1. Just noticeable differences in perceived concentration of a single odor are dC/C ~ 0.1.
2. The number of simultaneously perceived SIMPLE odors (composed only of one chemical) is about 12.
3. The number of simultaneously perceived COMPLEX odors is 1-2 (using an objective information theoretical measure).

We show that these observations can be understood using an on-off model for glomerular response. In this model the response of each glomerulus is binary. A more detailed model allows us to reproduce closely conditional probabilities obtained in human psychophysical experiments on perception of complex odors.