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The Role of Auditory Cues in Modulating the Perception of Electric Toothbrushes
1 Department of Experimental Psychology, University of Oxford, South Parks Road, Oxford, OX1 3UD, UK; and Correspondence: * corresponding authors, massimiliano.zampini{at}psy.ox.ac.uk or charles.spence{at}psy.ox.ac.uk
Recent research shows that what people hear can influence what they feel. We investigated whether the perception of an electric toothbrush might also be affected by the sound that it makes. Participants were required to make stereotypical brushing movements with a standard electric toothbrush while they rated either the pleasantness or the roughness of the vibrotactile stimulation they felt on their teeth. The results demonstrate that the perception of the sensations experienced during toothbrush use were systematically altered by variations in the auditory feedback elicited by the brushing action. Participants reported that the toothbrush felt more pleasant and less rough when either the overall sound level was reduced, or when just the high-frequency sounds were attenuated. These results highlight the significant role that auditory cues can play in modulating the perception and evaluation of everyday products in use, and provide a paradigm for future study in this area.
Key Words: multisensory • vibration • touch • hearing • perception
The sounds that are elicited when we touch or use many everyday objects often convey potentially useful information regarding the nature of the objects with which we are interacting (Lederman, 1979; Guest et al., 2002). A particularly dramatic example of the auditory modulation of tactile perception, the "parchment-skin" illusion, was described recently by Jousmäki and Hari (1998). In this study, people’s perception of the palmar surface of their own hands was changed simply by changing the sounds they heard when their hands were rubbed together. More recently, Guest et al.(2002) replicated these findings using a more rigorous psychophysical testing procedure. Such results provide a powerful example of the multisensory nature of much of our everyday perception. In fact, a growing body of research now shows that the information processed in one sensory modality can modulate our perception of stimuli in other sensory modalities (see Driver and Spence, 2000, for a review). In the present study, we investigated whether people’s perception of an everyday product in use could also be modified by changing the sounds associated with its operation. Participants used a standard electric toothbrush to brush their front teeth in a stereotypical manner and were required to rate both the pleasantness and roughness of the tactile sensations they felt using an anchored visual analog scale. We predicted that if auditory cues affect the perception and evaluation of real products in use, then our participants should perceive the "feel" of the electric toothbrush against the teeth to vary as the sounds made by the brushing action were manipulated, even though they were explicitly instructed to ignore the sounds that they heard.
Participants Twenty participants (two males and 18 females; mean age, 26 yrs) took part in this experiment as paid volunteers, and all were naïve as to the purpose of the study. The experiment lasted approximately 30 min. The participants gave their informed consent prior to the start of the experiment, and their rights were protected by the ethical review board of Department of Experiment Psychology, Oxford University.
Apparatus and Stimuli
The response scale was presented on a computer monitor situated outside the sound-attenuated booth and visible through a window in the side of the booth. The response scale had 100 scale divisions and semantic anchors at either end. For the unpleasant-pleasant scale, the ‘unpleasant’ anchor was placed on the left side of the bar, while for the rough-smooth scale the ‘rough’ anchor was placed on the left. Movement of the scale pointer was achieved by a pair of footpedals situated under the participant’s feet. Participants normally kept both footpedals depressed, and moved the pointer to the left by lifting their left foot, and to the right by lifting their right foot. When participants were satisfied with their subjective rating on the response dimension presented for a given trial, they were instructed to depress a button located next to the microphone. This registered the participant’s response and advanced the experiment to the next trial.
Design
Procedure
The data from the unpleasant-pleasant response scale are presented in Fig. 2a  . A two-way within-participants repeated-measures analysis of variance (ANOVA) performed on these data had the factors of Frequency Manipulation (attenuated, veridical, or amplified) and Overall Sound Attenuation (0 dB, 20 dB, or 40 dB). For all of the analyses reported here, post hoc comparisons used Bonferroni-corrected t tests (where p < 0.05 prior to correction), and each data cell reflects the average data value from 5 trials for each of the 20 participants tested. The analysis revealed a significant main effect of Frequency Manipulation (F2,18 = 26.19, p < 0.001), reflecting the fact that participants judged the toothbrush to feel significantly more pleasant when the high-frequency sounds were attenuated (mean score of 50) than when either the veridical sound was presented (M = 46) or when the high-frequency sounds were amplified (M = 36; the post hoc comparison between these latter two conditions revealed that the difference between the means was also statistically significant). The main effect of Sound Attenuation was also statistically significant (F2,18 = 25.55, p < 0.001), attributable to participants reporting that the toothbrush felt more pleasant at 40-dB attenuation (M = 55) than at 20-dB attenuation (M = 46), which in turn was rated as more pleasant than 0-dB attenuation (M = 31; all pairwise comparisons were significant).
The interaction between Frequency Manipulation and Overall Sound Attenuation also reached statistical significance (F4,16 = 5.15, p = 0.001), reflecting the fact that the frequency manipulation had a greater effect on pleasantness ratings at the two lower attenuation levels (i.e., at 0-dB and 20-dB attenuation) than at the 40-dB attenuation level. At 0-dB overall attenuation, participants rated high-frequency amplification as significantly more unpleasant (M = 22) than veridical feedback (M = 33) and high-frequency attenuation (M = 39; the comparison between these latter two conditions was also statistically significant). At 20-dB attenuation, the most pleasant sensations were reported when the high frequencies were attenuated (M = 55) as compared with the veridical feedback (M = 48) or high-frequency amplification conditions (M = 36; all pairwise comparisons were significant). The frequency manipulations had no significant effect on pleasantness ratings at the 40-dB attenuation level, presumably because the overall sound level approached threshold (i.e., no pairwise differences reached significance at this overall level of attenuation).
