1  

 

Can you tickle yourself if you swap bodies with someone else?

George Van Doorn1,*, Jakob Hohwy2 & Mark Symmons1

1School of Applied Media and Social Sciences, Monash University, Northways Road,

Churchill, Victoria, 3842, Australia; emails: george.vandoorn@monash.edu,

mark.symmons@monash.edu

2Department of Philosophy, Monash University, Clayton Road, Clayton, Victoria, 3800,

Australia; email: jakob.hohwy@monash.edu

Abstract

The effect of the body transfer illusion on the perceived strength of self- and externally-

generated “tickle” sensations was investigated. As expected, externally generated

movement produced significantly higher ratings of tickliness than those associated with self-

generated movements. Surprisingly, the body transfer illusion had no influence on the

ratings of tickliness, suggesting that highly surprising, and therefore hard to predict,

experiences of body image and first-person perspective do not abolish the attenuation of

tickle sensations. In addition, evidence was found that a version of the rubber hand illusion

exists within the body transfer illusion. We situate our findings within the larger debate over

sensory attenuation: (1) there is an attenuation of prediction errors that depends upon the

context in which sensory input is predicted (i.e., efference copy), and (2) sensory attenuation

is a necessary consequence of self-generated movement irrespective of context (i.e., active

inference). The results support the notion of active inference.

Keywords

Tickle, Body Transfer Illusion, Self- and Externally Generated Movement, Active Inference

                                                                                                                         * Corresponding author at: School of Applied Media and Social Sciences, Monash University, Northways Road, Churchill, Victoria, 3842, Australia. E-mail address: george.vandoorn@monash.edu. Phone: +61 (03) 5122 6746.  

2  

 

1. Introduction

Gibson’s (1962) work distinguishing touching from being touched provided insight into how

perceptual awareness may be modulated by self-generated actions (see also Blakemore,

Rees, & Frith, 1998). In Gibson’s experiments passive touch consisted of having a stimulus

pressed upon the skin with no (intended) subsequent movement. The term “passive touch”

has since been applied to situations where the stimulus is moved across the skin by an

experimenter, or where the hand of the participant is guided in such a way that the input can

resemble that of self-generated exploration, but with movement being externally generated

(Symmons, Richardson, & Wuillemin, 2008).

Self-generated movements involve the intent to move. They also involve the planning of,

the preparation for, and the execution of, movement (Grezes & Decety, 2001). Self-

generated movements may also involve the production of a copy of the efferent signal (a

copy of the motor commands), which can be used to predict the sensory consequences of

motor commands (e.g., the flow of proprioceptive input one would receive upon executing a

reach movement; see Blakemore, Goodbody, & Wolpert, 1998; Blakemore, Oakley, & Frith,

2002; Blakemore, Wolpert, & Frith, 1998). It is thought that this so-called efference copy is

used to make a comparison between the expected and actual consequences of movement.

These predicted consequences are known as ‘corollary discharge’ and are thought to enable

the fine-tuning and updating of action. In contrast, since externally generated movements do

not involve motor commands, no efference copies are generated, and thus the

consequences of those movements cannot be compared with a prediction.

The efference copy mechanism has been suggested as an explanation of how people

distinguish externally generated movements from their self-generated movements

(Blakemore et al., 2002). Specifically, if there is no (or minimal) discrepancy between the

predicted and actual sensory input, attenuation of the sensory information occurs (Voss,

Ingram, Haggard, & Wolpert, 2006); a nice example of this would be our inability to tickle

3  

 

ourselves. In contrast, since externally generated sensations are not predicted, they are not

attenuated, and hence give rise to the ability of others to tickle us (Blakemore, Wolpert, &

Frith, 2000).

A number of findings seem to favour the idea of such an efference copy mechanism. For

example, when a slight delay or trajectory perturbation (i.e., varying the direction of a tactile

stimulus’ movement as a function of the direction of a hand’s movement) is introduced to

self-generated touch, the self-tickle effect is weakened (i.e., self-tickle becomes stronger),

consistent with the notion of a fine-grained, spatiotemporally-sensitive prediction being at the

heart of the tickle effect (Blakemore, Frith, & Wolpert, 1999). In brief, the motor control

explanation for sensory attenuation equates percepts with prediction error rather than with

predictions (since the intensity of the tickle reflects the magnitude of the prediction error),

and explains sensory attenuation as the attenuation of prediction errors that depend upon

the accuracy and precision with which sensory inputs are predicted (via the corollary

discharge produced by the efference copy).

Attenuation of re-afferent signals has been investigated more generally, unearthing

evidence suggesting that self-generated movement is associated with non-specific gating of

a wider range of current input than just the specific input predicted on the basis of the

efference copy. It is still an open question how wide the range of gated input is, and various

phenomena have been studied, from the suppression of saccadic eye movements (Wurtz,

2008) to attenuation within the auditory system during self-generated ‘sound-making’ actions

(Poulet & Hedwig, 2002). In somatosensory research, Gallace, Zeeden, Röder, and Spence

(2010) argued that attenuating tactile stimuli during the execution of self-generated

movements is advantageous as it prevents potentially irrelevant stimuli (e.g., a fly landing on

one’s arm) from interfering with important movements (e.g., defending oneself from an

attacker). Accordingly, Juravle and Spence (2011) have shown that the execution of self-

generated movements during a juggling task suppresses task-irrelevant cutaneous inputs.

They argue that comparison of the anticipated and actual states attenuates the unnecessary

4  

 

sensory information (cutaneous inputs in their experiments) and enhances the essential

sensory information (kinesthetic information). Their finding is consistent with work (e.g.,

Hecht, Reiner, & Karni, 2008; Seki, Perlmutter, & Fetz, 2003) showing that self-generated

movements make it more difficult to notice tactile stimuli presented on a moving limb through

non-specific gating of incoming information.

The idea of non-specific gating for the purposes of ensuring focus on task-relevant stimuli

would explain the attenuation of self-generated tickling as a more general effect of self-

generated movement, rather than as an efference copy mechanism exclusively associated

with self-generated movement related to somato-sensation. Of relevance here is an earlier

study (Claxton, 1975) which showed that perceived tickle is diminished when visual input of

the dynamics of the tactile input is made available, suggesting that the tickle effect relies on

general predictability of the stimuli, rather than on efference copies alone. Problematically,

Claxton focused on passive touch conditions (similar to that of Gibson (1962), wherein an

experimenter brushed a feather across a subject’s stationary arm) and did not test whether

depriving participants of visual input had an influence on tickle ratings during self- and

externally generated movements. This account thus explains findings of attenuation that are

specific to the predictability of passively-received stimuli; it does not, however, help explain

whether predictability plays a role in self- and externally generated movements and, as such,

whether attenuation of sensory input occurs broadly (i.e., non-specifically) as a result of self-

generated movements.

