Figure 1. Electrocorticography (ECoG) signals in visual cortex.

(a) A medial view of right occipital cortex in S1. The location of the magnified view is shown in the inset. The positions of two strips of intracranial electrodes are shown in magenta and blue. One of the electrodes, indicated by the black arrow, is the source of the data plotted in panels c and d. (b) A schematic of the flicker during ‘on’ periods. The stimulus is a dartboard pattern which reverses in contrast (pattern ‘A’ to pattern ‘B’) at 7.5 Hz, such that there are 15 stimulus events per second. (c) The ECoG time series is plotted for an ‘on-off’ experiment (see Methods). The ‘on’ periods are plotted in light gray and the ‘off’ periods in dark gray. The signal fluctuations are larger during the ‘on’ than the ‘off’ periods. (d) Spectral representation of the same data plotted in panel b. Short-time Fourier analysis was used to calculate the spectral power in one-second non-overlapping windows across the 48-s experiment. The mean spectra during the two periods differ from one another in two salient ways. First, during the ‘on’ periods but not the ‘off’ periods, there are spectral peaks at even harmonics of the stimulus frequency (dashed vertical lines). Second, there is a broadband elevation of the signal during ‘on’ periods compared to ‘off’ periods, as seen by the offset in the fitted white lines. The broadband elevation spans the spectrum from below 10 Hz to above 150 Hz. Data at 60 Hz and 120 Hz are not plotted because signals at these frequencies are corrupted by electrical line noise. Data points with circles indicate the points used to fit a line to the broadband signal (dashed white lines). The inset shows the responses at 15 Hz, with the arrows indicating the stimulus-locked response (black) and the broadband response (white). See Figure S1 for example plots of the spectrum and the time series in response to a flickering contrast pattern within a bar aperture for the same electrode depicted in this figure.