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Exp Brain Res (2015) 233:375–383DOI 10.1007/s00221-014-4121-8

RESEARCH ARTICLE

Illusory movements induced by tendon vibration in right- and left-handed people

Emmanuele Tidoni · Gabriele Fusco · Daniele Leonardis · Antonio Frisoli · Massimo Bergamasco · Salvatore Maria Aglioti

Received: 26 June 2014 / Accepted: 29 September 2014 / Published online: 8 October 2014 © Springer-Verlag Berlin Heidelberg 2014

or spinal damage and developing prosthetics and virtual embodiment systems.

Keywords Tendon vibration · Kinesthetic illusion · Handedness · Body illusion · Proprioception

Introduction

Vibrating the peripheral tendons in a physically immobile limb increases the firing rate of the muscle spindles, which alters the normal processing of proprioceptive information. When a vibration of approximately 70–100 Hz is applied to a tendon of the biceps or triceps muscle of a limb that is obstructed from view, a sensation of arm displacement is generated (Goodwin et al. 1972; Gilhodes et al. 1986; Lackner 1988; Naito et al. 1999). Notably, the illusory movement of the arm can be perceived as either flexion or extension, depending on the directional stretching of the vibrated muscle. Interestingly, the firing frequency of mus-cle spindles has been known to increase with movement velocity (Grill and Hallett 1995; Ribot-Ciscar et al. 2002). Similarly, increasing the vibration frequency increases the velocity of the perceived illusory movement (Roll and Vedel 1982).

When the vibratory stimulation is interrupted the spin-dle’s discharge decreases, inducing the perception that the limb is returning toward its original position. This post-vibratory illusory displacement named “aftereffect” is partly due to the adaptation of muscle spindles that recover their resting activity ~30–40 s after vibratory stimulation (Ribot-Ciscar et al. 1998; Seizova-Cajic and Azzi 2010).

Recently, tendon vibration (TV) has been combined with virtual visual feedback to improve the control of a virtual body through brain computer interface (BCI; Leonardis

Abstract Frequency-specific vibratory stimulation of peripheral tendons induces an illusion of limb movement that may be useful for restoring proprioceptive informa-tion in people with sensorimotor disability. This potential application may be limited by inter- and intra-subject vari-ability in the susceptibility to such an illusion, which may depend on a variety of factors. To explore the influence of stimulation parameters and participants’ handedness on the movement illusion, we vibrated the right and left tendon of the biceps brachii in a group of right- and left-handed peo-ple with five stimulation frequencies (from 40 to 120 Hz in step of 20 Hz). We found that all participants reported the expected illusion of elbow extension, especially after 40 and 60 Hz. Left-handers exhibited less variability in report-ing the illusion compared to right-handers across the dif-ferent stimulation frequencies. Moreover, the stimulation of the non-dominant arm elicited a more vivid illusion with faster onset relative to the stimulation of the dominant arm, an effect that was independent from participants’ handed-ness. Overall, our data show that stimulation frequency, handedness and arm dominance influence the tendon vibra-tion movement illusion. The results are discussed in refer-ence to their relevance in linking motor awareness, improv-ing current devices for motor ability recovery after brain

E. Tidoni · G. Fusco · S. M. Aglioti IRCCS Fondazione Santa Lucia, Rome, Italy

E. Tidoni (*) · G. Fusco · S. M. Aglioti Dipartimento di Psicologia, Università di Roma La Sapienza, Rome, Italye-mail: emmanuele.tidoni@uniroma1.it

D. Leonardis · A. Frisoli · M. Bergamasco PERCRO (Perceptual Robotics) Laboratory, Scuola Superiore Sant’Anna, Pisa, Italy

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et al. 2012) and for hand rehabilitation in stroke patients (Rinderknecht et al. 2013). Virtual reality (VR) is a use-ful and valid system that can be used for physical therapy (Cameirão et al. 2012; Sveistrup 2004; Turolla et al. 2013a, b) and its combination with TV requires the synchroniza-tion of the illusory movement with the virtual visual feed-back. For example, if the perceptive qualities (e.g., vivid-ness and latency) of an illusion are strongly mismatched with the visual feedback, an illusory movement may occur while the observed virtual limb remains stationary, creating a temporal delay that can limit the efficacy or acceptance of the therapy.