Recent work in theoretical neuroscience has challenged the idea of motor commands,

and hence efference copies of motor commands (see Adams, Shipp, & Friston, 2012;

Brown, Friston, & Bestmann, 2011). This, simultaneously, puts pressure on the efference

copy account of sensory attenuation and provides a unified framework for findings

suggesting non-specific gating. This work is inspired by the free energy principle (Friston,

2010) which says that the brain is fundamentally engaged in minimising its free energy, or

average prediction error, over time. According to this theory both perceptual inference and

5  

 

action simply minimize prediction error. In perception, prediction error is minimised by

updating model parameters to fit the sensory input. For action, prediction error is minimised

by using action (such as eye movement and palpation) to change the actual sensory input to

fit with the predicted sensory input. Movement is not caused by descending motor

commands but by predictions of the proprioceptive consequences of an intended movement.

These predictions are fulfilled by classic reflex arcs, until the predicted proprioceptive state is

attained. Crucially, this explanation for self-generated movement is thought to require the

withdrawal of attention or precision from the sensory consequences of action. The argument

here is: if ascending (proprioceptive or exteroceptive) prediction errors were not attenuated,

they would subvert the predictions causing the intended movement. In other words, one has

to suspend attention to the actual sensory consequences of movement to allow movement

to unfold - otherwise, one would infer the intended movement was not occurring. If attention

is conceived as gain modulation (encoding expected precision), the attenuation of sensory

precision is entirely consistent with the withdrawal of attention from sensory inputs in the

non-specific sense reviewed above. This furnishes another explanation for the tickle effect

as a fundamental aspect of self-generated movement (Brown et al., 2011; Brown, Adams,

Parees, Edwards, & Friston, 2013). In this “active inference” account, prior beliefs about an

intended movement cause predictions and sensory attenuation. This is consistent with

findings suggesting non-specific gating and makes the stronger prediction that it is not a

limited range of sensory input but all current sensory input that is attenuated during action.

In summary, we can discern two alternative explanations for the tickle effect. First, there

is an attenuation of prediction errors that depends upon specific predictions in the form of

efference copy (Blakemore et al., 1999). Second, sensory attenuation is a necessary

consequence of predictions about an intended movement. There is a crucial distinction

between these accounts: In the first, sensory attenuation depends upon the context in which

sensory input is predicted. For example, predicted somato-sensation depends on who is

touched (the individual him or herself, or a stranger), how touch is delivered (incongruence

6  

 

between movement and tactile sensations delivered via delay or trajectory perturbation), and

where touch is delivered (a real hand or a non-hand object). In this way, a highly surprising

context for the touch should undermine predictions, and thus increase prediction error and

intensify somato-sensation. In the second, active inference account, sensory attenuation is

an inherent consequence of descending predictions and is context insensitive; here, a

surprising context should have no influence on somato-sensation. In what follows, we seek

to distinguish between these two accounts by changing the context in which predictions

about sensory consequences were manipulated - namely through illusory manipulations of

body image (i.e., the body transfer illusion and the rubber hand illusion). In this setting, if

prediction errors determine percepts, we would expect the self-tickle effect to be sensitive to

illusory context because the highly surprising experience of the illusions should undermine

confident processing of the prediction and thereby increase the prediction error and intensify

the tickle percept. Accordingly, if one swaps bodies with someone else, acquires new types

of limbs, or experiences asynchronous touch, the ratings of tickliness should be higher.

Alternatively, under active inference, sensory attenuation is a consequence of self-generated

movement irrespective of context - and we would anticipate the self-tickle effect to be robust

to illusory changes in body image and touch characteristics.

2. Method

The majority of rubber hand illusion-style experiments are designed to incorporate both

synchronous and asynchronous touch conditions. Since the present study utilises the rubber

hand illusion, it too was designed with these two conditions. In our complex experimental

design, synchronous and asynchronous touch were administered in each of the two different

body transfer illusion conditions. Undergoing this procedure (including intervening

modification of the equipment) would take an inordinate amount of time for each participant.

The study was therefore planned as a between-subjects experiment, where the participants

were split into two groups. One group was run through one set of conditions (i.e., synchrony

7  

 

in non-body transfer illusion and asynchrony in body transfer illusion) before the other group

was run through the other conditions (i.e., asynchrony in non-body transfer illusion and

synchrony in body transfer illusion). The data were then analyzed together.

3. Participants

3.1 Group 1

The participants were 12 volunteers with a mean age of 30.5 years (SD = 8.0 years), six

were female. They had no reported visual, tactile or muscular abnormalities. The data from

two additional participants were deemed unreliable and were not included in the statistical

analyses; the majority of their tickling scores were zero or very close to zero. Specifically,

eight of 12 “tickle” ratings were zero or close to zero (i.e., < 5 on a visual analogue scale

from 0 [not tickly] to 100 [very tickly]) for one subject, while nine of 12 were zero or close to

zero (i.e., < 2) for another.

3.2 Group 2

The participants were 11 volunteers with a mean age of 29.2 years (SD = 9.1 years), five

were female. They had no reported visual, tactile or muscular abnormalities.

The experiment was carried out in accordance with the regulations of the Monash University

Human Research Ethics Committee. The participants gave their written informed consent

before the start of the experiment.

4. Stimulus and Apparatus

The “tickle” device consisted of two triangular pieces of soft foam attached on either end of a

wooden rod (length = 65 cm). The rod was mounted on a cradle so that it could be moved

back-and-forth. The rod could be moved either by the participant using their right hand or,

8  

 

from the other end, by the experimenter. The foam made light contact with the participant’s,

and experimenter’s, left palm.

A CCTV camera (Hikivision QCPass N816) was mounted on a bicycle helmet;

participants sat directly opposite and facing the experimenter and wore a Head Mounted

Display (i-Glasses A501429C), which was connected to the CCTV camera. In the

“experimenter’s perspective” conditions, the helmet with the camera was worn by the

experimenter and presented his viewpoint (see Figure 1). This set-up allowed the

participants to see their physical bodies from slightly below the shoulders to the top of the

table, where their forearm and hand rested. As such, they could clearly recognize

themselves and distinguish between their own arm and the arm of the experimenter (which,

in some cases, belonged to a person of the opposite gender). This condition was expected

to lead to the body transfer illusion. In the “participant’s perspective” conditions the

participant wore the camera and could see the experimental set-up from their true point-of-

view.

