Fig. 3.
Glomerular layer circuitry and model architecture. (A) Schematic of the glomerular layer network. Triangles connote excitatory synapses; open circles indicate inhibitory synapses. OSN input is delivered onto mitral cells (Mi), ET cells, and a minority of PG cells. ET cells activate SA cells, and these two cell types form a recurrent excitatory network (red synapses). The output of this network is delivered to PG cells (blue synapses), which in turn inhibit mitral cells. (B) Illustration of the spatial distribution of ET/SA network projections in the model. To match experimental estimates of ET/SA connectivity (26), the probability of synaptic formation between a given SA cell and a distant target cell (Psyn) was the product of a baseline probability P = 0.1 and a Gaussian distribution fgauss(R, d) that declined with the distance d between the two cells. The breadth of the distribution of projection distances was determined by the standard breadth R. Experimental estimates were best matched by radius R = 2 glomerular diameters for inputs to SA cells and a radius of R = 7 glomerular diameters for SA outputs. (C) Raw glomerular activity maps were derived directly from patterns of 2-deoxyglucose uptake in the rat OB. Calculated normalization of these data by using a z score statistic yielded normalized activity maps that were less sensitive to concentration differences and more predictive of perceptual similarities. The raw glomerular activity maps also were processed by the OB model, in which patterns of mitral cell output activity reflect the normalizing effect of the recurrent excitatory network. The output patterns of mitral cell activity are compared directly with calculated z score normalizations.