Temporal coding in the olfactory bulb of awake behaving rats during active sampling.
Many sensory systems, including vision and audition, represent stimuli with temporally precise trains of action potentials. Might this also be the case in olfaction? Experiments in anesthetized animals have demonstrated that many Mitral and Tufted cells (MTCs) - the exclusive output neurons of the olfactory bulb - are entrained by respiration, and that odorant stimulation produces shifts in the timing of spikes within single breathing cycles. Olfaction in rodents, however, is an active sense much like vision: rats, for example, exhibit a rapid and stereotyped mode of breathing (~7-8 Hz), called sniffing, as they explore their olfactory environment. It is unclear, though, how olfactory processing relates to their rich and dynamic sampling behavior. Psychophysical experiments suggest a close relationship, as rats are capable of performing highly accurate odorant discriminations with just a single sniff.
Our general goal is to determine the odor coding principles of MTCs during exploratory sniffing in a behaving rodent. Towards this, we monitored the respiration behavior of rats while simultaneously recording the activity of multiple single MTCs while the animal engaged in an odor discrimination task in which they exhibit high frequency sniffing during odorant sampling. One aspect of our analysis explored the timing of spikes relative to their phase within the respiration cycle. Through the generation of respiration-phase histograms, we were able to identify phase-shifted responses in 21% of our data set (odor-neuron pairs). These responses are comprised of odor- and neuron-specific epochs of excitation and inhibition precisely locked to the respiration cycle. Importantly, these ‘phase’ responses often occur (83% of the cases) in the absence of a significant change in overall spike count.
The above analysis suggests that odorant information is contained in the spike trains of MTCs on timescales less than the duration of an individual sniffing bout (~140 ms at 7 Hz). To more explicitly determine the temporal resolution of response, we employed a linear discrimination analysis on the spike trains of individual MTCs to identify the optimal integration window for maximum classification of stimuli. The results of this analysis suggest that optimal discrimination occurs for windows of less than 20 ms.
Overall, these findings demonstrate an intricate relationship between sniffing and the activity of MTCs in awake, behaving rodents. The identification of responses on fine time scales that do not accompany changes in firing rate suggests that responses in the olfactory bulb are not as sparse as previously reported. Lastly, our finding on the optimal discrimination window strengthens the notion that the olfactory system exhibits temporal coding in the early stages, and provides insight into the putative decoding mechanisms utilized by downstream processing stages.
Conference:
Computational and systems
neuroscience 2009, Salt Lake City, UT, United States, 26 Feb - 3 Mar, 2009.
Presentation Type:
Poster Presentation
Topic:
Poster Presentations
Citation:
(2009). Temporal coding in the olfactory bulb of awake behaving rats during active sampling..
Front. Syst. Neurosci.
Conference Abstract:
Computational and systems
neuroscience 2009.
doi: 10.3389/conf.neuro.06.2009.03.006
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Received:
29 Jan 2009;
Published Online:
29 Jan 2009.