Several studies carried out on animals indicate the possibility of long-term memorisation of odours acquired during infancy. Infantile olfactory memory has been examined particularly in relation to the orientation of social and sexual preferences. In domestic dogs, maternal odour is retained by puppies, and even after two years of total separation, they remain capable of recognising their mother's odour (Hepper 1994). This 'olfactory imprinting' process has been analysed more thoroughly in rodents. Young female mice raised in the absence of adult males do not display, as adults, a normal attraction for male mice (Mainardi 1963). This effect is due to an olfactory mechanism: female mice whose mothers were odorised with Parma violet odour throughout the nursing period, when sexually mature, manifest a preference for males with this odour compared with test males. On the other hand, females raised in a normal environment prefer non-odorised males (Mainardi et al. 1965). This early effect of maternal odorising is not however to be found in young male mice: in the experiments conducted by Mainardi et al., young male mice do not seem to be differentially affected by the experimental odour. However, in rats, an olfactory experience associated with suckling modifies male reactivity (in terms of reducing ejaculation latency) towards a receptive female carrying the same odour (Fillion and Blass 1986). This effect appears to be specific to the odour-suckling association because an odour connected indirectly with suckling (applied to the back of the nursing female or in the nest) entails no modification of adult sexual behaviour. Control tests further reveal that acquisition of the odour-suckling association has no influence on sexual performance of males exposed to non-receptive females or females without the experimental odour; in addition, males not exposed to the odour-suckling association show no changes in sexual behaviour in the presence of an artificially odorised female. In these species, mouse and rat, the olfactory canal plays a predominant role in early behaviour and the olfactory imprint can have long-term effects. The validity of these mechanisms for humans has not been examined in depth, and the differences between rodents themselves (see the case of male mice and rats described above) calls for prudence regarding inter-specific extrapolation.
However, some findings that should be treated with caution suggest that infant olfactory learning could last until adulthood. For example, among 92 young adults (20-35 years old) surveyed using a questionnaire on the positive and negative odours that have affected them most, 38% declared the most pleasant olfactory memories go back to childhood (Lenti-Boero 1994). The olfactory ambience associated with the mother is mentioned by 37%, whilst only 27% refer to present-day close relationships, and 6.5% to sexual relationships. These long-lasting childhood olfactory memories are most often reported in relation to food perception (Lenti-Boero 1994). Another research work traces the aversions acquired during childhood (between 0 and 5 years) and which persist 50 years later (Garb and Stunkard 1974); among subjects aged 13-20 when responding to the questionnaire, 12% trace their aversion to the 0-5 age period, 58% to the 6-12 age period, and 30% to the 13-20 age period.
The early milk-based feeding context, either at breast or on bottle, can clearly be causal in the developmental trajectory of odour or flavour preference phenotype. Infants who have sucked a breast scented with chamomile during the first postnatal months are more attracted by a toy odorised with the same note, and suck more actively from a chamomile-scented bottle, when they are re-tested 21 months later (Delaunay-El Allam et al. 2006b). Another element in favour of persistent effects of early experience in milk, but this time in formula milk, has already been discussed above (Mennella and Beauchamp 2002). It is further consolidated by the fact that children with phenylketonuria, who received from the first months phenylalanine-free protein hydrolysate formulas with distinctive flavours, made up of unpleasant smell and sweet-sour taste, are not reluctant to consume such formulae as adolescents, after years of exposure discontinuation. Lastly, a quasi-experimental trial took advantage of the flavouring of infant milk formula. Before 1992, most milk formulas were flavoured with vanilla in Germany. Thus, subjects having been either exclusively breast-fed or bottle-fed before that date could be considered free of exposure to the vanilla flavour during nursing or to have massively experienced it in formula, respectively. Subjects (aged between 12 and 59 years, average age 28.8) were then asked to evaluate standard ketchup and the same ketchup slightly flavoured with vanilla (Haller et al. 1999). The first group preferred normal ketchup to vanilla-flavoured ketchup (70.9% compared to 29.1%), while those exposed to vanilla through bottle-feeding responded the other way round (40% compared to 60%). Although this approach did not account for exposure to vanilla since weaning, it does underline the fact that infantile memory for odours and flavours may remain accessible and then possibly influence the attitudes and decisions of older children, juveniles and adults.
Another research considered the problem from the opposite perspective: guessing the infancy environment of adults from 19 countries by direct examination of their pattern of flavour preferences and aversions. Teerling el al. (1994) endeavoured to determine the food context to which these persons had been exposed during the first 5 years of life. To accomplish this, a series of eight contrasting flavours with strong cultural connotations was defined.3 Based on the positive, negative and neutral responses to this series of aromatic indicators in national or ethnic flavour principles, each person was assigned to an infantile culinary context. Among the 74 tested subjects, 35% were correctly assigned from a cultural point of view, which is a significantly different proportion as compared to random distribution.
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