Figure 3. Characterization of the CS+-induced gamma activity. A, The mean spectrogram from all subjects across all recording sites and conditioning days. A brief broadband increase in spectral power occurs for 200 ms after the tone, followed by a relatively narrow activation in the gamma band from 40 to 120 Hz, which develops over several hundred milliseconds and appears to be strongest 0.5–4.0 s after tone onset. B, The mean z-score change in spectral power across all recording sites during conditioning. The blue line corresponds to the 4.0 s pretone baseline period, and the red line is the entire 10 s tone period. Shaded regions denote ±1 SD intervals. C, An example spectrogram from a single electrode, on a single trial. Note that gamma activation tends to occur in half-second bursts, which recur throughout the duration of the CS+. The saturated region between 11 and 12 s was caused by the shock artifact. D, Same data as shown in C, but instead the LFP waveform and several bandpass (10 Hz bandwidth around each center frequency, Butterworth filter) filtered traces are displayed. The LFP signal displays the hallmarks of low-voltage fast activity after tone onset, and the bandpassed traces show a decrease in power for a low-frequency band (9 Hz) and an increase in the higher frequency bands (40, 70, and 100 Hz). Each trace was z-normalized to its pre-CS+ baseline. The gray dotted line indicates the CS+ onset. E, Four magnified segments of the same data shown in D.