The effect of transcranial Direct Current Stimulation (tDCS) on perspective taking and the out-of-body experience

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Running head: THE EFFECT OF TDCS ON PERSPECTIVE TAKING

Monique Duizer University of Amsterdam

Monique Duizer, Research Master Psychology Student, University of Amsterdam. Correspondence concerning this article should be addressed to Monique Duizer. E-mail: moniqueduizer@gmail.com

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Abstract

Multisensory integration refers to the process whereby visual, vestibular, auditory, somatosensory and proprioceptive information is integrated by the brain. The right temporo-parietal junction (rTPJ) plays an important role in this process. Multisensory integration of bodily signals is important for perspective taking, and changes in this process may result in an out-of-body experience (OBE). The aim of this study was to investigate if the rTPJ is critically involved in spatial perspective taking. Participants performed a mental body rotation task while receiving transcranial direct current stimulation (tDCS). In addition, the effects of experimentally induced changes in the rTPJ on the sensitivity to the full body illusion (FBI) – an experimentally induced illusion in which a conflict between visual and tactile information results in a mislocalization of the perceived position of one’s body - and the relation with spirituality was assessed. It was expected that participants in the anodal stimulation condition would perform better on a mental body rotation task. Furthermore, it was hypothesized that this mental rotation task combined with anodal tDCS stimulation caused an increased sensitivity for the FBI.

However, in the present study we did not find that stimulation of the rTPJ enhances the ability to take the perspective of another person (i.e. third-person perspective taking) compared to first-person perspective taking. Nor increasing the excitability (anodal tDCS), neither decreasing the excitability (cathodal tDCS) had an effect on the reaction times and accuracy of perspective taking. Furthermore, individual differences in spirituality and hallucination-proneness did not affect perspective taking.

Our findings replicate previous research for the FBI: synchronous visual and tactile stimulation during the FBI caused a stronger identification with the virtual body. We did find some small effects of tDCS stimulation on congruency, suggesting an extension in peripersonal space to the virtual body.

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Recent findings from neuroscience and psychology have underlined the central role of the processing of bodily signals for a neurobiological model of the self. The processing, combining and integration of bodily signals such as visual, auditory, sensory and vestibular information is called multisensory integration. It is found that in neurological patients disturbed processes of multisensory integration can result in altered perceptions of the body and the self (Blanke et al., 2004; Heydrich & Blanke, 2013; Heydrich et al., 2010). Furthermore, experimentally induced multisensory conflicts can result in illusory body perception and corresponding alterations in the bodily self, e.g. body ownership of a rubber hand, or ownership of a virtual body (Aspell et al., 2010; Ehrsson, Holmes & Passingham, 2005; Lenggenhager et al., 2007). This ownership of a virtual body, causes a change in the perspective on your surroundings, from first-person perspective to third-person perspective. Research on

multisensory integration suggests a link between perspective taking, body perception and also spirituality. The present research focuses on the overlap of these three aspects in the brain.

Perspective taking

In a third-person, or allocentric, perspective people view the world from the perspective of another person. Third-person perspective taking is important in social interactions where people have to ascribe mental states to someone else. In a first-person, or egocentric, position people view the world from their own bodily perspective. Research on perspective taking has shown that the right temporo-parietal junction (rTPJ) is involved in both third-person and first-person perspective taking (Santiesteban et al, 2012; Vogeley, 2003).

The “Own-Body-Transformation” (OBT) task has often been used to measure spatial (third-person) perspective taking (Braithwaite & Dent, 2011; Blanke et al., 2005). In the OBT task participants are presented with a human avatar on a computer screen. The avatar is presented in different bodily positions and participants have to indicate whether a bracelet is on the left arm or the right arm as seen from the perspective of the avatar or as seen from their own perspective. Depending on the position of the avatar, participants have to mentally rotate their own body to a stronger or a lesser extent. When the avatar is presented face-to-face a prolonged body rotation is necessary and this has been associated with a longer activation of the rTPJ compared to when the avatar is seen from the

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back (Arzy et al., 2006). Also, increasing the neuronal excitability of the rTPJ with transcranial direct current stimulation (tDCS) affects the ability to take the visual perspective of another person, but not the ability to attribute mental states to the self or others (Santiesteban et al.,2012).

Out-of-body experience

Comparable to adopting a third-person perspective, during an out-of body experience (OBE) people view the world from a different perspective. An OBE is described as an experience in which a person is awake and seems to perceive the world from a location outside his physical body (Blanke et al., 2004; Blank et al., 2005; Braithwaite & Dent, 2011; Braithwaite et al., 2011; Ehrsson, 2007). Two other characteristics of an OBE are: disembodiment, and sometimes seeing the own body (autoscopy) (Blanke & Arzy, 2005). OBEs are often experienced by people with a clinical condition, e.g. epilepsy, but are also found in the normal population. Direct stimulation of the right angular gyrus (AG), which is located in the temporo-parietal junction (TPJ), elicits not only illusionary transformations of the arms and legs but also whole body displacements (Blanke et al., 2002). Also, researchers found overlap of lesions in the TPJ, the anterior part of the AG and the posterior part of the superior temporal gyrus (Blanke et al., 2004) and the rTPJ (Heydrich et al., 2011) in association with the occurrence of an OBE.