9  

 

Figure 1. (a) Experimenter’s perspective of experimental set-up with “real” limb; (b) Experimenter’s perspective of experimental set-up with “fake” limb; (c) Experimenter’s perspective of experimental set-up with “no” limb; (d) Participant’s perspective of experimental set-up with “real” limb; (e) Participant’s perspective of experimental set-up with “fake” limb; (f) Participant’s perspective of experimental set-up with “no” limb.

5. Design

Each group was subjected to a repeated-measures design. The independent variables were

“movement type” which had two levels (self-generated vs. externally generated),

“perspective” with two levels (participant’s vs. experimenter’s), and “limb type” which had

three levels (real vs. baseball bat vs. none). “Group” acted as a between-subjects variable

which had two levels (asynchrony in body transfer illusion vs. synchrony in body transfer

illusion). The dependent variables were magnitude estimates of “tickliness” and “strength of

body transfer illusion” - made on visual analogue scales - and responses to a questionnaire

(see below).

10  

 

6. Procedure

6.1 Group 1

Participants were shown the experimental set-up and how the “tickle” device worked prior to

testing. During the pre-experiment instructions, participants were also given an opportunity

to feel the tickle of the piece of foam in both self- and externally generated conditions. They

were told that they would feel the piece of foam move across the palm of their hand and that

sometimes they would move it and sometimes the experimenter would move it. They were

then told that their task was to rate “how tickly the piece of foam was” by making a mark

through a line on a visual analogue scale; no mention was made by the experimenter of

efference copy or an inability to tickle oneself. Further, they were told, in a manner similar to

Blakemore et al. (1999), that they should use the same scale throughout the experiment and

that the “emphasis was on the relative values of their judgments” (p. 558).

During the self-generated movement condition and while viewing the scene from the

participant’s perspective (i.e., without body transfer illusion), the participant was asked to

position their right arm in front of themselves and take hold of the rod, which they moved

repeatedly, and radially, back-and-forth for 10 sec. This movement produced a tactile

sensation on the palm of their left hand. In this condition, participants wore both the goggles

and the camera, and thus visual information was presented from the participant’s own first-

person perspective (see Figure 2).

In the self-generated movement condition viewed from the experimenter’s perspective

(i.e., during body transfer illusion), the participant viewed the scene through the goggles in

such a way that the perspective was that of the experimenter (i.e., the experimenter wore the

helmet and camera). Participants were, again, responsible for generating the action that

moved the stimulus across the skin of their palm. In this condition, participants should feel

the tickle on the experimenter’s hand due to the body transfer illusion, and may even believe

11  

 

that their own movement was performed with the experimenter’s hand, which was proximal

in visual space.

In the externally generated condition without body transfer illusion, the experimenter

moved the rod. This movement produced a tactile sensation on the palm of the participant’s

left hand. In this condition, participants wore the goggles and the camera.

In the externally generated movement condition during body transfer illusion, the visual

perspective was the same as that in the self-generated movement condition during body

transfer illusion (i.e., the participants were presented with visual input from the

experimenter’s perspective), but now participants may believe that the experimenter’s

movement was performed with their hand, which was distal in visual space.

These four conditions were repeated with the experimenter’s arm kept out of sight (i.e.,

behind the experimenter’s back), or replaced with a baseball bat that acted as a substitute

limb. Each participant experienced each combination of conditions once.

In this part of the experiment, the participant’s perspective conditions produced touch on

the proximal, own hand of the participant that was synchronous (i.e., when the foam was

seen to move away from the body it was also felt, tactilely, to be moving away from the

body). Conversely, the experimenter’s perspective conditions produced touch on the

proximal hand belonging to the experimenter that was asynchronous (i.e., when the foam

was seen to move away from the body it was felt, tactilely, to move towards the body).

Group 2 experienced synchrony and asynchrony across these conditions in a manner that

was opposite to the way Group 1 experienced them.

12  

 

Figure 2. Birds-eye view of the experimental set-up. (A) Participant’s perspective (i.e., camera and goggles are situated on the participant’s head), no body transfer illusion experienced (i.e., they see the scene from their perspective); (B) Experimenter’s perspective (i.e., goggles are situated on the participant’s head while the camera is placed on the experimenter’s head), body transfer illusion experienced (i.e., they see the scene from the experimenter’s perspective). In (A), the location of the tickle is felt to be on the visually proximal left palm of the participant’s own arm, i.e., where the tickle occurs in reality (“Tickle point”). In (B), the location of the felt tickle is again the visually proximal left palm, which now belongs to the experimenter. This condition then gives rise to a difference between the “Illusory tickle point” and the real “Tickle point”. In Group 1, there is visuotactile synchrony in (A) and asynchrony in (B); in Group 2, a delay line produced visuotactile asynchrony in (A) and synchrony in (B).

6.2 Group 2

The stimulus, apparatus, procedure and design were the same as those used for Group 1,

with one exception. In the first part of the experiment, the body transfer illusion resulted in

asynchrony between vision and touch (i.e., the foam was seen to move in a direction

opposite to the one it was felt to move on the proximal hand). The second part of the

experiment was designed so that there was synchrony between visual and haptic inputs

during the body transfer illusion, which was achieved via a delay line (Ovation Systems Ltd.),

with the delay set to 0.47 sec; setting the delay to 0.47 seconds presupposes that the tickling

movements were roughly sinusoidal - with the appropriate frequency to create synchronous

13  

 

stimuli. This provided a contrast to the asynchrony in the body transfer illusion during the

experimenter’s perspective condition experienced by Group 1, which ensured that the only

violation of overall prediction is in the body image, rather than in body image and temporal

congruency. Conversely, generating synchrony during body transfer illusion in the second

part of the experiment resulted in asynchrony between vision and touch during the non-body

transfer illusion participant’s perspective conditions, i.e., the exact opposite of that

experienced by Group 1. The structure of the two parts of the experiment in combination is

described in Table 1.

Table 1. Summary of the Procedure for Groups 1 and 2.

