(a) Example participant’s facial reactions at two time points, corresponding to high and low activation of action unit 12. (b) Action unit time series for five example participants (blue). Bold line corresponds to the participant shown in panel (a), and black arrows indicate time points corresponding to the representative pictures. The group mean time course across all participants is shown in red. Red arrows indicate funny moments in the stimulus, evoking sudden facial changes in individual participants. These changes are less prominent in the group mean time course. (c) Time-frequency representation for the same five participants, calculated as the amplitude of the continuous wavelet transform. Intuitively, the heatmap colour indicates how much of each frequency is present at each time point. Shading indicates the cone of influence – the region contaminated by edge effects. (d) Mean of all participants’ time-frequency representations. Red arrows correspond to time points with marked high-frequency activity above 1 Hz (e) Time-frequency representation of the group mean time course. (f) Difference between (d) and (e). Non-significant differences (p>0.05) are shown in greyscale. Common colour scale is used for (d–f).