Although numerous studies have been conducted on this topic, susceptibility to bodily illusions remains a contro-versial issue. Inter- and intra-subject variability highlights individual inconsistencies in the ability to perceive such bodily experiences. For example, having an expert knowl-edge of one’s own body, obtained, for example, by practic-ing yoga, does not modulate a person’s ability to embody a rubber hand (Botvinick and Cohen 1998; David et al. 2013), while handedness and interoceptive skills do (Ock-lenburg et al. 2011; Tsakiris et al. 2011).

Sensitivity to illusory movement is influenced by hand-edness, and it has been shown that position sense alteration following TV might be a function of the dominance of the limb (Adamo and Martin 2009). Asymmetries associated with handedness during proprioceptive-based tasks (Adamo et al. 2012; Goble et al. 2009) reveal that the dominant arm has an advantage in controlling limb dynamics (Sain-burg and Kalakanis 2000; Sainburg 2002). Similarly, brain imaging studies conducted on healthy participants have shown that vibration-induced stimulation of proprioceptive afferents activates the controlateral motor cortex and right fronto-parietal areas (Naito et al. 1999, 2002; Casini et al. 2006; Forner-Cordero et al. 2008; Cignetti et al. 2014).

Importantly, the ability to imagine motor actions (motor imagery, MI) correlates with the perceived movement illu-sion (Kitada et al. 2002; Shibata and Kaneko 2013), con-firming that functional activation during motor imagery involves neural regions at least partly activated during motor execution and illusory movement perception (Ste-phan et al. 1995; Porro et al. 1996; Schnitzler et al. 1997; Naito et al. 1999, 2002; Naito and Ehrsson 2001; McCor-mick et al. 2007; Keinrath et al. 2006).

The principal aim of the present study was to evaluate whether dominant arm preference affected the subjective features of illusory movement perception in right- and left-handers by comparing different stimulation frequencies using a force-based stimulator device. We measured the perceptive qualities of the illusory movement for each fre-quency, and we assessed the onset of the illusion and the resulting aftereffect through the frequency that best elicited the illusion.

Materials and methods

Participants

Twenty participants were pre-screened for right- and left-hand preference by means of the 10-point Edinburgh Handedness Inventory (Oldfield 1971). This allowed us to calculate a Hand Preference Index (HPI) ranging from −100 to +100 (10 right-handed (Rh), 4 males–6 females, mean ± s.e.m, age 27.5 ± 3.24, HPI > 20, 72.00 ± 6.29; 10 left-handed (Lh), 7 males–3 females, age 26.2 ± 3.91, HPI < −20, −60.00 ± 7.60). All participants had normal or corrected-to-normal visual acuity in both eyes and were naïve to the purposes of the study. The experimental proto-col was approved by the ethics committee of the Fondazi-one Santa Lucia and was carried out in accordance with the ethical standards of the 1964 Declaration of Helsinki. All participants gave their written informed consent to take part in the study. No discomfort or adverse effects during TV were reported or noticed.

Motor imagery assessment

Before the first experimental session, participants per-formed a motor imagery assessment with the Vividness of Motor Imagery Questionnaire (VMIQ; Isaac et al. 1986). The VMIQ is a 24-item scale consisting of movements that the subject is requested to imagine. Participants closed their eyes and were asked to indicate the vividness of the imagined movement on a 5-point Likert scale (1 = excel-lent imagination of the movement performance as lively as actual performance, 5 = no image at all) following two statements (in counterbalanced order across subjects): (1) “Imagine in a first-person perspective, the kinesthetic sen-sation associated with the given movement.” (kinesthetic subscale) and (2) “Imagine that someone else is executing the movements.” (visual subscale). Questionnaire scores ranged from 24 to 120, with 24 representing the highest vividness score. That is, the lower the score, the more vivid the imagery. To avoid order effects, the items were deliv-ered randomly.

