Brain and Language 100 (2007) 142–149 www.elsevier.com/locate/b&l Laterality in metaphor processing: Lack of evidence from functional magnetic resonance imaging for the right hemisphere theory Alexander M. Rapp a,b,¤, Dirk T. Leube b,c, Michael Erb b, Wolfgang Grodd b, Tilo T.J. Kircher c b a Department of Psychiatry, University of Tuebingen, Osianderstrasse 24, 72076 Tuebingen, Germany Section Experimental MR of the CNS, Department of Neuroradiology, University of Tuebingen, Hoppe-Seyler-Str. 3, 72076 Tuebingen, Germany c Department of Psychiatry and Psychotherapy, RWTH Aachen University, Pauwelsstr. 30, D-52074 Aachen, Germany Accepted 4 April 2006 Available online 4 May 2006 Abstract We investigated processing of metaphoric sentences using event-related functional magnetic resonance imaging (fMRI). Seventeen healthy subjects (6 female, 11 male) read 60 novel short German sentence pairs with either metaphoric or literal meaning and performed two diVerent tasks: judging the metaphoric content and judging whether the sentence has a positive or negative connotation. Laterality indices for 8 regions of interest were calculated: Inferior frontal gyrus (opercular part and triangular part), superior, middle, and inferior temporal gyrus, precuneus, temporal pole, and hippocampus. A left lateralised network was activated with no signiWcant diVerences in laterality between the two tasks. The lowest degree of laterality was found in the temporal pole. Other factors than metaphoricity per se might trigger right hemisphere recruitment. Results are discussed in the context of lesion and hemiWeld studies.  2006 Elsevier Inc. All rights reserved. Keywords: Language lateralisation; Metaphor nonliteral language; fMRI; Right hemisphere; Laterality; Schizophrenia; Metaphoric language; Proverb 1. Introduction Metaphoric language is a ubiquitous part of everyday communication, not just a poetic device (Gibbs, 1994). The neural basis behind the process of understanding a metaphor is thereby a relevant topic for our understanding of the neuroanatomy of language comprehension. In addition, metaphor, and proverb comprehension are of clinical relevance in neuropsychiatry. For instance, patients with disorders like schizophrenia and autism have a deWcit in comprehending metaphors and assessing the meaning of proverbs. There is some evidence that disturbed lateralisation processes may play a role in the pathophysiology of these disorders (Mitchell & Crow, 2005; Kircher et al., 2004; Sommer, Ramsey, Kahn, Aleman, & Bouma, 2001). * Corresponding author. Fax: +497071 29 4141. E-mail address: Alexander.Rapp@med.uni-tuebingen.de (A.M. Rapp). 0093-934X/$ - see front matter  2006 Elsevier Inc. All rights reserved. doi:10.1016/j.bandl.2006.04.004 Although metaphor and proverb comprehension skills are tested routinely in psychiatry, the contribution of the two cerebral hemispheres to their comprehension is not yet understood (Papagno & Carporali, 2007). During the last decade, enormous progress has been made in knowledge on hemispheric lateralisation of both literal and non-literal language processing. It is now clear that both hemispheres contribute to language processing when more complex processes such as whole sentences and texts come into play (Beeman, 1993; Kircher, Brammer, Tous-Andreu, Williams, & McGuire, 2001; Bookheimer, 2002; Xu, Kemeny, Park, Frattali, & Braun, 2005). Recent consensus has been that the right cerebral hemisphere plays a key role especially during processing of non-literal language and during processing of complex linguistic speech forms like humor, irony, sarcasm, metaphors, and proverbs (Burgess & Chiarello, 1996; Coulson & Wu, 2005; Mitchell & Crow, 2005). For metaphors, this is referred to as the “right hemisphere theory” of A.M. Rapp et al. / Brain and Language 100 (2007) 142–149 metaphor processing. One strong version of this theory predicts that metaphors are predominantly processed by the right hemisphere. The right hemisphere theory is mainly based on studies with patients who suVer from right cerebral hemisphere lesions, on hemiWeld investigations (Anaki, Faust, & Kravetz, 1998), and on research with positron emission tomography (Bottini et