airflow data. Infants slept during odor presentation to allow MRI scanning. We found that odors evoke robust fMRI activity in bilateral olfactory cortex and thalamus, and that fMRI response magnitudes in olfactory cortex differ across odors. However, in contrast with prior work in adults, we did not find compelling evidence that odor stimuli evoke discriminable fMRI activity patterns in olfactory cortex or thalamus using two different multivariate pattern analysis techniques. Finally, the average inhale airflow rate was higher for appetitive odors than aversive odors, which tentatively suggests that infants could modulate their respiration to reflect odor valence. Overall, these results show strong neural responses to odors at this early developmental stage, and highlight nasal airflow as a behavioral metric for assessing odor preference in infants. Significance statement The sense of smell facilitates several adaptive behaviors in infants (e.g., feeding, soothing), but the brain areas supporting infant olfaction are understudied. Here, we delivered appetitive and aversive odors to sleeping infants during functional magnetic resonance imaging. We show that odors evoke activity in olfactory brain regions and thalamus already at one month of age, and activity levels vary across odors in some of these regions (e.g., piriform cortex, amygdala). However, we did not find strong evidence for pattern-based odor information in the same brain areas. Finally, preliminary nasal airflow findings suggest that infants inhale more vigorously in response to appetitive compared to aversive odors. Taken together, these findings advance our understanding of the neural mechanisms of infant olfaction.