Fig. 3.

Gamma oscillations induced by ramp light stimulation. A: no. of recording sites that showed modulation in multiunit spiking activity (increasing and decreasing) to optical stimulation according to time elapsed since viral injection. B: LFP (<300 Hz, top; 40–80 Hz, bottom) from 2 representative microelectrodes during a 4-s-long light ramp. Insets show position of electrode. C: trial-averaged (n = 36) spectrograms showed elevated LFP power in the gamma band (40–80 Hz) during the late stage of the light ramp. D: evolution of gamma power during light ramps (blue solid line: mean across trials, shaded area: 95% CI). Onset of optogenetically induced gamma oscillations, detected as the lower CI bound crossing the 99% percentile (red line) of the baseline gamma power, occurred once a critical light stimulation level was reached (2.67 mW and 3.06 mW). Time differences in the onset of gamma oscillations in the 2 recording sites in this ramp stimulation condition could result from the progressive recruitment of more distant cortical regions, as the cortical volume affected by light increases with increasing light power.