the role of embodied simulation in mental transformation of whole-body images: evidence from...

Human Movement Science xxx (2013) xxx–xxx

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Human Movement Science

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The role of embodied simulation in mentaltransformation of whole-body images:Evidence from Parkinson’s disease

0167-9457/$ - see front matter � 2013 Published by Elsevier B.V.http://dx.doi.org/10.1016/j.humov.2013.10.006

⇑ Corresponding authors at: Neuropsychology Laboratory, Department of Psychology, Second University of NapEllittico 31, 81100 Caserta, Italy. Tel.: +39 0823 275327.

E-mail addresses: [email protected] (M. Conson), [email protected] (G. Santangelo).

Please cite this article in press as: Conson, M., et al. The role of embodied simulation in mental transformwhole-body images: Evidence from Parkinson’s disease. Human Movement Science (2013), http://dx.d10.1016/j.humov.2013.10.006

Massimiliano Conson a,⇑, Luigi Trojano a,b, Carmine Vitale c,d,Elisabetta Mazzarella e, Roberto Allocca f, Paolo Barone c,g, Dario Grossi a,Gabriella Santangelo a,c,⇑a Neuropsychology Laboratory, Department of Psychology, Second University of Naples, Viale Ellittico 31, 81100 Caserta, Italyb Salvatore Maugeri Foundation, IRCCS Institute of Telese Terme (BN), Italyc IDC ‘‘Hermitage-Capodimonte’’, Naples, Italyd University of Naples Parthenope, Naples, Italye Department of Neuromotor Physiology, Scientific Institute Foundation Santa Lucia, Via Ardeatina 306, 00179 Rome, Italyf Department of Neurological Sciences, University Federico II, Naples, Italyg Neurodegenerative Diseases Center, University of Salerno, Salerno, Italy

a r t i c l e i n f o a b s t r a c t

Article history:Available online xxxx

PsycINFO classification:2520 Neuropsychology & Neurology

Keywords:Embodied cognitionAction simulationMental transformationParkinson’s diseaseMental rotationMotor imagery

It has been repeatedly demonstrated that mentally performing anaction and mentally transforming body-parts entail simulation ofone’s own body movements, consistent with predictions of embod-ied cognition theories. However, the involvement of embodied sim-ulation in mental transformation of whole-body images is stilldisputed. Here, we assessed own body transformation in Parkin-son’s disease (PD) patients with symptoms most affecting the leftor the right body side. PD patients were required to perform left–right judgments on front-facing or back-facing human figures, anda letter rotation task. Results demonstrated that PD patients wereselectively impaired in judging the side of back-facing human fig-ures corresponding to their own most affected side, but performedas well as healthy subjects on mental transformation of front-facingbodies and on letter rotation. These findings demonstrate a parallelimpairment between motor and mental simulation mechanisms inPD patients, thus highlighting the specific contribution of embodiedcognition to mental transformation of whole-body images.

