(a) Example distribution of glutamatergic inputs (grey circles) and GABAergic inputs (blue) on the reconstructed cell (for clarity, some dendrites are not shown). Olfactory information was mediated by 20 excitatory and 40 inhibitory inputs marked by closed circles. Open circles indicate additional 120 background synaptic inputs that were activated at random times. Left, clustered distribution, where stimulus-encoding excitatory inputs are concentrated on a single postsynaptic branch. Right, dispersed excitatory distribution over all distal dendrites. (b) Example postsynaptic responses to stimulation with 10/20 (left) and 20/40 (right) presynaptic excitatory/inhibitory inputs over a 120 input background. Top, voltages recorded from the stimulated dendrite. Middle, somatic EPSPs. Bottom, the temporal presynaptic activation pattern of signal–bearing inputs, simulated to mimic typical odor responses of mitral cells in-vivo. The presynaptic firing trains were identical between clustered and dispersed distributions. (c) Peak somatic EPSPs as a function of input clustering for different number of synaptic inputs. Blockage of NMDA spikes with Ohmic NMDA channels abolished the preference for clustered excitatory drive (n = 100 repetitions for each stimulation intensity) (d) The simulated peak EPSP amplitude recorded at the soma as a function of the number of presynaptic inputs. (e-h) Dendritic odorant selectivity with NMDA spikes. (e) Distribution of excitatory inputs (black) from three glomeruli (A-C, six inputs each; A and B target the same dendrite, while C innervated a different branch) and odor-unselective inhibitory inputs (24 synapses), on a background of 120 randomly activated background inputs. (f) Example somatic voltage profiles following activation of inputs from two glomeruli that converge on the same distal apical dendrite (AB, left) and glomerular input on two different dendrites (AC, right). Grey-background activity in the absence of LOT activation, magenta-somatic potential following activation of a single glomerulus, dotted black – expected linear summation, solid black – actual response to simultaneous pairing of two glomerular inputs. Top, original traces, bottom, with background activity subtracted. (g) pairing nonlinearity, quantified as the ratio between the area under the curve of the background subtracted actual response to the pairing vs. the area under the curve of the expected linear summation. Pairing nonlinearity of 1 represents a linear system, larger/smaller values represent supra/sub-linear responses respectively. n = 1000 simulation repeats, p < 10−9 between AB and AC conditions, Kolmogorov-Smirnov test. (h) The peak (left) and the area under the curve (right) of the background-subtracted EPSPs in same (AB) vs. different (AC) dendritic pairing. Each data point represents a trail with similar spatial and temporal synaptic distribution of the background and stimuli inputs (save for the location of inputs B and C). n = 1000, p < 10−9, paired t-test. Shaded areas-SEM.