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Right hemisphere sensitivity to novel metaphoric relations:Application of the signal detection theory

N. Mashal a,*, M. Faust a,b

a The Leslie and Susan Gonda (Goldschmied) Multidisciplinary Brain Research Center, Bar-Ilan University, Ramat-Gan, Israelb Department of Psychology, Bar-Ilan University, Israel

Accepted 12 February 2007Available online 18 April 2007

Abstract

The present study used the signal detection theory to test the hypothesis that the right hemisphere (RH) is more sensitive than the lefthemisphere (LH) to the distant semantic relations in novel metaphoric expressions. In two divided visual field experiments, sensitivity (d 0)and criterion (b) were calculated for responses to different types of word pairs. In the first experiment, subjects were presented with unfa-miliar two-word novel metaphoric expressions (‘‘signal’’) and unrelated word-pairs (‘‘noise’’). In the second experiment, literal expres-sions (‘‘signal’’) and unrelated word pairs (‘‘noise’’) were presented. In line with the Coarse Semantic Coding Theory [Beeman, M. (1998).Coarse semantic coding and discourse comprehension. In: M. Beeman & C. Chiarello (Eds.). Right hemisphere language comprehension:

Perspectives from cognitive neuroscience. Mahwah, NJ: Erlbaum, pp. 255-284.] as well as with the Graded Salience Hypothesis [Giora, R.(2003). On our mind: Salience, context and figurative language. New York: Oxford University Press.], the findings suggest that the RH ismore sensitive than the LH to unfamiliar metaphoric relations. Furthermore, this RH advantage in processing distant semantic relationsdid not extend to familiar semantic relations.� 2007 Elsevier Inc. All rights reserved.

Keywords: Metaphors; Hemispheres; Signal detection; Salience; Sensitivity; Criterion

1. Introduction

The present study explored right hemisphere (RH) sen-sitivity to distant semantic relations by applying the signaldetection theory (Green & Swets, 1966) to data obtainedfrom two divided visual field experiments. The first exper-iment focused on the ability of the right and left (LH)hemispheres to discriminate unfamiliar metaphoric two-word expressions taken from poetry from unrelated expres-sions. The second experiment investigated whether thisunique RH sensitivity is limited to unfamiliar distant met-aphoric relations by additionally comparing the ability ofthe LH and RH to discriminate familiar literal from unre-lated two-word expressions.

A large body of research indicates that whereas LHsuperiority for most semantic processing tasks is indisput-able, a wider and less differentiated set of word meanings,including distant, unusual, nonsalient, subordinate, andfigurative meanings is activated within the RH than withinthe LH (for reviews, see Chiarello, 2003; Faust & Lavidor,2003). RH contribution to the processing of distant, nonsa-lient meanings of words has been especially emphasized inconnection with the processing of metaphoric expressions.Thus, metaphor comprehension may involve the retrievalof alternate, distantly related and even unrelated interpre-tations in order to process unusual or unfamiliar wordcombinations. Although this mode of meaning activationseems to be important for the successful processing of con-ventional, relatively familiar, metaphors, it may be evenmore critical for the comprehension of novel, less familiarmetaphors, such as metaphoric expressions taken frompoetry (Mashal, Faust, & Hendler, 2005; Mashal, Faust,

0093-934X/$ - see front matter � 2007 Elsevier Inc. All rights reserved.

doi:10.1016/j.bandl.2007.02.005

* Corresponding author. Fax: +972 3 5352184.E-mail address: n-mashal@northwestern.edu (N. Mashal).

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Hendler, & Jung-Beeman, 2007). RH unique semantic sys-tem may thus be especially suitable for processing thenewly created, unfamiliar semantic relations between thewords comprising a novel metaphoric expression.