A similar ANOVA performed on the rough-smooth response scale data (see Fig. 2b
The significant interaction between Frequency Manipulation and Overall Sound Attenuation (F4,16 = 4.72, p < 0.025), illustrated in Fig. 2b
The results of the experiment reported here clearly demonstrate that the perceived pleasantness and roughness of an electric toothbrush are sensitive to both the intensity and the frequency spectrum of the sound it makes. The vibrotactile stimulation elicited by the toothbrush was judged to be more pleasant when the overall sound level was reduced and/or when just the high-frequency components of the sound were attenuated. Increasing the sound and amplifying the high-frequency components both resulted in the vibratory sensations being rated as less pleasant. These results show that even though the toothbrush stimulation did not actually change (i.e., the toothbrush was not controlled by any external device), and the participants were instructed to pay attention only to the vibratory-tactile stimulation on their teeth, they could not ignore the task-irrelevant auditory information. The results of the roughness scale dimension are also consistent with those of our previous study (Guest et al., 2002), showing that amplifying high-frequency sounds or increasing the overall level of all sounds results in an increased perception of roughness (of the skin of the hands, or of sandpaper samples felt by the hand), while attenuating the high-frequency sounds or reducing the overall sound level leads to an increased perception of smoothness. Importantly, however, the present study extends those findings to the perception and evaluation of an everyday product in use (i.e., to an electric toothbrush). Comparison of the results of the two studies reveals the magnitude of the sound-induced change in tactile roughness perception to be somewhat greater in the present study than in the previous study. Given that the design and procedure of the two studies were very similar, this difference in effect size may reflect the fact that participants received more tactile information when rubbing their hands together in the former study than they did from using an electric toothbrush on their teeth in the present study. Researchers have argued for many years that the extent to which one sense dominates, or modulates, perception in another modality may depend on the relative strength, reliability, or amount of the information presented in the two modalities (Welch and Warren, 1980; Ernst and Banks, 2002). Given that sex differences have been shown to affect the perception of stimuli in all the senses (for review, see Baker, 1987; Velle, 1987), it should be noted that the majority of participants in our study were female. However, at present, we are not aware of any role for sex differences in modulating multisensory integration effects such as that reported here. [We assessed the possible role of sex differences in another experiment in which we investigated whether the auditory-induced change in the perception of an electric toothbrush (as reported here) would also be affected by the addition of cool mint flavor in the mouth. Twenty participants (eight male and 12 female) took part in the study. In one session, the conditions of stimulus presentation were similar to those of the present study with the sole exception that participants had to evaluate only the pleasantness of the toothbrush, and not its roughness. The analysis of the data from this study revealed no significant main effect of the Sex of the participant, nor any interaction between Sex and any of the other factors (all Fs < 1.5).] Our results support recent claims regarding the multisensory nature of human perception (Driver and Spence, 2000), as well as numerous previous studies showing that people cannot ignore one sensory attribute of a stimulus while trying to respond to another, as, for example, in the oft-cited McGurk illusion (McGurk and MacDonald, 1976). One reason why participants in our study may have found it so hard to ignore the sounds is that although the sounds were presented over headphones, participants reported subjectively perceiving them as if they were coming from the toothbrush itself, presumably due to audiotactile ventriloquism (Caclin et al., 2002). [The phenomenon of ventriloquism is more frequently experienced between auditory and visual stimuli, as, for example, at the cinema where the voices of the actors on the screen appear to come from the location of their lips, rather than from the loudspeakers situated elsewhere in the auditorium (see Bertelson and de Gelder, in press, for a review). However, recent studies have also demonstrated an audiotactile equivalent of this effect, whereby the perceived location of auditory stimulation is drawn toward simultaneously presented tactile stimulation (Caclin et al., 2002).] It is important to note that while reducing the noise made by an electric toothbrush enhanced pleasantness ratings in the present study, there are other situations in which reducing the sound made by a product may actually have a detrimental effect on people’s perception. For example, Froman (1953; cited in Cox, 1967, p. 327) describes the example of a noiseless food mixer that failed in the marketplace because "it didn’t seem to have any power—it didn’t make enough noise" (however, also see Keiper, 1999). It may be that while loud sounds signify efficiency and power in a food processor, amplifying high-frequency sounds in the present study reminded people of the electric drills used by dentists, and this may be why they found this condition so unpleasant (an association confirmed by many of the participants when debriefed at the end of the experiment). In summary, the present study provides a "proof of principle" demonstrating the effect of changing sound on the perception on electric toothbrushes. It will be particularly interesting in future research to investigate whether there are any specific frequency components that may be specifically associated with pleasantness/roughness judgments for electric toothbrushes, given the uncertain ecological validity of the particular frequency manipulations used in the present study (see Guest et al., 2002). It will also be interesting in future research to investigate whether changing the sound made by an electric toothbrush can also be used to provide an effective means of modulating the pressure with which people press against their teeth while cleaning them (cf. Miller et al., 1999). Such a finding would have obvious potential dental health implications if auditory feedback could be used to encourage people to apply a more appropriate force when cleaning their teeth.
This investigation was supported by a research grant from Unilever Research. Received for publication May 7, 2003. Revision received August 6, 2003. Accepted for publication September 8, 2003.
Journal of Dental Research, Vol. 82, No. 11,
929-932 (2003)
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ABSTRACT
INTRODUCTION
