A Neural Mechanism for Generalization over Equivalent Stimuli in the Olfactory System 1

The olfactory system in mammals poses an interesting problem in stimulus equivalence. On the one hand the input to the system is provided by an immense number of receptors (Le Gros Clark [1957]), roughly 10⁸, embedded in an intricately folded membrane exposed to the air in the nasal passages. The number of types of receptor specificity for odor quality has been estimated (Amoore [1971]) to lie between 10 and 10², suggesting that there may be 10⁶ or more receptors that are sensitive to any one odor. On the other hand the remarkable sensitivity of this system indicates that excitation of a small number of receptors, say 10¹ to 10², suffices for detection by a trained animal of the presence of an odor. The detection is consistent over multiple presentations, as shown by the performance of a tracking dog. Considering the turbulence of air flow through the nose, it is unlikely that an odorous substance falls on the same small subset of receptors on any two or more sniffs. In order to explain the perceptual invariance that is implied by the animal behavior, a neural mechanism must be postulated that gives a fixed output for all samples of an odor given to varying subsets of receptors among the set that is responsive to that odor.