Proprioceptive stimulator

Proprioceptive stimulation was provided through a device specifically designed to evoke an illusory extension of the forearm by TV (Leonardis et al. 2012). The device was composed of an electromagnetic linear actuator and fixed supports. The actuator had adjustable mountings that allowed us to set the position and orientation of the par-ticipants’ arm before the beginning of the experiment. A hemispherical tactor (15 mm diameter), mounted at the tip of the actuated moving shaft, was in contact with the skin.

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The applied force was controlled by embedded electronics and software (XVR media, Carrozzino and Tecchia 2005) running on a host PC. Each vibrating stimulus was gener-ated with a sinusoidal force reference plus a constant offset force of 2.4 N. The offset force was added in order to press the skin tactor perpendicular to the tendons, with constant and known average force.

Procedure and apparatus

Each participant was seated comfortably in a chair and received a brief and demonstrative explanation of the pro-prioceptive stimulator. After the Motor Imagery Assess-ment, we tested each participant’s right and left arm with TV in two separate sessions counterbalanced across sub-jects. Each session was composed of three blocks.

In Block-1, we gently placed the proprioceptive actuator on the participants’ right/left flexor tendon brachii with the elbow angular position of approximately 120° (Fig. 1a). We applied a 70 Hz stimulation with 2.4 N for 30 s (Fig. 1b). We asked participants to pay attention to any change in the physical and sensory perception of their arm. We took care not to influence the subjects about the possible illusory experience they might have. At a certain point, participants reported feeling “as if their elbow was extending”. This procedure helped us to find the optimal position to elicit an illusory sensation related to the proprioceptive stimulation.

In Block-2, we evaluated the illusory movement percep-tion using five different vibration frequencies. We applied different frequencies (from 40 to 120 Hz in steps of 20 Hz) with a constant force amplitude of 2.4 N. Each frequency was repeated 3 times for a total of 15 randomized trials. Each trial lasted ~60 s characterized by 15 s of TV and the

remaining time for questions and rest. Participants were asked to close their eyes and relax their arms. White noise, provided through headphones, started 500 ms before the tactile stimulation and lasted until the end of the stimu-lation itself. This procedure prevented participants’ dis-traction and ruled out any response bias induced by the frequency-specific sound generated by the actuator. After, participants opened their eyes and answered three illusion-related questions (IRQs; vividness, duration and extension; Naito et al. 1999) by choosing numerical values on a 0–100 scale (see Fig. 1c). For each frequency, the mean score of the IRQs was computed and a general index of the move-ment illusion (i-Mov index) was defined. In this way, we were able to identify for each individual the frequency that best elicited the illusory perception.

Data from Block-2 were analyzed using a mixed 2 × 5 × 2 factorial design with group (Rh and Lh) as the between-subject factor, and frequency (40-60-80-100-120 Hz) and stimulated arm (dominant, right and left arm, respectively, in right- and left-handers; non-dominant, right and left arm, respectively, in left- and right-handed) as within-subject factors.

In Block-3, we evaluated the onset of the illusory move-ment and the post-vibration aftereffect. Each participant was stimulated three times for 15 and 30 s, counterbal-anced across participants, at the individual best frequency (see Table 1), which was identified in Block-2. Two micro-phones were used to collect responses: The first recorded the proprioceptive stimulator sound, the second was held by participants in their non-stimulated arm. Participants signaled the onset of the illusion by hitting the microphone on the table. The recordings were saved in a single track in which the delay between the vibration sound onset and the

Fig. 1 a The proprioceptive stimulator was in contact with the tendon biceps brachii of the right/left arm. The elbow angu-lar position was approximately 120°. b Visual description of the events’ sequence during Block-1, Block-2 and Block-3 with c the final set of questions participants answered on a 0–100 scale

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hit indicated the time the illusion took to be noticed (due to technical failure, we lost only the 0.03 % of the trials). This latency was analyzed by means of ANOVA with group as the between factor and arm as the within factor. At the end of each trial, participants, keeping their eyes closed, reported the vividness of the aftereffect using a 0–100 scale (see Fig. 1c).

