(A) An example single trial current dipole waveform generated from a stationary square-wave grating, presented from 0 to 700 ms. MEG data was source localized to the pericalcarine region of the visual cortex (top). The corresponding time-frequency representation (bottom) shows transient bursts of induced gamma activity (bottom). Data is as in Pantazis et al. (2018) and described further in text. (B) Analogous waveforms from 100 trials overlaid (top) and corresponding averaged time-frequency spectrograms (bottom). Averaging in the spectral domain creates the appearance of a more continuous oscillation. (C) Parameters used to simulate the canonical PING rhythm in Figure 9 were adjusted (Steps 6 and 7) to test the hypothesis that a reduced rate of Poisson synaptic input to the pyramidal neurons could account for the bursty nature of the observed gamma data. The parameter set for this example is provided in the ‘gamma_L5weak_L2weak_bursty.param’ file. In this example, the Poisson input rates to the Layer 2/3 and Layer 5 cells were reduced to 4 Hz and 5 Hz, respectively, and the NMDA synaptic inputs were set to a small positive weight (dialog box not shown). Running the simulation will display a histogram of the Poisson drive (top), the current dipole waveform (middle), and the corresponding time-frequency spectrogram (bottom). With this parameter adjustment,~50 Hz bursty gamma activity is reproduced. A scaling factor of 5 was applied to the net current dipole to produce a signal amplitude comparable to the data (<0.1 nAm), suggesting that only ~1000 pyramidal neurons contributed the recorded signal. (D) Poisson input over 100 simulations (top), current dipole waveforms overlaid (middle), and the corresponding spectrogram average (bottom). Similar to the recorded data, averaging in the spectral domain creates the appearance of a more continuous oscillation. (E–F) Microcircuit details including cell spiking responses and layer specific current dipoles shows Layer 5 activity dominates the net current dipole signal, see text for further description.