Temporal Phase Differences Encode Tactile Motion from Static Vibrotactile Inputs

Motion perception in touch is traditionally attributed to spatially sequential stimulation or skin deformation caused by moving objects. Here, we demonstrate that directional tactile motion can instead be inferred purely from temporal phase differences between two spatially static vibrotactile inputs. Using psychophysical experiments in human participants (n = 54), we show that continuous vibrations delivered to separate fingertips, when offset in phase, evoke a robust and directionally consistent motion percept despite the absence of physical movement. Discrimination performance depended systematically on the temporal phase relationship between stimuli, following a sinusoidal relationship, consistent with a phase-based motion inference mechanism. Vibration amplitude had little influence on perceptual accuracy once stimuli exceeded detection threshold, although higher amplitudes modestly reduced response times. In contrast, envelope dynamics and spatial configuration significantly modulated performance: vibrations with exponential amplitude envelopes - reflecting the natural propagation of mechanical energy through a medium - enhanced phase sensitivity and sped responses, and bimanual stimulation across the two hands improved both accuracy and response time. These findings identify temporal phase integration as a fundamental mechanism for tactile motion perception, analogous to timing-based computations in auditory localisation and visual motion detection. Phase-coded vibrotactile stimulation therefore offers a minimal and efficient strategy for conveying directional motion in haptic interfaces without requiring spatial arrays or moving actuators.