Vibrotactile difference thresholds for intensity and the effect of a masking stimulus

Vibrotactile difference thresholds for intensity were measured at several intensity levels of a test stimulus in the absence of a masking vibration and in the presence of three different amplitudes of a masking vibration. The test stimulus was a 160-Hz vibration delivered to the right index finger. The masking stimulus was a 160-Hz vibration delivered to the right little finger. For the same amplitudes of the test stimulus, △I varied as a direct function of the amplitude of the masking vibration. The smallest △Is resulted from measurements made in the absence of the masking stimulus. The Weber fraction, △I/I, was constant only for the more intense test stimuli in the absence of any masking stimuli. Independent of the presence or level of the masker, the Weber fraction for all stimuli approached approximately the same value, .25, when the test stimuli were raised to 20-dB sensation level. A model is proposed to account for the increase in the Weber fraction as a function of masker intensity and to predict masked thresholds.