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Mental rotation of mirrored letters: Evidence from event-related brain potentials q

M. Isabel Núñez-Peña a,b,*, J. Antonio Aznar-Casanova b,c

a Department of Behavioral Science Methods, Faculty of Psychology, University of Barcelona, Passeig Vall d’Hebron, 171, 08035 Barcelona, Spainb Cognitive Neuroscience Research Group, Department of Psychiatry and Clinical Psychobiology, Faculty of Psychology, University of Barcelona,Passeig Vall d’Hebron, 171, 08035 Barcelona, Spainc Department of Basic Psychology, Faculty of Psychology, University of Barcelona, Passeig Vall d’Hebron, 171, 08035 Barcelona, Spain

a r t i c l e i n f o

Article history:Accepted 10 July 2008Available online 17 August 2008

Keywords:Event-related brain potentialsRotation-related negativityMirrored lettersParity-judgment task

a b s t r a c t

Event-related brain potentials (ERPs) were recorded while participants (n = 13) were presented with mir-rored and normal letters at different orientations and were asked to make mirror-normal letter discrim-inations. As it has been suggested that a mental rotation out of the plane might be necessary to decide onmirrored letters, we wanted to determine whether this rotation occurs after the plane rotation in mirrorrotated letters. The results showed that mirrored letters in the upright position elicited a negative-goingwaveform over the right hemisphere in the 400–500 ms window. A similar negativity was also present inmirrored letters at 30�, 60�, and 90�, but in these cases it was delayed. Moreover, the well-known orien-tation effect on the amplitude of the rotation-related negativity was also found, although it was more evi-dent for normal than for mirrored letters. These results indicate that the processing of mirrored lettersdiffers from that of normal letters, and suggest that a rotation out of the plane after the plane rotationmay be involved in the processing of mirror rotated letters.

� 2008 Elsevier Inc. All rights reserved.

1. Introduction

Mental rotation is a classical psychological process. It was firstreported by Shepard and Metzler (1971) in an experiment whereparticipants were presented with pairs of three-dimensional blockfigures at different orientations, and were required to determinewhether both figures were the same or one was a mirror reflectionof the other. Results showed that reaction time (RT) was longer forlarger angles of misorientation. It was proposed that this increasedRT was due to the fact that in order to perform the parity-judgmenttask the image had to be mentally rotated to put it in the uprightposition. Since then, the mental rotation effect has been reportedin studies with alphanumeric characters (Cooper & Shepard,1973; Koriat & Norman, 1985a), letter-like characters (Tarr & Pin-ker, 1989), left-right hands (Cooper & Shepard, 1975), and even in anaming task with natural objects (Jolicoeur, 1985, 1988, 1990).Although the mental rotation effect has been reported with differ-ent types of stimuli, it has been suggested that the form of these RTfunctions depends on the familiarity of the stimuli. When the stim-ulus is unfamiliar the RT function is linear (Shepard & Metzler,1971), whereas when the stimulus is familiar—i.e., alphanumericcharacters—the RT function departs from linearity and shows a

quadratic trend (Cooper & Shepard, 1973). The suggested explana-tion for this nonlinearity effect is that familiar stimuli are over-learned visual stimuli that achieve a certain degree ofindifference to small misorientations from their normal position(Cooper & Shepard, 1973; Koriat & Norman, 1985b). However,unfamiliar stimuli require rotation even for small deviations fromupright.

Similar mathematical functions relate the angle of misorienta-tion and the amplitude of an event-related brain potential (ERP)component in mental rotation tasks. This component, known as‘rotation-related negativity’, was first reported by Stuss, Sarazin,Leech, and Picton (1983), Peronnet and Farah (1989), and Wijers,Otten, Feenstra, Mulder, and Mulder (1989). It consists of a nega-tive-going waveform, maximum over parietal regions, whoseamplitude is modulated by the angle of misorientation: the greaterthe angle of misorientation, the larger the rotation-related negativ-ity. The rotation-related negativity has been reported in studieswith alphanumeric characters (Heil, Rauch, & Hennighausen,1998; Heil & Rolke, 2002; Milivojevic, Johnson, Hamm & Corbalis,2003), letter-like shapes (Núñez-Peña, Aznar, Linares, Corral & Es-cera, 2005), paper-folding stimuli (Milivojevic, et al., 2003), left-right hands (Thayer & Johnson, 2006), and geometric objects(Muthukumaraswamy, Johnson, & Hamm, 2003; Rösler, Heil, Baj-ric, Pauls, & Hennighausen, 1995). It has been suggested that thiscomponent is a neurophysiological correlate of the mental rotationprocess (Heil, 2002), because its amplitude is modulated by theamount of mental rotation needed to make a parity decision.Moreover, there are other evidences. First, Heil, Bajric, Rösler,