al., 1994). Patients with right hemisphere damage have diYculties in understanding humorous expressions (Gardner, Ling, Flamm, & Silverman, 1975, see Wild, Rodden, Grodd, & Ruch, 2003) and non-literal language (Brownell, Simpson, Bihrle, Potter, & Gardner, 1990). However, recent research has questioned the speciWty of some of these results for the right cerebral hemisphere, especially in the case of metaphor comprehension and idiom comprehension (Oliveri, Romero, & Papagno, 2004). For example Zaidel, Kasher, Soroker, and Batori (2002) found such severe impairments also in left hemisphere damaged patients. Complementarily, right hemisphere damaged patients had preserved metaphor comprehension skills and lesion extent in the left, but not the right hemisphere correlated with the ability to verbally explain the meaning of metaphors (Giora, Zaidel, Soroker, Batori, & Kasher, 2003). Another methodology to investigate contribution of the cerebral hemispheres to metaphor comprehension is hemiWeld research. As in the lesion studies, data from hemiWeld studies are heterogeneous: whereas a study using single word level suggested a right hemisphere advantage for metaphor comprehension (Anaki et al., 1998), a study from the same work group investigating processing of whole sentences did not (Faust & Weisper, 2000). Few studies have yet investigated metaphor comprehension using functional magnetic resonance imaging. Two functional imaging studies investigated processing of metaphoric words (Mashal, Faust, Hendler, & Jung-Beeman, 2005; Lee & Dapretto, 2006). One of these studies found right hemisphere involvement (Mashal et al., 2005) whereas the other did not (Lee & Dapretto, 2006). However processing of single metaphoric words may represent a diVerent cognitive task than processing phrasal metaphors. Noteworthy, lateralisation eVects during processing of single metaphoric words are diVerent from those of phrasal metaphors in previous investigations (Gagnon, Goulet, Giroux, & Joanette, 2003; Faust & Weisper, 2000). In addition, two recent fMRI investigations (Ahrens et al., 2007; Stringaris, Medford, Giampetro, Brammer, & David, 2007) investigated brain activation during comprehension of phrasal metaphors. Stringaris et al. (2007) found diVerential activation between metaphoric and literal sentences in the left hemisphere thalamus and left inferior frontal gyrus. Only small clusters of activation were found in the right cerebral hemisphere for both reading metaphors > literal sentences and the opposite contrast, reading literal sentences > metaphors. In contrast, Ahrens et al. (2007) found a small diVerence between conventional metaphors and literal sentences in the right inferior temporal gyrus, but the diVerences 143 between anomalous metaphors and literal sentences were quite large and involved bilateral activation. Further evidence from functional imaging against the right hemisphere theory of metaphor processing comes from a previous investigation carried out by our group. Using event-related functional magnetic resonance imaging, we directly compared brain activation during processing of metaphoric and carefully matched literal control sentences (Rapp, Leube, Erb, Grodd, & Kircher, 2004). DiVerence contrast between metaphoric > literal sentences revealed activation in a left lateralised network including the left inferior frontal gyrus and the left temporal lobe, but no signiWcant diVerences in right hemisphere homologues. This Wnding is in contrast to a positron emission tomography study on metaphor comprehension by Bottini et al. (1994). However, Bottini et al.’s study investigated only 6 subjects and diVerences in the results of the two studies may be well explained by methodological factors and stimulus selection (Rapp et al., 2004). In the present study we used a less conservative analysis than in our previous investigation on metaphor comprehension (Rapp et al., 2004). For the analysis presented in this paper, we used a region-of-interest analysis and a more liberal threshold. We calculated laterality indices for several regions of interest. The laterality index is a count between ¡1 and +1. (¡1 if only voxels in the left hemisphere are activated