Data from Block-3 were analyzed using a mixed 2 × 2 × 2 factorial design with group as the between fac-tor, and arm (dominant and non-dominant) and stimulation duration (15 and 30 s) as within factors.

Throughout Block-2 and Block-3, participants were reminded to relax their shoulders, arms and hands. No overt limb movements were observed by the experimenter.

Results

Motor imagery and illusory movement

Block-1

Right- and left-handers were comparable in their motor imagery ability as both the kinesthetic (Rh. 50.90 ± 4.00; Lh, 50.60 ± 3.27; t(18) = 0.058, p = 0.954, r = 0.013) and visual subscale (Rh. 49.80 ± 5.41; Lh, 49.40 ± 4.38; t(18) = 0.057, p = 0.955, r = 0.013) were similar.

Block-2

Each question was analyzed by ANOVA with frequency (40-60-80-100-120 Hz) and arm (dominant and non-dominant) as within factors and group (Rh and Lh) as the between factor effect. All significant main effects and

interactions were analyzed by means of Newman–Keuls Post hoc (refer to Table 2 for raw data of main effects and interactions with comparisons higher than two).

Vividness (Q1)

We found a main effect of frequency (F4,72 = 20.78; p < 0.001; η2 = 0.53). Vividness scores for 40 and 60 Hz did not differ (p = 0.417) and were higher compared to other frequencies (all p < 0.029). Scores for 80 and 120 Hz did not differ (p = 0.488) and were higher relative to 100 Hz (p < 0.001). The interaction between frequency and group was significant (F4,72 = 3.80; p = 0.007; η2 = 0.17; Fig. 2). Post hoc comparisons revealed that Rh’s score at 100 Hz was lower compared to all others scores (all p < 0.02) except for 100 Hz in the Lh group (p = 0.19). Scores at 80 and 120 Hz did not differ (p = 0.754) from one another but were lower relative to Rh-40 Hz and Rh-60 Hz (all p < 0.006). Moreover, Lh’s score at 100 Hz was lower relative to other Lh’s frequencies (all p < 0.012) and also to Rh-40 Hz and Rh-60 Hz (all p < 0.041). Other main effects (group and arm) or interactions were not sig-nificant (all F < 1.55; p > 0.23; all η2 < 0.08).

Duration (Q2)

The main effect of frequency was significant (F4,72 = 20.11; p < 0.001; η2 = 0.52). Post hoc comparisons showed that the illusion was perceived for a shorter amount of time after 100 Hz compared to all other frequencies (all p < 0.001). Moreover, 80 Hz vibration produced a com-parable duration of the illusion to 120 Hz (p = 0.79) but was shorter than the 60 Hz (p = 0.02), which did not differ from 40 Hz (p = 0.37). No other main effect (group and

Table 1 Distribution of the optimal frequency for inducing the illusion for the dominant (D) and non-dominant (nD) arm in right-handers (Rh) and left-handers (Lh)

40 Hz 60 Hz 80 Hz 100 Hz 120 Hz

nD D nD D nD D nD D nD D

Rh 4 3 5 5 0 0 0 1 1 1

Lh 6 2 1 2 2 3 0 0 1 3

Table 2 Raw data (mean ± standard error mean) for the significant main effects and interactions with more than 2 comparisons