0278-2626/$ - see front matter � 2008 Elsevier Inc. All rights reserved.doi:10.1016/j.bandc.2008.07.003

q This research was supported by Grants SEJ2006-000496/PSIC, SEJ2006-15095/PSIC, and Consolider-Ingenio 2010-CSD2007-00012 from the Spanish Ministry ofScience and Technology, and SGR2005-00953 from the Generalitat de Catalunya.

* Corresponding author. Fax: +34 93 402 13 59.E-mail address: [email protected] (M.I. Núñez-Peña).

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and Hennighausen (1996a) and Heil et al. (1998) found that therotation-related negativity was evoked by a misoriented stimulusonly if mental rotation is required to solve the task. Second, Heiland Rolke (2002) provided evidence that the onset of this negativecomponent is delayed by delaying the mental rotation process.

As regards the spatial distribution of the mental rotation pro-cess, neuroimaging and electrophysiological studies have reportedinconsistent results. Although positron emission tomography(PET) and functional magnetic resonance imaging (fMRI) studieshave reported clear evidence of the involvement of parietal re-gions in mental rotation, considerable debate remains as towhether mental rotation is a right parietal function or whetherneither hemisphere is dominant (Alivisatos & Petrides, 1997;Cohen et al., 1996; Harris et al, 2000; Jordan, Heinze, Lutz,Kanowski, & Jancke, 2001; Richter, Ugurbil, Georgopoulos, &Kim, 1997; Yoshino, Inoue, & Suzuki, 2000). Milivojevic et al.(2003) suggested that the type of task might account for someof these contradictory results: ‘the right hemisphere may be pref-erentially engaged when the task is simple and involves a singletransformation, but the left hemisphere is also engaged as thetask becomes more complex, as when a coordinated sequenceof transformations are required’ (Milivojevic et al., p. 1355). Thisexplanation agrees with that proposed by Corballis (1997), whodifferentiates between holistic and analytic mental rotation pro-cesses. According to his view, the right hemisphere is preferen-tially engaged in holistic mental rotation processes—when theentire image is mentally rotated in a unitary process—and the lefthemisphere is preferentially engaged in analytic processes—whenthe image is parsed into units, which are then rotated individu-ally. However, there is also some evidence that the right hemi-sphere contribution to spatial performance increases with thecomplexity of the task. Roberts and Bell (2003) reported greateractivation of the right parietal region in a three-dimensional men-tal rotation task than in a two-dimensional one. Alivisatos andPetrides (1997) provided evidence that activity in the left parietalcortex was more intense in a task that required active mentalrotation in the picture plane than in one that requires making amirror-normal decision regarding upright letters.

Whereas the change in orientation has been extensively inves-tigated with both behavioral and psychophysiological measures,the mirror-normal difference has attracted less interest among sci-entists. Behavioral studies have systematically shown that a mir-rored stimulus decision takes longer than a normal stimulusdecision (see for example, Bajric, Rösler, Heil, & Hennighaugen,1999; Hamm, Johnson, & Corballis, 2004; Milivojevic et al.,2003). A suggested explanation for this difference in RT is thatthe mirrored stimulus is rotated both in the picture plane, in orderto put it in the vertical upright position, and out of the plane, in or-der to put it in the normal upright position. This explanation issupported by several psychophysiological studies. First, Alivisatosand Petrides (1997), in a PET study, found that mirror-normaljudgment of upright letters activated similar brain areas to thoseactivated in a classical mental rotation task. This study suggeststhat both experimental tasks—the mirror-normal judgment taskwith upright letters and the mirror-normal judgment task withletters presented at different orientations—require visuo-spatialprocessing to identify misoriented stimuli. Second, in a recentERP study, Hamm et al. (2004) concluded that mirrored stimuliare not only rotated in the picture plane but are subsequently ro-tated out of the plane, involving a ‘flip’ to fully normalize the mir-rored stimuli.