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1. Introduction

2 M. Conson et al. / Human Movement Science xxx (2013) xxx–xxx

Classical psychophysical studies showed that when healthy participants have to judge whether ahand image with a specific spatial orientation is left or right (i.e., the hand laterality task) they imagine
their own hand moving to match the stimulus orientation for responding (Parsons, 1987a, 1994;Sekiyama, 1982). Moreover, making hand laterality judgments and executing a hand movement followthe same temporal profile and the same hand-specific joint-constraints (Conson, Mazzarella, &Trojano, 2011; de Lange, Helmich, & Toni, 2006; Parsons, 1987a, 1994; Sekiyama, 1982). Accordingly,specific impairments on the hand laterality task and spared mental transformation of object imageshave been reported in patients with severe motor disorders, such as locked-in syndrome (Conson,Pistoia, Sarà, Grossi, & Trojano, 2010; Conson et al., 2008), amyotrophic lateral sclerosis (Fiori et al.,2013a) or spinal cord injury (Fiori et al., 2013b). These observations consistently support the modelof embodied cognition according to which cognitive processes are grounded in bodily states (Gallese& Sinigaglia, 2011). In this view, the same sensorimotor representations activated when performing anactual action are also involved in different ‘‘action-related phenomena’’ such as motor imagery (i.e.,mental simulation of body-parts movements), action observation and imitation (Decety & Grèzes,2006; Gallese & Sinigaglia, 2011; Jeannerod, 2001).
No clear data are available instead on the role of embodied simulation in mental transformation ofwhole-body images. In a seminal neuroimaging study, Zacks, Rypma, Gabrieli, Tversky, and Glover(1999) presented participants with front-facing or back-facing schematic human figures with one out-stretched arm; in order to judge which arm was outstretched (i.e., left–right judgment), participantsimagined themselves in the position of the figure. This own body transformation led to increased cor-tical activity in the temporo-parietal junction, as well as in other areas including the frontal cortex. Sub-sequent neurofunctional studies employing the same paradigm (Arzy, Mohr, Michel, & Blanke, 2007;Arzy, Thut, Mohr, Michel, & Blanke, 2006; Blanke et al., 2005) confirmed the involvement of the temp-oro-parietal junction in whole-body processing and suggested that whole-body transformations implysome sort of ‘‘rotation of the self’’ (Arzy et al., 2006, 2007; Blanke et al., 2005). This evidence would sug-gest that, analogously to body-parts transformation, mental transformation of whole-body is groundedon embodied cognitive processes (Kessler & Thomson, 2010). However, other studies demonstratedthat whole-body transformation can also be accomplished by resorting to an object-based, visuospatialtransformation not related to one’s own body representation (Gardner, Brazier, Edmonds, & Gronholm,2013; Kessler & Wang, 2012). For instance, Kessler and Wang (2012) reported that healthy individualswith low empathic abilities were more prone to rely on object rotation strategies to solve the own bodytransformation task. These findings would undermine the idea that whole-body transformation isdependent on actual sensorimotor information available in the agent’s brain.

In synthesis, the role of embodied simulation in whole-body transformation is not supported con-sistently. Strong clues on this issue could be provided by a behavioral study on patients with a well-defined damage of the motor system, such as Parkinson’s disease (PD).

Neuropsychological studies investigating mental transformation of body-parts in PD patients bymeans of the hand laterality task reported motor imagery asymmetries: patients mentally simulatedmovements more slowly with their most affected hand (Amick, Schendan, Ganis, & Cronin-Golomb,2006; Dominey, Decety, Broussolle, Chazot, & Jeannerod, 1995). These results were confirmed by re-cent experiments in which PD patients performed the hand laterality task while keeping their arms indifferent postures (Helmich, de Lange, Bloem, & Toni, 2007; van Nuenen et al., 2012). Taken together,available evidence supported the strong relationships between motor disorders and mental transfor-mation of body-parts in PD patients. However, no evidence is available about mental transformation ofwhole-body images in this clinical population.

In the present study we required PD patients to perform own body transformation tasks requiringlaterality judgments on a schematic human figure. If sensorimotor information is causatively involvedin processing of whole-bodies, we can predict that PD patients would be impaired on own body trans-formation task, and that the side of motor impairment would affect behavioral performance, withopposite patterns in patients with left or right most affected side. More precisely, consistent with pre-vious studies on mental transformation of body parts (Helmich et al., 2007; van Nuenen et al., 2012),

Please cite this article in press as: Conson, M., et al. The role of embodied simulation in mental transformation ofwhole-body images: Evidence from Parkinson’s disease. Human Movement Science (2013), http://dx.doi.org/10.1016/j.humov.2013.10.006



M. Conson et al. / Human Movement Science xxx (2013) xxx–xxx 3

we could expect that PD patients are specifically impaired in judging the body side corresponding totheir own most affected side. Relevant here, we also assessed PD patients’ ability to perform mentalrotation of letters that is thought not to involve one’s own body representation (Conson et al., 2008,2010; Dalecki, Hoffmann, & Bock, 2012; Fiori et al., 2013a, 2013b; Kosslyn, Di Girolamo, Thompson,& Alpert, 1998). By these means we could ascertain whether any failure in own body transformationwas associated with a generalized deficit in performing mental transformation tasks.