and +1 if only voxels in the right hemisphere are activated). Calculating regional laterality indices from fMRI data has shown a high correspondence with results from intracarotid amytal testing, the gold standard in investigating language lateralisation (Spreer et al., 2002). The rationale behind this was as following: We predicted left lateralisation for metaphoric and for non-metaphoric (literal) sentences. In addition, we investigated the inXuence of the task performed by the subjects on language laterality. In our previous investigation, the task for the subjects during the experiment was to judge the connotation of the sentences by pressing one of the two buttons with their right index Wnger. However, judging the connotation of sentences could possibly itself alter laterality during language processing tasks. More speciWcally, the right hemisphere has been implicated as important both for ascertaining connotative meanings (Brownell, Potter, Michelow, & Gardner, 1984) and evaluating the emotional content of text (Borod, Bloom, Brickman, Nakhutina, & Curko, 2002; Ferstl, Rinck, & von Cramon, 2005). Our connotation judgement task could have balanced language lateralisation in favour of the left hemisphere. In this paper, we present data from another experiment in which the same subjects pressed a button when deciding whether a sentence is metaphoric or literal. Based on the results from our previous experiment, we predicted left lateralised activation for metaphors and literal sentences in both tasks. We investigated laterality indices for regions of interest that were activated in previous functional imaging studies on metaphor comprehension (Bottini et al., 1994; Rapp et al., 2004; Sotillo et al., 2005; Ahrens et al., 2007). More 144 A.M. Rapp et al. / Brain and Language 100 (2007) 142–149 speciWcally, we looked at the dorsolateral prefrontal cortex (Rapp et al., 2004, Ahrens et al., 2007), the superior temporal gyrus (Bottini et al., 1994; Kircher, Leube, Erb, Grodd, & Rapp, 2006), the middle temporal gyrus (Sotillo et al., 2005; Ahrens et al., 2007), the inferior temporal gyrus (Rapp et al., 2004), the precuneus (Bottini et al., 1994; Kircher et al., 2006), the temporal pole (Rapp et al., 2004), and the hippocampus. 2. Materials and methods 2.1. Subjects Seventeen healthy, right handed (Annett, 1970) subjects (6 female, 11 male), all native German speakers, participated in the study. Mean age was 29.1 (SD: 8.9, range: 20– 52) years. Exclusion criteria were past or present medical or psychiatric illness or psychiatric illness in Wrst degree relatives as well as impaired language skills (Lehrl, Triebig, & Fischer, 1995). Permission for the study was obtained from the local ethical committee. After complete description of the study, subjects gave their informed consent. 2.2. Experimental stimuli A set of 260 short German sentences were initially created de novo for the experiment. Half of these sentences had a metaphoric meaning which was literally implausible. Sentence pairs diVered only in their last one to three words and had either a metaphoric (e.g., “Die Worte des Liebhabers sind Harfenklänge” [the lovers’ words are harp sounds]) or a literal (“Die Worte des Liebhabers sind Lügen” [the lovers’ words are lies]) meaning. All stimulus sentences were simple statements (of the form “a” is a “b”). We chose this simple form of stimuli to exclude possible confounding factors such as complex syntax processing. Prior to the study, 21 raters, who did not take part in the fMRI experiment, rated each sentence on 6 point comprehensibility (1 D completely absurd; 6 D highly meaningful), metaphoricity, imageability, and sentence content (1 D very negative content; 6 D very positive content) scales. From the total pool, a set of 120 sentences (60 metaphors with their literal counterparts) were chosen as stimuli for the fMRI experiment. Post-hoc testing showed that 7 metaphoric sentences out of the 60 used in the fMRI experiment occurred in a large corpus (Google). All stimuli scored > 3.5 in the comprehensibility rating (metaphors mean 4.57, SD 0.56, literal sentences mean 5.67, SD 0.26, p < 0.001, MannU-Whitney-Test). There was a signiWcant diVerence in the estimated imageability (metaphors 4.33, SD 0.57, literal sentences 2.81, SD 0.87, p < 0.001, Mann-U-Whitney-Test) and metaphoricity (metaphors 5.39, SD 0.97, literal sentences 1.40, SD 0.78, p < 0.001, Mann-Whitney-U-Test), whereas they were matched for tense (past or present), number of words, word frequency of the last 3 words and positive connotation (metaphors 3.22, SD 0.79, literal sentences 3.76, SD 0.73, p D 0.13). 