Experimental manipulations of the congruency between perceived visual and tactile information related to one’s body can result in the illusion of seeing one’s own body from a third person

perspective (Ehrsson, 2007). This so-called ‘full body illusion’ (FBI) is induced by having healthy participants wear a head-mounted display (HMD), which is connected to a camera 1.5 meters behind the participant. This creates the visual illusion of sitting a few meters behind your bodily position. To maximize this body ownership illusion the experimenter synchronously applies visio-tactile

stimulation to the participant’s chest and the “illusory body”. This causes participants to report the experience of sitting behind their physical body (Ehrsson, 2007).

In a study of Ionta et al. (2011) participants were placed in the fMRI scanner during synchronous and asynchronous robotic stroking. The participants reported a stronger feeling of body ownership with an illusory body in the synchronous condition compared to the asynchronous condition.

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Furthermore, the TPJ was more or less activated during synchronous stroking compared to

asynchronous stroking, depending on if the participants reported looking up or down to the illusionary body.

These findings point to an overlap in brain processes involved in third-person perspective taking and the out-of-body experience (Braithwaite & Dent, 2011).

Spirituality

Another personality trait that has been associated to OBEs and perspective taking is spirituality. A pre- and post-neurosurgery study with personality assessments including spirituality, showed that people with lesions in the right angular gyrus showed an increase in spirituality (Urgesi et al., 2010). According to the researchers, this increase in spirituality may reflect a change in the ability to

transcend the boundaries of the physical body. This is necessary for focusing attention towards one’s own inner world and disconnect from the current body perception and actions (Urgesi et al., 2010). This supports an earlier study of Johnstone and Glass (2008) who found that spirituality was

negatively correlated with right parietal lobe functioning. These previous studies show that individual differences in spirituality are directly related to the excitability of the TPJ and accordingly, in the present study we propose to measure individual differences in spirituality as an important moderator of the effects of tDCS on perspective taking and the OBE.

Present research

In this research we combine tDCS with measures of perspective taking, the full body illusion and measures of spirituality. During tDCS a weak constant electric current is passed between an active and a reference electrode on the scalp. This causes enhanced (anodal) or decreased (cathodal) cortical excitability. A sham condition is added as a control. In the sham condition, the tDCS device stops after 1 minute of stimulation. The aim of this research is to investigate if the activity of the rTPJ can be modulated with anodal tDCS and compared to cathodal tDCS and a sham-stimulation condition in a well-established spatial perspective taking task, namely the Own Body Transformation task. This task consists of the perspective rotation task and the self-perspective taking task. In the other-perspective rotation task participants mentally rotate their body along three axes of rotation and

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indicate if a bracelet is on the left or right arm seen from the perspective of the avatar. In the self-perspective taking task participants have to indicate if the bracelet is on the left or right seen from their own perspective. The main hypothesis is that stimulating the rTPJ enhances the ability to take the perspective of another person (i.e. third-person perspective) compared to the first-person perspective (Santiesteban et al. 2012). More specifically it is expected that lowering the excitability threshold of the rTPJ (anodal tDCS) enhances the ability to take the visual perspective of another person, while raising the excitability threshold of the rTPJ (cathodal tDCS) decreases this ability. This enhancement in the ability to take the perspective of the other should be reflected in shorter reaction times and a higher accuracy on the other-perspective rotation task in the anodal tDCS group compared with the cathodal tDCS group and the sham group. This is a direct replication of the study of Santiesteban et al. (2012), with a different, though comparable dependent measure. In the Santiesteban study they used a spatial perspective task were participants had to take the perspective of a director on the computer screen and move objects from this perspective, while in our study we use a body rotation task to measure SPT.

Furthermore, it is expected that anodal tDCS combined with the other-perspective rotation task decreases the activation threshold of the rTPJ, resulting in an increased sensitivity for the full body illusion. Therefore, a stronger full body illusion is expected when participants receive anodal tDCS compared to cathodal and sham tDCS. The strength of the full body illusion is measured with a subjective rating, the full body illusion questionnaire, and also with the crossmodal congruency effect (CCE) which is explained in more detail in the methods section.

Finally, we expect individual differences in spirituality to moderate the effects of tDCS on the other-perspective rotation task and the full body illusion. People scoring high on spirituality have an enhanced ability to take the perspective of another person and an enhanced sensitivity for the full body illusion.

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Methods Questionnaires

Spiritual-transcendence scale

We used the Spiritual-transcendence scale as described in Piedmont (2002). The

spiritual-transcendence scale (STS) measures individual differences in spirituality. The questionnaire consisted of 24 items about spiritual experiences (e.g. “I feel on a higher level all of us share a common bond”; “I find inner strength and/or peace from my prayers or meditations”) with a 7-point answering scale ranging from ‘a totally disagree’ till ‘a totally agree’. The STS was normally distributed, with a mean of 80.63 (range 52 to 136) and a standard deviation of 21.04, p = .34 (Shapiro-Wilk), Cronbach’s α = .90.

Launay-Slade hallucination scale

We used the Launay-Slade hallucination scale as described in Launay & Slade (1981) and Braithwaite et al. (2011). The Launay-Slade Hallucination Scale (LSHS) measures the number and types of hallucinations that people have experienced in the past. The questionnaire consisted of 12 items about different types of hallucinations (e.g. “Sometimes my thoughts seem as real as actual events in my life”; “I often hear a voice speaking my thoughts aloud”) with a 7-point answering scale ranging from ‘a totally disagree’ till ‘a totally agree’. The LSHS was also normally distributed, with a mean of 31.22 (range 14 to 51) and a standard deviation of 8.57, p = .32 (Shapiro-Wilk), Cronbach’s α = .74.

