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1 This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier s archiving and manuscript policies are encouraged to visit:

2 Brain and Cognition 69 (2009) Contents lists available at ScienceDirect Brain and Cognition journal homepage: 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, Barcelona, Spain b Cognitive Neuroscience Research Group, Department of Psychiatry and Clinical Psychobiology, Faculty of Psychology, University of Barcelona, Passeig Vall d Hebron, 171, Barcelona, Spain c Department of Basic Psychology, Faculty of Psychology, University of Barcelona, Passeig Vall d Hebron, 171, Barcelona, Spain article info abstract Article history: Accepted 10 July 2008 Available online 17 August 2008 Keywords: Event-related brain potentials Rotation-related negativity Mirrored letters Parity-judgment task Event-related brain potentials (ERPs) were recorded while participants (n = 13) were presented with mirrored and normal letters at different orientations and were asked to make mirror-normal letter discriminations. As it has been suggested that a mental rotation out of the plane might be necessary to decide on mirrored letters, we wanted to determine whether this rotation occurs after the plane rotation in mirror rotated letters. The results showed that mirrored letters in the upright position elicited a negative-going waveform over the right hemisphere in the ms window. A similar negativity was also present in mirrored letters at 30, 60, and 90, but in these cases it was delayed. Moreover, the well-known orientation effect on the amplitude of the rotation-related negativity was also found, although it was more evident for normal than for mirrored letters. These results indicate that the processing of mirrored letters differs from that of normal letters, and suggest that a rotation out of the plane after the plane rotation may be involved in the processing of mirror rotated letters. Ó 2008 Elsevier Inc. All rights reserved. 1. Introduction Mental rotation is a classical psychological process. It was first reported by Shepard and Metzler (1971) in an experiment where participants were presented with pairs of three-dimensional block figures at different orientations, and were required to determine whether both figures were the same or one was a mirror reflection of the other. Results showed that reaction time (RT) was longer for larger angles of misorientation. It was proposed that this increased RT was due to the fact that in order to perform the parity-judgment task the image had to be mentally rotated to put it in the upright position. Since then, the mental rotation effect has been reported in studies with alphanumeric characters (Cooper & Shepard, 1973; Koriat & Norman, 1985a), letter-like characters (Tarr & Pinker, 1989), left-right hands (Cooper & Shepard, 1975), and even in a naming task with natural objects (Jolicoeur, 1985, 1988, 1990). Although the mental rotation effect has been reported with different types of stimuli, it has been suggested that the form of these RT functions depends on the familiarity of the stimuli. When the stimulus is unfamiliar the RT function is linear (Shepard & Metzler, 1971), whereas when the stimulus is familiar i.e., alphanumeric characters the RT function departs from linearity and shows a q This research was supported by Grants SEJ /PSIC, SEJ / PSIC, and Consolider-Ingenio 2010-CSD from the Spanish Ministry of Science and Technology, and SGR from the Generalitat de Catalunya. * Corresponding author. Fax: address: inunez@ub.edu (M.I. Núñez-Peña). quadratic trend (Cooper & Shepard, 1973). The suggested explanation for this nonlinearity effect is that familiar stimuli are overlearned visual stimuli that achieve a certain degree of indifference to small misorientations from their normal position (Cooper & Shepard, 1973; Koriat & Norman, 1985b). However, unfamiliar stimuli require rotation even for small deviations from upright. Similar mathematical functions relate the angle of misorientation 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 negative-going waveform, maximum over parietal regions, whose amplitude is modulated by the angle of misorientation: the greater the angle of misorientation, the larger the rotation-related negativity. The rotation-related negativity has been reported in studies with alphanumeric characters (Heil, Rauch, & Hennighausen, 1998; Heil & Rolke, 2002; Milivojevic, Johnson, Hamm & Corbalis, 2003), letter-like shapes (Núñez-Peña, Aznar, Linares, Corral & Escera, 2005), paper-folding stimuli (Milivojevic, et al., 2003), leftright hands (Thayer & Johnson, 2006), and geometric objects (Muthukumaraswamy, Johnson, & Hamm, 2003; Rösler, Heil, Bajric, Pauls, & Hennighausen, 1995). It has been suggested that this component is a neurophysiological correlate of the mental rotation process (Heil, 2002), because its amplitude is modulated by the amount of mental rotation needed to make a parity decision. Moreover, there are other evidences. First, Heil, Bajric, Rösler, /$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi: /j.bandc