While it seems clear that mirrored letters in the upright condi-tion need to be rotated out of the picture plane in order to make amirror-normal judgment, the case of a mirrored letter presented atan orientation different from upright has been less studied. Hammet al. (2004) suggested that the flipping of mirrored stimuli will

occur at whatever orientation and that it will occur after the planerotation. They stated that this flipping of mirrored stimuli ‘‘will oc-cur after the plane rotation because of the theoretical complica-tions that arise if one postulates it as occurring prior to the planerotation” (p. 819). If information about the mirror-normal statusof the stimuli is available before the plane rotation, then rotatingthe mirrored stimuli in the picture plane will be unnecessary.However, no evidence to support this sequential processing hasbeen brought forward so far.

The purpose of the present study was to add evidence in sup-port of the ideas that (1) mental rotation out of the picture planeis necessary to make a mirror-normal judgment in mirrored lettersand (2) this mental rotation occurs after the plane rotation in ori-entations different from the upright. Participants were presentedwith mirrored and normal letters at eleven different orienta-tions—the 0� orientation and the 30�, 60�, 90�, 120�, or 150� clock-wise or counterclockwise orientations—and were asked to performa parity-judgment task. The decision on the normal versions of let-ters requires only the mental rotation of the stimulus in the pictureplane, whereas the decision on the mirrored versions of letters re-quires mental rotation of the stimulus in the picture plane and anextra rotation out of the plane. Mirrored upright letters, where thedecision requires only rotation out of the picture plane, served as acontrol condition to isolate the flip effect. Once this effect had beenisolated, we performed a detailed analysis of the ERPs at different50-ms windows in order to study the mirror-normal difference inother rotated stimuli. We hypothesized that if a mirror rotated let-ter is rotated out of the picture plane after the plane rotation, thenthe flip effect (the difference mirror-normal) would be delayed inthe ERP pattern. Moreover, it was predicted that the typical mod-ulation of the amplitude of the rotation-related negativity wouldbe present in normal letters and that this ERP pattern would be dif-ferent for mirrored letters, where rotation in and out of the pictureplane would be needed.

2. Methods

2.1. Participants

Fifteen healthy volunteers were tested in this study (12 women;age 19–28 years, mean = 21.8, standard deviation = 2.5). All wereuniversity students and had normal or corrected-to-normal visualacuity. Because of a large number of artifacts, data from two partic-ipants were excluded from the ERP data analysis; this analysis wasthus performed with data from thirteen subjects (10 women; age19–28 years, mean = 21.9, standard deviation = 2.7). Subjects hadno history of neurological or psychiatric disorder, and gave writteninformed consent to participate after the nature of the study hadbeen explained to them.

2.2. Stimuli and procedure

The characters were the uppercase letters F, L, P, and R, whichwere presented at an orientation of 0� or at 30�, 60�, 90�, 120�, or150� clockwise or counterclockwise orientations. Fig. 1 showssome of the stimuli used in the experiment.

Stimuli were shown in green on a white background (luminance110 cd/m2), and at an orientation of 0� subtended a vertical visualangle of 2.3� and a horizontal visual angle of 1.37�. The programused to manage the experiment was developed by the authorsusing C++/Open GL (glut library).

Participants were seated in an electrically shielded, sound-attenuating room at a distance of 150 cm from the display screen,whose center was at eye level. They were monitored continuouslywith a closed circuit video camera. The experiment started with atraining period to familiarize participants with the procedure and

M.I. Núñez-Peña, J.A. Aznar-Casanova / Brain and Cognition 69 (2009) 180–187 181


the equipment. All subjects achieved a minimum of 90% correct an-swers in the practice trials.

During the recording period, subjects were instructed to relaxand to keep their eyes on the screen. They were encouraged tomake any eye-blinks during the presentation of a fixation pointor during the pauses. The sequence of events began with a redfixation point presentation at the center of the screen that re-mained in view for 500 ms. The character was then presentedfor 500 ms, after which the screen remained black until the sub-ject pressed one of the two response buttons. The task was to de-cide whether each character displayed was presented in a normalor a mirrored version, as quickly as possible, while keeping errorsto a minimum. Response fingers were counterbalanced acrosssubjects.