Cardiff anomalous perception scale

We used the Cardiff anomalous perception scale (CAPS) as described in Bell, Halligan and Ellis (2006) and Braithwaite et al. (2013). The CAPS consisted of 31 yes/no items and measures perceptual anomalies (e.g. “Do you ever sense the presence of another being, despite being unable to see any evidence?” or “Do you ever see shapes, lights or colors even though there is nothing really there?”). The CAPS was not normally distributed, p = .0024 (Shapiro-Wilks). There were no participants with high scores on the CAPS (range 0 to 15). This might be because all participants had a higher or university level education. Thus, the CAPS was removed prior to the analysis.

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False believe story task

We used the False Belief story task as described in Dodell et al. (2011). In this task participants are required to attribute mental states to others. The task consisted of 20 memory control items and 20 items in which participants are required to attribute mental states to others. An example of a false belief mental item is: “The morning of high school dance Sarah placed her high heel shoes under her dress and then went shopping. That afternoon, her sister borrowed the shoes and later put them under Sarah’s bed. Sarah gets ready assuming her shoes are under the dress.” and an example of false belief physical item is “When Jeff got ready this morning he put on a light pink shirt instead of a white one. Jeff is color blind, so he can’t tell the difference between subtle shades of color. In reality, Jeff’s shirt is pink”. The False believe story task was not normally distributed, p < .001 (Shapiro-Wilk), there were no differences in scores between participants on this task. Again, this might be because all participants had a higher or university level education. Thus, the False believe story task was removed prior to the analysis.

Full Body Illusion Questionnaire

We used the Full Body Illusion questionnaire as described in Ehrsson (2005). This questionnaire consisted of a total of 10 items with a 7-point answering scale ranging from ‘a totally disagree’ to ‘totally agree’. There were 2 target statements, 2 control statements and 6 filler statements. The 2 target statements were: “I experienced that the hand I was seeing approaching the cameras, was directly touching my chest.” and “I felt as if my head and eyes were located at the same place as the cameras, and my body just below the cameras.”. The 2 control statements were: “The visual image of me started to change appearance so that I became (partly) transparent.” and “I felt as if my head and body were at different locations, almost as if I had been ‘decapitated’.”.

Transcranial direct current stimulation

tDCS was applied with a neuroConn, dc-stimulator. Two 5*7 cm. electrodes were attached to the scalp with Ten20 conductive electrode paste (Weaver). The anodal electrode was placed on CP6 and the reference electrode on C3 according to the 10-20 EEG system. Stimulation was applied for 20 minutes with 1mA.

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For the OBT we used stimuli representing a human-like avatar. The avatar was designed with MakeHuman TM_{, an open source tool for making 3D characters, and Blender} TM_{, an open source 3D} animation program. The OBT task is a 3D version of Parsons’ Imagined Spatial Transformation Task (Parsons, 1987). The experiment was programmed with Presentation® software (NeuroBehavioral Systems, Berkeley).

The OBT task (figure 1a and 1b) consisted of the other-perspective rotation task and the self-perspective taking task. In the other-self-perspective rotation task, participants were instructed to indicate whether a bracelet was on the right or the left arm of the avatar on the screen. In the self-perspective taking task, the participants had to indicate whether the bracelet was on the left or the right side of the cross on the chest of the avatar with respect to the central fixation cross on the screen, as seen from their own perspective.

Figure 1. Examples of the Own Body Transformation task. The avatar presented on the screen in A) 0 degrees and B) 120 degrees rotated along the y-axis. For image A participants had to respond ‘right’ in the other-perspective rotation task

image and ‘left’ for the self-perspective taking task. For image B participants had to respond ‘left’ in the other-perspective

rotation task and ‘left’ for the self-perspective taking task.

To prevent that participants created a responding rule (e.g. respond with the same hand when the avatar is facing away and respond with the opposite hand when the avatar is face-to-face) we

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Running head: THE EFFECTS OF TDCS ON PERSPECTIVE TAKING

degrees (60 trials), 120 degrees (60 trials) and 180 degrees (60 trials) and along three different rotation axes: z-axis, x-axis, and y-axis (figure 1b.). Each task (i.e. other perspective rotation task and self-perspective taking task) consisted of 200 trials. The participants had 10 seconds to respond before the next stimulus appeared. Participants responded by pressing the left or right arrow key on a keyboard. The groups differed in their average reaction times for the OBT, F(2,44) = 3.49, p = .04; anodal (M = 31.2, sd = 4.5), cathodal (M=30.9, sd = 3.9), sham (M = 28.1, sd = 1.5). However, Tukey’s post-hoc multiple comparison showed that there was no significant difference between groups. For anodal vs. cathodal, p = .97; anodal vs. sham, p = .05; cathodal vs. sham, p = .09.

Out-of-body experience

We used the full body illusion task (FBI) as described in Ehrsson (2007). The experiment was programmed with custom-made software (ExpyVR, developed by LNCO, Lausanne, Switzerland). Video images were displayed through a head mounted display (HMD; Oculus Rift DK1).