2.3. Experimental task and procedure During two diVerent tasks (each involving either “metaphoricity judgements” or “connotation judgements”), subjects were instructed to read all sentences silently and then to respond by pressing one of two buttons. In the “metaphoricity” task, subjects had to decide whether the sentence had a metaphoric or a literal content. In the “connotation” task, they indicated whether the sentence had a positive or negative connotation (for example the metaphoric sentence “die Worte des Liebhabers sind Harfenklaenge” [the lovers’ words are harp sounds] has a positive connotation). During each of the two conditions, the subjects saw one version of the sentence pairs (e.g., during the connotation condition they saw the metaphoric one and during the metaphoricity condition they saw the literal counterpart). This sequence was counterbalanced between the subjects. Both tasks were practiced in a training session prior to the fMRI experiment with metaphors not used in the experiment. During the Wllers presentations subjects rested without response. During the fMRI scanning procedure, subjects lay supine in the MR-scanner, their head being secured by foam rubber to minimise movement artefacts. Sentences were presented as whole sentences, visually on a translucent screen viewed by the subjects via a mirror, each sentence in two lines with black letters on a grey background. A total of 75 stimuli (30 metaphors, 30 literal sentences, 15 Wllers (grey background without a sentence)) were presented within each scanning session, each for 5 s with a 3-s interstimulus interval of blank screen. Stimuli were presented in a pseudo-randomized order and counterbalanced across subjects, so that subjects saw only one version of each sentence pair (metaphor or literal) during each task. 2.4. Functional MRI acquisition Imaging was performed on a 1.5-T Scanner (Siemens, SONATA). Functional images were acquired with an echoplanar image sequence which is sensitive to BOLD-contrast (TE 40 ms, TR 2 s,  Xip angle D 90°). The measurement sequence used “mosaic” images to allow fast data storing and handling (Klose, Erb, Wildgruber, Muller, & Grodd, 1999). The volume covered the whole brain with a 64¤64 matrix and 22 slices (voxel size 3¤3¤5 mm3); the slice thickness was 5 mm with a 1 mm inter-slice gap. Two runs consisting of 310 volumes were acquired during the experiment. The Wrst eight volumes of each run were discarded to reach steady state magnetisation. A trigger signal from the scanner, the button presses of the subjects and the onset of the stimuli were registered in a protocol together with the timeline on a separate computer. 2.5. Data analysis Data analysis was performed with SPM 99 (Wellcome Department of Cognitive Neurology, London). In a Wrst A.M. Rapp et al. / Brain and Language 100 (2007) 142–149 step of data analysis, the functional images of each subject were corrected for motion and realigned by using the Wrst scan of the block as reference. T1 anatomical images were coregistered to the mean of the functional scans and spatial normalized to the SPM T1 template in the MNI space. The same transformation was applied to normalize the functional data. Finally, the functional images were smoothed with a 12 mm full-width, half-maximum (FWHM) Gaussian Wlter. Model functions/time courses were calculated by deWning SOA (stimulus onset asynchrony) from the protocol as events using a box car function (5 s) convolved with the hemodynamic response function (hrf) to specify the appropriate design matrix. Condition and subject eVects were estimated according to the general linear model at each voxel in brain space. A high pass Wlter of 80 and a low pass Wlter of 4 were used. SigniWcant signal changes for each subject and