3 M.I. Núñez-Peña, J.A. Aznar-Casanova / Brain and Cognition 69 (2009) and Hennighausen (1996a) and Heil et al. (1998) found that the rotation-related negativity was evoked by a misoriented stimulus only if mental rotation is required to solve the task. Second, Heil and Rolke (2002) provided evidence that the onset of this negative component is delayed by delaying the mental rotation process. As regards the spatial distribution of the mental rotation process, neuroimaging and electrophysiological studies have reported inconsistent results. Although positron emission tomography (PET) and functional magnetic resonance imaging (fmri) studies have reported clear evidence of the involvement of parietal regions in mental rotation, considerable debate remains as to whether mental rotation is a right parietal function or whether neither 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 some of these contradictory results: the right hemisphere may be preferentially engaged when the task is simple and involves a single transformation, but the left hemisphere is also engaged as the task becomes more complex, as when a coordinated sequence of transformations are required (Milivojevic et al., p. 1355). This explanation agrees with that proposed by Corballis (1997), who differentiates between holistic and analytic mental rotation processes. According to his view, the right hemisphere is preferentially engaged in holistic mental rotation processes when the entire image is mentally rotated in a unitary process and the left hemisphere is preferentially engaged in analytic processes when the image is parsed into units, which are then rotated individually. However, there is also some evidence that the right hemisphere contribution to spatial performance increases with the complexity of the task. Roberts and Bell (2003) reported greater activation of the right parietal region in a three-dimensional mental rotation task than in a two-dimensional one. Alivisatos and Petrides (1997) provided evidence that activity in the left parietal cortex was more intense in a task that required active mental rotation in the picture plane than in one that requires making a mirror-normal decision regarding upright letters. Whereas the change in orientation has been extensively investigated with both behavioral and psychophysiological measures, the mirror-normal difference has attracted less interest among scientists. Behavioral studies have systematically shown that a mirrored stimulus decision takes longer than a normal stimulus decision (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 that the mirrored stimulus is rotated both in the picture plane, in order to put it in the vertical upright position, and out of the plane, in order to put it in the normal upright position. This explanation is supported by several psychophysiological studies. First, Alivisatos and Petrides (1997), in a PET study, found that mirror-normal judgment of upright letters activated similar brain areas to those activated in a classical mental rotation task. This study suggests that both experimental tasks the mirror-normal judgment task with upright letters and the mirror-normal judgment task with letters presented at different orientations require visuo-spatial processing to identify misoriented stimuli. Second, in a recent ERP study, Hamm et al. (2004) concluded that mirrored stimuli are not only rotated in the picture plane but are subsequently rotated out of the plane, involving a flip to fully normalize the mirrored stimuli. While it seems clear that mirrored letters in the upright condition need to be rotated out of the picture plane in order to make a mirror-normal judgment, the case of a mirrored letter presented at an orientation different from upright has been less studied. Hamm et al. (2004) suggested that the flipping of mirrored stimuli will occur at whatever orientation and that it will occur after the plane rotation. They stated that this flipping of mirrored stimuli will occur after the plane rotation because of the theoretical complications that arise if one postulates it as occurring prior to the plane rotation (p. 819). If information about the mirror-normal status of the stimuli is available before the plane rotation, then rotating the mirrored stimuli in the picture plane will be unnecessary. However, no evidence to support this sequential processing has been brought forward so far. The purpose of the present study was to add evidence in support of the ideas that (1) mental rotation out of the picture plane is necessary to make a mirror-normal judgment in mirrored letters and (2) this mental rotation occurs after the plane rotation in orientations different from the upright. Participants were presented with mirrored and normal letters at eleven different orientations the 0 orientation and the 30, 60, 90, 120, or 150 clockwise or counterclockwise orientations and were asked to perform a parity-judgment task. The decision on