Figure 2 Full Body Illusion adapted from Guterstam & Ehrsson (2012). Figure 2 A) shows the participant’s through the Head Mounted Display and B) the position of the participant, the experimenter and the camera.

Figures 2 a and b show the setup of the FBI, which was similar as in Ehrsson (2007). The participant was seated on a stool, about 1.5 meters in front of a camera that was mounted on a tripod. The participants wore a set of HMDs. Display resolution = 1280 x 800; These were connected to a webcam (Logitech). The experimenter stood behind the participant’s left shoulder, between the

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participant and the camera. Figure 2 a shows the participant’s view through the HMD. The experimenter touched the participant’s chest with a small plastic rod (10 centimeters long, two centimeters diameter) with her left hand, which was hidden from view of the camera. With her right hand the experimenter repeatedly moved another similar rod towards the empty space below the camera (figure 2). This was visible for the participant through the camera.

Reaction time measures.

The effect of the synchronous and asynchronous stimulation during the FBI on the integration of visuo-tactile information was measured with the Crossmodal Congruency Task (CCT) We used the CCT as described in Aspell et al. (2010). Visuo-tactile stimulation for the CCT was applied by using custom made LEDs and buzzers. Participants wore a t-shirt with holes in the back. The buzzers were attached through the holes directly on the skin of the participant. The LEDs were attached just above or beneath the buzzers directly on the t-shirt with Velcro. The visuo-tactile stimulation could be congruent or incongruent. Figure 3 a and b show an example of two trials of the CCT. In a congruent trial, the light was displayed at the same location as the buzzer (e.g. both down; figure 3A), in an incongruent trial, the light was displayed at the opposite location from the buzzer (e.g. light down, buzzer up; figure 3B).

Lights could be displayed on the same side (e.g. light and buzzer both on the left side) or the opposite side (e.g. light on the left side, buzzer on the right side) of the body. This results in four possible trials: same side congruent (SSC), same side incongruent (SSI), different side congruent (DSC), different side incongruent (DSI), with a total of 80 trials (20 in each condition). Participants had to indicate whether they felt one of the ‘up’ buzzers or one of the ‘down’ buzzers, irrespective of the location of the light. They responded by pressing either the ‘up’ arrow key or the ‘down’ arrow key on a keyboard. Lights displayed at the same location as the buzzer (i.e. congruent trials) typically result in faster reaction times. Lights displayed at a different location as the buzzer (i.e. incongruent trials) typically result in slower reaction times. The reaction time difference between incongruent and congruent trials is known as the crossmodal congruency effect (CCE).

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Figure 3. Example of a trial in the Cross Modal Congruency Task of A) a same side congruent trial (SSC) and B) a different side incongruent trial (DSI).

The condition in which the displayed image of the participant was synchronous with the touches on the participant’s body was the target condition. In this condition simultaneous visuo-tactile

stimulation was applied with an irregular pattern. This so called synchronized condition is believed to maximize body ownership illusions (Ehrsson, 2007; Guterstam & Ehrsson, 2012; Blanke, 2012). In the asynchronous condition the touching pattern was the same, but the visual image of the camera in the HMD was delayed with 2500 ms. This resulted in asynchronous visuo-tactile stimulation (i.e. the observed touches on the virtual body and the felt touches on the subject’s body did not match). This condition has been shown to reduce the full body illusion (Ehrsson, 2007; Guterstam & Ehrsson, 2012). In both conditions the frequency of touching was approximately one Hertz. After one minute of repeated touching, the participants performed the Cross Modal Congruency Task. This whole

procedure was repeated three more times, after which the participants filled out a Full Body Illusion Questionnaire (figure 4). All participants took part in a synchronous and an asynchronous condition, and block order was counterbalanced across participants and conditions. All experiments took place at the University of Amsterdam.

Procedure Participants.

Participants were randomly assigned to either the anodal, cathodal or sham condition. In the experimental session participants started with 20 minutes tDCS stimulation while conducting the own

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body transformation task (OBT). After the tDCS stimulation and the OBT task, they participated in the Full Body Illusion task (FBI).

All participants gave their written informed consent before the start of the study. The procedures were approved by the Ethical Committee of the psychology department of the University of

Amsterdam. In this study we tested only male participants, to rule out the effects of hormonal fluctuations in woman on tDCS. A total of 58 men participated in the study, 11 participants dropped out because of failing equipment (e.g. tDCS stopped during stimulation). One participant was removed because of an error rate higher than 40 % on the OBT task. Thus for the final analysis we used 16 participants in the anodal group, 16 in the cathodal group and 15 in the sham group. All participants but one had a higher education or university level. The groups did not differ on age: anodal (M = 21.1, sd = 2.5), age range 18-28, cathodal (M = 21.3, sd = 3.3), age range 18-31, sham (M = 21, sd = 1.9), age range 18-25, F(2,43) =.069, p = .93.

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Screening session.

The experiment consisted of two sessions on separate days. The complete experiment is depicted in figure 4. In the first screening session of about 1 hour, the participants read the information

brochure of the study, gave their informed consent and filled out the Spiritual-transcendence Scale (STS), the Launay-Slade Hallucination scale (LSHS), the Cardiff anomalous perception scale (CAPS) and they conducted a false believe story task. After completing the questionnaires they received a 1 minute tDCS stimulation so participants could experience tDCS and had the opportunity to withdraw from the study if they felt too uncomfortable with the procedure. One participant decided not to take part in the second session after this first tDCS stimulation.

