Contrast enhancement generator in a single olfactory bulb glomerulus. Ai–Aiv. Odor-evoked activity in model mitral cells as a function of odor ligand-receptor affinity, in the absence of periglomerular inhibition and neglecting stimulus concentration. Increasing odor ligand-receptor affinity generates a monotonic increase in mitral cell activation. Av–Aviii. The addition of periglomerular inhibition upon local mitral cells creates a contrast enhancement generator element by first inhibiting (panel vii), and then exciting (panels vi, v), mitral cells as odor ligand-receptor affinity increases. Inhibition was held constant, and panels v–viii depict the same four odor ligand-receptor affinities as are shown in panels i–iv. Aix-Ax. Periglomerular cell activation by the two lower-affinity odorant stimuli. While current input to periglomerular and mitral cells is identical, the greater input resistance and smaller volume of PG spines compared to mitral dendrites result in a greater voltage deflection in and hence a greater activation of PG cells (compare panels iv and x). Additionally, low-threshold T-type calcium current  evokes a near-maximal burst response from PG cells even at low input levels (panel ix), which mediates the mitral cell inhibition shown in panel vii. B. Mitral cell spike count over a 1 sec stimulus in the absence and presence of PG cell-mediated NTCE. In order to illustrate the effects of mitral cell inhibition, a 150 pA depolarizing current was continuously injected into the mitral cell soma to elicit a baseline spike rate. Mitral cell spiking is employed solely as an index of mitral cell activation; the present mitral cell model does not include complex spike patterning mechanisms. With intact NTCE, as odor ligand-receptor affinity increases, the mitral cell activation level reflects a half-hat function (Figure 3, Mi
). Inset. Model architecture. OSN synaptic input activates mitral cell distal dendrite, periglomerular dendritic spine, and a combined ET-SA-PG function that projects inhibition onto all mitral cells other than that in the same glomerulus (shaded region). Synaptic weights were the same across all glomeruli, while odorant-receptor affinities differed. Filled triangles: excitatory synapses. Open triangles: inhibitory synapses. Lower case labels denote incoming processes originating in other glomeruli.