Two kinds of evidence, two species

The study ran along two tracks. 647 people took part in the survey and reported the pleasant smells of their childhood. Mice, for their part, were kept in an environment enriched with attractive odorants between postnatal days 23 and 33, the period corresponding to early childhood in humans.[1]

In the adult mouse, recall of the memory turned out to depend on granule cells formed on the first day after birth. The team labelled those cells with a lentivirus carrying halorhodopsin and silenced them with light at 561 nanometres; investigation time devoted to the odorant fell, the habituation slope steepened and the preference index declined. Group sizes were 18 and 12 animals.[1]

Two accounts in competition

The mechanism the authors propose is that the early odorant settles into the olfactory bulb through increased connectivity with the reward system. As Scientific American puts it, granule cells make privileged connections with pleasure and reward areas, so the early scent becomes imprinted in childhood.[1]

The behaviour that changes after silencing with light can be explained by the interruption of an imprint established through reward. The same outcome could also arise from the early odorant being merely familiar; when granule cells are shut off, the route that reads familiarity closes too and investigation time falls just the same. Separating the two accounts requires testing pleasure and familiarity independently of one another.[1]

Whose life this changes

Nothing in this research can be carried into the clinic today. The two cautions Scientific American gathered support that reading: Hervé This of AgroParisTech-INRAE says the work focused largely on positive memories, while Luca Turin of the University of Buckingham argues smell's distinctiveness may be only apparent. In the ageing mouse, the memory persists only if the odorant is presented again periodically, and as the postnatal cells disengage the limbic connections strengthen. That is the question with a human counterpart: does re-exposure to an early scent hold up a memory that ageing weakens?[1]