Figure 6

Non-topographical contrast enhancement replicates the canonical observation of olfactory contrast enhancement. A. A model mitral cell's response to the sequential presentation of a homologous series of nine different odorants. Nonspecific sinusoidal excitation was added to replicate the respiration-linked activity observed in the cell recorded by these authors (see Methods). The homologous odor series was simulated by altering the odor ligand-receptor affinity of the OSNs that project to the mitral cell depicted from near zero to a maximal value (at odor 4) and back along the trajectory of a normal distribution. The ligand-receptor affinities of the OSNs associated with the other nine glomeruli were sampled randomly from another normal distribution, as if they were exhibiting similar sensitivity profiles to unknown odor series. Odors were applied for 2 seconds (horizontal bar). NTCE fully replicated the Mexican hat contrast enhancement function observed in a mitral cell reported by Yokoi et al. ([10]; their Figure 2A). B. Response dependence on GABA_A receptors. Odor-evoked inhibition in mitral cells can be transformed into excitation when bulbar GABA_A receptors are blocked [10]. In the present model, blockade of GABA-ergic synapses from periglomerular cells effected this reduction of inhibition, replicating the results of Yokoi et al. ([10]; their Figure 5D). A constant background stimulation was applied in all cases to generate tonic spiking so that inhibition could be observed. Control, no odor stimulus was applied. Odor, a 4 second stimulus (using odor 6 from Figure 6A) was applied (horizontal bar), evoking an inhibitory response in the mitral cell. Bicuculline + odor, the same stimulus was applied after blocking all periglomerular synapses onto mitral cells. NTCE replicated the effects of bicuculline application as shown by Yokoi et al. [10], though the effect was mediated via periglomerular cells rather than granule cells as proposed by those authors.