SIAM-AMS Proceedings Volume 13 1981
WALTER J. FREEMAN
Introduction. 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.
A mechanism that can do this is based on the postulate that an animal forms a template of connections among those neurons in the olfactory bulb that are activated by a given odor during a training or familiarization experience. This postulate stems from observations on the spatial patterns of electrical waves generated by the olfactory bulb in cats and rabbits that were trained to identify particular odors.
© 1981 American Mathematical Society____________________
Supported by Grant MH06686 from the National Institute of Mental Health, USPHS.