the normal versions of letters requires only the mental rotation of the stimulus in the picture plane, whereas the decision on the mirrored versions of letters requires mental rotation of the stimulus in the picture plane and an extra rotation out of the plane. Mirrored upright letters, where the decision requires only rotation out of the picture plane, served as a control condition to isolate the flip effect. Once this effect had been isolated, we performed a detailed analysis of the ERPs at different 50-ms windows in order to study the mirror-normal difference in other rotated stimuli. We hypothesized that if a mirror rotated letter is rotated out of the picture plane after the plane rotation, then the flip effect (the difference mirror-normal) would be delayed in the ERP pattern. Moreover, it was predicted that the typical modulation of the amplitude of the rotation-related negativity would be present in normal letters and that this ERP pattern would be different for mirrored letters, where rotation in and out of the picture plane would be needed. 2. Methods 2.1. Participants Fifteen healthy volunteers were tested in this study (12 women; age years, mean = 21.8, standard deviation = 2.5). All were university students and had normal or corrected-to-normal visual acuity. Because of a large number of artifacts, data from two participants were excluded from the ERP data analysis; this analysis was thus performed with data from thirteen subjects (10 women; age years, mean = 21.9, standard deviation = 2.7). Subjects had no history of neurological or psychiatric disorder, and gave written informed consent to participate after the nature of the study had been explained to them Stimuli and procedure The characters were the uppercase letters F, L, P, and R, which were presented at an orientation of 0 or at 30, 60, 90, 120, or 150 clockwise or counterclockwise orientations. Fig. 1 shows some of the stimuli used in the experiment. Stimuli were shown in green on a white background (luminance 110 cd/m 2 ), and at an orientation of 0 subtended a vertical visual angle of 2.3 and a horizontal visual angle of The program used to manage the experiment was developed by the authors using C++/Open GL (glut library). Participants were seated in an electrically shielded, soundattenuating room at a distance of 150 cm from the display screen, whose center was at eye level. They were monitored continuously with a closed circuit video camera. The experiment started with a training period to familiarize participants with the procedure and

4 182 M.I. Núñez-Peña, J.A. Aznar-Casanova / Brain and Cognition 69 (2009) Fig. 1. Normal and mirrored versions of the F letter in each orientation (from 0 to 330 ). the equipment. All subjects achieved a minimum of 90% correct answers in the practice trials. During the recording period, subjects were instructed to relax and to keep their eyes on the screen. They were encouraged to make any eye-blinks during the presentation of a fixation point or during the pauses. The sequence of events began with a red fixation point presentation at the center of the screen that remained in view for 500 ms. The character was then presented for 500 ms, after which the screen remained black until the subject pressed one of the two response buttons. The task was to decide whether each character displayed was presented in a normal or a mirrored version, as quickly as possible, while keeping errors to a minimum. Response fingers were counterbalanced across subjects. Each participant was given ten blocks of 96 trials. A message indicating a 1-min pause appeared on the screen after each block, and a 5-min pause was provided to participants halfway through the experimental trials. The type of trials was controlled within each block in such a way that a block included trials resulting from the 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-reversed), and type of stimulus (four letters). The sequence of presentation in each block was randomized per participant. All participants were tested on 960 trials, 40 for each experimental condition resulting from the combination of orientation, direction of the angular disparity and version Electrophysiological recording EEG was recorded with the SynAmps/SCAN 4.3 hardware and software (NeuroScan, Inc., Herndon, VA) from 31 tin electrodes mounted in a commercial electro-cap (Electro-Cap International, Eaton, OH). Nineteen electrodes were positioned according to the International System: three electrodes were placed over midline sites at Fz, Cz, and Pz locations, along with 8 lateral pairs of 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 at Fpz and Oz, and ten electrodes were placed halfway between the following additional locations: fronto-central (FC1/FC2), frontotemporal (FT3/FT4), centro-parietal (CP1/CP2), temporo-parietal (TP3/TP4), and mastoids (M1/M2). The common reference electrode for EEG and EOG measurements was placed on the tip of the nose. EEG channels were continuously digitized at a rate of 500 Hz by a SynAmp TM amplifier (5083 model, NeuroScan, Inc., Herndon, VA). A band pass filter was set from 0.16 to 30 Hz, and electrode impedance was always kept below 5 kx. For monitoring eye movements an electrode placed at the external canthi of the right eye was used Data analysis Behavioral data Response times for correctly solved trials and error rate were analyzed with repeated-measures ANOVAs, taking version (normal and mirror-reversed) and orientation (0, 30, 60, 90, 120, and 150 ) as within-subjects factors. 