Perception, 2014, volume 43, pages 818 – 824

doi:10.1068/p7733

SHORT REPORT The Butcher’s Tongue Illusion

Charles Michel, Carlos Velasco, Alejandro Salgado-Montejo, Charles SpenceCrossmodal Research Laboratory, Department of Experimental Psychology, University of Oxford, 9 South Parks Road, Oxford OX1 3UD, UK; e‑mail: charles.michel@psy.ox.ac.uk Received 22 February 2014, in revised form 21 July 2014

Abstract. We report two experiments, based on a novel variant of the Rubber Hand Illusion (RHI), in which tactile stimulation is referred to an artificial (out-of-body) tongue. In the experiments reported here the participant’s tongue was stimulated while they looked at a mirrored dummy tongue. On average, the participants agreed with the statement that they felt as if they had been touched in the location where they saw the rubber tongue being touched (experiment 1), thus demonstrating visual capture. When the external tongue was illuminated with a laser pointer (experiment 2), a significant proportion of the participants reported feeling either tactile or thermal stimulation on their own tongue. These results therefore demonstrate that the multisensory integration of visual, tactile, and proprioceptive information that gives rise to the RHI can be extended to the tongue (a body part that is rarely seen directly).Keywords: perception, illusion, tongue, touch, vision, multisensory, oral somatosensory

1 Introduction “But the truth is they [visual and tactile perceptions] remain bound with one another …, and this thanks to a particular property by which visual perception, because it is the strongest, captures the other perception, and brings it to coincide with her more or less exactly.”

(Tastevin, 1937, page 60)

Multisensory illusions (often induced through situations of intersensory conflict) have long been used by psychologists to investigate the processes underlying perception in humans. Following Tastevin’s (1937) early experimental work on visual capture and proprioceptive drift using an artificial finger, Botvinick and Cohen’s (1998) report on the Rubber Hand Illusion (RHI) gave rise to a large (and currently still expanding) body of empirical research devoted to understanding this phenomenon (see Makin, Holmes, & Ehrsson, 2008, for a review). This illusion is associated with changes in activity in both motor and multisensory areas of the brain (Ehrsson, Spence, & Passingham, 2004; Tsakiris, Hesse, Boy, Haggard, & Fink, 2007). The illusion is typically induced by playing on the brain’s preference for binding synchronous, or better-said correlated, sensory stimulation (see Armel & Ramachandran, 2003; Parise, Spence, & Ernst, 2012) between various sensory modalities (Moseley, 2011) and represents a classic example of visual dominance over tactile perception (Ernst & Banks, 2002; Rock & Victor, 1964). Tactile sensations appear to be experienced in the visual representation of a body part, inducing a feeling of ownership (Tsakiris & Haggard, 2005).

Since the recent renaissance of interest in the RHI sparked by Botvinick and Cohen’s (1998) study, several variants of the illusion have been reported using different body parts such as the face (Tsakiris, 2008) or the head (Ramachandran, Krause, & Case, 2011), and even out-of-body illusions (eg Altschuler & Ramachandran, 2007; Ehrsson, 2007; Lenggenhager, Tadi, Metzinger, & Blanke, 2007). When the appropriate correlated sensory inputs are provided, a virtual limb can be made to feel as though it is part of an observer’s body. However, the latest research suggests that these illusions can be induced only with objects that resemble actual body parts (Tsakiris, Carpenter, James, & Fotopoulou, 2010).

Durgin, Evans, Dunphy, Klostermann, and Simmons (2007) conducted a series of experiments demonstrating how many people report feeling tactile sensations by seeing

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them being applied to only a body surface that has become incorporated into their body schema. Two thirds (66%) of the participants that they tested reported thermal or tactile sensations on seeing the light of a laser pointer moving over a rubber hand positioned near their own unseen hand, once the RHI had been established.

In fact, vision is so powerful in relation to the sense of touch that the very experience of touch (ie active touch or haptics) itself can appear to be modified (Ernst & Banks, 2002; Rock & Victor, 1964). Despite the extensive body of research that has now arisen around the RHI, the illusion has never been extended to the tongue (although an attempt to do so was conducted at a market a few years ago; see Guerilla Science, 2010), a part of our bodies that we rarely see directly. Given that the information we receive about the tongue normally comes from oral-somatosensory and proprioceptive inputs (Sessle, 2006), one might wonder whether visual dominance will still occur. Here, we outline a novel experimental version of the RHI, adapted to the tongue (see figure 1), that we call the Butcher’s Tongue Illusion (BTI).

(a)

(b)Figure 1. (a) Experimental design. (b) Participants’ view.