Each participant was given ten blocks of 96 trials. A messageindicating a 1-min pause appeared on the screen after each block,and a 5-min pause was provided to participants halfway throughthe experimental trials. The type of trials was controlled withineach block in such a way that a block included trials resulting fromthe factorial combination of the following variables: orientation (0�,30�, 60�, 90�, 120�, and 150�),1 direction of the angular disparity(clockwise and counterclockwise), version (normal and mirror-re-versed), and type of stimulus (four letters). The sequence of presen-tation in each block was randomized per participant. Allparticipants were tested on 960 trials, 40 for each experimentalcondition resulting from the combination of orientation, directionof the angular disparity and version.

2.3. Electrophysiological recording

EEG was recorded with the SynAmps/SCAN 4.3 hardware andsoftware (NeuroScan, Inc., Herndon, VA) from 31 tin electrodesmounted in a commercial electro-cap (Electro-Cap International,Eaton, OH). Nineteen electrodes were positioned according to the10–20 International System: three electrodes were placed overmidline sites at Fz, Cz, and Pz locations, along with 8 lateral pairsof electrodes over standard sites on frontal (FP1/FP2, F7/F8, F3/F4), central (C3/C4), temporal (T3/T4, T5/T6), parietal (P3/P4),and occipital (O1/O2) positions. Two electrodes were placed atFpz and Oz, and ten electrodes were placed halfway between thefollowing additional locations: fronto-central (FC1/FC2), fronto-temporal (FT3/FT4), centro-parietal (CP1/CP2), temporo-parietal(TP3/TP4), and mastoids (M1/M2). The common reference elec-trode for EEG and EOG measurements was placed on the tip ofthe nose. EEG channels were continuously digitized at a rate of500 Hz by a SynAmpTM amplifier (5083 model, NeuroScan, Inc.,Herndon, VA). A band pass filter was set from 0.16 to 30 Hz, andelectrode impedance was always kept below 5 kX. For monitoring

eye movements an electrode placed at the external canthi of theright eye was used.

2.4. Data analysis

2.4.1. Behavioral dataResponse times for correctly solved trials and error rate were

analyzed with repeated-measures ANOVAs, taking version (normaland mirror-reversed) and orientation (0�, 30�, 60�, 90�, 120�, and150�) as within-subjects factors.2 The repeated-measures ANOVAwas performed with the Greenhouse–Geisser correction for spheric-ity departures, which was applied when appropriate. The F value,the uncorrected degrees of freedom, the probability level followingcorrection, the e value and the g2 effect size index (Kirk, 1996) are re-ported. Whenever a main effect reached significance, pairwise com-parisons were conducted using t tests, and the Hochberg approachwas used to control for the increase in Type I error (Keselman,1998). Tests of simple effects were calculated in the presence of a sig-nificant interaction. Finally, trend analyses were also performed.

2.4.2. EEG analysisOnly trials on which the subjects responded correctly were in-

cluded in the ERP analysis. First, epochs for every subject in eachexperimental condition were averaged relative to a pre-stimulusbaseline consisting of the 100 ms of activity preceding the epochof interest. Second, trials with artifacts (voltage exceeding ±50 lVin FP1, FP2, FPz, or HEOG) and those with response errors were ex-cluded from the ERP average. The mean number of epochs includedin each ERP average varied between 45.9 and 58.5 for the varioustypes of stimuli used.

The orientation effect was studied by analyzing mean ampli-tude measures in the 400–500 ms window. This latency windowwas selected because according to visual inspection of ERP wave-forms it was representative of the orientation effect. A2 � 6 � 3 � 5 repeated-measures ANOVA was performed on theERP amplitudes at 15 electrodes (F7, F3, Fz, F4, F8, T3, C3, Cz, C4,T4, T5, P3, Pz, P4, and T6), taking as factors version (normal andmirror-reversed), orientation (0�, 30�, 60�, 90�, 120�, and 150�),frontality (frontal, central, and parietal), and laterality (five levelsfrom left to right). Statistical analyses were performed as describedfor behavioral data. Topographic maps were plotted using the EEP-robe 3.1 program (ANT Software BV, Enschede, The Netherlands).

The version effect was studied in a more detailed way in orderto detect whether the mirror-normal difference was delayed acrossorientations. Mean amplitude measures in 50-ms windows from400 to 700 ms at nine electrodes (F3, Fz, F4, C3, Cz, C4, P3, Pz,and P4,) were analyzed. Repeated-measures ANOVAs were per-formed at each orientation, taking as factors version (normal and

Fig. 1. Normal and mirrored versions of the F letter in each orientation (from 0� to 330�).

