Perceived intensity of vibrotactile stimuli: the role of mechanoreceptive channels.

The perceived intensity of vibrotactile stimuli was studied by means of free magnitude estimation. Eighty different sinusoidal stimuli ranging in frequency from 10 to 200 Hz, and in amplitude from 2.4 to 154 microns, were presented to the left index fingerpad of psychophysical observers through a 5-mm-diameter contactor. Estimates at a given frequency increased with amplitude in all four subjects, and estimates at a given amplitude increased with frequency in three. For the fourth subject, however, intermediate frequencies (25-75 Hz) produced the most intense sensations; the relative sensory effectiveness of different frequencies suggested that in her case, perceived vibrotactile intensity was determined largely by signals in Meissner afferents. From the data of this unusual subject, and from high-frequency (200-Hz) measurements on the normal subjects, quantitative descriptions were derived of the signals in Meissner and Pacinian channels, respectively, that could contribute to subjective intensity. Candidate algorithms by which the signals from the two channels might interact were then evaluated by comparison of modeled and empirically determined subjective intensity values. It was found that subjective intensity is given by the sum of (1) the stronger of the two channels' signals, and (2) half the weaker signal, the latter apparently being reduced by cross-channel suppression that occurs only at suprathreshold levels. Adapting to 25-Hz vibration selectively reduces the perceived intensity of low frequencies, whereas adapting to 200-Hz vibration has a corresponding effect at high frequencies. It is concluded that an understanding of perceived vibrotactile intensity requires knowledge of the signals in vibrotactile channels, and of the interactions between those channels.