A number of observations suggest that glomeruli of related, but distinct, molecular specificity are clustered together within the olfactory bulb. Odorants sharing molecular features activate overlapping regions of the glomerular layer, but the glomeruli stimulated within these areas are often different even for closely related odorants (Johnson et al., 1998, 1999; Johnson and Leon, 2000b). Most odorants activate clusters of adjacent glomeruli within their primary response regions rather than activating individual glomeruli, and the number of glomeruli in a cluster that are activated to any given extent increases with increasing odorant concentration (Johnson and Leon, 2000a). Along a homologous series of straight-chained odorants possessing the same functional group but increasing numbers of carbon atoms, the activated glomeruli shift gradually and systematically in location within the glomerular region that was responsive to molecules containing that functional group (Johnson et al., 1999, 2004). 

By comparing activity patterns of odorants differing in chemical structure by small increments, and by outlining the new glomerular clusters associated with these changes, we have attempted to define boundaries enclosing the glomeruli responding to the related features (Johnson and Leon, 2000a; Johnson et al., 2002). These areas, which we have termed "glomerular modules", have proven useful in testing predictions regarding the activity patterns that would be evoked by previously untested odorants (Johnson et al., 2002, 2004, 2005a). The specificity of these domains has been replicated with the use of optical imaging of endogenous signals from the glomerular layer of the olfactory bulb (Uchida et al., 2000; Takahashi et al., 2004). We use the boundaries of our proposed glomerular modules to reduce the complexity of our data matrices for statistical analyses (Linster et al., 2001; Johnson et al., 2004, 2005a, 2005b). On this website, we offer these modular boundaries as overlays on our activity patterns. However, despite their usefulness, the modules are works-in-progress. We expect to alter their boundaries as we collect additional activity patterns. It already is obvious to us that certain modules could easily be divided into sub-modules and that certain odorants stimulate activity centered on the borders between adjacent modules as they are drawn currently (Johnson et al., 2005a). Furthermore, interdigitated glomeruli within certain modules appear to respond to odorants not obviously sharing molecular features (e.g., modules "c" and "C" stimulated by ketones and aromatic odorants in Johnson et al., 2005b). While most of the original designations remain consistent with our present data, we continue to treat these modular designations as ongoing hypotheses that we will revise as new data become available.  

Current working definitions of glomerular modules used in our analyses of bulbar activity patterns.