Most previous research linking metaphor processing tothe RH has contrasted literal semantic relations with con-ventional, relatively familiar, metaphoric relations.Although some research using conventional metaphoricstimuli appears to support an enhanced role of the RH inprocessing metaphoric language (e.g., Beeman, 1998;Brownell & Martino, 1998; Brownell, Simpson, Bihrle, Pot-ter, & Gardner, 1990; Burgess & Chiarello, 1996; Chiarello,1991; Kaplan, Brownell, Jacobs, & Gardner, 1990; Kle-pousniotou & Baum, 2005; McDonald, 2000; Rinaldi,Marangolo, & Baldassari, 2004; Van Lancker, 1997; VanLancker & Kempler, 1987; Weylman, Brownell, Roman,& Gardner, 1989; Winner & Gardner, 1977), other studiesreport either LH superiority or no hemispheric differencesin metaphor comprehension (see, e.g., Chobor & Schwei-ger, 1998; Faust & Weisper, 2000; Lee & Dapretto, 2006;Olivery, Romero, & Papagno, 2004; Rapp, Leube, Erb,Grodd, & Kircher, 2004; Stringaris, Medford, Giampetro,Brammer, & David, 2007; Tompkins, 1990; Zaidel, Kasher,Soroker, & Batori, 2002).

As noted above, since metaphoric expressions mayinvolve the use of both previously acquired, familiar, andnewly created, unfamiliar, categories and conceptual link-ages (Cacciari & Glucksberg, 1994), the inconsistent find-ings may be due, at least partly, to differences infamiliarity of the metaphors used. According to this claim,there are processing differences between metaphors at dif-ferent levels of conventionality (see, e.g., the Career ofMetaphor hypothesis by Bowdle & Gentner, 2005), and,consequently, the extent of RH involvement in metaphorprocessing may depend on the cognitive processes underly-ing familiar vs. novel metaphor processing.

In line with this notion, most of the relatively few morerecent studies that examined unfamiliar or novel meta-phoric expressions seem to show strong and consistentRH involvement. Anaki, Faust, and Kravetz (1998) inves-tigated semantic priming for literal (stinging-mosquito) andmetaphoric (stinging-insult) associates presented to theRVF/LH and the LVF/RH across SOAs of 200 and800 ms. The metaphoric stimuli used in this study wereconventional metaphoric expressions, but they were onlymoderately related. For the short SOA condition, meta-phorically related targets were facilitated in both visualfields while literally related targets were facilitated only inthe RVF. For the long SOA condition, metaphoricallyrelated targets were facilitated in the LVF/RH, whereas lit-erally related targets were facilitated in the RVF/LH. Theseresults support an enhanced role of the RH in metaphorcomprehension that was interpreted as reflecting theunique ability of this hemisphere to maintain activationof distantly related meanings for a longer period of time.Another study that used positron emission tomography(PET, Bottini et al., 1994) explored novel metaphoric and

literal sentence processing in normal participants using aplausibility judgment task. The main finding was increasedactivation in the right prefrontal cortex during metaphorcomprehension, but not literal sentence comprehension.Sotillo et al. (2005) used event related potential (ERP) mea-sures to study brain responses to target words that could orcould not be defined by a previously presented novel met-aphoric sentence. Analyses of the data using source locali-zation algorithms showed differential activity for the twotypes of target words in the right middle/superior temporalareas, supporting selective RH involvement in at least onephase of metaphor processing.