IRQ Effect Frequency

40 Hz 60 Hz 80 Hz 100 Hz 120 Hz

Q1 Frequency / 74.00 ± 3.38 71.09 ± 3.35 63.16 ± 4.01 44.68 ± 4.91 60.68 ± 3.72

Q1 Groups × Frequency Rh 75.02 ± 5.06 75.75 ± 4.20 56.62 ± 5.65 39.57 ± 6.57 55.03 ± 4.85

Lh 72.98 ± 4.73 66.43 ± 5.01 69.70 ± 5.14 49.78 ± 7.25 66.32 ± 5.28

Q2 Frequency / 65.85 ± 3.31 68.71 ± 3.02 60.24 ± 4.38 42.80 ± 4.79 59.40 ± 4.02

Q3 Frequency / 61.78 ± 3.07 62.78 ± 3.48 52.23 ± 4.64 33.47 ± 4.81 52.14 ± 4.33

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arm) or interaction was significant (all F < 1.72; p > 0.15; η2 < 0.09).

Extension (Q3)

We found a main effect of group (F1,18 = 5.90; p = 0.025; η2 = 0.24) with higher scores for Lh (59.37 ± 4.21) rela-tive to Rh (45.58 ± 3.80) and a main effect of frequency (F4,72 = 17.13; p < 0.001; η2 = 0.48). Perceived positions did not differ after 40 Hz and 60 Hz (p = 0.80) but were higher relative to other frequencies (all p < 0.049). Scores at 100 Hz were lower compared to 80 Hz and 120 Hz, which did not differ from each other (p = 0.98). No other main effect (arm) or interaction was significant (all F < 2.09; p > 0.09; η2 < 0.10).

Illusory movement at best frequency

We performed an additional ANOVA on the subjective response to the frequency with the highest i-Mov index with arm as the within factor and group as the between fac-tor. All significant main effects and interactions were ana-lyzed by means of Newman–Keuls Post hoc.

Vividness (Q1)

We found a main effect of arm (F1,18 = 13.18; p = 0.002; η2 = 0.42; Fig. 3a) with higher vividness of the illusion in the non-dominant (85.16 ± 2.95) relative to the dominant arm (77.18 ± 2.46). Main effect of group (F1,18 = 0.12; p = 0.72; η2 < 0.01) and interaction (F1,18 = 0.65; p = 0.42; η2 = 0.03) were not significant.

Duration (Q2)

We found no significant main effects or interactions (all F1,18 < 1.51; all p > 0.233; all η2 < 0.07).

Extension (Q3)

We found a nearly significant main effect of group (F1,18 = 3.45; p = 0.079; η2 = 0.16; Rh, 65.58 ± 3.42; Lh, 75.83 ± 4.32) and arm (F1,18 = 3.84; p = 0.065; η2 = 0.17; non-dominant, 74.48 ± 3.79; dominant, 66.93 ± 3.12). The interaction was not significant (F1,18 < 0.01; p = 0.989; η2 < 0.01).

These results expand previous analysis by showing that arm dominance plays a role in how vividly the illusion is perceived. In particular, we observed that participants had higher susceptibility to the movement illusion when the TV occurred in their non-dominant limb. Inter- and intra-indi-vidual variability was highlighted by the fact that the same frequency did not elicit the best illusory perception across participants and arms (Table 1), suggesting that handedness and within-subject differences may affect the different per-ceptual features (vividness, duration and extension) of the illusory movement.

Aftereffect and illusion onset

Aftereffect

The vividness of the aftereffect (Block-3) was analyzed with an ANOVA with group as the between-subject fac-tor, and arm and stimulation duration (15 and 30 s) as within-subject factors. We found neither a main effect (F1,18 < 0.11; p > 0.793; η2 < 0.01) nor an interaction (F1,18 < 0.60; p > 0.446; η2 < 0.03). The aftereffect illusion was generally low (31.68 ± 2.60), and mean scores after 15 s (31.90 ± 3.86) and 30 s (31.45 ± 3.55) stimulation were similar.