2. Material and methods

2.1. Participants

We screened consecutive PD patients attending the Movement Disorders Unit of the University ofNaples Federico II (Naples, Italy) from February to June 2012. Patients were enrolled in the study ifthey met the following inclusion criteria: diagnosis of idiopathic PD according to United Kingdom Par-kinson’s Disease Society brain bank (Gibb & Lees, 1988); clinical and history evidence of asymmetricmotor disturbances; lack of PD-associated dementia (PDD) as diagnosed according to an algorithm forclinical diagnosis of PDD recommended by the MDS Task Force (Emre et al., 2007); lack of majordepression according to DSM IV criteria (American Psychiatric Association, APA, 1994).

All PD patients underwent a neurological examination including the Unified Parkinson’s DiseaseRating Scale motor subscale (UPDRS-III; Fahn, Elton, & Members of the UPDRS DevelopmentCommittee, 1987) to evaluate severity of motor symptoms, and Hoehn & Yahr Scale (H&Y; Hoehn &Yahr, 1967) to assess PD stage. Age, level of formal education, age at onset, side of onset of PD, diseaseduration, and type and dosage of pharmacological treatment were recorded; Levodopa equivalent dailydose (LEDD) was calculated (Tomlinson et al., 2010). As a screening for general cognitive impairmentwe used an Italian version of the Mini Mental State Examination (MMSE), and excluded from the studyPD patients with a total age- and educational-adjusted score <23.8, that is the standard cut-off for thenormal range (Folstein, Folstein, & McHugh, 1975; Measso et al., 1993).

Twenty-nine right-handed patients (10 females and 19 males) matched inclusion and exclusion cri-teria. Mean patients’ age was 67.6 ± 7.7 years (range 54–83 years) and mean educational level was10.9 ± 4.9 (range 3–18 years). In the total sample, mean age at onset was 60.7 ± 8.5 (range 44–77 years)and mean disease duration was 7 ± 4.3 years (range 2–21 years), respectively. Mean UPDRS-III scoreswere 14.1 ± 4.9 (range 6–28). H&Y rating scale ranged from Stage I to Stage III. Mean total LEDD was621.5 ± 295.6 mg (range 150–1300). Side of onset of motor symptoms and of most severe motor distur-bances on repeated clinical assessment (most affected side) was the left in 14/29 patients (left-mostaffected PD: 48.3%) and the right in 15/29 (right-most affected PD: 51.7%) patients. No significant dif-ferences between the two groups were found on demographic and clinical features (Table 1). PD pa-tients underwent the experimental tasks (see below) when they were in the ‘‘on’’ phase.

Thirty right-handed healthy subjects (10 females and 20 males; mean age 49.7 years, SD 7.3, range46–83 years; mean education 14.7, SD 2.5, range 11–18 years) took part in the experiment as healthycontrols. Exclusion criteria were the following: (1) diagnosis of PD or any other neurologic or psychi-atric disorder; (2) clinically evident dementia or major depression, both diagnosed by means of DSMIV criteria (APA, 1994); (3) general intellectual impairment, defined by MMSE score below the normalcut-off, as above.

The study was conducted in accordance with the ethical standards of Helsinki Declaration and aninformed consent was obtained from all participants after the nature of the study was fully explainedto them.

2.2. Experimental tasks

Participants underwent the classical own body transformation task (OBT; Arzy et al., 2006; Blankeet al., 2005; Parsons, 1987b; Zacks et al., 1999), requiring left–right judgments on a human imagewhose left or right hand was marked to appear as wearing a black glove. After imagining themselvesto be in the figure’s body position and to have its visual perspective, participants had to judge whether




Table 1Mean (±SD) of demographic and clinical features of PD patients.