1 The letters presented at 0� were presented twice as often as letters at the otherorientations because the 30�, 60�, 90�, 120�, and 150� clockwise and counterclockwiseare usually treated as equivalent.

2 Response times were subjected to an initial analysis to test for symmetry about0�. No asymmetries were detected, so data were collapsed into six orientations (0�,30�, 60�, 90�, 120�, and 150�) for all analyses.

182 M.I. Núñez-Peña, J.A. Aznar-Casanova / Brain and Cognition 69 (2009) 180–187


mirror-reversed), frontality (frontal, central, and parietal), and lat-erality (left, middle, and right).

3. Results

3.1. Behavioral data

Mean response times for normal and mirrored letters as a func-tion of stimulus orientation are plotted in Fig. 2. Overall responsetime increased with angular deviation from upright(F(5,70) = 44.47, p < .001, e = .23, g2 = .76), and responses to normalletters were faster than responses to mirrored letters(F(1,14) = 25.20, p < .001, g2 = .64). However, the ANOVA alsoyielded a significant Orientation � Version interaction (F(5,70) =4.44, p = .012, e = .51, g2 = .24), so the orientation effect variedaccording to the letter version. A more detailed analysis of theinteraction showed that the orientation effect in normal letterswas described by a linear (F(1,14) = 73.31, p < .001, g2 = .84) anda quadratic (F(1,14) = 34.97, p < .001, g2 = .71) trend, whereas thesame effect in mirror-reversed letters was described by a lineartrend (F(1,14) = 33.08, p < .001, g2 = .70). Moreover, tests of simpleeffects demonstrated that responses to mirrored stimuli wereslower than to normal stimuli at all the orientations (all p-values< .001) except for 150 degrees (p = .07). Therefore the advantageof normal over mirrored letters decreases gradually with increas-ing angular deviation from upright.

Error rate ANOVA showed a significant effect for orientation(F(5,70) = 24.54, p < .001, e = .29, g2 = .64), version (F(1,14) = 9.26,p = .009, g2 = .40) and the interaction Orientation � Version(F(5,70) = 15.17, p < .001, e = .29, g2 = .52). Overall, error rate in-creased with greater angular disparity from the upright. The orien-tation effect was described by a linear and a quadratic trend bothin mirrored (F(1,14) = 8.10, p = .013, g2 = .37 for linear trend, andF(1,14) = 17.75, p = .001, g2 = .56 for quadratic trend) and normalletters (F(1,14) = 26.42, p < .001, g2 = .65 for linear trend, andF(1,14) = 31.22, p < .001, g2 = .69 for quadratic trend). As for the

interaction Orientation � Version, tests of simple effects showedthat accuracy was worse in normal than in mirrored letters at150� angular disparity from upright (F(1,14) = 18.10, p < .001,g2 = .56). However, there were no differences in accuracy betweenthe two versions of letters for the other orientations.

3.2. Event-related potentials

Fig. 3A and B show the grand-average ERPs for each orienta-tion of normal and mirror reversed letters at P3, Pz, and P4. Theorientation effect is evident for normal letters. As can be seenin Fig. 3A, the rotation-related negativity becomes more nega-tive with increasing angular disparity from upright, the effectbeing more evident for larger deviations. However, the orienta-tion effect is not so clear for mirror-reversed letters (see Fig.3B): although the voltage tends to be more negative the greaterthe degree to be rotated, these differences seem not to be aslarge as those for normal letters. Fig. 4 shows the amplitudemeans in the 400–500 ms window for normal and mirror re-versed letters as a function of stimulus orientation at P3, Pzand P4. These plots show again that the orientation effect isdifferent for normal and for mirror-reversed letters: the orienta-tion effect over the ERP amplitude is more evident for normalletters than for mirrored letters. Voltage maps in Fig. 5A andB showed the spatial distribution of the orientation effect innormal and mirror-reversed letters over all electrodes at thescalp surface in the 400–500 ms window. These voltage mapsshow that the orientation effect has a centro-parietal scalp dis-tribution in normal letters and is not so clear for mirroredletters.