820 C Michel, C Velasco, A Salgado‑Montejo, C Spence

2 Results and discussionWe report on the results of two experiments performed with the same experimental materials (see figure 1). Overall, the results reported here on the BTI are comparable, at least to a certain extent, with the RHI. Participants reported feeling the touch that they saw being applied to an external tongue as having been applied to their own tongue, thus demonstrating visual capture over the felt position of the tongue for the very first time. The effects of capture, ownership, and agency over the fake tongue were significantly higher when the touch was applied synchronously to the real and fake tongues, as compared with when applied asynchronously with a 0.5 s delay. The participants also reported feeling thermal and tactile sensations on their own tongue, while seeing the fake tongue being illuminated by the beam of a laser pointer.

2.1 Experiment 1: when the rubber tongue ‘feels’ the touch that the eyes seeSeparate one-sample t-tests were performed on each of the scales in order to determine whether participants’ ratings were significantly different from the mid-point of the scale (the end points were labelled with 1 = ‘disagree strongly’ and 100 = ‘agree strongly’). In particular, the scores for question 2 in the synchronous condition (visual capture) were significantly higher from the mid-point of the scale. In addition, paired-sample t-tests were performed in order to determine whether the participants’ responses to the questions differed as a function of whether they were touched synchronously or asynchronously. Intriguingly, ratings were significantly higher in the synchronous than in the asynchronous condition for questions 2, 3, and 4 (visual capture, agency, and ownership, respectively). See figure 2 for a summary of the results.(1)

The results of our first experiment therefore provide initial support for the claim that this new experimental paradigm bears comparison with the RHI, as participants have the illusion of their own tongue being touched when seeing a dummy tongue being touched. The asynchronous control condition utilized in the present study was similar to that used by Botvinick and Cohen (1998) in their classic study of the RHI. The participants in the present experiment gave significantly higher ratings under synchronous than under asynchronous stimulation in three of the perceptual effects that we tested.

(1) The results of the t-tests are not Bonferroni corrected. The debate as to whether Bonferroni corrections are necessary has been ongoing for a few years (eg see Perneger, 1998). Should the reader be interested in the Bonferroni-corrected p-values, use the (0.05) corrected p = 0.01 as a benchmark for significance for the paired-sample t-tests, and p = 0.005 for the one-sample t-tests in experiment 1.

Figure 2. The results of experiment 1. Black lines represent mean ratings and SD for each question in the synchronous condition. Dashed lines represent mean ratings and SD for each question in the asynchronous condition. On the left, the asterisks indicate whether the scores differ significantly from the mid-point (50) of the visual analogue scale for each question and condition, with their respective p-values. On the right, the asterisks indicate whether the scores differed between the synchronous and asynchronous conditions for each question, again with their respective p-values.

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2.2 Experiment 2: when the rubber tongue ‘feels’ the touch of lightHaving established an oral-somatosensory version of the RHI in our first experiment, we then went on to assess whether, as shown by Durgin et al.’s (2007) previous study, the fake tongue would also ‘perceive’ any kind of sensation when ‘touched’ by the light of a laser beam.(2) The participants in our second experiment were asked a binary yes/no question in order to determine whether they had perceived any sensation on their tongue while seeing a red laser pointer being moved across the fake tongue. Out of thirty-two participants, twenty two (68.75%) said that they had felt some kind of sensation on their own tongue. Eight of these participants reported feeling a cold sensation, seven warm, ten tingling, and eleven tactile. The participants were allowed to give more than one descriptor. In an open-ended question, the participants clearly stated the tactile consequences of the illusion that they had just experienced: “I felt vibrations on my tongue moving in synchrony with the light movement” (participant 19); “the light was like rubbing the side of my own tongue” (participant 32). Thermal effects were also reported: “it was a sensation of warmth; I could sense where the light was” (participant 20); “when the light hit the tongue, it felt like the temperature was lower at that spot” (participant 24).

Our results are consistent with those reported by Durgin and his colleagues (2007). Two thirds of our participants felt either a tactile or thermal sensation thanks to the purely visual stimulation. Interestingly, although the percentage of those ‘feeling the touch of light’ matched Durgin et al.’s findings, more participants reported feeling ‘cold’ sensations in the present experiment. While Durgin et al. found that people reported mostly ‘warm’ sensations elicited by the light, we found that the predominant response was that of a cold sensation (even if the light’s colour was red in both experiments). One possible explanation is related to the fact that the tongue is typically moist and hence anything that causes that moisture to evaporate (or cause people to think that it might evaporate) might conceivably lead to a lowering, rather than a raising, of skin temperature. Moseley and Arntz (2007) tested whether the same noxious stimuli, applied to the hand, would be perceived as having a different intensity, unpleasantness, or temperature when associated with a red or blue light. Their results showed that the perceived temperature of the stimuli was affected by the colour of the light, though in a manner that was somewhat different from that seen here.

