Fig 1.
Each trial began with the presentation of a fixation cross that was followed by an annular grating that remained static (0°/s) or drifted inward for 1.2–3 s at one of the three velocities: 1.2, 3.6, 6.0°/s. The 100% and 50% contrast gratings were presented in separate sessions. Participants responded to the change in the stimulation flow (disappearance of the static grating or termination of motion) with a button press. Short animated cartoon characters were presented randomly between every 2–5 stimuli to maintain vigilance and reduce visual fatigue.
Fig 2.
Grand average statistical maps of the cortical GRs to drifting visual gratings.
The maps are given for the weighted peak gamma power, separately for the two contrasts and four motion velocities. Positive signs of the t-statistics correspond to stimulation-related increases in gamma power. Note the different scales for the 50% and 100% contrasts and that the magnitude of the GR was affected by both contrast and velocity.
Fig 3.
Grand average spectra of cortical GRs to the gratings of 100% (A) and 50% (B) contrasts drifting at four motion velocities. Here and hereafter, the GR power change is calculated as (post-pre)/pre ratio, where ‘pre’ and ‘post’ are the spectral power values in the -0.9 to 0 and 0.3 to 1.2 s time windows relative to the stimulus onset.
Fig 4.
Individual spectra of cortical GRs.
The plots are the same as in Fig 3, see Methods for further details.
Fig 5.
Grand average peak frequency (A) and power (B) of cortical GRs as a function of Contrast and motion Velocity. Vertical bars denote 0.95 confidence intervals. *** p<0.001; ** p<0.01; * p<0.05.
Fig 6.
Effect of contrast on the gamma suppression transition point: individual variability.
A. The centre of gravity of visual motion velocity that approximates the GR suppression transition point (‘suppression transition velocity’—STVel). The STVel increased with decreasing contrast in all subjects (p<1e-6). B. The centre of gravity that approximates the peak frequency of the GR at the suppression transition point (‘suppression transition frequency’—STFreq) in 14 subjects for whom frequency could be assessed in all experimental conditions (see Materials and Methods for details). The STFreq slightly, but significantly, decreased with decreasing the contrast (p = 0.007).
Fig 7.
Rank-order consistency of the GR power across conditions.
A. Correlations between GR power measured at 50% and 100% contrasts. B. Correlations between GR power values measured in different velocity conditions at 100% contrast (see S1 Fig in Supporting Information for a similar figure for the 50% contrast stimuli). Blue dots in A and B denote individual GR power values. C. Correlations between STVel values at the two contrasts. Green squares denote individual STVel values. The linear regression is shown in red. Dashed lines in all plots correspond to the axis of symmetry.
Fig 8.
Grand average power changes in the alpha-beta range.
A. Statistical maps of the stimulation-related changes in the two luminance contrasts and four motion velocity conditions. Blue color corresponds to suppression of alpha-beta power. B. Spectral changes at a selection of ‘maximally suppressed’ voxels. Dashed lines show absolute power spectra (right y-axis, arbitrary units) corresponding to the periods of visual stimulation. The dot -dashed lines of the same colors show the power spectra for the respective pre-stimulus intervals (also right y-axis). Solid lines show respective stimulation-related changes in power ([post-pre]/pre ratio, left y-axis, arbitrary units). Neither contrast nor velocity had significant effects on the magnitude of alpha-beta suppression.