Fig. 2 Reports of vividness after vibration of Block-2. Participants reported high illusory sensation of movement after 40 and 60 Hz. Less variability was observed in left-handers relative to right-handers. Bars denote standard errors

Fig. 3 a Block-2 vividness (Q1) mean scores of best frequency vibration. b Onset latency of the illusion indicated by participants during Block-3. Bars denote standard errors

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Onset illusion

Illusion latency (i.e., the mean between 15 and 30 s illu-sion’s onset) was analyzed with group as the between fac-tor and arm as the within factor. We found no main effect of group (F1,18 = 0.07; p = 0.787; η2 < 0.01) with compara-ble illusion’s latency in the Lh (5.70 ± 0.68) and Rh group (6.07 ± 0.77). We also found a nearly significant main effect of arm (F1,18 = 4.10; p = 0.057; η2 = 0.18; Fig. 3b) with shorter latency for the non-dominant (5.27 ± 0.60 s) relative to the dominant (6.51 ± 0.81 s). The interaction was not significant (F1,18 = 0.47; p = 0.497; η2 = 0.02; see Table 3 for raw data).

Finally, we performed two separate correlation analyses between the aftereffect and the illusion onset for the domi-nant and non-dominant arm. Here, we did not replicate data from a previous study showing a relationship between the extension illusion and the aftereffect (Kito et al. 2006). Our results were indeed not significant (dominant: r = −0.12, p = 0.585; non-dominant: r = −0.30, p = 0.197).

Discussion

A better understanding of the illusory features generated by TV can optimize the advancements in the field of virtual and physical technologies for rehabilitation.

Recent studies paired the visualization of virtual move-ments with proprioceptive feedback to improve motor imagery for BCI (Leonardis et al. 2012) and enhance the perception of finger illusory movements (Rinderknecht et al. 2013).

In the present study, we sought to assess the subjective experience of the illusory extension of the upper limbs in left- and right-handed subjects. Both right- and left-hand-ers reported a stronger illusory movement sensation after 40 and 60 Hz compared with other frequencies. We found less variability in left-handers, who showed compara-ble illusion vividness in all frequencies except for 100 Hz vibrations. In contrast, right-handers’ illusory perception during 80 and 120 Hz was higher relative to 100 Hz, but lower relative to 40–60 Hz. Moreover, analysis on the best frequency showed that subjective vividness of the illusion

was higher for the non-dominant arm than for the dominant arm. This is also suggested by the nearly significant differ-ence between the illusion’s onsets. Indeed, non-dominant arm stimulation appeared to require a shorter time window to elicit the illusory perception of movement. Finally, no asymmetries were found between subjects in experiencing the aftereffect illusion. Note that participants were stimu-lated using the frequency specifically tested for the illusory movement’s perception. It is likely that the low aftereffect scores indicate that the frequencies adopted for the vibra-tory stimulation were not effective, hinting that the afteref-fect may be modulated by a different subset of frequencies.

Movement illusion

In our setup, we manipulated the stimulation frequency (from 40 to 120 Hz in steps of 20 Hz), maintaining a fixed force (2.4 N), which was applied to the biceps tendon in both the right and left arm.

Previous studies have shown that arm stimulation at low frequencies (i.e., 2–48 Hz) can induce position errors with no illusory movement, suggesting that position and move-ment sensations can be differently represented within the same muscle (McCloskey 1973; Jones 1988).

Typically, higher frequencies (between 70 and 110 Hz) are used to elicit a vivid illusion of movement (Naito et al. 1999; Feys et al. 2006; Forner-Cordero et al. 2008). In the present study, we observed a more vivid illusory percep-tion at 40 and 60 Hz vibration, while there was a selective decrease in movement illusion at the 100 Hz vibration. This seems to contradict the study by Goodwin et al. (1972). Moreover, Naito et al. (1999) reported that the illusion increases from 10 to 70 Hz and declines with higher fre-quencies, while in our study we found higher illusory per-ception for 80 and 120 Hz but not for 100 Hz.