condition were assessed using t-statistics. For each subject, diVerential contrasts were calculated between metaphoric sentences and baseline as well as between literal sentences and baseline. Results of reading each sentence type versus baseline showed robust activation mainly in the visual cortex, the left hemisphere motor cortex and a left lateralised network including the temporal lobes and prefrontal cortex in all the subjects in both tasks. The results for group statistics and diVerential contrasts of the “connotation” task have been published previously (Rapp et al., 2004). For the second part of the analysis, eight regions of interest were deWned using the automated anatomic labelling tool box (AAL) (Tzourio-Mazoyer et al., 2002). These regions were selected because of diVerential contrasts found there in our previous investigation or in the literature on neural correlates of metaphor processing. Regions of interest were: The inferior frontal gyrus (opercular part) (AAL: F3O) (Rapp et al., 2004), the inferior frontal gyrus (triangular part) (AAL: F3T) (Rapp et al., 2004; Ahrens et al., 2007), the superior temporal gyrus (AAL: T1) (Bottini et al., 1994), the middle temporal gyrus (AAL: T2) (Sotillo et al., 2005), the inferior temporal gyrus (AAL: T3), the precuneus (AAL: PQ) (Bottini et al., 1994; Kircher et al., 2006), the temporal pole (middle temporal gyrus) (AAL: T2P) (Rapp et al., 2004), and the hippocampus (AAL: HIP). After that, the laterality index was calculated. This was done by counting supra-threshold voxels in each hemisphere (a threshold of P < 0.005 was used) and calculating a laterality index: (R¡L)/(L+R), where L is the number of suprathreshold voxels in the left hemisphere and R is the number of suprathreshold voxels in the right hemisphere. The result of this laterality index ranges between ¡1 and +1. A negative value indicates that more voxels are activated in the left than in the right cerebral hemisphere. For example, if only voxels in the left cerebral but no in the right hemisphere are activated, the index is ¡1. Four laterality indices were calculated for each subject and region of interest: for each of the two tasks (metaphoricity judgement and connotation judgement), one for reading of metaphors and one for reading of literal sentences. Mean values were 145 calculated for each condition between the 17 subjects. Statistical signiWcance was assessed using student’s t-test at a p D 0.05 level. 3. Results Whole brain analysis of the “connotation” task from 15 out of the 17 subjects of the current study has been published previously (Rapp et al., 2004). Reaction time was deWned as the time between the onset of the sentence and the button press of the subject. For the metaphoricity judgement task, mean reaction time was 2.27 s (SD 0.37) for the metaphoric sentences and 2.39 (SD 0.40) for the literal sentences (p D 0.41). Accuracy of response was 94.0% (SD 7.15, range 78–100%). For the connotation judgement task, mean reaction time was 2.28 s (SD 0.44) for the metaphoric sentences and 2.11 (SD 0.39) for the literal sentences (p D 0.20). Accuracy of response was 94.1% (SD 4.5, range 85–98%). DiVerences in reaction time between the metaphoricity and connotation task were not signiWcant. The results for the laterality indices for each task, sentence type and region are shown in Table 1. 4. Discussion No signiWcant diVerences in laterality across literal and metaphoric stimuli were found in the regions of interest under investigation. Relative to carefully matched literal control sentences, no signiWcant diVerences in laterality were found in the superior temporal gyrus, the middle temporal gyrus, the inferior temporal gyrus, the triangular and the opercular part of the inferior frontal gyrus, the precuneus, the temporal pole, and the hippocampus. Two diVerent tasks were used in our experiment, metaphoricity judgement and connotation judgement. In nearly all regions of interest, the fMRI activation was clearly left lateralised with only small group diVerences between the two tasks. Marked diVerences in laterality between diVerent tasks were found during metaphor comprehension in previous studies, for example, in the seminal