The statistical analysis performed on the 400–500 ms windowsupports these observations. The overall ANOVA showed signifi-cant effects of orientation (F(5,60) = 11.40, p < .001, e = .54,g2 = .49), Orientation � Version (F(5,60) = 2.68, p = .03, e = .77,g2 = .18), Orientation � Version � Frontality (F(10,120) = 4.37,p = .003, e = .42, g2 = .27) and Orientation � Version � Laterality(F(20,240) = 2.46, p = .035, e = .29, g2 = .17). A more detailed analy-sis of the Orientation � Version effect was carried out by perform-ing ANOVAs at frontal, central and parietal sites. TheOrientation � Version effect reached statistical significance at cen-tral (F(5,60) = 2.79, p = .025, e = .76, g2 = .18) and parietal sites(F(5,60) = 3.98, p = .003, e = .71, g2 = .25). The Orientation � Ver-sion � Laterality interaction reached statistical significance at cen-tral sites (F(20,240) = 2.48, p = .032, e = .21, g2 = .17).

The analysis performed at parietal sites revealed that the orien-tation effect was significant for mirrored (F(5,60) = 3.56, p = .024,e = .60, g2 = .23) and normal letters (F(5,60) = 17.14, p < .001,e = .65, g2 = .59). A linear trend could be fitted for both mirrored(F(1,12) = 7.87, p = .016, g2 = .40) and normal letters (F(1,12) =48.20, p < .001, g2 = .80): the more the letter was rotated, the morenegative the potential. However, when paired contrasts were per-formed in order to determine whether there were specific differ-ences between the different orientations the results were asfollows: while there were no differences between orientations formirrored letters (all adjusted p-values > .05), differences werefound between orientations for normal letters, specifically, be-tween the orientations 0–120, 0–150, 30–120, 30–150, 60–120,60–150, and 90–150 degrees (all adjusted p-values < .05).

The analysis performed at central sites yielded significant ef-fects for orientation (F(5,60) = 9.87, p < .001, e = .61, g2 = .45) andOrientation x Laterality (F(20,240) = 5.07, p = .001, e = .24,g2 = .30) only for normal letters. There were no significant effectsfor mirrored letters. The Orientation � Laterality interaction wasanalyzed by performing separate ANOVAs for each central elec-trode and taking orientation as a factor. With normal letters theorientation effect reached statistical significance at all the central

Fig. 2. Response time means (in milliseconds) for normal and mirrored letters as afunction of stimulus orientation (in degrees).



electrodes (all p-values < .002), and a linear trend could be fitted forall of them (all p-values < .004). Again, the voltage became morenegative the more the letter was rotated.

Fig. 6 shows the spatial distribution of the version effect at eachorientation in 50-ms windows from 400 to 700 ms. It can be seenthat at 0�, where no rotation in the picture plane is required at all,

Fig. 3. (A) Grand-average ERPs (n = 13) elicited by normal letters in each orientation at the P3, Pz, and P4 electrodes. (B) Grand-average ERPs (n = 13) elicited by mirroredletters in each orientation at the P3, Pz, and P4 electrodes.

Fig. 4. Amplitude mean (in microvolts) in the 400–500 ms window for normal (solid line) and mirrored letters (dotted line) as a function of stimulus orientation at the P3, Pz,and P4 electrodes.

Fig. 5. (A) Spatial distribution of the orientation effect in normal letters over all electrodes at the scalp surface (the voltage difference between 400 and 500 ms). From left toright, voltage differences between 30�, 60�, 90�, 120�, 150�, and the 0� normal upright. (B) Spatial distribution of the orientation effect in mirrored letters over all electrodes atthe scalp surface (the voltage difference between 400 and 500 ms). From left to right, voltage differences between 30�, 60�, 90�, 120�, 150�, and the 0� mirrored upright.



the voltage is more negative in mirrored than in normal letters inthe 400–450 and the 450–500 ms windows and that this effect isright lateralized. The version effect is also present at 30�, 60�,and 90� but in those cases the effect seems to be delayed. This ef-fect is not present at large orientations.

