Broad topographic mapping

The lack of point to point topography in the projection of the olfactory epithelium to the olfactory bulb and the early recognition that olfactory bulb glomeruli appear to represent anatomically discrete units has led to the widespread view that glomeruli play an important role in olfactory coding. The precise relationship of olfactory receptor axons to individual glomeruli has, thus, drawn considerable attention. 

Axons from the olfactory epithelium travel in parallel fascicles (Daston et al. 1990) but form a dense plexus after they pass through the cribriform plate. Individual glomeruli receive fascicles entering from different directions (Le Gros Clark, 1951 1956). A similar plexus of fibers is seen in amphibians (Scalia, 1976 Kauer, 1981 Duncan et al. 

These early observations of Le Gros Clark were confirmed by later studies using degeneration or tracer methods to label the fibers arising from restricted regions of the 

In the second model suggested by Le Gros Clark, each glomerulus receives a sample of many different receptor types. Random distribution of individual fibers would result in a broad representation of different fiber types within glomeruli. Each glomerulus would respond to many possible odors. In this case small portions of the bulb would be capable of representing all possible odors. 

Experiments by Slotnick et al. (1987) have shown that a learned olfactory discrimina- 

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