Another type of chemotopy involves overall similarities in the patterns of activity across the entire glomerular layer, which can be judged by calculating pattern dissimilarity indices across data matrices of glomerular activity (23,27,28) or Pearson correlation coefficients (29,30,31) between pairs of odorant-evoked activity patterns. These analyses show that odorants of the most similar chemistry in any series evoke the most similar overall activity patterns. Thus, odorants of the most similar carbon number in a homologous series evoke the most similar overall patterns (28,30).  

In a series of odorants possessing the same carbon number and the same functional group, but differing in the position of the functional group within the molecule, odorants having the functional group in the most similar position evoked the most similar overall activity patterns (31). In a survey of 55 odorants differing in functional group and hydrocarbon structure, odorants sharing both functional group and aspects of hydrocarbon structure evoked the most similar overall patterns (29).  

If the pattern of activity in the glomerular layer is related to the perception of odor, then overall pattern similarities should predict that rats would perceive similarities in odor. Indeed, behavioral measures of odorant similarity appear to correlate well with pattern similarity for a number of odorant series (7,18,36,37). The ability to make such predictions is one of the greatest benefits of using 2DG uptake as a method for mapping bulbar activity.