| Learning can be either non-associative or associative, though the molecular mechanisms underlying both remain enigmatic. The nematode Caenorhabditis elegans can adapt to both the AWC sensed odorants benzaldehyde (Bnz) and isoamyl alcohol (IsoA) and reciprocally cross-adapt. Surprisingly, however, these four adaptation permutations actually represent two distinct forms of learning: non-associative habituation and associative learning by pairing with starvation. Conditioning to the single odorant IsoA leads to both associative and non-associative memory traces, which can be preferentially accessed by choice of a Bnz or IsoA retrieval stimulus, respectively. Furthermore, we show that the molecular mechanisms underlying each form of memory can be genetically double dissociated, with insulin signalling and egl-4 being required for associative learning and osm-9 and arr-1 being essential for IsoA olfactory habituation. This represents the first demonstration where the form of learning displayed after conditioning to a single stimulus is a function of the retrieval stimulus employed. |
