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Leslie M. Kay

University of Chicago
Olfactory system dynamics and the physiology of context
The computational architecture of the olfactory bulb is intriguing, as it combines a relatively ordered set of inputs from the peripheral olfactory nerve with a massive array of inputs from many other brain areas. When animals are anesthetized the activity of the principal neurons (mitral cells) appears to be driven by a somewhat ordered set of relationships dependent on similarities in chemical structure, which is suggestive of feed-forward chemotopy. Neural recordings from animals, trained to associate a behavioral meaning with an odor stimulus, present a different picture of odor ?representation.? In these experiments we find that the activity of individual mitral cells is driven primarily by the behavioral requirements of the stimulus. Odor representation, when seen, is also affected by changes in meaning. These context-dependent changes are most likely driven by input from other brain areas, one candidate of which we have found to be the entorhinal cortex. Changes in a mitral cell?s firing characteristics occur at the same time as distinct changes in the frequency profile of the local field potential, a measure of cooperative activity of thousands of individual neurons. At this time the entorhinal cortex sends input to the olfactory bulb, the characteristic olfactory bulb gamma bursts (40-100 Hz) all but disappear, mitral cells uncouple from the ongoing respiratory (theta) rhythm, and the theta rhythm increases frequency from a resting level of 2-4 Hz to a motivated level of 5-10 Hz. Knockout studies in mice that selectively disrupt mutual inhibition in the olfactory bulb also increase the amplitude and frequency of olfactory bulb gamma bursts in animals engaged in exploratory behavior. These changes are linked to subtle differences in the ability of these animals to identify odors as compared to homozygous controls.
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