Differences between devices and experimental setups used for eliciting the illusory movement may help to rec-oncile our current results with those reported in previous studies. It is worth noting, for example, that several stud-ies used actuators based on fixed-displacement vibration. The association between the mechanical energy and the frequency of the stimulus is different if the moving shaft of the skin tactor is applied with constant force or with constant amplitude. In our study, we applied different fre-quencies at constant force, while other studies maintained constant amplitude (Calvin-Figuière et al. 1999; Naito et al. 2002, 2005, 2007). Furthermore, we used a single stimu-lator, different from other studies that used two actuators (Casini et al. 2006, 2008; Fuentes et al. 2012; Shibata and Kaneko 2013). The total mechanical energy applied to the tendons and the adopted frequency may differently modu-late the peripheral and central mechanisms for illusory movement perception. Thus, differences in results might

Table 3 Raw data (mean ± standard error mean) of the illusory movement onset for the dominant and non-dominant arm in right- and left-handers

Arm

Dominant Non-dominant

Rh 6.90 ± 1.21 5.24 ± 0.95

Lh 6.11 ± 1.12 5.30 ± 0.81

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arise depending on whether constant amplitude or constant force criteria are used. This may explain why we found higher illusory sensations during 40–60 Hz relative to the other frequencies.

Interestingly, we found a greater perception of arm extension in left- rather than right-handed subjects. Although this finding is somewhat limited due to the fact that we used subjective reports, this result still hints that there is less lateralization of sensorimotor functions in left-handers relative to right-handers (Kim et al. 1993; Vinger-hoets et al. 2012; Solodkin et al. 2001; Begliomini et al. 2008).

Moreover, perceptual qualities of the illusion may vary within and between people. TV does not produce illusions of movement and arm position in all people. Previous stud-ies screened participants for susceptibility to the illusions (Roll et al. 2009; Thyrion and Roll 2009; Fuentes et al. 2012), while in this study, we were able to induce a move-ment illusion in all participants. Our results are thus not limited to a subset of people whose spindles are sensitive to activation via superficial vibration; rather our data can be used to make improvements in the selection of vibratory parameters for the coupling of virtual and proprioceptive feedback in physical rehabilitation.

Rinderknecht et al. (2013) studied how illusory move-ment of fingers was enhanced in healthy participants by receiving TV, while at the same time, seeing a virtual hand simulating fingers’ extension. In the same vein, Gay et al. (2007) reported how vibratory stimulation of the hand and wrist produced pain relief in patients with complex regional pain syndrome type I (CRPS-I). Specifically, after the stimulation treatment, the experimental group reported an improved range of motion and reduced pain scores relative to the control group, highlighting the potential value of this technique as a new and valid physical therapy.

Aftereffect and onset latencies

Previous studies suggested that proprioceptive sensitivity is influenced by handedness and gender. It has been dem-onstrated, for example, that if right- and left-handers are asked to reproduce a movement elicited by TV, males show a greater movement’s velocity for the dominant relative to the non-dominant arm while females exhibit a symmetric sensitivity to vibration (Adamo et al. 2012).

Interestingly, such asymmetry has been observed not only at the behavioral level but also at the neural one. For example, it has been shown that a preferentially right-sided fronto-parietal circuit is called into play during the illusion (Naito et al. 2007; Cignetti et al. 2014).