Left-most affectedPD (n = 14)

Right-most affectedPD (n = 15)

F/U test P

Age (yr) 62.9 ± 4.7 66 ± 8.6 1.347 .256Education (yr) 10.8 ± 5.8 11 ± 4.8 .015 .904Sex (M/F) 8/6 11/4 .840 .359Age at PD onset (yr) 61.8 ± 8.4 59.7 ± 8.7 .442 .512PD duration (yr) 7.5 ± 5.1 6.6 ± 3.5 .310 .582UPDRS-III score in on-state 12.9 ± 4.1 15.4 ± 5.6 1.767 .195H&Y score 1.9 ± 0.6 1.7 ± 0.6 93 .621Total-LEDD (mg/d) 626.8 ± 328.5 616.7 ± 278 .008 .929Dopamine agonist LEDD (mg/d) 483.3 ± 360.5 340 ± 218.1 1.490 .235Patients receiving levodopa monotherapy 5 2Patients receiving dopamine agonist monotherapy 3 2Patients receiving combination agonist and levodopa therapy 6 11

Yr = years; PD = Parkinson’s disease; UPDRS-III = Unified Parkinson’s disease rating scale; H&Y = Hohen & Yahr; TotalLEDD = Total L-Dopa equivalent daily dose.


the left or the right hand of the human figure was marked. Human images were displayed facing to-ward (front-facing-OBT) or away from the observers (back-facing-OBT). The back-facing orientation iscompatible with the participants’ perspective, whereas in the front-facing orientation the observershave to imagine their own bodies into the position of the front-facing human figure to perform theleft/right judgment. Following previous studies (Arzy et al., 2006; Blanke et al., 2005; Zacks et al.,1999), task instructions explicitly required participants to mentally simulate one’s own body move-ment. By these means, we could induce subjects to resort to one’s own body representation duringtask performance (see Hétu et al., 2013, for a discussion on explicit versus implicit motor imagery).Participants also completed a letter rotation task (LRT), requiring whether a capital letter was pre-sented in canonical or mirror-reversed form (Conson et al., 2008). The three tasks (i.e. front-facing-OBT, back-facing-OBT and LRT) were administered by means of a computerized procedure, and werearranged to be as similar as possible in presentation and response modalities, although specificinstructions differed.

2.3. Stimuli and procedure

Stimuli consisted of line drawings depicting front-facing or back-facing body images whose right orleft hand was marked in black, or of letters in canonical or mirror-reversed form. All stimuli were pre-sented in four spatial orientations: 0�, 90� clockwise (cw), 90� counterclockwise (ccw) and 180�(Fig. 1). Stimuli were large approximately 8 cm along the widest axis (about 7.6� of visual angle ata viewing distance of 60 cm from a 15’’ computer screen) and were presented at the centre of the mon-itor until response completion; each stimulus was preceded by a fixation point (1000 ms).

Patients and healthy controls gave their responses by pressing one of two centrally located keys (Band H keys on QWERTY keyboard) with their index and middle fingers of the right (dominant) hand;the stimulus-response association for each task was counterbalanced across participants. The lefthand was placed in a comfortable position, palm down next to the keyboard. Following classical stud-ies (e.g., Sekiyama, 1982), both hands were covered with a black cloth in order to avoid that visual cuesprovided by one’s own hands could facilitate left–right judgments on body images (for instance seeIonta & Blanke, 2009 for recent data on the role of visual familiarity on mental transformation ofbody-parts). Participants were encouraged to respond as fast and correctly as possible; we recordedboth Reaction Times (RTs, in milliseconds) and accuracy. Stimulus presentation and data collectionwere controlled by a PC using Cedrus SuperLab v.4.

