Abstract
Human newborns do not have a fixed set of olfactory likes or dislikes because they acquire them mainly by evaluative conditioning. However, odorants elicit differential preferential responses, even without prior experience.
Odorant stimuli generally convey olfaction per se as well as some degree of intranasal chemesthesis. This tactile confound of the odour sensation mediated by the trigeminal system, whose stimulation may result into neurological airway protection processes, could be a potential predictor of spontaneous preferential responsiveness to unknown odorants.
In our study we explored if unfamiliar odorants with contrasted trigeminal intensity led to stronger defensive response (heart rate (HR) acceleration) indicative of arousal magnitude and perceived unpleasantness in newborns. To fifty of them (two-to-three-days old) we administered three randomized order odorants; two of them unfamiliar with different trigeminal pungency (one strong, one weak) and one blank.
Repeated-measures ANOVA revealed that the newborns' HR exhibited a significant interaction of the odorant with the course of HR variation in the first trial. Further, repeated planned contrasts showed a significant difference between odorants across repeated measures circa half a minute after the presentation.
However, paired samples ttests have shown that the significant difference was for the blank stimulus only, the strong trigeminal odorants induced a slight HR increase compared to which the weak ones induced a stronger HR increase; blank stimuli a marked drop. Further, no such effects were found in the two, consequent trials.
At this moment, our findings do not support asymmetric processing of odours with different trigeminal component at the HR level. However, we do provide further evidence that newborns in irregular sleep respond differentially to very low dilutions of odorants.