In our study, a generalized low vividness characterized the aftereffect with no asymmetry between subjects. The aftereffect represents the perceptual counterpart following

peripheral TV and can be considered an adaptation index of the proprioceptive system (Seizova-Cajic et al. 2007). Considering the rapid and transient nature of the phenom-enon (Goodwin et al. 1972), a vibration of 15 and 30 s could have been too short to evoke a strong aftereffect or to highlight any difference between arms. Moreover, the aftereffect did not correlate with the onset latency although a previous study reported a correlation between illusory movement and aftereffect (Kito et al. 2006). We stimulated participants’ arms using the best frequency determined at the individual level for eliciting the illusion of a downward movement of the forearm. Thus, it is likely that frequen-cies that can elicit the aftereffect may differ from frequen-cies that generate the illusory movement. It is worth noting that previous studies reported that only 60 % of participants show an aftereffect (Seizova-Cajic et al. 2007) and that it mainly occurs after 60 s of stimulation, relative to 10 and 30 s (Kito et al. 2006). Thus, we cannot exclude that a dif-ferent setup, frequency and vibratory duration along with the absence of participants’ preselection could have con-tributed to our results.

Importantly, we found that there was a shorter latency for the illusion after the non-dominant arm was stimulated. In fact, participants tended to perceive the onset earlier in the non-dominant arm than in the dominant one. Descrip-tive analysis in previous studies showed a similar trend (Naito and Ehrsson 2006; Naito et al. 2007), and great variability was observed (from a minimum latency of 0.3 s to a maximum of 6.3 s) when comparing the onset laten-cies, as was reported in several studies (Kitada et al. 2002; Gilhodes et al. 1986; Seizova-Cajic and Azzi 2011; Good-win et al. 1972; Naito et al. 2007; Carr et al. 2013). Our relatively high latency may have been influenced by the absence of a sample preselection according to an early illu-sion’s onset (Naito and Ehrsson 2001). Overall, this result may corroborate previous findings indicating that the domi-nant arm relies more on vision, while the non-dominant arm more on proprioception (Goble and Brown 2008).

Conclusion

In the last thirty years, many studies explored several fea-tures of the TV illusion, such as intensity (Naito et al. 1999; Kitada et al. 2002), temporal duration and resolution (Seizova-Cajic et al. 2007; Fuentes et al. 2012), movement speed (Casini et al. 2008), and direction and movement tra-jectory (Calvin-Figuière et al. 2000; Roll et al. 2009; Thy-rion and Roll 2009). Recent advances in VR technologies tried to pair illusory movements of arms and fingers with congruent visual feedback (Rinderknecht et al. 2013; Leon-ardis et al. 2012) to improve the illusory sensation and BCI performances. We tested the dominant and non-dominant

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arms in both right- and left-handed people. We used a single stimulator, contrary to other studies that used two simultaneous actuators (Casini et al. 2006, 2008; Fuentes et al. 2012; Shibata and Kaneko 2013) and found that 40 and 60 Hz stimulation with 2.4 N amplitude elicited more intense illusions relative to higher stimulation frequencies (from 80 to 120 Hz). Although the present study relied mostly on subjective experiences of the illusion, we believe that parameters related to stimulation frequencies, setup and applied force might be potential factors that influence the vividness, latency and duration of the illusory move-ment. Thus, it is entirely possible that the difference in set-tings may explain why some individuals with functioning muscle spindles have been reported not to perceive any illusion (Cignetti et al. 2014). Moreover, we found that the frequency used to evoke the best illusory movement varies within and between subjects. Thus, future studies should consider whether to manipulate the subjective qualities of the illusion (e.g., speed, vividness, duration and extension) or the vibration parameters (e.g., applied force, frequency and moving shaft amplitude).

Overall our data show that handedness and arm domi-nance influence the TV movement illusion. The results and methodological considerations presented here are relevant for TV studies aimed at exploring important issues ranging from the neural correlates of movement awareness to the use of motor imagery for improving the control of exter-nal devices (Leonardis et al. 2012) and from the recovery of motor capabilities after having a stroke (Rinderknecht et al. 2013) to providing relief from pain (Gay et al. 2007) in addition to developing prosthetics and virtual embodiment systems.

Acknowledgments Financial support from the EU Informa-tion and Communication Technologies Grant (VERE project, FP7-ICT-2009-5, Prot. Num. 257695), the Italian Ministry of Health (RF-2010-2312912).

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