Experimental session.

Perspective taking.

In the second session, participants entered the experimentation room. They were asked to put on a t-shirt with holes in the back. They sat down on a chair and the buzzers were applied to the skin and the lights were attached on the t-shirt of the participant. Next, the tDCS electrodes were applied at the correct positions on the scalp. The positions of the electrodes were determined by measurement of the head. The tDCS device was placed behind the participant, so they were unable to infer the

experimental condition they were in. The stimulation was started and the participants read the instructions for the other-perspective rotation task on the computer screen. The participants were instructed to mentally rotate their body in the other-perspective rotation task. After the experimenter checked if the instructions were clear, the participants started with 8 practice trials for the other-perspective rotation task and 8 practice trials for the self-other-perspective taking task. The experimenter asked again if everything was clear. Additionally, the participants were told to take breaks if they felt fatigued and to call if anything was unclear during the task. After the instructions, the experimenter told the participant he could start the task and the experimenter left the room.

Out-of-body experience.

After completion of the OBT task of about 30 minutes, the tDCS electrodes were removed from the scalp. The participant was asked to sit down on a stool and the camera was placed 1.5 meters

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behind the participant and adjusted along the right position. The experimenter told the participant he would be observing his own body. Participants were instructed to look at a cross that was located at the center of the t-shirt, and to sit strait. After the instructions the HMD was placed on the participant’s head and adjusted. Also, headphones with white noise were placed on the ears. After the camera was started the visuo-tactile stimulation (e.g. synchronous) was applied with an irregular pattern. After one minute of tactile stimulation, the experimenter stepped out of sight of the camera (and thus the

participant) and the CCT started. This was repeated three more times after which the camera was stopped. After 4 times of stimulation and 4 times the CCT, the experimenter read the statements of the Full Body Illusion Questionnaire to the participant. Next, the camera was started again and the

procedure was repeated for the other condition (e.g. asynchronous). After completion of the FBI the participants filled out a questionnaire about side effects of tDCS and they were debriefed and sent home.

Analysis

Perspective taking.

We performed a repeated measures ANOVA with conditions (anodal vs. cathodal vs. sham) as a between-subjects factor, and three rotation angles (60 vs. 120 vs. 180) and two perspective taking tasks (self vs. other perspective) as within-subjects factors. The significance threshold was set at .05, and multiple comparisons were Bonferroni-corrected.

Out-of-body experience.

On the reaction time data from the CCT that was measured during the FBI, we performed a repeated measures ANOVA with condition (anodal vs. cathodal vs. sham) as a between-subjects factor, and synchrony (synchronous vs. asynchronous), side (same side vs. other side) and congruency (congruent vs. incongruent) as within-subjects factors. The significance threshold was set at .05, and multiple comparisons were Bonferroni-corrected.

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Results Perspective taking

To have sufficient observations per cell 1, we collapsed the analysis along the three rotation axes (i.e. x, y and z). As predicted, participants were overall faster for the self-perspective taking task (mean RT = 634 ms, sd = 33.6) compared to the other-perspective rotation task, (mean RT = 1199 ms, sd = 67.0), F(1,44) = 115.26, p < .001, η2 = .72. Table 1 shows the reaction times for the different rotation angles for the other-perspective rotation task and the self-perspective taking task. Participants became slower with increased rotation angles as reflected in a main effect of rotation angle, F(2,88) = 41.78, p < .001, η2 = .49. Furthermore, as can be seen in figure 5, this effect was qualified by a significant interaction between task and rotation angle, F(2,88) = 58.03 , p < .001, η2 = .57, indicating that the effects of rotation angle on reaction times was only observed for the other-perspective taking task and not for the self-perspective taking task.

Contrary to our hypothesis, the three tDCS groups did not differ in reaction times on both the self and the other perspective taking task, F(4,88) = .56, p=.69, η2 = .03 (Appendix A). Also, the three groups did not differ in accuracy.

Task Angle Mean sd

Other-perspective 60 1081 59.4 120 1209 74.8 180 1307 69.1 Self-perspective 60 620 31.7 120 665 37.5 180 617 33.2

Table 1. Mean RT and sd for other-perspective rotation task and self-perspective taking task for angle = 60, 120 and 180 degrees.

1

mean observations per cell

Other Self

Angle 0 60 120 180 0 60 120 180

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The scores on the spiritual transcendence scale (STS) and the Launay-Slade hallucination scale (LSHS) were included as covariate in the repeated measures analysis. No effects were found for the score on the STS and the LSHS.

In sum, there was no effect of tDCS stimulation on the ability to take the perspective of another person. Therefore, our hypothesis that stimulating the rTPJ enhances the ability to take the perspective of another person was not confirmed. Also, individual differences in spirituality did not predict the ability to take the perspective of another person.

Figure 5. Boxplot of reaction times for other-perspective rotation task and self-perspective taking task on four rotation angles: 0, 60, 120 and 180 degrees. The upper whisker extends from the third quartile of the box to the highest value that is within 1.5 * IQR, where IQR is the inter-quartile range, or distance between the first and third quartiles. The lower whisker extends from the first quartile of the box to the lowest value within 1.5 * IQR. The median is depicted as the band inside the

box. Data beyond the end of the whiskers are outliers and plotted as points.

