Snake Effect: A Novel Haptic Illusion.

We present a novel, movement-based haptic illusion called the "snake effect." Unlike apparent motion or sensory saltation, the snake effect feels wavy and creepy as though the belly of a slithering snake is making and breaking contact with the skin. This illusion is achieved by modulating the amplitudes of vibrotactile pulses sent successively to an array of tactors. Pilot testing established the following signal parameters for creating the snake effect: a minimal pulse duration of 1.69 s, carrier frequency in the range of 200-300 Hz, amplitude modulation of the carrier with a sine, sine-squared or Gaussian waveform (shown to be more effective than a linear up-and-down ramp), and a peak amplitude of 30 dB above detection threshold. The main experiment examined the most effective signal onset asynchrony (SOA) ranges by estimating the upper and lower SOA thresholds using a one-up one-down adaptive procedure with interleaved ascending and descending series. The results indicate an optimal SOA range from 271.5 ms to 798 ms with a midpoint of 535 ms. The snake effect is a vivid illusion that can be used as a distinctive signal for encoding information and to enhance immersion and engagement in gaming and entertainment.