investigation by Winner and Gardner (1977), but only when pictorial probes were used. It is possible that both of our tasks bias processing demands in the cerebral hemispheres towards the same direction. The lowest degree of cerebral lateralisation was found in the temporal pole. Whereas a moderate left-lateralisation was found during connotation judgement in this region, there were more activated voxels during metaphoricity judgement of metaphoric sentences in the right than in the left hemisphere. However, this diVerence was not statistically signiWcant, presumably because of the marked inter-individual diVerence in hemispheric laterality between the subjects in our study. Large inter-subject diVerences in laterality indices have been constantly described in functional imaging studies using laterality indices (see Ramsey, Sommer, Rutten, & Kahn, 2001). Little is known about the reasons of this large 146 Mean values from data of 17 healthy subjects (threshold of p < 0.005). No signiWcant diVerence were evident between metaphoric and literal sentences in any of the above regions. (* student’s t-test). Literal sentences 0.67 0.94 0.99 0.69 0.77 0.53 0.51 0.47 0.98 0.49 0.60 0.94 0.44 0.15 0.70 0.63 0.80 0.67 0.99 0.77 0.72 0.15 0.70 0.29 ¡0.57 (SD: 0.46) ¡0.39 (SD: 0.57) ¡0.42 (SD: 0.60) ¡0.54 (SD: 0.45) ¡0.39 (SD: 0.54) ¡0.06 (SD: 0.71) ¡0.46 (SD: 0.59) ¡0.06 (SD: 0.69) ¡0.53 (SD: 0.46) ¡0.31 (SD: 0.47) ¡0.42 (SD: 0.68) ¡0.59 (SD: 0.48) ¡0.45 (SD: 0.48) ¡0.24 (SD: 0.60) ¡0.54 (SD: 0.47) ¡0.32 (SD: 0.50) 0.86 0.90 0.59 0.86 0.12 0.20 0.52 0.46 ¡0.41 (SD: 0.48) ¡0.50 (SD: 0.48) ¡0.42 (SD: 0.67) ¡0.61 (SD: 0.48) ¡0.34 (SD: 0.42) ¡0.23 (SD: 0.81) ¡0.60 (SD: 0.44) ¡0.24 (SD: 0.64) ¡0.44 (SD: 0.56) ¡0.52 (SD: 0.58) ¡0.28 (SD: 0.73) ¡0.58 (SD: 0.57) ¡0.57 (SD: 0.45) 0.16 (SD: 0.85) ¡0.46 (SD: 0.63) ¡0.43 (SD: 0.63) Inferior frontal gyrus (triangular part) Inferior frontal gyrus (opercular part) Superior temporal gyrus Middle temporal gyrus Inferior temporal gyrus Temporal pole Hippocampus Precuneus Connotation task Metaphoricity task Table 1 Laterality indices for eight regions of interest Metaphoric sentences Literal sentences p value* Metaphoric sentences Literal sentences p value* Metaphoric sentences Statistics Metaphoricity task vs connotation task (p value*) A.M. Rapp et al. / Brain and Language 100 (2007) 142–149 variation and the cerebral regions in which they occur. In fact, in some regions there was a clear left-lateralisation in some of the subjects, whereas others had right lateralised activation. Handedness is a factor that is associated with the degree of language lateralisation (Sommer, Aleman, Bouma, & Kahn, 2004), however handedness is no explanation for the interindividual diVerences in our study, since all subjects were carefully selected right-handers. Future research should evaluate the reasons for the interindividual diVerences in laterality among subjects. Several factors may have confounded our result. One critical point is whether the applied methodology—calculating laterality indices—is sensitive enough to detect the diVerences between metaphoric and literal sentences. However, modiWcation of the threshold level did not change this main result even at a very low signiWcance level of p < 0.01. In addition, laterality indices sensitively detected diVerences between tasks in previous investigations (see Ramsey et al., 2001). Two recent functional imaging studies found signiWcant right hemisphere activation in metaphoric sentences relative to literal stimuli (Bottini et al., 1994, Ahrens et al., 2007). Ahrens et al. (2007) found right hemisphere activation for both salient and non-salient metaphoric sentences relative to literal sentences, but their stimuli were not balanced for syntax and diYculty. Bottini et al. (1994) used more complex stimulus material. In contrast to these two studies, the stimulus sentences in our investigation were more similar to each other, as most of them had the form “an a is a b”. Furthermore, these studies used a block design, whereas in our study the sequence of the stimuli was unforeseeable (event-related design, see Rapp et al., 2004). Stringaris et al. (2007), using an event-related design and