Integration of auditory and vibrotactile stimuli: effects of phase and stimulus-onset asynchrony

The perceptual integration of 250 Hz, 500 ms vibrotactile and auditory tones was studied in detection experiments as a function of (1) relative phase and (2) temporal asynchrony of the tone pulses. Vibrotactile stimuli were delivered through a single-channel vibrator to the left middle fingertip and auditory stimuli were presented diotically through headphones in a background of 50 dB sound pressure level broadband noise. The vibrotactile and auditory stimulus levels used each yielded 63%-77%-correct unimodal detection performance in a 2-I, 2-AFC task. Results for combined vibrotactile and auditory detection indicated that (1) performance improved for synchronous presentation, (2) performance was not affected by the relative phase of the auditory and tactile sinusoidal stimuli, and (3) performance for non-overlapping stimuli improved only if the tactile stimulus preceded the auditory. The results are generally more consistent with a "Pythagorean Sum" model than with either an "Algebraic Sum" or an "Optimal Single-Channel" Model of perceptual integration. Thus, certain combinations of auditory and tactile signals result in significant integrative effects. The lack of phase effect suggests an envelope rather than fine-structure operation for integration. The effects of asynchronous presentation of the auditory and tactile stimuli are consistent with time constants deduced from single-modality masking experiments.