Figure 1.

Schematic representation of two theories of odor processing. A simplified olfactory system (without lateral connections) is presented, with five different receptor neurons. Below each graph, the molecular receptive range of chosen receptor neurons is represented, according to a putative chemical dimension along which chemically similar odorants would be placed near each other on the scale. An example of such a dimension would be the number of carbon atoms in an aliphatic chain. (A) Labeled-line: each receptor has a narrow molecular receptive range, i.e., it is activated by a single (or very few) odorant(s). Two different odors, A and B, are each detected by only one receptor, which activates only one neuronal unit. Differentiation between A and B does not need further processing, but only five different odorants can be thus coded. (B) Across-fiber pattern: each receptor has a broad molecular receptive range, i.e., it can be activated by a range of different odors. The five different receptors have different – but broad – receptive ranges. In our example, odor A will activate several neuronal units, although with different intensities depending on the receptor. Receptor 2 will be highly activated by odor A, but only slightly by odor B. Receptor 3 shows the opposite response profile. Among the other receptors, some will be equally activated by the two odors (receptor 4), others will show a contrasted response (i.e., responding to A but not to B; receptor 1), while others will not be activated at all by either odor (receptor 5). This system allows the fine coding of many odors, but differentiation among odors needs additional downstream processing as the representation of each odor is contained in the combination of activations of the different neuronal units.