Oscillatory gamma activity in humans and its role in object representation

Gamma and beta neural activity evoked during a sensory gating paradigm: Effects of auditory, somatosensory and cross-modal stimulation

“…Time-varying, band-evoked amplitude was then computed as the absolute value of the resulting complex waveform that was the linear combination of the real and imaginary component waveforms (i.e., amplitude was computed as the square root of the sum of squares of the real and imaginary parts). For the purpose of comparison to existing literature it should be pointed out that time-series waveforms generated by the present analysis provide similar, though non-identical information to those obtained by convolution with a short "wavelet" of a selected frequency (Gurtubay et al, 2001;Herrmann, 2003;Makinen et al, 2004;Tallon-Baudry and Bertrand, 1999). Also, the present method collapses activity across a specified frequency band, selected a priori, whereas wavelet approaches separate activity into discrete frequency channels.…”

“…Neither of these findings depended upon ISI. The overall lack of stimulus-induced activity may reflect the absence of top-down attention directed towards the stimuli, because selective attention has been shown to strongly modulate stimulusinduced activity in this temporal window (Muller et al, 2000;Posada et al, 2003;TallonBaudry et al, 2005;Tallon-Baudry and Bertrand, 1999;Vernon et al, 2005). We were unable to test this hypothesis directly, however, because all recordings here were conducted during an alternate task (watching a silent video) that did not involve the stimuli.…”

“…Later (200-400 ms) induced oscillations were detected in that study, but beta activity preceded gamma activity in that latency window. Also, stimulus-induced gamma oscillations in this latency range are generally thought to relate to conscious perception and cognitive processing of a stimulus (Herrmann et al, 2004;Muller et al, 2000;Tallon-Baudry and Bertrand, 1999), and thus were not expected to be significantly modulated by ISI for task-irrelevant (i.e., ignored) stimuli here. A final reason for focusing on early, stimulus-locked activity was that we wanted to compare our findings to more standard, ERP-based studies of sensory gating.…”

“…Like the computation of stimulus-evoked activity described above, the stimulus-induced oscillation amplitude was computed for the interval -100 ms to 500 ms post-stimulus. However, the latency range of analysis (i.e., the determination of peak activity) for the stimulus-induced activity was restricted to a range of 200-400 ms poststimulus, as this window typically contains significant stimulus-induced oscillation in both gamma (Tallon-Baudry and Bertrand, 1999) and beta (Haenschel et al, 2000) frequency bands. Although induced activity has also been demonstrated after 600 ms (see also Kranczioch et al, 2006), we did not assess this because the neural response elicited by the short-ISI stimuli presented here (i.e., 500 ms) would overlap with this window.…”

“…Grand average stimulus-induced (non phase-locked) activity for the auditory-only condition. Based on previous studies (Haenschel et al, 2000;Tallon-Baudry and Bertrand, 1999), the planned analysis window for this activity was between 200 and 400 ms. However, as illustrated here, stimulus-induced gamma activity in that latency range was not substantially different than random noise, and stimulus-induced beta activity showed an apparent suppression (i.e., reduction below pre-stimulus levels).…”