Self - identification with another person ’ s face : 1 the time relevant role of multimodal brain areas in the enfacement illusion 2 3 4 5

53 Illusory subjective experience of looking at one’s own face while in fact looking at another 54 person’s face can surprisingly be induced by simple synchronized visuo-tactile stimulation of 55 the two faces. Recently, Apps and colleagues (Cerebral Cortex, 2014) investigated for the 56 first time the role of visual unimodal and temporo-parietal multimodal brain areas in the 57 enfacement illusion, and suggested a model in which multisensory mechanisms are crucial 58 to construct and update self-face representation. 59 60 61 Neuro Forum 62 63 The topic of how self-representation is built, maintained and constantly updated in the 64 human brain has recently become a highly debated issue in cognitive neuroscience. 65 At a very basic level the sense of self is built upon the sense of bodily self, for which the face 66 holds special importance being the most distinctive feature of one’s own physical 67 appearance. A coherent visual representation of one’s own face is formed and maintained 68 by matching felt and observed sensorimotor experiences in the mirror. This process finally 69 allows self-face mirror recognition, an ability that ontogenetically precedes higher forms of 70 self-consciousness and social behavior (Tsakiris, 2010). 71 Due to its importance, it was classically held that self-face representation is fundamentally 72 stable and can rarely be disrupted, unless in severe neurological degenerative or psychiatric 73 disorders (Feinberg and Keenan, 2005). 74 However, recent evidence shows that self-other distinction can be easily blurred by 75 synchronous interpersonal visuo-tactile stimulation of one’s own and another person’s face, 76 an effect we named enfacement illusion (Sforza et al., 2010). Participants incorporate facial 77 features of the other into the self-face representation and, at a phenomenological level, 78 report to be feeling the tactile stimulus observed on another person's face and to be looking 79 at themselves in the mirror. 80 81 Apps and colleagues (2013) were the first to investigate the neural correlates of this illusory 82 felling of ‘I felt I was looking at my face' while instead looking at another person's face, 83 induced by interpersonal multisensory stimulation (IMS). 84 Participants’ brain activity was recorded via fMRI while they were stimulated on their upper 85 cheek and concomitantly observed matching or mismatching stimulation on another 86 person’s face. Mismatch between felt and observed stimulation was spatial (incongruent 87 stimulation position, e.g. the chin) and/or temporal (1 second asynchrony) and gave rise to 4 88 experimental conditions: synchronous-congruent (SC), synchronous incongruent (SI), 89 asynchronous congruent (AC), asynchronous incongruent (AI). 90 After each block, participants rated how much they felt they were looking at their own face. 91 At the behavioral level, results show that participants felt to look at their own face more 92 during the SC condition, and progressively less in the other conditions, i.e. SI, AC and AI. The 93 authors, unfortunately, do not report whether these conditions differed from one another, 94 which would have allowed to test if temporally synchronous but spatially incongruent IMS is 95 sufficient to generate the illusion, even if to a weaker extent. 96 At a neural level, the right Inferior Occipital Gyrus (IOG), the right Inferior Parietal Sulcus 97 (IPS) and the right Temporo Parietal Junction (rTPJ) showed a significant interaction between 98 congruency and synchronicity in both whole brain factorial and small volume correction 99 analysis, performed around the coordinates of the same areas previously found to be 100 involved in self-face visual recognition and in bodily illusions (Apps et al., 2013). Activity in 101 these regions was also found to vary parametrically with the phenomenological experience 102 of the illusion (‘looking at my face’), regardless of IMS condition. 103 Thus, the authors proposed that the interplay between unimodal and multimodal brain 104 areas drives dynamic self-identification with a face whose sensations match one’s own. Very 105 interestingly, Apps and colleagues (2013) argue that the processes of constructing and 106 updating a mental representation of one’s own face may conform to the principles of 107 predictive coding within the free-energy theoretical model. The authors explain the 108 enfacement illusion accordingly and suggest that congruency between observed and felt 109 touch initially generates surprise, since participants can not move to prove themselves the 110 observed face is not theirs, and then that the brain attempts to minimize surprise by 111 changing self-face representation to include the other person’s facial features. 112 In particular, the authors propose that rIPS processes ‘multisensory driven predictions about 113 upcoming somatosensory input' and that rTPJ responds as a 'function of the extent to which, 114 self or other, perspectives are being processed'. 115 116 We find the interpretation provided by the authors appealing however, at times, it does not 117 seem to closely describe their results. We therefore would like to raise few controversial 118 points and attempt to suggest a more comprehensive interpretation of the neural correlates 119 sub-serving the 'looking at my face’ illusory experience. 