Trends in Neurosciences
OpinionFrom the top down: flexible reading of a fragmented odor map
Section snippets
Lateral interactions could allow flexible readout of the fragmented chemotopic odor map
Sensory systems must optimize the processing of input to permit timely and efficient extraction of information. One elegant solution to this challenge is to organize information into a spatial map. In vision, for instance, the cornea focuses a spatial representation of an image onto the retinal surface in the eye, while in hearing the representation of sound is organized as a frequency map in the cochlea. Odor maps also appear to have a gross chemotopic arrangement. For example, carboxylic
Mitral cell odor responses are influenced by learning, behavior, and context
Evidence for top-down regulation of processing in the olfactory bulb was first provided by Kerr and Hagbarth [15] who showed that excitation of centrifugal fibers enhances the local field potential (LFP) activity of the olfactory bulb. The LFP, first described by Adrian, is a field potential recorded extracellularly in the olfactory bulb that reflects the oscillatory synchronous activity of neurons aligned on the average in the same direction 16, 17. Since 1955 other groups have shown that
Local processing in the olfactory bulb is intrinsically dynamic
As discussed in Box 1, the spatio–temporal information contained in odor maps is processed by the interplay between the principal neurons of the bulb (tufted, T, and mitral, MT, cells) and the interneurons in the glomerular and external plexiform layers (EPL). This interplay gives rise to lateral inhibition 12, 14, a potential mechanism for synchronizing MT cell firing. Synchronization of MT cells through reciprocal connections to granule cells has been demonstrated in OB slices [32], and
Mechanisms for top-down regulation of mitral cell responsiveness
Modulation of MT cell responsiveness by adrenergic, cholinergic and serotonergic fibers is fairly well established [40]. In a recent study, Shea and co-workers found that odor-evoked increases in MT cell firing are suppressed in anesthetized mice when odor stimulation is paired with activation of the locus coeruleus (LC), the brainstem nucleus that houses the adrenergic neurons that innervate the olfactory bulb [41]. In another study, bulbar acetylcholine enhanced learning to discriminate
Acknowledgements
We would like to thank Drs. Peter Brunjes, Tom Finger, Kurt Illig, Laura López-Mascaraque, Nathan Schoppa, Michael Shipley, John Scott and Ben Strowbridge and Mr. David Gire for enlightening discussions. We would like to acknowledge funding by NIH grants DC00566 and HD041697 (D. Restrepo), DC008066 (W. Doucette), DC009369 (T. McTavish) and DC008679 (Salcedo).
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