similar stimuli like in our investigation found diVerences between metaphoric and literal stimuli in the right cerebral hemisphere, but they were not a speciWc eVect of metaphors because right hemisphere diVerences were also found for reading literal sentences > metaphoric sentences. Taken together, the results from imaging studies are heterogeneous. DiVerences in laterality may occur in regions that were not under investigation in our study. However, we chose the regions of interest based on activations found in previous imaging studies in phrasal metaphor comprehension (Bottini et al., 1994; Rapp et al., 2004; Sotillo et al., 2005; Ahrens et al., 2007) and we had no hypothesis for regions apart from the ones under investigation in our study. Also, the number of subjects was high and the stimuli were presented pseudo randomised in an event-related design, so it is unlikely that diVerent processing strategies were used for metaphoric and literal sentences within the tasks by the subjects in our experiment. One strong version of the right hemisphere theory predicts a “dichotomy” of hemispheres between literal and nonliteral language. Based on our Wndings, we argue against a strong hemispheric dichotomy between literal and nonliteral language. This view is compatible with the results from divided visual Weld studies which suggest that A.M. Rapp et al. / Brain and Language 100 (2007) 142–149 both hemispheres have the ability to process metaphoric meanings (Faust & Weisper, 2000; Schmidt, DeBuse, & Seger, 2007; Kacinik & Chiarello, 2007). Our results are compatible with lesion studies on metaphor processing. Patients with right-hemisphere lesions have preserved ability to understand phrasal metaphors (Rinaldi, Marangolo, & Baldassarri, 2004; Winner & Gardner, 1977). In a recent study by Giora et al. (2003), lesion extent in the left, but not the right cerebral hemisphere correlated with performance in explaining the meaning of metaphors verbally (Giora et al., 2003; Zaidel et al., 2002). The preserved ability of right hemisphere lesioned patients to understand metaphors correctly suggests that the left hemisphere has the ability to process phrasal metaphors correctly. This is also supported by some recent hemiWeld research (Kacinik & Chiarello, 2007). Spence, Zaidel, and Kasher (1990) investigated performance on the “right hemisphere communication battery” in four patients with complete commissurotomy As a group, the commissurotomy patients performed signiWcantly below normals on all tasks except verbal metaphor comprehension (Spence et al., 1990). In a recent study, Sotillo et al. (2005) used source-localisation EEG algorithms to investigate neural activity associated with metaphor comprehension. More speciWcally, they investigated the N400 induced by words which were or were not metaphorically related to previously presented metaphoric sentences. Source-localisation EEG (LORETA analysis) for metaphorically related minus unrelated words revealed a maximum of activation in the right middle / superior temporal gyrus. This Wnding is in contrast to our Wndings, in which no right hemisphere “shift” was found in this region. However, the priming paradigm used in the Sotillo et al. investigation may trigger right hemisphere contribution and this could possibly explain the diVerence in laterality between the two studies. Our study investigated metaphor comprehension without a context, however semantic context could possibly inXuence the underlying comprehension process (Giora et al., 2003; Robertson et al., 2000). Other N400 investigations with metaphoric sentences used sentence ending paradigms more similar to the task used in our study. They found no diVerence in the lateralisation between metaphoric and literal sentence endings (Coulson & Van-Petten, 2002; Pynte, Besson, Robichon, & Poli, 1996). However, a serious limitation of the former study is the inclusion of left-handers among the participants. Another factor that could possibly play a role in right hemisphere recruitment during metaphor comprehension is the stimulus material used. In our study, only short, simple sentences (an “a” is a “b”) were used and selection of stimulus material may play a role