Pairwise correlations between LGN input neurons were generated according to Equation 3 and Equation 4 (Experimental Procedures). Each presynaptic LGN cell generated a change in conductance in the postsynaptic simple cell in proportion Palbociclib cost to its firing rate. In other words, the total stimulus-evoked change in conductance

in the simple cell (Δgexc  ) was taken to be proportional to the total spike rate in the presynaptic simple cells. The visually-evoked depolarization in the simple cell then becomes equation(Equation 1) ΔVm=ΔgexcEexc+grestErestgexc+grestwhere grest   is the resting or leak conductance of the cell, and Erest   it’s reversal potential. Dividing through by grest   and expressing all potentials relative to Erest  , this can be rewritten as equation(Equation 2) ΔVm=ΔgexcgrestEexc′1+Δgexcgrestwhere Eexc′ is the excitatory reversal potential relative to Erest. The scale factor between BMN 673 in vivo total LGN spike rate and Δgexc/grest was set such that high-contrast, optimally oriented stimuli evoked an average peak depolarization of 20 mV in the simple cell ( Finn et al., 2007). For example, for a simple cell with an input resistance of 80 MΩ (grest = 12 nS), high contrast gratings would evoke

an increase in conductance of ∼6 nS, which reduces the input resistance to 55 MΩ. This conductance increase is in the range of previous observations from cortical intracellular recordings ( Monier et al., 2003, Anderson et al., 2000 and Berman

et al., 1991). Synaptic efficacy was modulated by short-term synaptic depression, modeled after Boudreau and Ferster (2005) ( Equation 5 and Experimental Procedures). Mean Vm responses at high-contrast for one iteration of the model are overlaid (black lines) on the mean responses of the 16 LGN inputs in Figure 5A (red, ON-center; blue, OFF-center). Single-cycle and mean response amplitudes as only a function of orientation at low and high contrasts closely matched actual data recorded intracellularly from a simple cell (Figures 5B and 5C, data in C reproduced from Finn et al., 2007). The model qualitatively matched many features of the data, such as orientation tuning width, the extent of the trial-to-trial Vm variability, the relative orientation independence of trial-to-trial variability and the dependence of trial-to-trial variability on contrast. To explore the range of the model’s behavior, we simulated the responses of 50 simple cells, each receiving 16 LGN inputs whose properties were drawn from a different subset of recorded LGN cells. We measured the Vm response variability as trial-to-trial SD at the peak of the depolarization, and plotted variability at high contrast against variability at low contrast for preferred and null stimuli (Figures 6A and 6B, black) for each of the 50 model cells.