Learning Modulation of Odor-Induced Oscillatory Responses in the Rat Olfactory Bulb: A Correlate of Odor Recognition?

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Abstract
In the first relay of information processing, the olfactory bulb (OB), odors are known to generate specific spatial patterns of activity. Recently, in freely behaving rats, we demonstrated that learning modulated oscillatory activity in local field potential (LFP), in response to odors, in both β (15-40 Hz) and γ (60-90 Hz) bands. The present study further characterized this odor-induced oscillatory activity with emphasis on its spatiotemporal distribution over the olfactory bulb and on its relationship with improvement of behavioral performances along training. For that purpose, LFPs were simultaneously recorded from four locations in the OB in freely moving rats performing an olfactory discrimination task. Electrodes were chronically implanted near relay neurons in the mitral cell body layer. Time-frequency methods were used to extract signal characteristics (amplitude, frequency, and time course) in the two frequency bands. Before training, odor presentation produced, on each site, a power decrease in γ oscillations and a weak but significant increase in power of β oscillations (∼25 Hz). When the training was achieved, these two phenomena were amplified. Interestingly, the β oscillatory response showed several significant differences between the anterodorsal and posteroventral regions of the OB. In addition, clear-cut β responses occurred in the signal as soon as animals began to master the task. As a whole, our results point to the possible functional importance of β oscillatory activity in the mammalian OB, particularly in the context of olfactory learning.