2 The repeated-measures ANOVA was performed with the Greenhouse Geisser correction for sphericity departures, which was applied when appropriate. The F value, the uncorrected degrees of freedom, the probability level following correction, the e value and the g 2 effect size index (Kirk, 1996) are reported. Whenever a main effect reached significance, pairwise comparisons were conducted using t tests, and the Hochberg approach was used to control for the increase in Type I error (Keselman, 1998). Tests of simple effects were calculated in the presence of a significant interaction. Finally, trend analyses were also performed EEG analysis Only trials on which the subjects responded correctly were included in the ERP analysis. First, epochs for every subject in each experimental condition were averaged relative to a pre-stimulus baseline consisting of the 100 ms of activity preceding the epoch of interest. Second, trials with artifacts (voltage exceeding ±50 lv in FP1, FP2, FPz, or HEOG) and those with response errors were excluded from the ERP average. The mean number of epochs included in each ERP average varied between 45.9 and 58.5 for the various types of stimuli used. The orientation effect was studied by analyzing mean amplitude measures in the ms window. This latency window was selected because according to visual inspection of ERP waveforms it was representative of the orientation effect. A repeated-measures ANOVA was performed on the ERP 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 and mirror-reversed), orientation (0, 30, 60, 90, 120, and 150 ), frontality (frontal, central, and parietal), and laterality (five levels from left to right). Statistical analyses were performed as described for behavioral data. Topographic maps were plotted using the EEProbe 3.1 program (ANT Software BV, Enschede, The Netherlands). The version effect was studied in a more detailed way in order to detect whether the mirror-normal difference was delayed across orientations. Mean amplitude measures in 50-ms windows from 400 to 700 ms at nine electrodes (F3, Fz, F4, C3, Cz, C4, P3, Pz, and P4,) were analyzed. Repeated-measures ANOVAs were performed at each orientation, taking as factors version (normal and 1 The letters presented at 0 were presented twice as often as letters at the other orientations because the 30, 60, 90, 120, and 150 clockwise and counterclockwise are usually treated as equivalent. 2 Response times were subjected to an initial analysis to test for symmetry about 0. No asymmetries were detected, so data were collapsed into six orientations (0, 30, 60, 90, 120, and 150 ) for all analyses.

5 M.I. Núñez-Peña, J.A. Aznar-Casanova / Brain and Cognition 69 (2009) mirror-reversed), frontality (frontal, central, and parietal), and laterality (left, middle, and right). 3. Results 3.1. Behavioral data Mean response times for normal and mirrored letters as a function of stimulus orientation are plotted in Fig. 2. Overall response time increased with angular deviation from upright (F(5,70) = 44.47, p <.001, e =.23, g 2 =.76), and responses to normal letters were faster than responses to mirrored letters (F(1,14) = 25.20, p <.001, g 2 =.64). However, the ANOVA also yielded a significant Orientation Version interaction (F(5, 70) = 4.44, p =.012, e =.51, g 2 =.24), so the orientation effect varied according to the letter version. A more detailed analysis of the interaction showed that the orientation effect in normal letters was described by a linear (F(1,14) = 73.31, p <.001, g 2 =.84) and a quadratic (F(1,14) = 34.97, p <.001, g 2 =.71) trend, whereas the same effect in mirror-reversed letters was described by a linear trend (F(1,14) = 33.08, p <.001, g 2 =.70). Moreover, tests of simple effects demonstrated that responses to mirrored stimuli were slower than to normal stimuli at all the orientations (all p-values <.001) except for 150 degrees (p =.07). Therefore the advantage of normal over mirrored letters decreases gradually with increasing angular deviation from upright. Error rate ANOVA showed a significant effect for orientation (F(5,70) = 24.54, p <.001, e =.29, g 2 =.64), version (F(1,14) = 9.26, p =.009, g 2 =.40) and the interaction Orientation Version (F(5,70) = 15.17, p <.001, e =.29, g 2 =.52). Overall, error rate increased with greater angular disparity from the upright. The orientation effect was described by a linear and a quadratic trend both in mirrored (F(1,14) = 8.10, p =.013, g 2 =.37 for linear trend, and F(1,14) = 17.75, p =.001, g 2 =.56 for quadratic trend) and normal