120 121 Although, rIPS and rTPJ regions show an overall dissimilar pattern of modulations relative to 122 the different IMS conditions (see histograms of beta values resulting from factorial analysis 123 reported in Figure 2 by Apps et al., 2013), they both correlate with the perceived strength of 124 the illusion. Even if it is difficult to interpret beta values as deactivations or activations due 125 to the absence of an active baseline condition, and to know if they differ between conditions 126 due to the absence of post-hoc comparisons, by looking at the graph it seems that IPS is less 127 deactivated in SC condition, where participants report a stronger illusion, and progressively 128 more deactivated in those conditions (SI, AC, AI) where participants report progressively 129 weaker illusion. In contrast to rIPS, rTPJ activity maximally differs between SC and AC, with 130 AI and SI having similar and intermediate activation levels. Still, the parametric analysis 131 shows a linear correlation between rTPJ activity and the strength of illusory feeling. 132 Surprisingly, although factorial and parametric analysis are coherent for rIPS but not for 133 rTPJno clear interpretation of the correlation between rIPS activation and illusion strength 134 is provided by Apps and colleagues (2013), while they discuss the role of TPJ as crucial for 135 the illusion. 136 137 IPS is a large and multifaceted region involved in a wide range of diverse cognitive processes. 138 From our point of view, it is interesting to note, that IPS, and in particular its ventral part, 139 integrates multisensory body-related signals (Bremmer et al., 2001), since it receives 140 afferent sensory projections from somatosensory and visual areas and forms with the 141 putamen a visual–somesthetic network that processes the space on and near the body 142 (Graziano and Botvinick, 2002). Thanks to its anatomical and functional properties, IPS 143 activity is crucial to maintain a coherent body representation during conflicts that arise from 144 incongruent multisensory stimuli coming from one’s own body (caudo-medial IPS: Hagura et 145 al., 2007; inferior parietal lobule: Bufalari et al., 2013), or from congruent multisensory 146 signals coming from one’s own and another’s body (ventral and medial IPS: Ehrsson et al., 147 2004; Petkova et al., 2011). This second scenario is for example apparent when tactile 148 spatially and temporally congruent signals are delivered to one’s own unseen hand and to a 149 rubber hand spatially congruent with respect to one’s own arm. The initial conflict between 150 seen and felt position of the hand was found to be resolved mostly by anterior left IPS which 151 recalibrates (aligns) the peri-hand space representation toward that of the seen rubber hand 152 so that tactile, visual, and proprioceptive signals fuse in a single coherent percept (Brozzoli 153 et al., 2012). Accordingly, IPS is more active in the temporal window associated to this 154 remapping process, which usually precedes the illusory sense of ownership (Ehrsson et al., 155 2004). 156 Analogously, we hint that during the enfacement illusion ventral IPS could integrate 157 multisensory congruent stimuli and remap the space around the face (as seen in a mirror). 158 Such role is also supported by a recent study which found that ventral IPS is involved in 159 remapping visual information about touch applied to another person’s face on processing of 160 tactile stimuli concomitantly applied to the self-face (Cardini et al., 2011). 161 This remapping of the space around the self-face may result from a central process that 162 detects conflicts between tactile afferences and visual signals that, although temporally and 163 spatially congruent with self-percept, instead originate from another person’s face. Thus, 164 the spatial remapping process suppresses such conflict and prompts the insurgence of the 165 illusory perceptual experience of looking at one’s own face. Following this line of reasoning, 166 we believe that only once the illusion is at play (and thus just one identity is represented) 167 the multisensory integrative nature of IPS can subserve the function -as proposed by Apps 168 and colleaguesof predictive upcoming of congruent somatosensory stimuli (such as when 169 looking at one’s self in the mirror). 170 171 We further suggest that the brain area responsible to inform IPS about such mismatch 172 between self and other tactile sensations could be rTPJ. 173 This interpretation seems congruent with the study by Ionta and colleagues (2011) in which 174 synchronous IMS to participants in the fMRI scanner and to a virtual body resulted in 175 perceived shifts of self-location toward the virtual body. Interestingly, TPJ activity was 176 differentially modulated in those participants that perceived the virtual body as floating over 177 their real body in a spatially-congruent manner, compared to those participants that 178 experienced to be looking down on the virtual body, which implied a spatial-relocation of 179 the self, i.e. feeling as if lying prone on the scanner bed. It seems therefore that TPJ activity 180 is related to the presence/absence of a mismatch between real afferent proprioceptive 181 information and illusion-induced feelings (e.g. perceive the virtual body floating over or 182 under one’s self) (Aglioti & Candidi, 2011). 183 The trend of TPJ activations in the different IMS conditions as found in the present study, fits 184 also well with this interpretation of TPJ's role. As reported above, rTPJ beta values show 185 unexpected similar activations during SI and AI while its activity maximally differs between 186 SC and AC. This activation pattern, not discussed by Apps and colleagues, might relate to the 187 fact that participants were touched always on the same location. Congruency was then just 188 determined by the observed touch location, which could be inferred from the hand's 189 trajectory before the touch occurred. While rTPJ response might not be needed to 190 differentiate spatially incongruent conditions because they both imply immediate self/other 191 difference (due to different touch locations), a differential rTPJ response might be needed to 192 code for self/other difference when the other face is being touched in the same location (as 193 predicted from the approaching hand's direction) and therefore it may become crucial to 194 detect temporal a/synchrony. 