Population sparsening and temporal sharpening of olfactory representations in the honeybee mushroom bodies Summary

We characterized odor-evoked network activity in an insect brain at three consecutive neural compartments. Using Ca imaging, we recorded activity in the dendrites of the projection neurons that connect the antennal lobe with the mushroom body, a higherorder integration center. Next, we recorded the presynaptic terminals of these projection neurons. Finally, we characterized their postsynaptic partners, the intrinsic neurons of the mushroom body, the clawed Kenyon cells. Odors evoked combinatorial activity patterns at all three processing stages, but the spatial patterns became progressively sparsened along this path. Furthermore, Kenyon cells integrated projection neuron activities within 200 ms and transformed complex temporal patterns into brief phasic responses. Thus, two types of transformations occurred: Sparsening of the combinatorial code, depending on preand postsynaptic processing within the MB microcircuits, and temporal sharpening of postsynaptic Kenyon cell responses, probably involving a broader loop of inhibitory recurrent neurons.