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Why does pseudo-haptics work?

Your brain doesn't have a separate "touch" module — it builds the sensation of a surface by integrating vision, audio, and proprioception. When those signals agree on what you're touching, the brain commits to a single percept, even if one of the channels is missing or fake.

The three illusions at work here

1. Cross-modal binding (audio ⇄ touch). Each ping is a micro-impulse. When pings appear at the right spatial density and timbre for a given drag, the auditory cortex triggers activity in somatosensory regions (S1/S2). You literally "feel" what you hear. Jitter, transient bursts, and amplitude variation are essential — perfectly identical pings are flagged as synthetic and the illusion collapses.

2. Control/Display (C:D) gain mismatch. When the puck moves slower than your finger, your brain resolves the conflict by inferring friction. Lécuyer (2004) showed this is enough to make people report "resistance" with zero physical force. C:D < 1 = sticky/heavy, C:D > 1 = slippery/light. Stick-slip motion adds dry-friction realism on top.

3. Visual-motor prediction error. Puck wobble, ripples, and procedural surface patterns give the motor system predictive visual targets. Any mismatch between the expected and actual motion is interpreted as physical interaction with the surface.

What each control targets

Motion Coupling — velocity→brightness mimics friction exciting high harmonics; velocity→density creates washboard acceleration; direction→timbre creates anisotropy (brushed metal).

Pattern (audio) — temporal distribution of pings. Uniform = grid, random = natural, double = corduroy, sparse = gravel.

Transient ping + inharmonicity — the "impact" of contact before the tone. Crucial for crisp surfaces (glass, metal).

Reverberation + early reflections — signals the acoustic distance of the imagined surface.

Visual Feedback panel — all the classic Lécuyer-style cross-modal illusions.

Tips for the strongest illusion

• Use headphones. Laptop speakers destroy the stereo pan and low-end cues.
• Start with a Library preset (Sandpaper, Ice, Gravel) — they're hand-tuned to combine effects that reinforce each other.
• Try A/B: store two surfaces, then press A/B on the keyboard while dragging. The contrast makes each feel more "real."
• The Blind Test is the best way to convince yourself it works.

Blind Test

A random surface is loaded. Drag on the pad, listen and feel — then pick what it is.