Fig. 2.

(A) Spectrogram of EEG signal in humans during repetition experiments [adapted with permission from Gruber and Muller (6) (Copyright 2002, Elsevier)]. The power of the EEG signal in the gamma frequency band (40–80 Hz) was stronger for the first presentation of the stimuli and decreased during the second presentation of the stimuli. (B) Rastergram of spike times of the excitatory neurons of P1–P5 populations. The first presentation of a stimulus started at 200 msec and terminated at 1,000 msec (black bar beneath the rastergram). The second time, the stimulus was presented at 1,200 msec and removed at 2,000 msec. Populations P1 and P2 received stronger external inputs than populations P3, P4, and P5. The first presentation of the stimulus induced spike time synchrony among neurons of all five populations; however, during the second presentation, the synchrony decreased in populations P3–P5 compared with that in populations P1 and P2. (C) LFP was measured as the total synaptic current to excitatory neurons of all five populations. Amplitude of the LFP was higher during the first presentation than during the second presentation. This was because all five populations synchronized their spikes during the first presentation, which resulted in high-amplitude LFP oscillations. During the second presentation, spike synchrony was observed mostly among neurons in populations P1 and P2, which resulted in a lower amplitude LFP. (D) Spectrogram of the LFP revealed a strong response in the gamma frequency range (30–40 Hz) during the first presentation, which weakened during the second presentation. Power was also found at the first harmonic at 60–70 Hz.