Full body illusion

The first 13 participants were removed prior to the analysis of the FBI, because due to a

programming error the LED in the crossmodal congruency task in the asynchronous condition was not presented prior to the buzzer. Thus, for the analysis of the subjective ratings and the reaction time data we used data from 11 participants in the anodal group, 10 in the cathodal group and 13 in the sham group. All participants were male and the groups did not differ on age: anodal (M = 20.9, sd = 1.8),

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age range 18-24, cathodal (M = 20.9, sd = 2.0), age range 18-25, sham (M = 20.8, sd = 2.0), age range 18-25, F(2,31) = .004, p = .996. For all participants it was the first time they participated in a full-body illusion experiment and the first time they looked through a Head Mounted Display (HMD).

Full body illusion: Subjective ratings.

Figure 6 and table 2 show the mean subjective ratings for the full-body illusion questionnaire on the two target statements (S1 and S2) and the two control statements (C1 and C2). Only the target statement S2, “I experienced that the hand I was seeing approaching the cameras was directly touching my chest.”, differed significantly between synchronous and asynchronous condition, F(1,31) = 10.8, p < .01, η2 = .26. No differences between groups were found for both the target statements and the control statements; S1: F(2,31) = .32, p = .73, η2 = .02; S2: F(2,31) = .38, p = .69, η2 = .02; C1: F(2,31) = .17, p = .84, η2 = .01; C2: F(2,31) = .05, p = .95, η2 = .003. Thus, participants in the synchronous condition experienced that the hand they saw approaching the cameras was directly touching their chest. This experience was not modulated by tDCS stimulation. So tDCS had no effect on the subjective questionnaire measure of the full body illusion.

S1 S2* C1 C2 Anodal synchronous 4.2 (.6) 6.0 (.5) 3.0 (.5) 2.4 (.5) asynchronous 3.7 (.5) 4.3 (.7) 2.6 (.4) 2.5 (.4) Cathodal synchronous 4.1 (.6) 5.1 (.6) 2.5 (.5) 2.2 (.5) asynchronous 4.1 (.6) 3.9 (.7) 2.0 (.4) 2.2 (.5) Sham synchronous 3.9 (.5) 4.8 (.5) 2.6 (.5) 2.6 (.5) asynchronous 3.5 (.5) 3.9 (.7) 2.5 (.4) 2.5 (.4)

Table 2. Mean and standard deviation (in brackets) of the full body illusion questionnaire for the three tDCS conditions: anodal, cathodal and sham. For the synchronous condition and the asynchronous control condition. S1= “I felt as if my head

and eyes were located at the same place as the cameras, and my body just below the cameras.”; S2= “I experienced that the hand I was seeing approaching the cameras was directly touching my chest.”; C1= “I felt as if my head and body were at different locations, almost as if I had been ‘decapitated’.”; C2= “The visual image of me started to change appearance so that

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Figure 6. Subjective ratings on the full body illusion questionnaire for the synchronous condition and the asynchronous control condition. Only the target statement S2 is significant, F(1,31) = 10.8, p < .01, η2 = .26. S1= “I felt as if my head and eyes were located at the same place as the cameras, and my body just below the cameras.”; S2= “I experienced that the hand I

was seeing approaching the cameras was directly touching my chest.”; C1= “I felt as if my head and body were at different locations, almost as if I had been ‘decapitated’.”; C2= “The visual image of me started to change appearance so that I became

(partly) transparent.”

Reaction time measure.

On the crossmodal congruency task, participants were faster for the synchronous condition (mean RT = 545 ms, sd = 25.1) compared to the asynchronous condition (mean RT = 700 ms, sd = 41.4), F(1,31) = 42.97, p < .001, η2 = .58 (figure 7). Furthermore, we found that participants were faster for the congruent stimuli (mean RT = 574 ms, sd = 30.9) compared to the incongruent stimuli (mean RT = 671, sd = 33.9), F(1,31) = 109.55, p < .001, η2 = .78. A significant interaction between Congruency and Side was observed, F(1,31) = 32.84, p < .001, η2 = .51, reflecting that the congruency effect (i.e. the difference between incongruent and congruent trials) was stronger when both stimuli were

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Figure 7. Reaction times for Cross Modal Congruency task on side and congruency for the A) synchronous condition and B) asynchronous condition. The congruency effect (difference between incongruent en congruent trials) for both the

synchronous condition, t(33) = 4.1, p < .01 and the asynchronous condition, t(33) = 3.9, p < .0.1 was significant.

Next, we found an interaction effect between side and condition, F(2,31) = 3.58, p = .04, η2 = .19. This was reflected in slower responses of the cathodal group on the different side stimuli compared to the same side stimuli, t(39) = -2.17, p = .04; (see Table 3), while for the other groups RTs did not differ between same and different side stimuli (anodal: t(43) = -.03, p = .98; sham: t(51) = .46, p = .65).

Condition Side Mean sd

Anodal same side 605 55.3

different side 605 57.8

Cathodal same side 669 58.0

Sham same side 574 50.9

Table 3. Mean RT and sd for same side and different side stimuli of the anodal, cathodal and sham group, combined for synchronous and asynchronous condition.