3 Materials and methodsA box in which a rubber tongue (C&C Ltd, Lytham, UK, http://www.discountmagic.co.uk/product_info.php/products_id/3298) was mirrored, and where both the participants’ and the mirrored tongues could be stimulated by the experimenter, was specially designed for use in this study (see figure 1). A piece of foam board placed horizontally at nose level was used to prevent the participants from seeing what was happening in the region of their own mouths. The experiments were performed at the Crossmodal Research Laboratory, at Oxford University’s Department of Experimental Psychology, with the participants being mostly students. All of the participants signed a consent form and were compensated with five pounds sterling in return for their time.

3.1 Experiment 1Thirty-two participants were recruited (nineteen females, M = 26.18 years, SD = 7.92 years, range = 18–59 years). The participants read and agreed the information sheet and consent form before taking part in this experiment. The experiment was reviewed and approved by the Central University Research Ethics Committee of the University of Oxford.(2) Durgin et al. (2007) also demonstrated that their participants were unable to reliably detect when the laser pointer was shining on their hands, while having their eyes closed.

822 C Michel, C Velasco, A Salgado‑Montejo, C Spence

The participants were told how to position themselves in the experimental set-up and instructed to stick their tongue out, holding it gently between their teeth in order to limit its movement. The experimenter told them they would be asked to stick their tongue out for several trials involving various different types of stimulation.

The first segment of this study consisted of an adaptation of Botvinick and Cohen’s (1998) RHI experiment. Two consecutive trials, lasting 40 s each, were performed. In the first the experimenter stimulated both tongues, synchronizing as accurately as possible the location, timing, and pressure of the strokes, at a rate of one stroke per second. In the second trial both tongues were stimulated asynchronously; the stimuli (visual and tactile) were delayed by approximately 0.5 s. The presentation of the trials was counterbalanced across participants.

Following each trial, the participants were asked to rate the intensity of five perceptual effects by placing a mark on a horizontally arrayed visual analogue scale (VAS), presented on a computer screen. The order of appearance of the statements was randomized to avoid any order effects. The VAS was a 100-point scale, with the end points being labelled ‘strongly disagree’ on the left and ‘strongly agree’ on the right (see figure 2).

3.2 Experiment 2After answering the questions relevant to experiment 1, the participants were told that another trial was about to begin, and they were asked once again to assume the position in the experimental box by sticking their tongue out. This time, the fake tongue was stimulated with a red laser beam (650 nm, 1.3 mW). The light was moved randomly over the tip and upper surface of the participant’s tongue for 20 s. The experimenter asked the participants whether they felt anything on their tongue. If they answered in the affirmative, they were then asked to openly describe their experience. Finally, the participants were asked to tick one or more sensations that matched their experience (cold, tactile, tingling, warm, other, none of these) on a computer screen.

4 LimitationsWhile the stroking was presented to both tongues, the participants held their tongue gently between their teeth in order to keep it as still as possible. However, the tongue is still prone to movement, which provides some proprioceptive feedback and could therefore have affected the strength of the illusion. The experimenter tried to compensate for this as much as possible so the timing, pressure, and position of the stimulation on the real tongue would match as precisely as possible that seen on the fake tongue. Despite the difficulties associated with doing this, the results provided evidence that the illusory multisensory experience had indeed taken place. Future experiments might certainly consider the use of an automatized stroking device that would allow for the delivery of more precise, controlled stroking.

5 ConclusionsThe experiments reported in the present study clearly demonstrate that a perceptual illusion similar to the RHI can be obtained on the tongue, an illusion we name the Butcher’s Tongue Illusion. Our participants reported visual capture of a tactile stimulation, as well as feeling the touch of a light playing on an external dummy tongue. The particular interest in extending this illusion to the mouth comes, in part, from the fact that we rarely see our own tongues. Hence, it could have been anticipated that tactile/proprioceptive cues might play a somewhat greater role in determining the results of multisensory integration (cf Tipper et al., 2001).

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The mouth (or tongue) appears to most people to be the place where the flavour of food is experienced (a phenomenon known as ‘oral referral’; Lim & Johnson, 2012). From an evolutionary perspective, the tongue, which might be referred to as the organ of flavour, plays a very important role in determining the organization of the brain. The suggestion has been made that the brain appears to have evolved as the gut’s way of controlling its intake through the mouth, ultimately guiding cortical development (Allman, 2000; Young, 1968).

Demonstrating that the RHI can be extended to the tongue could potentially offer the opportunity to deliver (and study) taste/flavour experiences ‘outside the mouth’, if the appropriate (ie correlated) sensory stimulation is provided, as well as a way of understanding multisensory integration.

Acknowledgments. CV and AS would like to thank COLFUTURO, Colombia, for financial support toward their PhDs. CM is the first ‘Chef in residence’ at the Crossmodal Research Laboratory. CS is funded by the Rethinking the Senses grant from the AHRC.

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