Each task (front-facing-OBT, back-facing-OBT and LRT) consisted of 48 trials: in both front-facing-and back-facing-OBT, six trials were presented for each combination of hand laterality (left or right)




Fig. 1. Instances of stimuli used in the three mental transformation tasks: front-facing-OBT (first row), back-facing-OBT (secondrow), and letter rotation task (LRT, third row), in the four spatial orientations: 0�, 90� clockwise (cw), 90� counterclockwise(ccw) and 180� (schematic human figures with their left hand marked in black, and letters in mirror-reversed form are notreported here).


and spatial orientation; in the LRT, six trials were presented for each combination of type of stimulus(canonical or mirror-reversed) and spatial orientation. Trials were randomized within each task,which was divided in two blocks, with a 3-min pause allowed between the two blocks. A training per-iod preceded the experiment. Before starting each task, eight practice trials were given; if a wrong re-sponse was provided, feedback appeared on the monitor screen and the trial was repeated.Experimental session started only if the participants provided at least six consecutive correct re-sponses. Testing was conducted in a quiet room and in a single session that lasted about 20 min;the order of the three tasks was counterbalanced across participants.

2.4. Statistical analysis

First, Pearson’s correlations between accuracy and RTs were calculated for each mental rotationtask in each group to assess whether participants’ performance showed any speed-accuracy trade-off (Sanders, 1998). Then, overall mean RTs and accuracy were analyzed by means of separate two-way mixed Analysis of Variance (ANOVA), with task (front-facing-OBT, back-facing-OBT and LRT) asa within-subject factor and with group (healthy participants, left-most affected and right-most af-fected PD patients) as a between-subject factor.





To ascertain the involvement of embodied processing in whole-body transformation, we testedwhether the side of prevalent motor impairment in PD patients specifically affected performanceon the corresponding side of the human figure in the OBT tasks. To this aim, a four-way mixed-designANOVA was performed on correct RTs and accuracy, with task (front-facing-OBT or back-facing-OBT),side of marked hand (left or right) and spatial orientation (0�, 90� cw, 90� ccw or 180�) as within-sub-ject factors, and group (healthy subjects, left-most affected PD patients or right-most affected PD pa-tients) as a between-subject factor.

Finally, following previous studies (Jola & Mast, 2005; Kosslyn et al., 1998), we tested whether theparticipants used a rotation strategy to mentally transform both body images and letters by perform-ing planned linear contrasts on participants’ correct RTs for the four stimulus orientations, i.e. 0�, 90�cw, 90� ccw, and 180�. This analysis was conducted on each experimental task, separately in the threegroups.

3. Results

3.1. Overall performance

Pearson’s correlations showed significant negative correlations between overall accuracy and RTs ineach group for all experimental tasks (healthy subjects: front-facing-OBT, r = �.432, p = .017; back-fac-ing-OBT, r = �.456, p = .011; LRT, r = �.404, p = .025; left-most affected PD patients: front-facing-OBT,r = �.574, p = .032; back-facing-OBT, r = �.540, p = .041; LRT, r = �.493, p = .045; right-most affectedPD patients: front-facing-OBT, r = �.528, p = .043; back-facing-OBT, r = �.539, p = .031; LRT, r = �.574,p = .024). These findings ruled out a trade-off between speed and accuracy (e.g. Sanders, 1998).

Patients and healthy subjects performed the three tasks accurately, with mean accuracy and cor-rect RTs comparable across groups (Table 2). The two-way mixed ANOVA performed on accuracyshowed a significant effect of task, F(2,112) = 35.708, p = .0001, g2

p = .389, with higher accuracy forback-facing-OBT (mean = .92, SEM = .01) and LRT (mean = .91, SEM = .02) relative to front-facing-OBT (mean = .79, SEM = .02). The effect of group, F(2,56) = .732, p = .485, g2

p = .025, and the task bygroup interaction, F(4,112) = .336, p = .853, g2

p = .012, were not significant.An analogous pattern was obtained by applying the same ANOVA on correct RTs, with a significant

effect of task, F(2,112) = 92.126, p = .0001, g2p = .622, with faster RTs for back-facing-OBT

(mean = 2522.86, SEM = 72.64) and LRT (mean = 2087.82, SEM = 49.52) relative to front-facing-OBT(mean = 3138.87, SEM = 86.04). The effect of group, F(2,56) = .051, p = .950, g2

p = .002, and the taskby group interaction, F(4,112) = 1.355, p = .254, g2

p = .046, were not significant.

