Fig. 7.

Temporal tuning of excitatory and inhibitory conductances for OFF-center parasol cells: high sensitivity is mediated by synaptic excitation. (A) Left, intracellular voltage recording of an OFF-parasol to 10-, 20-, and 30-Hz temporal frequency modulation (50% contrast, 1-mm stimulus diameter). Membrane potential depolarizes during OFF-phase and hyper-polarizes during ON-phase after a latency to a stimulus onset of ~35 ms (spikes removed for illustrative purposes). Right, a plot of spike rate as a function of stimulus temporal frequency (50% contrast; 1-mm stimulus diameter; first harmonic amplitude) before (solid circles) and after (open circles) application of GABA_A (GABAzine, 5 µM) and GABA_C (TPMPA, 50 µM) and glycine (strychnine; 1 µM) receptor antagonists. (B–D) Family of stimulus-evoked synaptic currents to 10 Hz (B), 20 Hz (C) and 30 Hz (D) temporal modulation before (upper left) and after (upper right) the addition of GABAzine, TPMPA, and strychnine. Lower left and right, mean excitatory (blue) and inhibitory (red) synaptic conductances derived from sample number indicated for each associated stimulus condition. (E) Plot of percentage peak crossover inhibitory relative to excitatory conductance [peak inhibition/peak (excitation + inhibition)] as a function of temporal frequency. Percent of synaptic inhibition was calculated on a cell-by-cell basis. Crossover inhibition declines with increasing temporal frequency and is largely absent at 30 Hz, the highest temporal frequency measured. (F) Average peak OFF conductances before (solid circles) and after (open circles) the block of synaptic inhibition plotted as a function of temporal frequency.