     Experiment # Perspective Touch type Movement type

Group 1

Participant’s perspective Synchronous

Self-generated

Externally generated

Experimenter’s perspective Asynchronous

Self-generated

Externally generated

Group 2

Participant’s perspective Asynchronous

Self-generated

Externally generated

Experimenter’s perspective Synchronous

Self-generated

Externally generated

     

14  

 

7. Dependent measures

After each condition, observers rated the tickliness of the stimuli on a visual analogue scale

from 0 (not tickly) to 100 (very tickly). They also explicitly rated the strength of the body

transfer illusion on a similar scale (i.e., 0 [no illusion] to 100 [very strong illusion]); it was

explained to participants that “no illusion” meant that they felt as though they were occupying

their own body, while “very strong illusion” should be marked if they felt as though they were

occupying the experimenter’s body (It would have been unsurprising if this question was

scored higher in the experimenter’s perspective condition, as it makes comparatively little

sense in the participant’s perspective. However, it allows exploration of differences between

different limb types, movement types, and synchronous vs. asynchronous touch.). Further,

their experience was measured via a type of questionnaire commonly used in studies of the

rubber hand illusion, adapted for this experiment (see Table 2). Statements from Botvinick

and Cohen (1998) [S1, S2, S3, S4, S6, S7], Longo et al. (2008) [S8], Petkova and Ehrsson

(2008) [S5], and Hohwy and Paton (2010) [S9] were selected and adapted to reflect our

experimental paradigm, specifically with respect to the experience of the body transfer

illusion.

15  

 

Table 2. Randomised questionnaire presented after each trial; participants were asked to rate their agreement with each statement on a visual analogue scale anchored at the ends with ‘strongly disagree’ and ‘strongly agree’.

         

   

Statement No# Statement Text  

 

S1 It seemed as if I was feeling the touch of the piece of foam on the hand/bat or in the empty space that seemed closest to me in the goggles

 

 

S2 It seemed as though the touch I felt was caused by the piece of foam I could see touching the hand/bat/empty space that seemed closest in the perspective of the goggles

   

S3 It felt as if the hand/bat/empty space was part of me  

 S4 It seemed as if I might have more than one left hand or arm

 

 S5 It seemed as if I was in two different locations at the same time

   

S6 It felt as if I had lost control of my (real) hand  

 

S7 The experimenter's hand/the bat/the area of empty space began to resemble my own (real) hand in terms of shape, skin tone, freckles or some other visual feature

 

 

S8 I found that the touch of the piece of foam on my own hand was pleasant

   

S9 The room temperature changed during the experiment  

 

     

Statements S1-S3 are normally used to gauge the strength of the rubber hand illusion,

which is here potentially elicited by the foam touching the hand in synchrony with the visual

input (in both the participant’s perspective condition and the experimenter’s perspective

condition). Thus, S1 asks about the felt location of the touch, S2 asks about the felt cause of

the touch, and S3 is about whether it feels as if the hand is one’s own. S1-S3 were expected

to be rated higher in the participant’s perspective condition than the experimenter’s

perspective condition, since research shows that a more familiar, realistic, hand-like object

elicits a stronger rubber hand illusion than those looking less like one’s hand (Hohwy &

Paton, 2010; Tsakiris, Carpenter, James, & Fotopoulou, 2010; Tsakiris & Haggard, 2005). It

was further expected that the ratings would be higher in synchronous than asynchronous

touch, i.e., higher in participant’s perspective in Group 1 than in participant’s perspective in

Group 2. Further, if a rubber hand illusion is felt while experiencing the body transfer illusion,

then the ratings for S1-S3 would be higher in experimenter’s perspective in Group 2,

16  

 

because of the synchrony between stimuli in this condition, than in experimenter’s

perspective in Group 1, during which there is asynchronous touch.

Statements S4-S7 reflect the kinds of experiences that would be characteristic of the

body transfer illusion, when one has the illusion of looking at oneself while occupying

another’s body, but without asking so directly that the question makes little sense in the non-

body transfer illusion conditions. These statements thus probe the body transfer illusion

experience more implicitly than the “strength of illusion” question. S4 asks about duplication

of the arm, S5 asks about duplication of self-location, and S6 asks about the sense of control

as one sees oneself moving but experiences moving another’s arm. S7 is sometimes used

to gauge the rubber hand illusion but here we wanted to see if a stronger response was

elicited during the body transfer illusion, in the experimenter’s perspective, since in the

participant’s perspective the seen hand is already the participant’s own. Responses to S4-S7

were expected to be higher in the experimenter’s perspective than in the participant’s

perspective. Further, if synchronous touch strengthens the body transfer illusion, then S4-S7

should be rated higher in experimenter’s perspective in Group 2 than in experimenter’s

perspective in Group 1.

Finally, S8 and S9 are control questions that we do not expect to vary with conditions.

Responses to S8 should remain relatively high, while responses to S9 should remain low.

Combined, we thus expected this complex picture to result in a three-way interaction of

questionnaire responses, perspective and group: S1-S3 capturing variation in rubber hand

illusion, S4-S7 capturing variation in body transfer illusion, and S8-S9 assuring that

variations of interest are condition-specific.

8. Results

8.1 Tickle ratings

A 2 (perspective: participant vs. experimenter) x 2 (movement type: self-generated vs.

externally generated) x 3 (limb type: real vs. baseball bat vs. none) x 2 (group: 1 vs. 2)

17  

 

mixed-model ANOVA on subjective ratings of how tickly the stimuli were revealed that the

main effect of the movement type factor was significant [F(1,21) = 8.19, p = .009], with a

large effect size, i.e., partial eta-squared (ηp2 = .28). The tickles associated with externally

generated movements (M = 45.35, SE = 4.51) were consistently rated as stronger than

those associated with self-generated movements (M = 39.39, SE = 4.86). No other main

effects or interactions achieved significance. Together, this implies that the tickle effect is not

abolished nor significantly changed by manipulation of first-person perspective, by limb type,

or by the difference between synchronous and asynchronous touch.

8.2 Illusion ratings

A 2 (perspective: participant vs. experimenter) x 2 (movement type: self-generated vs.

externally generated) x 3 (limb type: real vs. baseball bat vs. none) x 2 (group: 1 vs. 2)

mixed-model ANOVA on subjective ratings of how strong the body transfer illusion was in

each condition revealed a significant main effect of perspective [F(1,21) = 46.92, p < .001],

with a large effect size (ηp2 = .69). Not surprisingly, the illusion was consistently rated as

stronger when subjects viewed the scene from the experimenter’s perspective (M = 48.07,

SE = 5.33) as compared to when they viewed the scene from the participant’s perspective

(M = 14.87, SE = 3.43). The main effect of limb type was also significant [F(2,42) = 16.11, p

< .001], with a large effect size (ηp2 = .43). Bonferroni-corrected post-hoc comparisons

established that the illusion associated with the real hand (M = 39.58, SE = 3.66) was

consistently rated as stronger than the illusion associated with a bat (M = 27.14, SE = 3.82;

p < .05) and the illusion associated with an empty space (M = 27.68, SE = 4.57; p < .05). No

other post-hoc comparisons were significant.