As depicted in figures 8 and table 4, there was a three way interaction between synchrony, side and condition, F(2,31) = 5.75, p = .008, η2 = .27. We performed separate repeated measures

ANOVA’s for each group, with synchrony (synchronous vs. asynchronous) and side (same side vs. different side) as within-subject variables. However, this revealed that the side * synchrony interaction effect was not significant within each separate group; anodal: F(1,10) = .26, p = .62, η2 = .03;

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Figure 8. Reaction times for the three conditions, anodal, cathodal and sham on side for the synchronous and asynchronous condition.

We observed a significant three way interaction between synchrony, congruency and condition, F(2,31) = 6.26, p < .01, η2 = .29 . To explore this 3-way interaction, we performed separate repeated measures ANOVAs for each group, with synchrony (synchronous vs. asynchronous) and congruency (congruent vs. incongruent) as within-subject variables. As can be seen in figure 9, the congruency * synchrony interaction effect was only significant for the anodal group, F(1,10) = 6.56, p = .03, η2 = .40. For the anodal group the difference between congruent and incongruent trials was stronger for the synchronous condition (M = 117, sd = 60.2) compared to the asynchronous condition (M = 64, sd = 53.3), t(10) = 2.56, p = .03.

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Condition Synchrony Side Congruency Mean sd

Anodal

sync

same side congruent 471 41

incongruent ₆₂₄ ₄₉

different side congruent 511 45

incongruent ₅₉₄ ₄₈

async

incongruent ₆₉₇ ₇₇

incongruent ₆₈₈ ₇₆

Cathodal

sync

incongruent ₆₅₃ ₅₁

incongruent ₆₂₉ ₅₀

async

incongruent ₈₅₈ ₈₁

incongruent ₈₆₆ ₈₀

Sham

sync

incongruent ₅₄₃ ₄₅

incongruent ₅₁₅ ₄₄

async same side

congruent ₅₈₈ ₆₄

incongruent ₇₂₃ ₇₁

different side congruent ₆₁₈ ₆₉

incongruent ₆₅₈ ₇₀

Table 4. Mean RT and sd for side and congruency of the anodal, cathodal and sham group for both the synchronous and asynchronous condition.

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Figure 9. Reaction times for the three conditions, anodal, cathodal and sham on congruency for the synchronous and asynchronous condition.

The four-way interaction between synchrony * side * congruency * condition was not significant, F(2,31) = 2.75, p =.08, η2 = .15.

Conclusion Summary

The present study assessed the role of the right TPJ in multisensory integration, spatial perspective taking and the full body illusion. We hypothesized that anodal stimulation of the rTPJ would enhance the ability to take the perspective of another person. This would be reflected in faster reaction times and higher accuracy for the anodal group on the other-perspective rotation task compared to the own perspective taking task. Also we hypothesized that anodal tDCS combined with the other-perspective rotation task would decrease the activation threshold of the rTPJ, resulting in an increased sensitivity for the full body illusion. Furthermore, we expected that individual differences in spirituality would moderate the effects of tDCS on the other-perspective rotation task and the full body illusion.

We found that both increasing the excitability (anodal tDCS) and decreasing the excitability (cathodal tDCS) had no effect on perspective taking for both reaction times and accuracy as measured with the Own Body Transformation task. So, our hypothesis that stimulating the rTPJ enhances the

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ability to take the perspective of another person (i.e. 3rd person perspective taking) compared to the first person perspective was not confirmed.

Furthermore, individual differences in spirituality and hallucination scores did not predict perspective taking. We did find an increase in reaction times with increased rotation angle when participants were instructed to take the perspective of the avatar on the screen. In addition, we observed an overall reaction time difference between the self-perspective taking task and the other-perspective rotation task. These findings are in line with previous studies on spatial other-perspective taking (Santiesteban et al, 2012; Vogeley, 2003) and confirm the validity of our dependent measures.

The lack of tDCS effects on perspective taking in our study are not in line with previous research on tDCS and perspective taking. In the study of Santiesteban et al. (2012) an effect of anodal tDCS on the accuracy of perspective taking was found. It is possible that the absence of an effect is due to differences between online and offline tDCS effects. Online effects are effects measured during the actual tDCS stimulation and offline effects are effects measured after the tDCS stimulation has ended. Online effects have not always been found in previous studies (Nitsche et al., 2005) while other studies do report online effects, but no offline effects (Wirth et al, 2011). In the Santiesteban study the tasks were performed offline. So, it is possible that applying tDCS before the task, causes an effect on perspective taking and tDCS during the task does not. This is in line with our offline results of tDCS on the reaction time measures of the CCT.

It is also possible that in the present study the current that was applied was too low. Although, 1mA is the usual current for this type of research, a recent extensive review on tDCS shows that tDCS generates only small effects on motor evoked potentials (MEPs) (Horvath, Forte & Carter, in press). Most tDCS research in psychology uses this low 1mA current. Increasing the current might generate more extensive neurological effects. For example, a double-blind control study of tDCS on depression with 1 mA stimulation did not find effects (Loo et al., 2010), where a double-blind clinical trial with 2 mA did find effects on depression (Boggio et al., 2008).

