Critical period is defined as a time during development, when an organism has heightened sensitivity to external stimuli, which are required for the acquisition of a particular skill. Many animals have to acquire certain skills early in life and if they don’t, they may never be able to learn them when they are older. Critical periods have been observed in learning of birdsong, plasticity induced by monocular deprivation, language learning in humans, etc.

Previous work has shown that if flies are exposed to odor immediately after eclosion, long term habituation can be induced after 4 days of odor exposure.(Devauld et. al. 2003, Sachse et. al. 2008,  Das et. al. 2011) But if 4 day odor exposure is started on the 8^th day after eclosion, flies don’t show behavioral habituation or structural plasticity associated with it. Thus, there is a critical period for long term olfactory habituation. (Devauld et. al. 2003, Sachse et. al. 2008) To find the earliest time, when the critical period is closed, we tested flies exposed to odor for 4 days starting within 0-12 hours after eclosion, 24-36 hours after eclosion, and 48-60 hours after eclosion. We found that, if 4 day odor exposure starts at 48-60 hours after eclosion, long term habituation can’t be induced. Structural plasticity (i.e. increase in volume) of the glomeruli responding to that odor is also not observed in flies exposed to the odor 48-60 hours after eclosion. Thus, the critical period for olfactory long term habituation, is closed at 2 days after eclosion.

When flies are exposed to odor for 4 days immediately after eclosion, we observe an increase in post-synaptic processes of projection neurons of odorant selective glomeruli. This increase is only observed during the critical period. Previous studies have shown that, rutabaga is necessary and sufficient in LN1 subset of neurons to induce habituation. (Das et. al. 2011) The increase in PN post-synaptic terminals is not observed in rutabaga mutants and expressing rut only in LN1 neurons, is sufficient to rescue the odor-induced increase in PN post-synaptic terminals.

When we expressed Kir2.1 (which decreases the probability of firing action potentials) in sensory neurons for 48 hours immediately after eclosion, and then exposed the same flies to EB for 4 days, there was an increase in volume of odorant-selective glomeruli. Thus, the critical period can be extended. We are now planning to look at the effect of activity on closing the critical period.