Odor evokes activity in a sparse and distributed set of pyramidal cells in the piriform cortex. (a) Cartoon representation of the mouse brain with the region of the piriform cortex shown in a box. (b) Odor activates a small number of pyramidal cells (around 10%). Neurons in orange have more calcium and are active. (c) Glomerular projections (via the lateral olfactory tract, red) can activate a few pyramidal cells directly (orange) and also activate GABAergic interneurons in layer 1 (green), which in turn suppress other pyramidal cells.