195 Also, Apps and colleagues found peak activation in the anterior subdivision of rTPJ, part of 196 the ventral attentional and not of the social cognition network (Mars et al., 2012), which is in 197 line with this interpretation of rTPJ role in the enfacement illusion given that attentional 198 reorienting could play a role to compare mismatching internal (self) and external (other) 199 stimuli as those present in the current study. 200 201 Additional brain areas might as well follow the free-energy principle and show attenuated 202 responses linked to multisensory processing of visual and tactile information when only one 203 face (the self-face) is represented (namely during the illusory experience), as opposed to 204 when, in the control conditions, separate visual and tactile information need to be 205 processed about one’s own and another person's face. We would expect, in line with studies 206 showing reduced activity in somatosensory cortices during predictable self-produced tactile 207 stimulation versus externally unpredictable stimulation (Blakemore et al., 2000) to find a 208 modulation of activity also in early somatosensory and visual areas once, in the illusion 209 condition, the stimulation has become predictable. Interestingly, the authors report an 210 interaction effect in the right parietal operculum (secondary somatosensory cortex), but 211 unfortunately we do not know which IMS condition drives this effect, since post-hoc tests 212 and plot of signal change in this region are not reported. 213 214 Along this line of reasoning based on Apps and colleagues initial findings, we suggest that, to 215 provide further insights into the dynamic process inducing the illusion, future studies should 216 compare activity in unimodal and multimodal brain regions before and after the appearance 217 of the illusory feeling of looking at one’s own face. Indeed, Apps and colleagues reported 218 that this illusory feeling intervenes with a great inter-subject variability (a mean delay of 13 219 seconds, with a range between 5.56–32.65 s), however they analyzed brain activity during 220 each kind of IMS stimulation independently of participants’ perceptual experience. 221 Furthermore, peri-personal space remapping processes could be at play in the time window 222 preceding the onset of the illusory sensation of ‘looking at me in the mirror’ and thus, 223 measuring the subjective experience of referral of touch would provide additional fruitful 224 information. Unfortunately, participants were not asked to rate mislocalization of their 225 tactile experience (e.g. 'It seemed as though the touch I felt was caused by the paintbrush 226 touching the other’s face.') although it is a significant component of the enfacement illusion 227 (Sforza et al., 2010) and it predicts the hand’s objective mislocalization in the RHI (Longo et228 al., 2008).229230As results of the aforementioned analysis we would expect (Figure 1) IPS to be involved in231integrating multisensory input and resolving multimodal conflict before the illusion onset232and, in analogy with the rubber hand studies, its activity to correlate with the subjective233experience of referral of touch. Only once the illusory sensation of looking at oneself is234established, we would expect IPS to inform TPJ of the likelihood of upcoming somatosensory235stimulation since at that point just one face (the self face) is represented. In addition, we236would expect TPJ to predict and monitor the congruency of the upcoming multisensory237stimulation and, probably conjointly with IPS, to modulate the activity of low-level unimodal238brain structures (i.e. somatosensory and visual cortices).239Lastly, we believe that approaches that allow investigating effective and functional240connectivity between brain areas (e.g. Granger and dynamical causal modeling; Friston et al.,2412013) would prove very useful to help determine the time relevant role of multimodal brain242areas in the enfacement illusion.243244245246References2471. Aglioti SM, Candidi M. Out-of-place bodies, out-of-body selves. Neuron 70: 173–5,2482011.249 2. Blakemore SJ, Wolpert D, Frith C. Why can’t you tickle yourself?.Neuroreport 11:250R11–6, 2000.251 3. Bremmer F, Schlack A, Shah NJ, Zafiris O, Kubischik M, Hoffmann K, Zilles K, Fink252 GR. 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In the case294of synchronous stimulation, conflict initially arises (red line, before the illusion onset)295between tactile afferences and spatially and temporally congruent visual signals coming296from the other person’s face. TPJ detects the conflict while IPS integrates multisensory297congruent stimuli and remaps the space around the face (as seen in the mirror). This finally298results in updating of the self-face representation to include facial features of the299synchronously stimulated other. Once the self-face representation is updated, the illusion300 (i.e., the perceptual experience of looking at one’s own face) emerges (blue line). TPJ now301detects less conflict and IPS can predict (and inform TPJ of) the likelihood of upcoming302 sensory tactile stimuli on the self-face based on those observed on the other’s face.303 Concomitantly TPJ and IPS modulate the activity of low-level unimodal brain structures (i.e.304somatosensory and visual cortices), which has an effect on the perceived multisensory305stimuli. The end result of synchronous IMS is therefore a biased perception of the other’s306 face as self.307308309310311312 Updating of the self-face representation