Unique RH involvement in processing novel meta-phoric expressions was found in two additional recentstudies that compared the processing of familiar andunfamiliar metaphoric stimuli. Schmidt, DeBuse, andSeger (2007) varied metaphorical and literal sentencefamiliarity in three divided visual field experiments. Theyfound RH advantage for both metaphorical and literalunfamiliar sentences containing distant associates andLH advantage for familiar sentences containing closelyrelated words. Furthermore, a recent fMRI study(Mashal et al., 2005) compared the patterns of brainactivation induced by processing the meanings of literal,conventional metaphoric, novel metaphoric and unre-lated word pairs. Participants in this study performed asemantic judgment task and the Principal ComponentsAnalysis (PCA) technique was applied in order to finddifferent functional networks corresponding to the differ-ent types of linguistic expressions. The results indicatedthat the right homologue of Wernicke‘s area in involvedin processing unfamiliar novel metaphors taken frompoetry but not in processing the other types of linguisticexpressions. The results provided evidence for the uniquerole of the right homologue of Wernicke’s area in pro-cessing novel metaphors. In particular, it was shown thatthe processing of novel metaphoric expressions that havenonsalient linguistic meanings requires the recruitment ofa special neural network consisting of the right homo-logue of Wernicke’s area, Broca’s area, left and rightinsula, and left and right premotor areas. Moreover, sig-nal changes in the homologues of Wernicke’s area weresignificantly higher for the novel metaphoric expressionsthan for both unrelated word pairs and literal expres-sions (Mashal et al., 2007). In addition, in accordancewith the fMRI coordinates of the right Posterior Supe-rior Temporal Sulcus (PSTS), a recent transcranial mag-netic stimulation (TMS) experiment (Pobric, Mashal,Faust, & Lavidor, submitted) that used the same stimuliand task, showed a selective increase in response timesfor the novel metaphoric expressions following magneticstimulation of the right PSTS. In contrast, responses tothe literal and the conventional metaphoric expressionswere not affected by rTMS in right PSTS. Furthermore,the results are consistent with the findings of an ERPstudy that also used the same stimuli and experimentaltask and applied source localization analyses to the

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N400 data. The findings indicated a unique involvementof RH regions in processing novel metaphoric expres-sions (Arzouan, Goldstein, & Faust, 2007).

RH advantage for the processing of unfamiliar meta-phors may not be particular to metaphors, but rathermay be one aspect of the unique semantic coding of theRH. An explanation of the role of the RH in metaphorprocessing has been provided by the coarse semantic cod-ing theory developed by Beeman (Beeman, 1998; Jung-Bee-man et al., 2004). According to this model, immediatelyafter encountering a word, the LH engages in relatively finesemantic coding, strongly focusing on few closely relatedword meanings or semantic features, whereas the RHengages in coarse semantic coding, weakly and diffuselyactivating large semantic fields containing multiple alterna-tive meanings and more distant associates. The larger andless discriminate the semantic field, the more likely it is tooverlap with semantic fields activated by other inputwords, including metaphorically related words. Since themetaphorical meaning of a word is usually more semanti-cally distant than its literal meaning, the processing of ametaphoric expression may depend on the ability to acti-vate a broader, more flexible set of semantic associationsin order to integrate the meanings of the weakly relatedwords into a meaningful linguistic expression. Thus, thecoarse semantic coding theory predicts RH semantic pro-cesses as generally being better suited for metaphor com-prehension. Although this theory does not explicitlyconsider conventional vs. novel metaphors, the comprehen-sion of novel metaphoric expressions, such as those takenfrom poetry, may rely even more on RH ability to link sev-eral distant associates in order to combine the meanings ofseemingly unrelated individual words.

Selective RH advantage for the processing of unfamiliarmetaphoric expressions is consistent with another currentmodel, the graded salience hypothesis (GSH) developedby Giora (1997, 2002). According to the GSH, the degreeof semantic salience (i.e., familiarity, frequency, promi-nence, conventionality, prototypicality, contextual inde-pendence) determines semantic processing. This modelhas some interesting implications for the processing of met-aphoric language and for the roles of the LH and RH inprocessing literal vs. metaphoric expressions. Thus, theGSH predicts that the RH will benefit more from the unfa-miliar semantic relationship between the meanings of singlewords appearing in a novel metaphoric expression whereasthe LH may show an advantage when comprehensiondepends on the ability to process familiar, either literal orconventional metaphoric, semantic relationship. The inte-gration of the coarse semantic coding theory with theGSH could imply that while salient, closely related mean-ings are processed mainly by the LH, the RH activatesand maintains nonsalient meanings.