The interaction between perspective and limb type achieved significance [F(1.37,28.77) =

5.19, p = .01], with a large effect size (ηp2 = .20). A Fisher’s LSD revealed that the illusion

was consistently rated as stronger when participants viewed the scene from the

experimenter’s perspective and saw a real hand (M = 60.51, SE = 5.09) relative to all other

18  

 

conditions (i.e., experimenter’s perspective with baseball bat [M = 43.19, SE = 5.93; p < .05],

experimenter’s perspective with empty space [M = 40.50, SE = 6.41; p < .05], participant’s

perspective with real hand [M = 18.66, SE = 4.35; p < .05], participant’s perspective with

baseball bat [M = 11.10, SE = 2.97; p < .05], participant’s perspective with empty space [M =

14.86, SE = 3.99; p < .05). The post-hoc comparisons also revealed that the illusion was

consistently rated as stronger when participants viewed the scene from the experimenter’s

perspective and saw a baseball bat relative to the participant’s perspective with real hand (p

< .05), participant’s perspective with baseball bat (p < .05), and participant’s perspective with

empty space (p < .05) conditions. Finally, the Fisher’s showed that the illusion was rated as

stronger when participants viewed the scene from the experimenter’s perspective and saw

an empty space relative to the participant’s perspective with real hand (p < .05), participant’s

perspective with baseball bat (p < .05), and participant’s perspective with empty space (p <

.05) conditions. None of the other comparisons achieved significance. This suggests that the

body transfer illusion was not rated significantly differently depending on whether the seen

and felt touch in the body transfer illusion conditions in Group 1 and Group 2 were in

synchrony or not. Further, it means the body transfer illusion was rated more strongly when

experienced for a hand than for a non-hand, or nothing.

8.3 Questionnaire

A 3 (statements: S1-S3 vs. S4-S7 vs. S8-S9) x 2 (perspective: participant vs. experimenter)

x 2 (movement type: self-generated vs. externally generated) x 3 (limb type: real vs. baseball

bat vs. none) x 2 (group: 1 vs. 2) mixed-model ANOVA on mean responses revealed a

significant three-way interaction between statements, perspective and group [F(2,42) = 5.95,

p = .005], with a large effect size (ηp2 = .22). A Fisher’s LSD revealed that responses to S1-

S3 were greater in the participant’s perspective condition of Group 1 (M = 89.48, SE = 4.94),

which had synchronous touch, than in any other condition except for the S1-S3 participant’s

perspective condition of Group 2 (M = 74.53, SE = 5.16; see Figure 3), which had

19  

 

asynchronous touch. As expected the rubber hand illusion statements were rated higher for

touch when there was no body transfer illusion. A significant pattern emerged in the

responses to S1-S3 between the experimenter’s perspective in Group 2 (M = 58.31, SE =

7.05) and the experimenter’s perspective in Group 1 (M = 35.45, SE = 6.75), with

participant’s responses to S1-S3 being significantly larger in Group 2 than in Group 1. This

suggests that a rubber hand illusion was experienced even when experiencing the body

transfer illusion, such that visuotactile stimuli coinciding spatiotemporally result in a stronger

rubber hand illusion during body transfer illusion. The pattern for S4-S7 differed from the

pattern exhibited by S1-S3. S4-S7 was rated higher in the experimenter’s perspective of

Group 2 (M = 38.27, SE = 6.51) relative to the participant’s perspective of Group 1 (M =

11.55, SE = 4.09). This suggests that, as expected, the body transfer illusion was

experienced but, importantly, and contrary to expectation, that the body transfer illusion was

not strengthened by synchrony, or weakened by asynchrony; a finding congruous with the

ratings of illusion strength outlined in the previous section. Finally, responses to the two

control questions S8-S9 were similar across conditions, which implies that the manipulations

discriminated between statements in the expected manner (a comprehensive outline of other

significant “Questionnaire” results is listed in Appendix A).

20  

 

Figure 3. Averaged questionnaire responses across conditions. Error bars (1 + SE) are

shown (Note: G1 = Group 1, G2 = Group 2; S1-S3 = Statements 1-3, S4-S7 = Statements 4-7, S8-S9 = Statements 8-9).

9. Discussion

The tickle effect refers to our inability to tickle ourselves. The main issue explored in this

study was whether the tickle effect is influenced by changes in context, here in body image

and spatiotemporal touch characteristics. Remarkably, the tickle effect is not abolished by

the dramatic changes in context induced by the body transfer illusion and rubber hand

illusion. Even as participants shift their first-person perspective to someone else’s, or

experience having a baseball bat as a hand, or an invisible hand, there is no change in the

characteristic pattern of feeling less tickle sensation when producing the touch themselves,

and more tickle sensation when the touch is produced by someone else. Clearly, in asserting

that the self-tickle effect is context insensitive we are accepting the null hypothesis of no

interaction with (illusory) context. However, given the efficiency or sensitivity of our design to

detect the main effects reported (i.e., the tickle effect), we can conclude that the interaction

(context sensitive) effects are very small in relation to the (context invariant) main effects.

21  

 

This supports increasingly influential theories based on prediction error minimisation (cf.

the active inference account; Friston, 2010). On this approach, action is thought to require

withdrawal or suspension of attention from the sensory consequences of movement in order

to permit fulfilment of proprioceptive predictions via classical reflex arcs – rather than

through updating of the inferred state of the (motionless) body (Brown et al., 2011, 2013).

Wherever there is agency there must be attenuation (i.e., down-weighting of interoceptive

input), irrespective, as we find, of more transient, contextual attributes such as those

involved in body image or visuotactile synchrony. This result is also in line with previous

findings of non-specific gating of the sensory consequences of action, albeit without appeal

to mechanisms pertaining to task relevance. In contrast, it speaks against theories that

explain the tickle effect solely in terms of the size of prediction error in self-generated

movement (cf. Blakemore, 1999).

