substances (13). Also important, in addition to such qualitative properties, are odor intensity (concentration of odor substances) and intermittency. A requirement for interruption or intermittency of stimulation for effective olfactory function is found in diverse animals, from crustaceans and insects to air-breathing vertebrates (1).

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.

Different odor substances stimulate different patterns of ORCs in the olfactory epithelium, owing to the different sensitivity spectra of the ORCs (28). The pattern of activity in the epithelium evoked by a particular odor substance constitutes the first molecular image of that stimulus, which represents the determinants of the stimulating molecules (13). Thus, although olfaction is not a spatial sensory modality, in contrast, for example, to vision and somatosensation, the initial representation of an odor stimulus in the olfactory pathway does have spatial structure.

At subsequent levels in the olfactory pathway, new molecular images of the odor stimulus are formed as patterns of activity across an array of neural elements. For example, in the olfactory nerve, which carries ORC axons to the olfactory bulb or lobe of the brain, the pattern of activity across the array of fasciculated primary-afferent fibers constitutes the second molecular image of the stimulus. In the olfactory bulb or lobe, another molecular image takes shape as the pattern of activity across the array of glomeruli, and yet another molecular image is generated as the pattern of activity across the array of projection-neuron axons emanating from the glomeruli. Each molecular image of a particular odor stimulus exemplifies the way neural space is used at that level in the pathway to represent information about the stimulus.

An important insight from many studies (28) is that the response patterns—the molecular images—at various levels in the central olfactory pathway are set up by the differential responses of the ORCs in the peripheral receptor epithelium. These studies also suggest that functional modules, which may correspond to recognizable structural units such as individual glomeruli with their associated cells, in the olfactory bulb or lobe participate in the analysis of olfactory information conveyed to them

Front Matter (R1-R10)

The Chemistry of Defense: Theory and Practice (1-16)

The Chemistry of Poisons in Amphibian Skin (17-28)

The Chemistry of Phyletic Dominance (29-40)

The Chemistry of Social Regulation: Multicomponent Signals in Ant Societies (41-50)

The Chemistry of Eavesdropping, Alarm, and Deceit (51-66)

Polydnavirus-Facilitated Endoparasite Protection Against Host Immune Defenses (67-86)

The Chemistry of Gamete Attraction: Chemical Structures, Biosynthesis, and ... (87-102)

The Chemistry of Sex Attraction (103-118)

The Chemistry of Sexual Selection (119-132)

The Chemistry of Signal Transduction (133-146)

Chemical Signals in the Marine Environment: Dispersal, Detection, and ... (147-160)

Analysis of Chemical Signals by Nervous Systems (161-182)

Chemical Ecology: A View from the Pharmaceutical Industry (183-202)

List of Abbreviations (203-204)

Index (205-214)

Supplementary Images (215-221)