3.2. Effect of patients’ most affected side on whole-body transformation

The four-way mixed-design ANOVA performed on correct RTs showed significant effects of task,F(1,56) = 48.793, p = .0001, g2

p = .466, with faster RTs in the back-facing-OBT (mean = 2561.06,

Table 2Mean accuracy and RTs (SEM) of healthy subjects, left-most affected and right-most affected PD patients, separately for the threeexperimental tasks.

Healthy subjects Left-most affected PD Right-most affected PD

Mean SEM Mean SEM Mean SEM

AccuracyFront-facing-OBT .80 .03 .78 .04 .76 .04Back-facing-OBT .95 .01 .90 .02 .93 .02LRT .91 .01 .92 .02 .90 .02

RTsFront-facing-OBT 3185.61 122.42 3061.14 179.20 3117.94 173.13Back-facing-OBT 2415.95 101.66 2638.22 148.81 2629.02 143.77LRT 2093.28 70.54 2117.61 103.26 2049.10 99.75





SEM = 76.85) than in front-facing-OBT (mean = 3121.56, SEM = 92.54), and of spatial orientation, withslower RTs for 180� oriented bodies (mean = 3154.94, SEM = 85.96) with respect to the other orienta-tions (0�: mean = 2739.35, SEM = 84.92; 90� cw: mean = 2756.32, SEM = 94.95; 90� ccw:mean = 2714.64, SEM = 84.29). There was a significant first-order interaction between task and spatialorientation, F(3,168) = 8.319, p = .0001, g2

p = .129, showing that the influence of stimulus orientationon RTs was different in the OBT tasks (see below analysis of the effect of stimulus orientation on men-tal transformation). More relevant here, there was a significant first-order interaction between side ofmarked hand and group, F(2,56) = 3.548, p = .035, g2

p = .112, that was further qualified by the signif-icant second-order interaction among task, side of marked hand and group, F(2,56) = 3.939, p = .025,g2

p = .123. All remaining main effects and interactions were not significant (p > .05).Post-hoc comparisons (paired t-tests) on the interaction among task, side of marked hand and

group (Fig. 2, upper row) showed that the side of marked hand did not affect RTs of the three groupson front-facing-OBT (healthy subjects: t = .114, p = .910; left-most affected PD patients: t = .046,p = .964; right-most affected PD patients: t = .090, p = .929), whereas it specifically modulated perfor-mance of both left- and right-most affected PD groups on back-facing-OBT. More precisely, while left-most affected PD patients were slower in judging back-facing-bodies with left than right marked hand(t = �3.321, p = .006), the opposite was true for right-most affected PD patients (t = 2.774, p = .028);healthy subjects’ RTs, instead, did not differ between left and right marked hand (t = 1.042, p = .306).

Fig. 2. Mean RTs and accuracy (bars are SEM) of the three groups on front-facing-OBT and back-facing-OBT plotted against theside of the figure’s marked hand.




Fig. 3. Mean RTs (bars are SEM) of the three groups on front-facing-OBT, back-facing-OBT and LRT plotted against the fourstimulus orientations.


The same four-way mixed-design ANOVA as above was performed on accuracy and showed a sig-nificant main effect of task, F(1,56) = 45.530, p = .0001, g2

p = .448, with higher accuracy in the back-facing-OBT (mean = .92, SEM = .01) than in front-facing-OBT (mean = .79, SEM = .02), whereas no othermain effect or interaction was significant (p > .05). However, it is worth underlining here that, consis-tent with RTs, left-most affected PD patients were less accurate in judging back-facing-bodies with leftthan right marked hand, whereas the opposite was true for right-most affected PD patients. This pat-tern was absent in healthy participants and in the performance of the three groups on the front-fac-ing-OBT (Fig. 2, lower row).

