Phantom touch illusion, an unexpected phenomenological effect of tactile gating in the absence of tactile stimulation

We report the presence of a tingling sensation perceived during self-touch without physical stimulation. We used immersive virtual reality scenarios in which subjects touched their body using a virtual object. This touch resulted in a tingling sensation corresponding to the location touched on the virtual body. We called it “phantom touch illusion” (PTI). Interestingly, the illusion was also reported when subjects touched invisible (inferred) parts of their limb. We reason that this PTI results from tactile gating process during self-touch if there is no tactile input to supress. The reported PTI when touching invisible body parts indicates that tactile gating is not exclusively based on vision, but rather on multi-sensory, top-down input involving body schema. This supplementary finding shows that representations of one's own body are defined top-down, beyond the available sensory information.

Experimental setup and virtual reality scenes.We used Unity (Unity Technologies, San Francisco, USA) version 2021.3.8f1 and Oculus Quest 2 (Meta, California, USA) head-mounted display (HMD) in hand tracking mode.The scenes took place in a simulated virtual environment where several realistically scaled daily objects were placed on a table in front of the participant (Fig. 1B).Among them there were two sticks, approximately 30 cm long and 2 cm wide.The subjects could only see their virtual hands but not their forearms or other body parts.
Procedure.At the beginning, the HMD was put on the subjects and we ensured they could see the scene and their virtual hands clearly.Then, subjects familiarized themselves with the scene such as freely moving and touching the objects on the table.After they appeared acclimatized to the VR environment, the experimental phase began.We asked the subjects to grab one of the virtual sticks by the end with their right hand, and repeatedly stroke their left virtual hand with the stick's end (See Fig. 1B and Supplementary Video 1).We did not further instruct the subjects about any detailed way of stroking the hand such as the exact location to start touching the hand or rhythm.We said that the stick can immerse a bit in the virtual skin but told them explicitly to be careful to not put the stick through the touched hand and to keep the hands apart (at stick length) and avoid touching their own body at all times.After approximately 30 to 60 s we asked the subjects "if they could feel anything".If they responded negatively, we let them touch their hand with the stick longer and repeated the question after a while.We did not record the exact time of asking these questions nor their number.Importantly, we did not cue the subjects as to what the sensation in the "receiving" hand could be.
If the subjects responded positively, we asked them to describe the feeling and noted down their response.We then asked if the sensation moved consistently with the position of the stick on the skin.Then, we asked the subjects to stroke the middle of the inner side of their palm and rate the intensity of the sensation on a scale from 0 to 10 (0 meant no sensation; we did not further specify what these levels exactly meant).Afterwards we asked them to touch the middle of their fingers (phalanges) and rate the sensation there (on the same scale as above).Lastly, we asked them to rate the sensation's intensity in the fingertips.Next, we asked subjects to touch the top of their hand and repeat stroking all three parts mentioned earlier.After obtaining ratings, we asked the subjects to stroke their (not visible) forearm.We then repeated the procedure for the right hand (with the left hand holding the stick).If subjects spontaneously stroked their hand in a different position than the above order, we did not instruct them to change it, instead we continued the procedure as above until probing all described locations (Fig. 1C).
Control experiment.We also tested whether the PTI would happen also when not predicted by the tactile gating model, that is without having a visual object touching the hand, for example due to the influence of suggestibility and demand characteristics 10 .For this, we designed a control experiment in which we recreated the basic procedures of the main task albeit without a stimulation to the hand delivered by a realistic object.The procedures mirrored those from the main experiment except from that the subjects were not using a virtual reality headset and, instead of a virtual stick, they held a small laser pointer in their right hand.The laser pointer could project a small (ca. 5 mm diameter) point of red light onto the skin of participant's left hand.There were two conditions: "Laser on", in which we asked the subjects to stroke their hand with the laser point of light and "Laser off ", in which we asked the subjects to move the pointer as if they were stroking their hand with the point of light.The condition order was counter-balanced and randomized across participants.See Supplement 1 for a detailed description and results.

