Olfactory coding

Pioneering efforts to understand the nature of olfactory coding were reported by Adrian (24-27). His work introduced the ideas that different odors activate ORCs in different regions of the olfactory epithelium and that spatiotemporal patterns of ORC firing would suffice to encode different odors. Subsequent studies by many investigators and involving various recording methods (reviewed in refs. 13 and 28) led to the conclusion that, at various levels of the pathway, the olfactory system uses distributed neural activity to encode information about olfactory stimuli. 

Laurent, G. (1999). A systems perspective on early olfactory coding Science 286 723-726. 

Sachse, S., Rappert, A. and Galizia, C. G. (1999). The spatial representation of chemical structures in the antennal lobe of honeybees steps towards the olfactory code. European Journal of Neuroscience 11 3970-3982. 

Vickers N. J., Christensen T. A., Baker T. C. and Hildebrand J. G. (2001) Odour-plume dynamics influence the brain s olfactory code. Nature 410, 466 470. 

Hekmat-Scafe D., Steinbrecht R. A. and Carlson J. R. (1997) Coexpression of two odorantbinding protein homologs in Drosophila implications for olfactory coding. J. Neurosci. 17, 1616-1624. 

If odor-evoked slow temporal patterns actually provide higher brain centers with information about the odor quality, identification and discrimination cannot be instantaneous as many of the temporal features in the response profiles appear late or even after offset of odor exposure. Honeybees need 500 ms for a response to (non-sexual pheromone) odors but at least 1 second of stimulation is required for a correct discrimination (J. Klein, unpublished, cited in Galizia el al., 2000a). Thus, it appears that time is an important factor in discrimination tasks involving non-pheromonal odors and the slow temporal patterns could theoretically contribute to an olfactory code. In contrast, these temporal patterns would be too slow to encode information about sexual pheromones. Male moths, for example, must be able to respond to rapid changes in stimulus intermittency when moving upwind in pheromone plumes in search of a calling female. 

If a spatial olfactory code underlies a subsequent identification of an odor, then we would expect concentration-dependent variations of odor maps to have an impact on the perceived odor quality. Stimulus concentration can indeed influence perception of odor quality and behavior of insects. For example, olfactory responses of the fruit fly, Drosophia melanogaster, shifted from attraction to repulsion as the concentration increased (Siddiqi, 1983 Stensmyr et al., 2003). Future studies should thus involve correlations of behavior with glomerular activity at different concentrations. 

Amoore, J. E. (1963). Specific anosmia A clue to the olfactory code. Nature 214,1095-1098. 

Peele P, Ditzen M, Menzel R, Galizia CG (2006) Appetitive odor learning does not change olfactory coding in a subpopulation of honeybee antennal lobe neurons. J Comp Physiol A Neuroethol Sens Neural Behav Physiol [A] 192 1083-1103... 

If I can ascertain what another organism detects via olfaction, then I can perform experiments upon it, which cannot be performed on human subjects. The objective of such experiments—to find out how odor is coded—has yet to be achieved. Suppose the olfactory code were unraveled. Reproducing an odor would become a matter of replicating the pattern of neural responses without having to duplicate the chemical stimulus (much as cinematography appears to reproduce color without necessarily matching the complete spectroscopic profile of the original scene) (Robertson, 1992). 

If the odors of specific objects translate into unitary percepts, which constitute the basic entities in linguistic descriptions of olfaction, then the question follows as to whether these unitary percepts take shape at the level of the receptor neurons or in the olfactory bulb or elsewhere in the brain. That question remains unanswered, as of this writing. Because the sense of smell does not correlate perfectly with externally monitored patterns of electrical response from the receptor neurons or the olfactory bulb, the nature of olfactory coding remains unknown. Outside the laboratory unitary percepts rarely equate to pure compounds. Two vocabularies coexist, one of smells (which varies from individual to individual, and which refers to other inputs besides olfaction) and the other of chemical structures. 

The paucity of acknowledged impossibilities hampers understanding of how chemical structure translates into a sequence of nerve impulses. Not until experiment falsifies many more plausible suggestions can a coherent theory of olfactory coding take shape. 

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