A detailed analysis of the version effect confirmed these obser-vations. First, the ANOVAs in the upright condition showed that theinteraction Version � Laterality reached statistical significance inthe 400–450 ms window (F(2,24) = 7.43, p = .003, g2 = .38) andthe 450–500 ms window (F(2,24) = 7.67, p = .003, g2 = .39). Testsof simple effects showed that the amplitude was more negativein mirrored than in normal letters over the right sites in both win-dows (F(1,12) = 6.10, p = .03, g2 = .34 in the 400–450 ms windowand F(1,12) = 4.64, p = .05, g2 = .28 in the 450–500 ms window).Second, when the analysis was performed for misoriented stimulithe results were as follows: (1) the analysis in the 30� orientationshowed that the version effect reached statistical significance inthe 400–450 ms windows over right sites (F(1,12) = 4.91, p = .04,g2 = .29) and in the 450–500 ms windows over the middle andright sites (F(1,12) = 5.35, p = .039, g2 = .31 and F(1,12) = 7.27,p = .019, g2 = .38, respectively): again the amplitude was more neg-ative in mirrored than in normal letters; (2) the analysis in the 60�orientation showed that mirror-normal difference was delayedcomparing to the upright and the 30� conditions and, moreover,has a different scalp distribution: amplitude was more negativefor mirrored than for normal letters at parietal sites in the 450–500 ms window (F(1,12) = 5.23, p = .013, g2 = .30); (3) the analysis

in the 90� orientation again showed a delay in the mirror-normaldifference over the scalp: the same pattern of differences as previ-ously described was found in the 500–550 ms window at centraland parietal sites (F(1,12) = 6.1, p = .03, g2 = .34 and F(1,12) =4.79, p = .04, g2 = .29, respectively); (4) no mirror-normal differ-ences were found for the 120� and 150� orientations in anywindow.

4. Discussion

Previous studies have shown that RT is longer for mirrored thanfor normal letters in mental rotation tasks (Bajric et al. 1999;Hamm et al., 2004; Milivojevic et al., 2003). This increase in RThas been attributed to the fact that only rotation in the pictureplane is involved in misoriented normal letters, whereas rotationin and out of the picture plane is involved in misoriented mirroredletters. Hamm et al. (2004) and Alivisatos and Petrides (1997) pro-vided psychophysiological evidence that an extra rotation out ofthe plane is involved in mirrored letters in the upright position.However, to our knowledge, this extra rotation in mirrored lettershas not been studied to date in orientations other than the upright.The present study aimed (1) to examine whether mental rotationof normal and mirrored letters differs in a letter discriminationtask, and (2) to study the extent to which these differences canbe explained by the fact that an extra rotation after the plane rota-tion is involved in parity judgment on mirror rotated letters. There-fore, we focused our attention on (1) the orientation effect in

Fig. 6. Spatial distribution of the version effect over all electrodes at the scalp surface. Voltage differences mirrored minus normal letters at 0�, 30�, 60�, 90�, 120�, and 150� in50-ms windows from 400 to 700 ms post-stimulus.



normal and mirrored letters and (2) the version effect in differentorientations.

First, the orientation effect was studied. Our results replicatedthe classical orientation effect both in RT and for the amplitudeof the rotation-related negativity in normal letters (Hamm et al.2004; Heil & Rolke, 2002): RT increases and the rotation-relatednegativity becomes more negative with angular deviation from up-right. The rotation-related negativity showed a centro-parietalscalp distribution without hemispheric asymmetry. These resultssuggest that mental rotation in the picture plane is used in misori-ented normal letters. Moreover, the typical indifference of normalletters to small deviations from upright was also found in RT,where a quadratic function between orientation and RT could befitted. For rotations of 60 degrees or less there was no differencefrom the upright condition, which suggests that mental rotationis not necessary to decide on the parity of most of these stimuli.Concerning mirrored letters, the classical orientation effect wasagain found both in RT and for the amplitude of the rotation-re-lated negativity. However, the effect over the rotation-related neg-ativity was not as evident as that found in normal letters. Althougha linear trend between orientation and amplitude could be fitted,differences between orientations were not found when pairedcomparisons were performed. ERP differences between normaland mirrored letters suggest that the mental rotation process inthe two types of stimuli is different. As suggested by Hamm et al.(2004), mirrored letters might require the mental rotation in thepicture plane and out of the picture plane in order to fully normal-ize the stimuli. This fact could explain why the orientation effect isless evident in mirrored than in normal letters, because ‘‘the fliprotation ERP effect would cancel the planar rotation ERP effectfor mirror rotated stimuli” (Hamm et al., p. 816).

