*2.1. Participants*

Five adult participants aged between 21 and 35 (all females) took part in this experiment on a voluntary basis and gave informed consent prior to the beginning of the experiments. They had normal or corrected-to-normal vision and had no history of neurological diseases. This study was conducted in accordance with the principles of the Declaration of Helsinki and the "Comitato Etico San Ra ffaele".

#### *2.2. Stimuli and Tasks*

Observers were seated 57 cm in front of a laptop screen. Visual stimuli (Figure 1) consisted of (i) a video clip (1280 × 720 @ 30 Hz) depicting sea ripples on a beach that we had previously used [2]; and (ii) a random dot kinematogram (RDK) (3000 black and white dots, 67 ms lifetime, 5 deg/s maximal speed, 0% coherence). In order to modulate video speed, the frame-rate of the visual stimulus was sinusoidally modulated (frequency, 1, 2, or 4 Hz; amplitude, 10%, 30%, or 50% of the original frame-rate) by controlling in real time the video frame flipping on the graphics board (Nvidia GTX 1060, Santa Clara, CA, USA). As a result, videos acquired a pulsatory rhythm. To ensure uniform conditions in extracting cortical oscillations (see below), in each trial there were 50 speed-modulation cycles regardless of the modulation frequency. Therefore, trials lasted 50, 25, or 12.5 s, respectively, for speed modulations of 1, 2, and 4 Hz. Observers kept their gaze in central fixation and watched the videos while wearing an electroencephalographic (EEG) recording headset (18 trials: 3 frequencies × 3 amplitudes for each visual stimulus, randomly interleaved). They were asked to mentally focus on the pulsation of the visual stimulus. Overall, the session lasted about 20 min.

**Figure 1.** A snapshot of the dynamic visual stimuli used in this study. They were displayed with various sinusoidal speed modulations, which conferred to them a pulsating appearance. In the experiment, there was also a central fixation dot. RDK—random dot kinematogram.

In a second experimental session, administered on a di fferent day, observers rated in a 9-point scale the perceived strength of the pulsation (108 trials: 3 frequencies × 4 amplitudes × 9 repetitions—including catch trials with zero amplitude, i.e., no speed modulation). Observers were specifically instructed to rate the perceived intensity (strength) of sinusoidal speed modulation. Phenomenally, such rhythmic speed modulation appeared as a pulsation, periodically speeding up and slowing down the stimulus at the experimentally imposed frequency (i.e., 1, 2 or 4 Hz). Stimuli and other conditions were the same as in the first session, except that observers could respond whenever they wished. Overall, the session lasted about 30 min.
