Page 40 - Spring2020

Music Beyond Sound
process of inhibition, wherein activity from neighboring receptor areas was dampened. However, it is important to note that despite the similarities across modalities in regard to frequency-tuned mechanoreceptors and inhibition, pitch discrimination thresholds obtained with vibrotactile stimuli tend to be about five times greater than those obtained with auditory stimuli (Verrillo, 1992).
Detection thresholds for vibrotactile stimuli show a peak sensitivity around 250 Hz, exhibiting a sharp decline below 100 Hz and above 1,000 Hz (Verillo, 1992). Sensitivity has also been found to increase as a function of the number of mechanoreceptors that are stimulated (Morioka and Griffin, 2005). The number of mechanoreceptors stimulated will be influenced by the type of skin and the contact area between
the source of vibrotactile stimulation and the skin. Mechano- receptor density is higher in smooth areas of the skin (e.g., palms of the hands), which leads to higher overall sensitivity in smooth skin compared with hairy skin (Verrillo and Bolanowski, 1986). A commensurate, albeit speculative, point to make about sensi- tivity to vibrotactile stimulation in smooth skin is that observers may experience annoyance sooner over smooth skin because of its heightened sensitivity.
Several studies have investigated the ability to discriminate timbre on the basis of vibrotactile stimulation alone. Russo et al. (2012) found that both deaf and hearing observers were able to accurately distinguish instrument timbres on the basis of vibrotactile input. Deaf and hearing partici- pants were also able to distinguish timbre on the basis of vibrotactile input when stimuli consisted of synthetic tones that differed only with respect to spectral tilt — whether the envelope was weighted toward low or high frequencies. Based on these findings, Russo et al. (2012) proposed that the vibrotactile perception of timbre involves the cortical integration of activity across the frequency-tuned mechano- receptors. The relative activity across channels would allow for perceptual coding of the spectral shape in the same way that has been proposed for the critical bands in the auditory system (Makous et al., 1995). It would only take two such channels to allow for the coding of spectral tilt. Moreover, a follow-up study revealed that deaf participants are able to discriminate sung vowels and that the extent of difference in the spectral tilt between pairs strongly predicted their discriminability (Ammirante et al., 2013).
In addition to the role of vibrotactile stimulation in percep- tion of music, it seems that vibrotactile feedback arising
Figure 4. Pitch, timbre, and rhythm can be perceived on the basis of touch alone.
during music performance provides valuable information about timbre that may be used by the performer (Marshall and Wanderley, 2011). Perhaps, not surprisingly, the per- ception of sound quality as evaluated by the performer has been shown to be positively influenced by microvibra- tions as feedback from the strings that can be felt through the keys (Fontana et al., 2017). The ability to incorporate vibrotactile feedback in performance is further supported by the observation that vibrotactile detection thresholds are reduced when stimulation arises from actively touching a vibrating surface compared with receiving the vibration on a stationary hand (Papetti et al., 2017).
Several lines of evidence exist to suggest that touch can influence our perception of rhythm. In one study, research- ers asked deaf and hearing participants to synchronize their movements to a vibrotactile beat that was delivered through a vibrating platform on which the participants were asked to stand (Tranchant et al., 2017). The synchronization of movements was achieved by attempting to bounce in time with the beat. Hearing participants were asked to perform an additional task in which they were asked to synchronize their bounces to the same beat when it was delivered through sound alone. The results showed that most participants were able to bounce to a vibrotactile beat. However, for hearing participants, synchronization performance was better in the auditory condition, presumably due to the years of experience they had amassed tapping or dancing to music. The study did not, however, consider individual differences such as formal dance training or the extent of experience in moving to music.
Empirical evidence for synchronization to a beat has also been found using vibrotactile stimulation applied to the fin-
40 | Acoustics Today | Spring 2020