Physical determinants of perceived vibration intensity: Insights from haptic feedback given at the wrist

Understanding how controlled mechanical vibrations applied to the skin translate into tactile sensations is key in advancing our understanding of somatosensory perception and using this in haptic applications. Prior research has established psychophysical thresholds and discrimination levels for vibrotactile stimuli, yet less is known about tactile intensity perception. Understanding the signaling of tactile intensity and how mechanical dynamics of skin transmission shape this is critical for providing effective graded haptic feedback. We investigated how modifying the frequency, amplitude, and waveform shape of vibration at the wrist influence free ratings of perceived tactile intensity. Two accelerometers recorded mechanical signals: one embedded within the vibrotactile wristband and another placed 1 cm away on the skin, enabling tracking of vibration propagation through the tissue. We found that perceived intensity was determined by all three parameters, but each had a distinct role. Vibration frequency exerted a non-linear influence, with perceived intensity peaking around 150 Hz, which also corresponded to the actuators mechanical resonance, with lower secondary peaks for square and sawtooth waveforms in-line with vibratory harmonics. Moreover, square waves were consistently rated as most intense. Vibration amplitude provided a consistent scaling of perceived strength, which was strongly correlated with both actuator and skin acceleration, indicating that skin-transmitted accelerations retained the stimulation profile. We find that the skin acts as a non-linear, frequency-dependent mechanical filter that selectively modulates vibration transmission. Understanding these determinants provides insights for the design of wearable haptic devices and how these can be used to provide graded feedback during tactile interactions.