letters (F(1,14) = 26.42, p <.001, g 2 =.65 for linear trend, and F(1,14) = 31.22, p <.001, g 2 =.69 for quadratic trend). As for the Fig. 2. Response time means (in milliseconds) for normal and mirrored letters as a function of stimulus orientation (in degrees). interaction Orientation Version, tests of simple effects showed that accuracy was worse in normal than in mirrored letters at 150 angular disparity from upright (F(1, 14) = 18.10, p <.001, g 2 =.56). However, there were no differences in accuracy between the two versions of letters for the other orientations Event-related potentials Fig. 3A and B show the grand-average ERPs for each orientation of normal and mirror reversed letters at P3, Pz, and P4. The orientation effect is evident for normal letters. As can be seen in Fig. 3A, the rotation-related negativity becomes more negative with increasing angular disparity from upright, the effect being more evident for larger deviations. However, the orientation effect is not so clear for mirror-reversed letters (see Fig. 3B): although the voltage tends to be more negative the greater the degree to be rotated, these differences seem not to be as large as those for normal letters. Fig. 4 shows the amplitude means in the ms window for normal and mirror reversed letters as a function of stimulus orientation at P3, Pz and P4. These plots show again that the orientation effect is different for normal and for mirror-reversed letters: the orientation effect over the ERP amplitude is more evident for normal letters than for mirrored letters. Voltage maps in Fig. 5A and B showed the spatial distribution of the orientation effect in normal and mirror-reversed letters over all electrodes at the scalp surface in the ms window. These voltage maps show that the orientation effect has a centro-parietal scalp distribution in normal letters and is not so clear for mirrored letters. The statistical analysis performed on the ms window supports these observations. The overall ANOVA showed significant effects of orientation (F(5,60) = 11.40, p <.001, e =.54, g 2 =.49), Orientation Version (F(5,60) = 2.68, p =.03, e =.77, g 2 =.18), Orientation Version Frontality (F(10,120) = 4.37, p =.003, e =.42, g 2 =.27) and Orientation Version Laterality (F(20,240) = 2.46, p=.035, e =.29, g 2 =.17). A more detailed analysis of the Orientation Version effect was carried out by performing ANOVAs at frontal, central and parietal sites. The Orientation Version effect reached statistical significance at central (F(5,60) = 2.79, p =.025, e =.76, g 2 =.18) and parietal sites (F(5,60) = 3.98, p =.003, e =.71, g 2 =.25). The Orientation Version Laterality interaction reached statistical significance at central sites (F(20,240) = 2.48, p =.032, e =.21, g 2 =.17). The analysis performed at parietal sites revealed that the orientation effect was significant for mirrored (F(5, 60) = 3.56, p =.024, e =.60, g 2 =.23) and normal letters (F(5,60) = 17.14, p <.001, e =.65, g 2 =.59). A linear trend could be fitted for both mirrored (F(1,12) = 7.87, p =.016, g 2 =.40) and normal letters (F(1,12) = 48.20, p <.001, g 2 =.80): the more the letter was rotated, the more negative the potential. However, when paired contrasts were performed in order to determine whether there were specific differences between the different orientations the results were as follows: while there were no differences between orientations for mirrored letters (all adjusted p-values >.05), differences were found between orientations for normal letters, specifically, between the orientations 0 120, 0 150, , , , , and degrees (all adjusted p-values <.05). The analysis performed at central sites yielded significant effects for orientation (F(5,60) = 9.87, p <.001, e =.61, g 2 =.45) and Orientation x Laterality (F(20,240) = 5.07, p =.001, e =.24, g 2 =.30) only for normal letters. There were no significant effects for mirrored letters. The Orientation Laterality interaction was analyzed by performing separate ANOVAs for each central electrode and taking orientation as a factor. With normal letters the orientation effect reached statistical significance at all the central

6 184 M.I. Núñez-Peña, J.A. Aznar-Casanova / Brain and Cognition 69 (2009) 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 mirrored letters in each orientation at the P3, Pz, and P4 electrodes. Fig. 4. Amplitude mean (in microvolts) in the 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 to right, 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 at the 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. electrodes (all p-values <.002), and a linear trend could be fitted for all of them (all p-values <.004). Again, the voltage became more negative the more the letter was rotated. Fig. 6 shows the spatial distribution of the version effect at each orientation in 50-ms windows from 400 to 700 ms. It can be seen that at 0, where no rotation in the picture plane is required at all,