for right hemisphere recruitment (see Rapp et al., 2004). Several authors recently claimed that the salience of metaphors might be the critical factor for right hemisphere recruitment of metaphoric expressions (see Giora et al., 2003). Our study was not designed to address this issue speciWcally. We did not 147 compare salient and non-salient metaphors. The metaphors used in our investigation were created de novo for the experiment; however post hoc testing showed that 7 out of the 60 metaphors used in our study occurred in a large corpus (internet Google search). By this, our study included processing of salient as well as non-salient stimuli, although the majority of the metaphors were nonsalient. Future research is needed to speciWcally investigate the eVects of salience during metaphor comprehension. Besides scientiWc, hemispheric lateralisation during metaphor processing is also of clinical interest, since some patient populations show altered metaphor comprehension skills. These are for example patients with Asperger Syndrome (Dennis, Lazenby, & Lockyer, 2001), subjects with high expression of schizotypal personaity traits (Nunn & Peters, 2001; Langdon & Coltheart, 2004), and patients with schizophrenia (De-Bonis, Epelbaum, DeVez, & Feline, 1997). Patients with schizophrenia show a marked tendency toward the literal meaning while processing metaphors and proverbs (Gorham, 1956; Langdon, Coltheart, Ward, & Catts, 2002). This phenomenon is referred to as schizophrenic “concretism”. Impaired metaphor and metonymy comprehension may play a role in the origin of delusions (Rhodes & Jakes, 2004). It has been suggested that impaired lateralisation of cortical activation during language processing plays a key role in the pathophysiology of this deWcit (Kircher et al., 2006; Kircher et al., 2001; Kircher et al., 2004; Langdon et al., 2002). Thereby, clarifying the issue of laterality during metaphor comprehension may help to better understand the pathophysiology of the above named disorders. In the scientiWc literature, metaphor comprehension is often mentioned as a “right hemisphere” language function (Langdon & Coltheart, 2004; Xu et al., 2005; Bottini et al., 1994; Kircher et al., 2001; Mitchell & Crow, 2005). Our study speciWcally addressed the issue of laterality during metaphor comprehension. No signiWcant laterality eVects could be demonstrated for metaphoric relative to literal sentences even at a liberal threshold in several regions of interest in two diVerent tasks. We conclude from this that other factors than metaphoricity per se may be signiWcant for right hemisphere involvement. This view is compatible with other recent theories on metaphor comprehension, which predict that right hemisphere contributions to metaphor comprehension varies depending on factors such as salience, novelty or semantic distance (Giora et al., 2003; Kacinik & Chiarello, 2007). Future investigations should systematically investigate the inXuence of these factors on laterality in nonliteral language processing. Acknowledgments The authors would like to thank Tilo Kellermann and Christine Schulte for technical assistance. 148 A.M. Rapp et al. / Brain and Language 100 (2007) 142–149 Appendix A Typical examples for stimuli used in the experiment Metaphoric sentence Der Kanarienvogel ist eine Alarmglocke Der Wecker ist ein Folterknecht Der Taxifahrer ist ein KamikazeXieger Die Worte des Liebhabers sind Harfenklänge Lisas Lächeln ist ein Frühlingsstrauß Der Chef war eine Planierraupe Der Termin beim Direktorium war eine Kreuzigung Literal sentence the canary is a tocsin the alarm clock is a torturer the cabdriver is a kamikaze the lover’s words are harp sounds lisa’s smile was a bouquet of Xowers the director was a bulldozer the meeting at the directory board was a cruciWxion References Ahrens, K., Liu, H., Lee, C., Gong, S., Fang, S., & Hsu, Y. Y. (2007). Functional MRI of Conventional and Anomalous Metaphors in Mandarin Chinese. Brain and Language, 100, 163–171. Anaki, D., Faust, M., & Kravetz, S. (1998). Cerebral hemispheric asymmetries in processing lexical metaphors. Neuropsychologia, 36, 691–700. Annett, M. 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