Results
All but four of the subjects reported a tactile sensation when touching parts of their hand with a virtual stick.The sensation was usually described as "tingling/static/prickle/electric" or "like wind passing through the hand" (see Fig. 2B and Supplementary Table 1).We call this sensation "phantom touch illusion" (PTI).All subjects that reported this sensation likewise reported that it was consistent with the position of the stick on the avatarized hand.Interestingly, some subjects spontaneously reported they "can feel something" before we asked them for the first time.All subjects described feeling surprised by the sensation caused by PTI.One subject even asked if we were using some device for remote tactile stimulation.The proportion of subjects reporting any sensation (89%) was significantly higher than for control condition without visual stimulation (44%) as evident from proportion test, Z = 3.97, p < 0.001 (PTI compared to condition "Laser Off ", see Supplement 1 for details on control experiment).Most importantly, the proportion of subjects consistently reporting "Tingling" responses was significantly higher in the main condition (50%) than in both control conditions: "Laser On" (13%; Z = 3.3, p < 0.001) and "Laser Off " (9%; Z = 3.62, p < 0.001).
Most importantly, most subjects (Fig. 2 A, Supplementary Fig. 1; Supplementary Table 1) also reported the sensation to be present when touching their non-visible forearm.Two subjects were visibly startled by this feeling.
The reported intensity of the stimulation differed across subjects (see Fig. 2) but in our analysis did not significantly differ across stimulation sites and positions, as shown by repeated-measures non-parametric (Friedman) ANOVA for the left hand (χ 2 [5] = 7.83, p = 0.16).As we did not control exact stimulation spots, we did not run more detailed tests regarding distinct intensities of the phantom sensation at different spots of the hand surface within each area of interest.
Lastly, several subjects reported PTI when actively touching objects available on the scene (see Supplementary Table 1).Unfortunately, we added this condition only in the course of the experiment and did not consistently measure the strength of the sensation and whether it corresponded to the touching hand parts.We also did not provide a consistent instruction on whether subjects had to just touch or grasp the object which might have further influenced the results (i.e., the number of subjects who reported PTI).