7 M.I. Núñez-Peña, J.A. Aznar-Casanova / Brain and Cognition 69 (2009) 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 in 50-ms windows from 400 to 700 ms post-stimulus. the voltage is more negative in mirrored than in normal letters in the and the ms windows and that this effect is right lateralized. The version effect is also present at 30, 60, and 90 but in those cases the effect seems to be delayed. This effect is not present at large orientations. A detailed analysis of the version effect confirmed these observations. First, the ANOVAs in the upright condition showed that the interaction Version Laterality reached statistical significance in the ms window (F(2,24) = 7.43, p =.003, g 2 =.38) and the ms window (F(2,24) = 7.67, p =.003, g 2 =.39). Tests of simple effects showed that the amplitude was more negative in mirrored than in normal letters over the right sites in both windows (F(1,12) = 6.10, p =.03, g 2 =.34 in the ms window and F(1,12) = 4.64, p =.05, g 2 =.28 in the ms window). Second, when the analysis was performed for misoriented stimuli the results were as follows: (1) the analysis in the 30 orientation showed that the version effect reached statistical significance in the ms windows over right sites (F(1, 12) = 4.91, p =.04, g 2 =.29) and in the ms windows over the middle and right sites (F(1,12) = 5.35, p =.039, g 2 =.31 and F(1,12) = 7.27, p =.019, g 2 =.38, respectively): again the amplitude was more negative in mirrored than in normal letters; (2) the analysis in the 60 orientation showed that mirror-normal difference was delayed comparing to the upright and the 30 conditions and, moreover, has a different scalp distribution: amplitude was more negative for mirrored than for normal letters at parietal sites in the ms window (F(1,12) = 5.23, p =.013, g 2 =.30); (3) the analysis in the 90 orientation again showed a delay in the mirror-normal difference over the scalp: the same pattern of differences as previously described was found in the ms window at central and parietal sites (F(1,12) = 6.1, p =.03, g 2 =.34 and F(1,12) = 4.79, p =.04, g 2 =.29, respectively); (4) no mirror-normal differences were found for the 120 and 150 orientations in any window. 4. Discussion Previous studies have shown that RT is longer for mirrored than for normal letters in mental rotation tasks (Bajric et al. 1999; Hamm et al., 2004; Milivojevic et al., 2003). This increase in RT has been attributed to the fact that only rotation in the picture plane is involved in misoriented normal letters, whereas rotation in and out of the picture plane is involved in misoriented mirrored letters. Hamm et al. (2004) and Alivisatos and Petrides (1997) provided psychophysiological evidence that an extra rotation out of the plane is involved in mirrored letters in the upright position. However, to our knowledge, this extra rotation in mirrored letters has not been studied to date in orientations other than the upright. The present study aimed (1) to examine whether mental rotation of normal and mirrored letters differs in a letter discrimination task, and (2) to study the extent to which these differences can be explained by the fact that an extra rotation after the plane rotation is involved in parity judgment on mirror rotated letters. Therefore, we focused our attention on (1) the orientation effect in

8 186 M.I. Núñez-Peña, J.A. Aznar-Casanova / Brain and Cognition 69 (2009) normal and mirrored letters and (2) the version effect in different orientations. First, the orientation effect was studied. Our results replicated the classical orientation effect both in RT and for the amplitude of the rotation-related negativity in normal letters (Hamm et al. 2004; Heil & Rolke, 2002): RT increases and the rotation-related negativity becomes more negative with angular deviation from upright. The rotation-related negativity showed a centro-parietal scalp distribution without hemispheric asymmetry. These results suggest that mental rotation in the picture plane is used in misoriented normal letters. Moreover, the typical indifference of normal letters to small deviations from upright was also found in RT, where a quadratic function between orientation and RT could be fitted. For rotations of 60 degrees or less there was no difference from the upright condition, which suggests that mental rotation is not necessary to decide on the parity of most of these stimuli. Concerning mirrored letters, the classical orientation effect was again found both in RT and for the amplitude of the rotation-related negativity. However, the effect over the rotation-related negativity was not as evident as that found in normal letters. Although a linear trend between orientation and amplitude could be fitted, differences between orientations were not found when paired comparisons were performed. ERP differences between normal and mirrored letters suggest that the mental rotation process in the two types of stimuli is different. As suggested by Hamm et al. (2004), mirrored letters might require the mental rotation in the picture plane and out of the picture plane in order to fully normalize the stimuli. This