As part of this result, we did not find any effect of visuotactile asynchrony on tickle

attenuation. Blakemore et al. (1999) found that when the tickle sensation was delayed

relative to the movement made to generate this sensation, the attenuation disappeared. This

part of the Blakemore et al. study is consistent with active inference and our results since

active inference predicts attenuation of sensory input only as a movement unfolds, not after

movement. In our study, and in contrast to Blakemore et al. (1999), we delayed the visual

input instead of the tickle sensation; in this situation the active inference account does

predict attenuation – which is what we found. Blakemore et al. also found that trajectory

perturbation abolishes the tickle effect. In our study, a tickle effect was still observed even

though the trajectory of the felt movement was opposite (by 180 degrees) to the seen

movement. These findings are more difficult to reconcile. Notice that a direct comparison is

made complicated by the fact that Blakemore et al. perturbed the trajectory between the

kinaesthetic and tactile inputs, whereas we perturbed the trajectory between the visual and

felt inputs (maintaining congruence between kinaesthetic and tactile input). It does remain a

question why Blakemore et al. did not find attenuation of self-tickle in perturbed trajectory

22  

 

conditions. One interesting possibility is that intrahaptic incongruence (cf., Van Doorn,

Hohwy, Symmons 2012), between kinaesthetic and tactile input, undermines the attempt to

withdraw attention from proprioceptive input and thus interferes with action initiation, as

conceived under active inference. This would predict longer response latencies and also

decreased confidence that self-generated action is truly self-generated, potentially leading to

experiences akin to delusions of alien control (cf. Brown et al., 2013). This raises the

interesting possibility that the tickle effect, along with other more subtle elements of general

sensory attenuation, may be one marker that assists in distinguishing self from other; in this

sense it would be a key element of our self-awareness.

The body transfer illusion itself was rated as stronger for a real, if foreign-looking, hand

than for either a fake body part or an empty space occupying the position of the hand. This is

consistent with preference for body-like objects in previous findings in rubber hand illusion

experiments (Hohwy & Paton, 2010; Tsakiris et al., 2010; Tsakiris & Haggard, 2005).

Interestingly, although participants rated the magnitude of the illusion as less intense in the

baseball bat and empty space conditions viewed from the experimenter’s perspective,

relative to viewing a real hand from this perspective, they felt tickle (at least to some degree)

on the baseball bat and at the empty space; and the self-generated attenuation of tickle was

not abolished even in that highly unusual context.

The study also explored further characteristics of the body transfer illusion. Most

interestingly, a version of the rubber hand illusion appears to be experienced during the

body transfer illusion, where the relevant questionnaire items were given a higher score

during synchronous touch relative to asynchronous touch. The rubber hand illusion can thus

arise when one is occupying another person’s first-person perspective, and observing

oneself from “the outside” (namely during the body transfer illusion). In contrast, there was

no significant difference in the scores on the rubber hand illusion-relevant questionnaire

items between synchronous and asynchronous manipulation in the participant’s perspective

condition, suggesting that, unexpectedly, no rubber hand illusion was experienced in that

23  

 

condition. However, the relevant questionnaire items were rated much higher in both the

synchronous and asynchronous conditions in the participant’s perspective than in any other

condition. This may potentially be explained by the overall congruency of limb position in

personal space and the knowledge that the hand one is seeing is in fact one’s own. This

knowledge may make the disambiguating signal from the synchrony less important for

sensory integration (see Maselli & Slater, 2013 for discussion relevant to such scenarios).

Responses to questionnaire items S4-S7, which relate to the body transfer illusion

experience, were higher during the synchronous body transfer illusion condition than during

the synchronous non-body transfer illusion condition. This is evidence that, as expected, the

body transfer illusion was in fact experienced: participants were inclined to experience the

situation in terms of a body transfer experience. Interestingly, whereas the rubber hand

illusion within the body transfer illusion was modulated by synchronous or asynchronous

touch, the body transfer illusion itself did not seem either strengthened by synchrony, or

weakened by asynchrony. The body transfer illusion did however seem to be weakened

when the illusory limb was not hand-like. This kind of complicated relation between

spatiotemporal overlap of the perceived and actual limbs, limb type congruency, and

synchrony is also seen in recent studies of full body ownership illusions (Maselli & Slater,

2013). Similarly, a recent study (Rognini et al., 2013) introduced a delay between self-

generated hand movements and seen hand movements in a crossmodal congruency task

and found that delayed visual information caused people to feel less like they embodied, or

controlled the movements of, virtual hands. This result was interpreted in terms of how

asynchronous visuotactile input may have led to uncertainty, and that this uncertainty

suggested to participants that the virtual hand was not their own. In our experiment, illusory

body ownership (i.e., the body transfer illusion) was not modulated by synchronous or

asynchronous information, but seemed instead to be modulated by limb type. It may be that

the highly unusual body transfer illusion-experience, where one simultaneously observes

both one’s own and the experimenter’s body, introduces differences in how uncertainty is

24  

 

processed. Again, there seems to be a complex interplay here suggesting that sensory

integration exploits cues in a manner sensitive to their potential for disambiguating confusing

input.

No effect of movement type (self- or externally-generated touch) was found within the

rubber hand illusion component, nor within the body transfer illusion component. In this way,

self-generated movement did not make it more difficult to create the illusory aspects

associated with the experimental set-up. This is consistent with previous findings (Tsakiris,

Prabhu, & Haggard, 2006; White, Aimola Davies, & Halleen, 2010) showing the rubber hand

illusion, and related illusions, can be experienced under self-generated movement, though

self-generated movement may give rise to a more integrated body image, as revealed in

proprioceptive drift measures, i.e., a displacement in the perceived position of a person’s

hand towards a rubber hand (Rohde, Di Luca, & Ernst, 2011; Tsakiris & Haggard, 2005).

10. Conclusion

We asked “can you tickle yourself if you swap bodies with someone else?” The short answer

is “no”. We show that the attenuation of self-generated tickle sensations is remarkably robust

across contexts that introduce various kinds of surprise and diminished predictability. This is

consistent with findings of non-specific sensory gating during self-generated movement and

supports the active inference hypothesis. This is remarkable because it suggests that body

image and first-person perspective play no role in sensory attenuation, and thus may be

independent of key cognitive processes involved in shaping self-awareness.

11. Acknowledgements

This research was supported by a Monash University Small Grant Research Support

Scheme grant (GV), a Monash University Emerging Research Excellence Fellowship (GV),

an Australian Research Council Future Fellowship (JH), and a Monash University Research

25  

 

Accelerator grant (MS). We wish to thank the editor and anonymous reviewers for numerous

helpful suggestions.