3.3. Effect of stimulus orientation on mental transformation tasks

Fig. 3 shows RTs of the three groups on front-facing-OBT, back-facing-OBT and LRT plotted againstthe four stimulus orientations. Planned linear contrasts on front-facing-OBT showed that in all thethree groups RTs did not increase with increasing stimulus orientation (healthy subjects,F(1,29) = .048, p = .828, g2

p = .002; left-most affected PD patients, F(1,13) = 1.274, p = .279,g2

p = .089; right-most affected PD patients, F(1,14) = .378, p = .548, g2p = .026). The same analysis on

back-facing OBT showed that the linear trend was highly significant in all the three groups (healthysubjects, F(1,29) = 37.625, p = .0001, g2

p = .565; left-most affected PD patients, F(1,13) = 46.527,p = .0001, g2

p = .782; right-most affected PD patients, F(1,14) = 15.097, p = .002, g2p = .519). However,

results showed that quadratic and cubic trends were also significant in healthy subjects (quadratictrend, F(1,29) = 21.267, p = .0001, g2

p = .423; cubic trend, F(1,29) = 12.848, p = .001, g2p = .307), and

that the quadratic trend was significant in both PD groups (left-most affected PD patients:F(1,13) = 4.794, p = .048, g2

p = .198; right-most affected PD patients: F(1,14) = 11.859, p = .004,g2

p = .459). On the contrary, planned linear contrasts on LRT showed a clear, significant linear trendin all the three groups (healthy subjects, F(1,29) = 74.666, p = .0001, g2

p = .720; left-most affectedPD patients, F(1,13) = 21.213, p = .0001, g2

p = .620; right-most affected PD patients, F(1,14) = 29.395,p = .0001, g2

p = .677), without any other significant trend.

4. Discussion

We assessed PD patients on OBT tasks in order to investigate the contribution of embodied simu-lation to mental transformation of whole-body. To this aim, we tested whether patients’ most affectedside influenced the ability to mentally manipulate whole-body images. The results demonstrated thatPD patients were specifically impaired in judging laterality of the hand corresponding to their ownmost affected side when presented with back-facing human figures (back-facing-OBT). The correspon-dence between the most affected side and mental transformation performance was not found for





judgments on the human figure presented in a front view (front-facing-OBT). Nonetheless, overall per-formance of both left- and right-most affected PD patients on OBT and LRT was comparable to that ofhealthy subjects, although healthy participants were substantially younger and better educated thanPD patients.

The present data were obtained from the analysis of responses that participants provided withtheir right dominant hand. Measuring RTs in PD patients can be problematic because of the motorsymptoms of the disease (Amick et al., 2006), but in particular our methodological choice might havebiased the results because some PD patients responded with their most affected hand, whereas othersresponded with their least affected hand. However, if this was the case, we should have found that theright-most affected PD patients were slower than left-most affected patients, but we did not find anymain effect of the group on RTs in any experimental task or condition. Moreover, as reported above,we also found that mean accuracy was comparable across groups in the three experimental tasks,and the pattern of accuracy data strongly overlapped with RTs on both front- and back-facing OBT.Consistently with previous studies on hand laterality judgments (Helmich et al., 2007; van Nuenenet al., 2012), these findings would rule out a general response bias related to the side of the responsehand, and allowed us to interpret data in the framework of embodied simulation mechanisms.