fact could explain why the orientation effect is less evident in mirrored than in normal letters, because the flip rotation ERP effect would cancel the planar rotation ERP effect for 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 letters in the upright condition suggests that the rotation out of the picture plane has an impact on the electrophysiological activity. A negative-going waveform, right lateralized, and with a latency between 400 and 500 ms was elicited by mirrored letters in the upright condition. This negative waveform was nearly identical to the well-known rotation-related negativity. Both were similar in latency and polarity but showed a different scalp distribution. The negativity elicited by mirrored upright letters was right lateralized whereas the negativity elicited by misoriented normal letters did not show hemispheric asymmetry, suggesting that the neural response to the two types of rotations is different. These results agree with those obtained by Alivisatos and Petrides (1997) who reported the role of the right parietal cortex in a mirror-normal discrimination task of upright letters and the role of right and left parietal cortex in a task that required active rotation in the picture plane. Our results are also consistent with those of Roberts and Bell (2003), who suggests that rotation of simple two-dimensional stimuli, can lead to greater activation of the left parietal area than of the right parietal area. Although mirrored letters in the present study were two-dimensional stimuli, they involve a three-dimensional mental rotation because the flipping strategy needs a mental transformation of the stimuli out of plane. In contrast, normal letters involve a two-dimensional mental rotation because they are believed to be rotated only in the picture plane. These differences in the hemispheric lateralization of mirror and normal letters may be explained in terms of different mental rotation processes. Whereas the rotation out of the picture plane may need a holistic mental rotation process, which preferentially engaged the right hemisphere, the rotation in the picture plane may also need an analytic, piecemeal mental rotation process, which preferentially engaged the left hemisphere (Corballis, 1997). Differences between mirrored and normal letters were also found at 30, 60, and 90. At30, 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 was elicited by mirrored letters. This result was predictable because it is generally agreed that the planar rotation is not needed to make a mirror-normal decision for small departures from upright. Thus, mirrored letters at 30 only have to be rotated out of the picture plane in order to make a mirror-normal decision and, therefore, differences between 0 and 30 were not expected. As we have previously mentioned, our data confirmed that there was no difference either in reaction time or in the rotation-related negativity between these two orientations. In contrast to the findings at 0 and 30, when the stimuli were presented at orientations of 60 and 90, a delay in the mirror-normal differences was observed. At 60 the difference was found in the ms window and at 90 it was found in the ms window. This pattern of results is consistent with the idea that the rotation out of the picture plane involved in mirrored rotated letters might occur after the plane rotation, because the mirror-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-lateralized, the second was centro-parietally distributed and without hemispheric differences. As for large misorientations, the mirror-normal difference was not found. There are two possible explanations for this result. First, large misorientations may place heavy visuo-spatial demands on normal letters. The reaction time and error rate results support this interpretation. Response time analysis showed that the advantage of normal over mirrored letters decreases with an increment in misorientation; moreover, participants were less accurate in normal than in mirrored letters at large misorientations. A similar pattern of results has been reported by Heil, Bajric, Rösler, and Hennighausen (1996b), who found no RT differences between mirrored and normal letters at large misorientations. The second explanation for the absence of mirror-normal ERP difference for large misorientations is that the mental rotation in and out of the picture plane for mirrored letters may occur in parallel. If both mental rotation processes were assumed to occur sequentially, differences between mirrored and normal letters should be found even at large misorientations. In summary, two main conclusions can be drawn from the present study. First, we found evidence that the processing of normal and mirrored letters in a letter discrimination task has a different impact on brain activity. The presence of a mental rotation out of the plane might account for this difference, because this extra rotation might cancel the plane rotation in mirrored letters. 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