12. References

Adams, R., Shipp, S., & Friston, K. (2012). Predictions not commands: Active inference in the motor system. Brain Structure and Function, 6, 1-33.

Blakemore, S-J., Frith, C. D., & Wolpert, D. M. (1999). Spatio-temporal prediction modulates the perception of self-produced stimuli. Journal of Cognitive Neuroscience, 11, 551-559.

Blakemore, S-J., Goodbody, S. J., & Wolpert, D. M. (1998). Predicting the consequences of our own actions: The role of sensorimotor context estimation. The Journal of Neuroscience, 18, 7511-7518.

Blakemore, S-J., Oakley, D. A., & Frith, C. D. (2002). Delusions of alien control in the normal brain. Neuropsychologia, 41, 1058-1067.

Blakemore, S-J., Rees, G., & Frith, C. D. (1998). How do we predict the consequences of our actions? A functional imaging study. Neuropsychologia, 36, 521-529.

Blakemore, S-J., Wolpert, D. M., & Frith, C. D. (1998). Central cancellation of self-produced tickle sensation. Nature Neuroscience, 1, 635-640.

Blakemore, S-J., Wolpert, D., & Frith, C. (2000). Why can't you tickle yourself? NeuroReport, 11, 11-16.

Botvinick, M., & Cohen, J. (1998). Rubber hands ‘feel’ touch that eyes see. Nature, 391, 756-756.

Brown, H., Friston, K. J., & Bestmann, S. (2011). Active inference, attention and motor preparation. Frontiers in Psychology, 2, 218.

Brown, H., Adams, R., Parees, I., Edwards, M., & Friston, K. (2013). Active inference, sensory attenuation, and illusions. Cognitive Processing. doi: 10.1007/s10339-013-0571-3.

Claxton, G. (1975). Why can’t we tickle ourselves? Perceptual and Motor Skills, 41, 335-338. Friston, K. (2010). The free-energy principle: A unified brain theory? Nature Reviews

Neuroscience, 11, 127-138. Gallace, A., Zeeden, S., Röder, B., & Spence, C. (2010). Lost in the move? Secondary task

performance impairs tactile change detection on the body. Consciousness and Cognition, 19, 215-229.

Gibson, J. J. (1962). Observations on active touch. Psychological Review, 69, 477-490. Grezes, J., & Decety, J. (2001). Functional anatomy of execution, mental simulation,

observation, and verb generation of actions: A meta-analysis. Human Brain Mapping, 12, 1-19.

Hecht, D., Reiner, M., & Karni, A. (2008). Enhancement of response times to bi- and tri-modal sensory stimuli during active movements. Experimental Brain Research, 185, 655-665.

Hohwy, J., & Paton, B. (2010). Explaining away the body: Experiences of supernaturally caused touch and touch on non-hand objects within the rubber hand illusion. PLoS ONE, 5, e9416.

Juravle, G., & Spence, C. (2011). Juggling reveals a decisional component to tactile suppression. Experimental Brain Research, 213, 87-97.

Longo, M. R., Schüür, F., Kammers, M. P., Tsakiris, M., & Haggard, P. (2008). What is embodiment? A psychometric approach. Cognition, 107, 978-998.

Maselli, A., & Slater, M. (2013). The building blocks of the full body ownership illusion. Frontiers in Human Neuroscience, 7, 83.

26  

 

Petkova, V. I., & Ehrsson, H. H. (2008). If I were you: Perceptual illusion of body swapping. PLoS ONE, 3, e3832.

Poulet, J. F., & Hedwig, B. (2002). A corollary discharge maintains auditory sensitivity during sound production. Nature, 418, 872-876.

Rohde, M., Di Luca, M., & Ernst, M. O. (2011). The rubber hand illusion: Feeling of ownership and proprioceptive drift do not go hand in hand. PLoS ONE, 6, e21659.

Rognini, G., Sengul, A., Aspell, J. E., Salomon, R., Bleuler, H., & Blanke, O. (2013). Visuo-tactile integration and body ownership during self-generated action. European Journal of Neuroscience, 37, 1-10.

Seki, K., Perlmutter, S. I., & Fetz, E. E. (2003). Sensory input to the primate spinal cord is presynaptically inhibited during voluntary movement. Nature Neuroscience, 6, 1309-1316.

Symmons, M. A., Richardson, B. L., & Wuillemin, D. B. (2008). Components of haptic information: Skin rivals kinaesthesis. Perception, 37, 1596-1604.

Tsakiris, M., Carpenter, L., James, D., & Fotopoulou, A. (2010). Hands only illusion: Multisensory integration elicits sense of ownership for body parts but not for non-corporeal objects. Experimental Brain Research, 204, 343-352.

Tsakiris, M., & Haggard, P. (2005). The rubber hand illusion revisited: Visuotactile integration and self-attribution. Journal of Experimental Psychology: Human Perception and Performance, 31, 80-91.

Tsakiris, M., Prabhu, G., & Haggard, P. (2006). Having a body versus moving your body: How agency structures body-ownership. Consciousness and Cognition, 15, 423-432.

Van Doorn, G., Hohwy, J., & Symmons, M. A. (2012). Capture of kinesthesis by a competing cutaneous input. Attention, Perception, & Psychophysics, 74, 1539-1551.

Voss, M., Ingram, J. N., Haggard, P., & Wolpert, D. M. (2006). Sensorimotor attenuation by central motor command signals in the absence of movement. Nature Neuroscience, 9, 26-27.

White, R. C., Aimola Davies, A. M., Halleen, T. J., & Davies, M. (2010). Tactile expectations and the perception of self-touch: An investigation using the rubber hand paradigm. Consciousness and Cognition, 19, 505-519.

Wurtz, R. H. (2008). Neuronal mechanisms of visual stability. Vision Research, 48, 2070-2089.

13. Appendix

The 3 (statements: S1-S3 vs. S4-S7 vs. S8-S9) x 2 (perspective: participant vs.

experimenter) x 2 (movement type: self-generated vs. externally generated) x 3 (limb type:

real vs. baseball bat vs. none) x 2 (group: 1 vs. 2) mixed-model ANOVA on mean responses

revealed a significant main effect of statements [F(2,42) = 75.09, p < .001], with a large

effect size (ηp2 = .78). Bonferroni-corrected post-hoc tests revealed that the averaged

responses to S1-S3 (M = 64.44, SE = 2.88) were different from the averaged responses to

S4-S7 (M = 23.99, SE = 3.24; p < .05) and the averaged responses to S8-S9 (M = 36.15, SE

= 3.87; p < .05); the averaged responses to S4-S7 were also significantly lower than the

averaged responses to S8-9 (p < .05).

