Comparing 20 Hz Steady-State Somatosensory Neural Responses for Contact Vibrotactile and Ultrasound Mid-Air Haptic Stimulation

Ultrasound mid-air haptics (UMAH) can create touch sensations without physical contact, but it remains unclear whether they evoke steady-state somatosensory evoked potentials (SSSEPs) that could serve as objective markers relevant to evaluating haptic perception in UMAH interfaces. This study tested whether 20 Hz UMAH delivered with a commercial device at maximum available intensity elicited SSSEPs comparable to those produced by vibrotactile stimulation (VTS). Electroencephalography was recorded from 26 participants during three stimulation conditions: full-intensity VTS, subjectively matched-intensity VTS, and full-intensity UMAH. Signal-to-noise ratio (SNR) and power spectral density (PSD) at 20 Hz were analyzed over contralateral and ipsilateral somatosensory regions using linear mixed-effects models, with baseline estimates derived from no-stimulation intervals. Full-intensity VTS produced clear contralateral SSSEPs, and subjectively matched-intensity VTS yielded weaker but significant responses. In the present setup, UMAH did not yield a detectable 20 Hz SSSEP relative to baseline in either SNR or PSD. These findings support SSSEPs as sensitive markers of contact vibrotactile stimulation, but suggest that, with the present apparatus and analysis approach, they are not yet a robust objective measure for evaluating UMAH interface experience.