  We present a model for olfactory coding based on spatial representation of
glomerular responses. In this model distinct odorants activate specific subsets
of glomeruli, dependent upon the odorant's chemical identity and concentration.
The glomerular response specificities are understood statistically, based on
experimentally measured distributions of detection thresholds. A simple version
of the model, in which glomerular responses are binary (the on/off model),
allows us to account quantitatively for the following results of human/rodent
olfactory psychophysics: 1) just noticeable differences in the perceived
concentration of a single odor (Weber ratios) are dC/C ~ 0.04; 2) the number of
simultaneously perceived odors can be as high as 12; 3) extensive lesions of
the olfactory bulb do not lead to significant changes in detection or
discrimination thresholds. We conclude that a combinatorial code based on a
binary glomerular response is sufficient to account for the discrimination
capacity of the mammalian olfactory system.
