SIFI

The experiment was programmed using MATLAB 2021 with Psychtoolbox 3.1.7 (Kleiner et al., 2007). Visual stimuli were displayed on a Lenovo IdeaCentre AIO 3 all-in-one computer with a screen resolution of 1024 × 768 pixels and a refresh rate of 60 Hz. The visual flashes consisted of white disks (RGB: 255, 255, 255; radius = 2°) presented simultaneously on both the left and right sides, each positioned 7° from the central fixation point against a black background (RGB: 0, 0, 0). Each flash was displayed for 17 ms.Auditory stimuli were pure tones (beeps) of 3.5 kHz presented at 75 dB SPL for 17 ms, delivered binaurally through headphones. The experiment followed a 3 (sound: 0, 1, or 2 beeps) × 2 (vision: 1 or 2 flashes) within-subject design, yielding six conditions in total: B0F1, B0F2, B1F1, B1F2, B2F1, and B2F2. Here, “B” denotes the number of auditory beeps and “F” denotes the number of visual flashes. B0F1 and B0F2 served as visual-only baseline conditions, whereas B1F1, B2F2, B1F2, and B2F1 were the four audiovisual combinations shown in Figure 1b. For example, B2F1 indicates one simultaneous visual flash accompanied by two auditory beeps. The inter-beep and inter-flash intervals were both 66 ms. To improve consistency, we use uppercase condition codes throughout the manuscript: B0F1 and B0F2 denote the visual-only baseline conditions, B1F1 and B2F2 denote the audiovisually congruent conditions, B1F2 denotes the fusion configuration, and B2F1 denotes the fission configuration.