Discussion
In this study we used immersive virtual reality to simulate scenes of self-touch with a tool without accompanying tactile stimulation.Our subjects reported a tingling sensation at the spots where the virtual stick touched their virtual hand.This happened despite the lack of any physical stimulation.Interestingly, the majority of the subjects reported feeling this phantom sensation when touching their invisible forearm too.We called this sensation "phantom touch illusion" to relate to an apparently similar phenomenon reported in the VR community for social touch 9 .We ruled out that the sensation was caused by heat, airflow or other physical sensation caused by the moving hand by: (1) ensuring separation of both hands by about 30 cm, that is twice the distance at which temperature sensation is typically perceived on the skin in normal conditions 11 ; (2) testing the procedure during pilot sessions with a plastic sheet separating both hands which did not affect the sensation.
While our results suggest that touch gating may be predominantly guided by vision because it was induced by a virtual stick and required subjects to look at it, the presence of PTI when subjects touched their invisible forearm is puzzling.It may indicate that tactile gating uses input from other senses than vision, for example proprioception, or top-down input from the body schema.The influence of other modalities and input sources (such as body schema) on tactile gating needs to be investigated.
The very overlapping descriptions of the PTI quality across some of our subjects for both passive (stick) and active (object) touch suggest that PTI does not represent predictions about specific features of object texture or that it does not differ between passive (self) and active (object) touch.Moreover, recent work from Kilteni and Ehrsson 12 suggests that tactile attenuation and gating may be distinct processes aimed at modulating intensity and precision of touch, respectively, and they may affect PTI differently.It is also worth mentioning that although we did not investigate touch quality or precision in detail, we believe there may be subtle differences in tactile gating depending on predicted touch qualities such as suggested by Fuehrer et al. 6 .On the other hand, the same study showed that tactile gating is not limited to predictable stimuli.This lack of prediction-specific gating speaks in favor of the fact that our reported PTI findings may likewise represent stimulus-unspecific cortical events.
An important question remains about the source of neural activity causing PTI.Electrophysiological data shows multi-level processing of tactile gating: cancellation signals are present in the spinal cord and in cortical somatosensory, motor and premotor areas 1,4 .This multi-level processing likely reflects top-down, predictive control of tactile gating processes before movement is initiated 4,13 .The reported conscious experience of PTI suggests that visual cues for self-generated touch modulate activity in the somatosensory cortex which is the site for tactile sensations 14 .However, the fact that some subjects felt PTI when stimulating their invisible forearm, suggests that its nature may be originating more from top-down modulation of somatosensory cortex, possibly through the body schema, rather than being purely sensory-driven.Posterior parietal cortex has been often suggested to be a critical site for both the body schema and action predictions 15,16 and a state comparator across different sensori-motor modalities 17 .As such, it may provide top-down input to the somatosensory cortex in the absence of direct vision of the stimulated skin surfaces.
Difference to embodiment illusions.While our finding involves feeling touch that is not physical, it does not seem to relate to widely-described embodiment illusions, such as rubber hand 18 .These illusions use simultaneous visual stimulation delivered to an artificial body part (which for example looks like a hand) and tactile stimulation to the user's own corresponding body part which results in an induced feeling that stimulating the artificial hand is like that of the real one.Interestingly, and in line with our results, such induced embodiment can also be applied to invisible body parts 19 .The main difference of phantom touch to these other embodiment illusions is that PTI did not involve any sort of tactile stimulation that would then be "transferred" to the virtual hand.We can not completely rule out, however, that the phenomenological effects reported in these illusions involve some sort of predictive tactile gating like we believe our PTI results from.This needs more research.
Open questions.Four subjects did not report having PTI, suggesting at least some context specificity that may depend on attentional or other cognitive processes.Our control experiment, together with spontaneous anecdotical reports of PTI in virtual environments, do not indicate that the tingling sensation was caused by the experimental procedure.It remains open, however, how top-down cognition influences the specific quality of illusory sensations.Such cognitive influences may be similar to effects reported before, where context seemingly affected object grasping, reflecting qualitative features (temperature) of objects that users grasp in VR 20,21 .Interestingly, one subject not reporting PTI still reported an unspecified feeling when touching a virtual cup, which indicates there may be differences between active and passive touch as suggested by previous research 12 .
Another interesting issue is whether self-reported values reliably represent the perceived intensity of phantom touch and putative differences across areas.While we report lack of differences in reported intensity values across areas, we believe that our experiment was not optimized for answering this question as it lacked an objective control of touch.We believe, nonetheless, that this issue is worth pursuing further with a more sensitive procedure.
It was recently suggested 10,22,23 that embodiment illusions such as the rubber hand may be prone to the influence of demand characteristics of the experimental conditions and phenomenological traits such as suggestibility or phenomenological control.In the case of our experiment, we minimized this confound as we did not provide subjects with any cues about what (especially what touch quality) they should expect and all subjects who had PTI expressed surprise when they felt it.Likewise, the high level of cross-subject congruence in verbal descriptions of the sensation further suggests PTI does not arise from demand characteristics.Such congruence was missing from our "Laser Off " control experiment.It is noteworthy that the actual influence of suggestibility on embodiment illusions has been also challenged 22,24,25 .In any case, possible modulatory effects of cognitive demands on PTI, such as mentioned above for temperature perception or sharpness, seem worth investigating.

Conclusions
We demonstrated an illusory phantom touch sensation emerging during self touch in the absence of actual tactile stimulus.This PTI was present also when subjects stimulated an invisible, inferred body part.We believe that PTI reflects cortical process of tactile gating resulting in conscious tactile sensation.Moreover, its reported presence in the absence of the vision of the body parts suggests that tactile gating is not just sensory-driven but additionally influenced by the body schema.

Figure 1 .
Figure 1.(A) Schematic depiction of stimulation with the virtual stick in prone and supine positions.(B) A screenshot of the VR stimulation scene in prone and supine positions.(C) Stimulation sites: fingertips, fingers (phalanges), palm and forearm.The forearms were invisible to subjects (B).

Figure 2 .
Figure 2. (A) Left-hand results of phantom touch intensity ratings across different locations on the hand and the forearm.Box colors represent the relevant parts of the hand as depicted on the right; gray dots represent individual data points; horizontal line represents the median; whiskers represent 1.5 times the interquartile range.Compare Supplementary Fig. 1 for the whole range of results across all positions in both hands.(B) Frequencies of verbal descriptions of the PTI sensation.