Excitation–inhibition sequence during optical MLI activation. (A1) Upper trace: Loose-seal cell-attached recording from an MLI soma. Lower traces: Raster plots for three consecutive runs at 1 min intervals. Spike rate increases during the 1-s-long light pulse (purple rectangle). After the light pulse there is a period with no spike activity. (A2) Black: PSTH for the three runs. The blue trace, with corresponding y axis on the right, displays the integral of the PSTH. The yellow line is an extrapolation of a linear fit of the blue trace during the pre-stimulus period, where blue and yellow traces superimpose. The distance between blue and yellow traces, representing spike number excess, increases during the light pulse and decreases thereafter. (B1 and B2) Same experiment, but using a 200 ms light pulse, which results in a similar increase in spike rate and a shorter post-stimulus silent period. (C) The biphasic effect of optical stimulation on spike rate was observed for 24 MLIs tested with 1-s-long light pulses. Pre denotes the average spike rate during the period preceding the pulse, Stim corresponds to the spike rate averaged over the 1-s-long pulse, and Post denotes the spike rate during the 1 s subsequent to the light pulse. Wilcoxon paired test P < 1.2e7 for both changes. (D) Pooled data on a subset of experiments in which 200 ms and 1-s-long optical stimulations were performed (three to seven repetitions per cell, alternating durations). Significant increases in the spike rate are observed during the light pulse for both durations (left) as well as a significant decrease in the spike rate compared to pre-stimulus values during the post-stimulation period (right). (E) Spike number excess is significantly larger at the end of the light pulse (labeled “peak”) compared to steady state (SS), both for 0.2 s long pulses (Wilcoxon paired test P = 0.01) and for 1 s long pulses (Wilcoxon paired test P = 0.004).