Second, we performed a detailed analysis of the version effect.Our analysis of the ERP data comparing mirrored and normal let-ters in the upright condition suggests that the rotation out of thepicture plane has an impact on the electrophysiological activity.A negative-going waveform, right lateralized, and with a latencybetween 400 and 500 ms was elicited by mirrored letters in the up-right condition. This negative waveform was nearly identical to thewell-known rotation-related negativity. Both were similar in la-tency and polarity but showed a different scalp distribution. Thenegativity elicited by mirrored upright letters was right lateralizedwhereas the negativity elicited by misoriented normal letters didnot show hemispheric asymmetry, suggesting that the neural re-sponse to the two types of rotations is different. These results agreewith those obtained by Alivisatos and Petrides (1997) who re-ported the role of the right parietal cortex in a mirror-normal dis-crimination task of upright letters and the role of right and leftparietal cortex in a task that required active rotation in the pictureplane. Our results are also consistent with those of Roberts and Bell(2003), who suggests that rotation of simple two-dimensionalstimuli, can lead to greater activation of the left parietal area thanof the right parietal area. Although mirrored letters in the presentstudy were two-dimensional stimuli, they involve a three-dimen-sional mental rotation because the flipping strategy needs a mentaltransformation of the stimuli out of plane. In contrast, normal let-ters involve a two-dimensional mental rotation because they arebelieved to be rotated only in the picture plane. These differencesin the hemispheric lateralization of mirror and normal letters maybe explained in terms of different mental rotation processes.Whereas the rotation out of the picture plane may need a holisticmental rotation process, which preferentially engaged the righthemisphere, the rotation in the picture plane may also need ananalytic, ‘‘piecemeal” mental rotation process, which preferentiallyengaged the left hemisphere (Corballis, 1997).

Differences between mirrored and normal letters were alsofound at 30�, 60�, and 90�. At 30�, the ERP pattern was similar to

that observed at the upright position: a negative-going waveform,right lateralized, and with a latency between 400 and 500 ms waselicited by mirrored letters. This result was predictable because itis generally agreed that the planar rotation is not needed to makea mirror-normal decision for small departures from upright. Thus,mirrored letters at 30� only have to be rotated out of the pictureplane in order to make a mirror-normal decision and, therefore,differences between 0� and 30� were not expected. As we have pre-viously mentioned, our data confirmed that there was no differ-ence either in reaction time or in the rotation-related negativitybetween these two orientations.

In contrast to the findings at 0� and 30�, when the stimuli werepresented at orientations of 60� and 90�, a delay in the mirror-nor-mal differences was observed. At 60� the difference was found inthe 450–500 ms window and at 90� it was found in the 500–550 ms window. This pattern of results is consistent with the ideathat the rotation out of the picture plane involved in mirrored ro-tated letters might occur after the plane rotation, because the mir-ror-normal difference is delayed across the orientations. However,scalp distribution of the negative-going waveform at 0� and 30�differed from that at 60� and 90�. Whereas the first was right-lat-eralized, the second was centro-parietally distributed and withouthemispheric differences.

As for large misorientations, the mirror-normal difference wasnot found. There are two possible explanations for this result. First,large misorientations may place heavy visuo-spatial demands onnormal letters. The reaction time and error rate results support thisinterpretation. Response time analysis showed that the advantageof normal over mirrored letters decreases with an increment inmisorientation; moreover, participants were less accurate in nor-mal than in mirrored letters at large misorientations. A similar pat-tern of results has been reported by Heil, Bajric, Rösler, andHennighausen (1996b), who found no RT differences between mir-rored and normal letters at large misorientations. The secondexplanation for the absence of mirror-normal ERP difference forlarge misorientations is that the mental rotation in and out of thepicture plane for mirrored letters may occur in parallel. If bothmental rotation processes were assumed to occur sequentially, dif-ferences between mirrored and normal letters should be foundeven at large misorientations.

In summary, two main conclusions can be drawn from the pres-ent study. First, we found evidence that the processing of normaland mirrored letters in a letter discrimination task has a differentimpact on brain activity. The presence of a mental rotation out ofthe plane might account for this difference, because this extra rota-tion might cancel the plane rotation in mirrored letters. Futuremental rotation research should take this difference into accountand study mirror and normal letters separately. Second, our dataprovide evidence that the rotation out of the plane in rotated mir-rored letters may occur after the plane rotation, because mirroredletters elicit a negative-going component whose amplitude wasdelayed across orientations.

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