Fig. 5.

Effects of the probability and spatial distribution of SA output connections. (A) Effect of separately varying the probability of synaptic connections (P) and the radius of their distribution (R) of connection probabilities (Fig. 3B) on the number of synapses received from SA neurons by any given cell (SA synaptic density). (B) SA synaptic density determined the regional variability in SA activation levels across the network (standard deviation of SA activity), reaching an asymptotic minimum at approximately four inputs per glomerulus independent of stimulus concentration. Experimental estimates of ET/SA connectivity (26) (dashed lines) correspond to the minimum degree of synaptic connectivity that can effectively achieve uniform feed-forward inhibition and compute a global normalization. Data shown are for a concentration of 75 ppm; similar results were observed at other concentrations (SI Fig. 7). (C) The quality of the normalization performed by the glomerular network (compared with a calculated z score) depends quasi-linearly on the uniformity of the computation of the mean across the ET/SA recurrent excitatory network. Indices of dissimilarity between model output and calculated z score normalizations of afferent data were directly proportional to the standard deviation of activity levels across this interglomerular network.