LAUSR

Haptic illusions serve as important tools for studying neuro-cognitive processing of touch and can be utilized in practical contexts. We report a new spatio-temporal haptic illusion that involves mislocalization when the order of vibrotactile intensity is manipulated. We tested two types of motors mounted in a 4 x 4 array in the lower thoracic region. We created apparent movement with two successive vibrotactile stimulations of varying distance (40, 20, or 0 mm) and direction (up/down/same), while changing the temporal order of stimulation intensity (strong-weak vs. weak-strong). Participants judged the perceived direction of movement in a 2-alternative forced-choice task. The results suggest that varying the temporal order of vibrotactile stimuli with different intensity leads to systematic localization errors: When a strong intensity stimulus was followed by a weak intensity stimulus, the probability that participants perceived a downward movement increased, and vice versa. The illusion is so strong that the order of the strength of stimulation determined perception even when the actual presentation movement was the opposite. We then verified this Intensity Order Illusionusing an open response format where observers judged the orientation of an imaginary line drawn between two sequential tactor activations. The intensity order illusion reveals a strong bias in vibrotactile perception that has important implications for the design of haptic information systems.