27  

 

The main effect of perspective was significant [F(1,21) = 5.10, p = .035], with a large

effect size (ηp2 = .20). Responses to questions were consistently greater when participants

viewed the scene from their perspective (M = 44.49, SE = 1.91) relative to when they viewed

it from the experimenter’s perspective (M = 38.56, SE = 3.83).

The main effect of the movement type factor was not significant (externally generated: M

= 42.10, SE = 2.92; self-generated: M = 40.95, SE = 2.60; p > .05), but the main effect of

limb type was significant [F(2,42) = 11.00, p < .001], with a large effect size (ηp2 = .34).

Bonferroni-corrected post-hoc comparisons established that the averaged responses to

questions were larger with the real hand (M = 44.95, SE = 3.02) than those associated with

either a bat (M = 39.79, SE = 2.58; p < .05) or an empty space (M = 39.84, SE = 2.84; p <

.05). The between-subjects main effect of group failed to achieve significance (group 1: M =

40.35, SE = 3.77; group 2: M = 42.71, SE = 3.94; p > .05).

The interaction between group and perspective achieved significance [F(1,21) = 6.80, p =

.016], with a large effect size (ηp2 = .25). A Fisher’s LSD revealed that mean responses were

consistently lower when participants viewed the scene from the experimenter’s perspective

in Group 1 (M = 33.96, SE = 5.30) relative to when they viewed the scene from the

participant’s own perspective in Group 1 (M = 46.74, SE = 2.64; p < .05). No other

comparisons were significant.

The interaction between statements and perspective achieved significance [F(2,42) =

38.93, p < .001], with a large effect size (ηp2 = .65). A Fisher’s LSD revealed that responses

to S1-S3 were significantly higher in the non-body transfer illusion (i.e., participant’s

perspective) condition (M = 82.00, SE = 3.58) than they were in the body transfer illusion

(i.e., experimenter’s perspective) condition (M = 46.88, SE = 4.88; p < .05), and were higher

than answers to S4-S7 and S8-9 in both body transfer illusion (S4-S7: M = 32.80, SE = 4.51;

S8-9: M = 36.01, SE = 4.01; p < .05) and non-body transfer illusion conditions (S4-S7: M =

15.18, SE = 2.96; S8-9: M = 36.29, SE = 3.83; p < .05). The post-hoc tests also revealed

that responses to S1-S3 in the body transfer illusion condition were significantly higher than

28  

 

answers to S4-S7 in the non-body transfer illusion condition (p < .05). No other comparisons

were significant.

The interaction between perspective and limb type was significant [F(1.41,29.62) = 10.17,

p = .001], with a large effect size (ηp2 = .33). A Fisher’s LSD revealed that mean responses

were significantly higher when participants viewed the scene from the experimenter’s

perspective and saw a real hand (M = 45.02, SE = 4.06) relative to when they viewed the

scene from the experimenter’s perspective and saw a baseball bat (M = 35.81, SE = 3.75; p

< .05), or when they viewed the scene from the experimenter’s perspective and saw an

empty space (M = 34.85, SE = 4.23; p < .05). The post-hocs tests also revealed that the

mean responses were consistently lower when participants viewed the scene from the

experimenter’s perspective and saw a baseball bat relative to when they viewed the scene

from the participant’s perspective and saw a real hand (M = 44.87, SE = 2.47; p < .05), and

when they viewed the scene from the participant’s perspective and saw an empty space (M

= 44.84, SE = 1.90; p < .05). Finally, mean responses were lower when participants viewed

the scene from the experimenter’s perspective and saw an empty space relative to when

they viewed the scene from any of the participant’s perspective conditions (i.e., first-person

perspective with real hand, first-person perspective with baseball bat (M = 43.77, SE = 1.68),

first-person perspective with empty space; p < .05).

The interaction between statements and movement type was significant [F(1.55,32.62) =

6.84, p = .006], with a large effect size (ηp2 = .25). A Fisher’s LSD revealed that responses to

S1-S3 were significantly higher in the self-generated movement condition (M = 65.72, SE =

3.05) than answers to S4-S7 and S8-9 in both self-generated (S4-S7: M = 21.99, SE = 3.01;

S8-9: M = 35.13, SE = 3.82; p < .05) and externally generated conditions (S4-S7: M = 25.99,

SE = 3.65; S8-9: M = 37.17, SE = 3.99; p < .05). The Fisher’s also revealed that responses

to S1-S3 were significantly higher in the externally generated condition (M = 63.16, SE =

2.88) than answers to S4-S7 and S8-S9 in both self- (p < .05) and externally-generated

conditions (p < .05). The post-hoc tests also revealed that responses to S4-S7 in the self-

29  

 

generated condition were significantly smaller than answers to S8-S9 in both self- (p < .05)

and externally-generated conditions (p < .05). Finally, the Fisher’s revealed that responses

to S4-S7 in the externally generated movement condition were significantly smaller than

answers to S8-S9 in both the externally generated (p < .05) and self-generated movement

conditions (p < .05). No other comparisons were significant.

The three-way interaction between questions, movement type and group was significant

[F(2,42) = 7.58, p = .002], with a large effect size (ηp2 = .27). The interaction between the

questions and limb type variables was significant [F(2.82,59.17) = 5.67, p = .002], with a

large effect size (ηp2 = .21). The three-way interaction between questions, limb type and

group was significant [F(4,84) = 3.14, p = .018], with a medium effect size (ηp2 = .13). The

three-way interaction between the questions, perspective and limb type variables was

significant [F(4,84) = 4.05, p = .005], with a large effect size (ηp2 = .16). The three-way

interaction between the questions, movement type and limb type variables was significant

[F(4,84) = 4.40, p = .003], with a large effect size (ηp2 = .17).

The four-way interaction between the questions, movement type, limb type and group

variables was significant [F(4,84) = 3.27, p = .015], with a medium effect size (ηp2 = .14). The

four-way interaction between the questionnaire responses, movement type, limb type and

perspective variables was significant [F(4,84) = 3.32, p = .014], with a medium effect size

(ηp2 = .14). None of the other interactions achieved significance.