Previous studies on healthy subjects revealed some behavioral differences between performance onback- and front-facing-OBT. In particular, it has been repeatedly demonstrated that judgments on a hu-man figure presented in a front view perspective take considerably longer (and are more error prone)than when the body image is shown in back view. Moreover, degree of rotation angle often affects RTsfor back view stimuli but not for front view stimuli (e.g., Jola & Mast, 2005; Parsons, 1987b; Stegge-mann, Engbert, & Weigelt, 2011). Accordingly, in the present study both PD patients and healthy sub-jects showed: (i) faster (and more accurate) judgments on back- than front-facing-OBT, and (ii) aninfluence of rotation angle on RTs in back-facing-OBT and letter rotation, but not in front-facing-OBT. In the LRT we found a linear increase of RTs with increasing stimulus orientation in both patientsand healthy subjects, consistent with classical studies on mental rotation of non-corporeal stimuli(Cooper & Shepard, 1973; Fischer & Pellegrino, 1988). On the contrary, in back-facing-OBT we observeda more complex pattern of relationships between spatial orientation and RTs, suggesting that non-rota-tional components could have affected whole-body transformation. Zacks, Mires, Tversky, and Hazel-tine (2002) hypothesized that non-linear relationships between orientation and RTs when processingwhole-bodies could reflect constraints arising from the kinematics of the body. Here, the influence ofthe PD patients’ most-affected side when processing back-facing stimuli supported the involvementof embodied simulation processes in mental transformation of back-facing whole-bodies.

Recently, Gardner and Potts (2010) required right- and left-handed healthy subjects to performthe classical OBT task. Results showed that left-handers were faster in judging the left side, whereasright-handers were faster in judging the right side of the image. These data suggested that embodiedsimulation might contribute to whole-body transformation in the absence of actual or implied ac-tions (Gardner & Potts, 2010). Moreover, Steggemann et al. (2011) required healthy participantswith or without motor expertise for rotational movements to perform back-facing- and front-fac-ing-OBT. Results demonstrated an advantage (shorter RTs and higher accuracy) in making judgmentson back- than front-facing human figures independently from motor expertise. Steggemann et al.(2011) suggested that to solve the back-facing-OBT, participants simply had to mentally ‘‘take a stepforward’’ to imagine themself into the position of the person presented. On the contrary, mentaltransformation of front-facing bodies would imply a turn around the longitudinal body axis, likelyinvolving complex visuospatial perspective processes. This interpretation fits well with the presentdata showing an effect of Parkinson’s disease on cognitive performance in the back- but not in thefront-facing-OBT. Thus, two kinds of mental transformations seems to be involved in whole-bodyprocessing: an ‘‘embodied transformation’’, that implies simulation of one’s own body movementsand is mainly activated when dealing with a back-facing body; and a ‘‘perspective transformation’’,that does not necessarily implies simulation processes and is mainly activated when observing afront-facing body (see also Gardner et al., 2013; Kessler & Thomson, 2010; Kessler & Wang,2012). This interpretation is consistent with the distinction between embodied and visuospatialtransformation mechanisms hypothesized for mental transformation of body-parts (Brady, Maguin-ness, & Ní Choisdealbha, 2011; Conson, Mazzarella, & Trojano, 2009; Ní Choisdealbha, Brady, &





Maguinness, 2011). Two recent neurofunctional studies on mental transformation of hand images inright-most affected PD patients (Helmich et al., 2007; van Nuenen et al., 2012) found that the pa-tients’ failure in judging images of the affected hand was related to an increased compensatoryactivity in the right extrastriate body area. On this basis, the present results would prompt furtherinvestigation aimed at verifying whether this compensatory mechanism is also involved in whole-body mental transformation in PD, and whether it changes over the course of the disease.

5. Conclusions

Here we demonstrated that the side of stronger motor impairment in PD patients can selectivelyaffect whole-body mental transformations. We found comparable patterns of performance in PD pa-tients and in healthy subjects on front-facing-OBT task and on LRT, consistent with evidence of unim-paired visuospatial transformations in this clinical population (Amick et al., 2006; Dominey et al.,1995). On the contrary, left- and right-most affected PD patients showed opposite patterns of impairedperformance on the back-facing-OBT. These findings demonstrated that one’s own body representa-tion is causatively involved in processing whole-bodies, but only when the posture of the body image(i.e., back-facing) activates embodied simulation processes.

Acknowledgments

We are grateful to Italia Pagano and Daniela Culiers for their help in collecting the data.

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