Illustration of the FT-EEG paradigms. a, Principle of the oddball paradigm. Top panel, Base stimuli are presented at 6 Hz base rate while oddball stimuli are inserted at every fifth cycle (i.e., 1.2 Hz oddball rate). Bottom panel, Base stimuli elicit responses at the base frequency (6 Hz). Left, When oddball stimuli are not perceived as different from the base stimuli, the stimulus presentation will only elicit responses at the base frequency (6 Hz) and harmonics (n × 6 Hz). Right, When oddball stimuli are perceived as different from the base stimuli, they elicit an additional oddball response at 1.2 Hz and harmonics (n × 1.2 Hz). b, Oddball paradigm for stimulus discrimination. Top panel, During EEG recording in the implicit oddball paradigm, different “between” (e.g., L4–L7) and “within” (e.g., L1–L4) category trials with their respective reversed base–oddball stimulus pairs (e.g., L7–L4 and L4–L1) are randomly presented. Size variations of 20% (20% smaller, normal size, 20% larger size) are randomly implemented with a different relative size at every consecutive presentation to abolish the impact of low-level (pixel) confounds. The base and oddball stimuli are presented using a sinusoidal contrast modulation at the presentation frequency. Bottom panel, For each oddball pair, the oddball response is calculated, and each corresponding response can be plotted along a spectrum. Note that we expect that “between”- versus “within”-category pairs result in systematically larger oddball responses (indicating a more prominent discrimination). This indicates the presence of spontaneous categorical perception. c, Sweep oddball paradigm for category classification. Left panel, Throughout a trial, the endpoint of a morph continuum is systematically presented as base stimulus (at 6 Hz), and in 10 s segments all different morph levels are systematically presented (from each endpoint) at the oddball frequency of 1.2 Hz. Right panel, For each sweep step, the oddball response is calculated (top), and each corresponding response can be plotted (bottom). Note that we expect that from a certain threshold (deviance from linearity) physical differences (on the x-axis) will result in systematically larger oddball responses (on the y-axis). We test this by investigating whether there is a distinct increase in the baseline-subtracted oddball amplitude between consecutive sweep steps. This threshold would represent the implicit category boundary.