Alternatively, it could be that the other-perspective rotation task does not directly rely on activation of the rTPJ. For example, in a study of David et al. (2006) increased neural activity during

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third-person perspective taking compared to first-person perspective taking was found in the left IPL (angular gyrus) with sub clusters in, among others, the right superior parietal lobe. Also, in a review of Zacks (2008) it was found that in conditions that favor motor stimulation (e.g. rotation of body parts) , mental rotation activates the medial superior precentral cortex. It was found that mental rotation of body parts depends on motor stimulation. In the present research participants had to mentally rotate their whole body, so it is possible that in our study more medial superior precentral parts of the cortex were activated and not the rTPJ. On the other hand Blanke et al. (2005) showed that TMS on the TPJ has a disruptive effect on the OBT task, specifically in front-facing figures were a prolonged body rotation is necessary compared with back-facing figures and compared to the special rotation of letters.

Full body illusion

Subjective measures.

We expected a stronger full body illusion for the participants in the anodal group, compared with the participants in the cathodal and sham group. However, we found no effect of tDCS stimulation on the subjective measures. As expected, participants reported in the synchronous condition, that they felt that the hand that was approaching the camera was touching their chest. This finding indicates that the basic experimental manipulation of synchronous visuo-tactile stimulation was successful in inducing the full body illusion. No differences on the control statements were found between the synchronous and asynchronous condition. These findings are partly in line with previous studies on out-of-body experiences (e.g. Guterstam & Ehrsson, 2012). In these studies participants also reported that they felt as if their head and eyes were located at the same place as the cameras, and their body just below the cameras. It is not entirely clear why we were unable to replicate this specific finding. It is possible that the lower screen resolution or the lack of stereoscopic vision in this study resulted in a less strong effect on image location. Also, in the synchronous condition, a minimal delay was reported by the participants. While this delay may not have affected the experience that the hand was touching their chest, it is possible that this small delay prevented participants from experiencing a change in the spatial perspective and the location of their physical body.

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Reaction time measures.

On the reaction time measures of the CCT, we did find an overall effect of synchrony. When visual and tactile information were in synchrony, participants responded faster on the buzzers compared to when visual and tactile information were out of synchrony. Furthermore, we found that participants were faster for the congruent stimuli compared to the incongruent stimuli and the

difference between incongruent and congruent trials was stronger when both stimuli were presented at the same side compared to when presented at different sides. These finding are in line with previous studies using the full body illusion paradigm (Aspell et al, 2010; Ehrsson, 2007; Guterstam & Ehrsson, 2012) and confirm the validity of our dependent measures.

We did find a stronger congruency effect for the anodal group compared to the cathodal group specifically in the synchronous condition. The size of the congruency effect is a direct measure for peripersonal space. The peripersonal space is the space around the body that people perceive as belonging to their own body. Extension of this space is also found following tool use (Farnè &

Làdavas, 2000). The congruency effect in this study suggests a stronger peripersonal space extension to the virtual body for the anodal group in the synchronous condition compared to the asynchronous condition. The finding of a stronger CCE is in line with our expectations of a stronger full body illusion when participants receive anodal tDCS compared to cathodal tDCS. Stronger identification with the illusionary body in the synchronous condition results in stronger differences in reaction time measures between congruent en incongruent visuo-tactile stimulation. We also observed an effect on side for the cathodal group in the asynchronous condition. The differences in side for the cathodal group in the asynchronous condition a difficult to interpret. It is not entirely clear what the underlying mechanism is for this effect.

Individual differences on spirituality and hallucination did not predict the strength of the full body illusion. It is possible that the simultaneous visuo-tactile stimulation causes such a strong effect on body ownership, that it is not possible to increase this effect with tDCS. This would imply that visuo-tactile stimulation alone is sufficient to create a ceiling effect, at least in normal, healthy participants.

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In conclusion, it seems that in the present research all dependent measurements indicated that the experimental paradigms showed effects in the expected direction and confirm findings that have been reported in the literature. However, no effects of tDCS were found on perspective taking. We did find some effects of tDCS on the out-of-body experience. The synchronous visual and tactile information during the Full body illusion were sufficient to create a feeling that the body was in a different location. In the present study tDCS does seem to strengthen this effect to some extent for the anodal group, suggesting an extension in peripersonal space to the virtual body and suggesting that the rTPJ is somehow involved in spatial perspective taking, although effects of tDCS are small.

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Appendix A

Condition Task Angle Mean sd

Anodal self-perspective 60 ₆₀₈ ₅₄ 120 ₆₄₉ ₆₄ 180 ₆₀₁ ₅₇ other-perspective 60 ₁₁₆₈ ₁₀₂ 120 ₁₃₂₄ ₁₂₈ 180 ₁₄₃₂ ₁₁₈ Cathodal self-perspective 60 ₇₂₄ ₅₄ 120 ₇₉₃ ₆₄ 180 ₇₃₇ ₅₇ other-perspective 60 ₁₁₄₃ ₁₀₂ 120 ₁₂₇₄ ₁₂₈ 180 ₁₃₆₃ ₁₁₈ Sham self-perspective 60 ₅₂₇ ₅₆ 120 ₅₅₃ ₆₆ 180 ₅₁₄ ₅₉ other-perspective 60 ₉₃₂ ₁₀₅ 120 ₁₀₂₈ ₁₃₂ 180 ₁₁₂₇ ₁₂₂

Mean RT and sd for the three conditions, anodal, cathodal and sham for the self-perspective taking task and the