A recent study directly compared the processing of sali-ent and nonsalient meanings by the two cerebral hemi-spheres using the divided visual field paradigm (Faust &Mashal, 2006). In two experiments, participants were pre-

sented with four types of two word linguistic expressions;literal (e.g., ‘‘loyal friend’’), conventional metaphoric(e.g., ‘‘sweet dream’’), novel metaphoric taken from poetry(e.g., ‘‘conscience storm’’) and unrelated (e.g., ‘‘processscreen’’). The first word of each two-word expression waspresented centrally and the second word was presented lat-erally to the left or right visual field. Participants wereasked to decide whether the second word was semanticallyrelated to the first word, i.e., whether the two-word expres-sion was meaningful or meaningless. Results showed aclear right RH advantage in judging novel metaphoricexpressions as meaningful expressions. The RH was foundto be faster and more accurate than the LH in identifyingnovel metaphoric expressions taken from poetry. Thisadvantage was not found for either unrelated word pairs,literal expressions, or conventional metaphoric expres-sions. In addition, the findings of both experiments indi-cated that novel metaphoric expressions were judgedfaster as meaningful than as meaningless in the RH,whereas the opposite pattern, i.e., slower ‘‘meaningful’’ ascompared to ‘‘meaningless’’ responses were found forRVF/ LH presented target words. Furthermore, the deci-sion that unrelated word pairs are meaningless was fasterthan the decision that novel metaphors are meaninglessin both hemispheres, but the difference in RT betweenthe two types of expressions was larger in the LVF/RH.Thus, it seems that the RH took relatively longer to decidethat a novel metaphoric expression is meaningless.

However, the findings of these studies could not deter-mine whether RH superiority for the processing of novelmetaphoric expressions points to a special sensitivity of thishemisphere to novel metaphoric semantic relations or to adifferent response criterion adopted by the RH. The pres-ent study used the signal detection theory to further eluci-date whether RH selective advantage for the processing ofunfamiliar metaphoric expressions found in previousresearch that used the same two-word expressions (Arzou-an, Goldstein, & Faust, submitted; Faust & Mashal, 2006;Mashal et al., 2005; Mashal et al., 2007; Pobric et al., sub-mitted) reflects unique semantic sensitivity to unfamiliarsemantic relations and/or a response bias, i.e., a generaltendency to decide that unfamiliar linguistic expressionsare meaningful.

The first experiment aimed to test the ability of the LHand RH to discriminate between novel metaphoric (NM)expressions and unrelated word pairs (UR). Both typesof expressions (NM, UR) were unfamiliar, hence represent-ing nonsalient meanings. Since previous research hasshown that the RH specializes in the processing of weakor distant semantic relationships, we hypothesized thatthe RH will demonstrate higher sensitivity, i.e., higher abil-ity to discriminate unfamiliar meaningful from unfamiliarmeaningless expressions than the LH. Furthermore, in linewith the coarse semantic coding and the GSH we predictedthat this RH sensitivity is beyond criterion, i.e., beyond thegeneral tendency to decide that unfamiliar linguistic expres-sions are meaningful. In this experiment, subjects were pre-

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sented with unfamiliar two-word expressions, half of whichwere novel metaphoric expression taken from poetry andhalf, unrelated word-pairs. Subjects were asked to decidewhether the second word of each pair that was presentedeither to the RVF/LH or the LVF/RH constitutes a mean-ingful expression with the first word. The second experi-ment aimed to investigate whether the predicted higherability of the RH as compared to the LH to discriminatebetween unfamiliar meaningful and meaningless expres-sions is specific or extends to familiar semantic relationsas well. Whereas metaphor comprehension involves somekind of meaning extension through the establishment ofcorrespondence between concepts from disparate knowl-edge domains, the comprehension of literal comparisonsinvolves two things that are clearly alike (Cacciari &Glucksberg, 1994; for a review, see Bowdle & Gentner,2005). Thus, if the higher sensitivity of the RH as com-pared to the LH reflects specific advantage for the process-ing novel semantic relations, we expected to find RH equalor even lower ability to discriminate between meaningfuland meaningless linguistic combinations when UR arereplaced with familiar literal expressions (LIT).

In order to measure sensitivity and criterion for theresponses to each type of word pair presented to theRVF/LH or LVF/RH we applied the signal detection the-ory. The signal detection model enables the separation ofsensitivity from response bias (i.e., criterion), and to com-pute estimates of each of these two measures (Green &Swets, 1966). Thus, the signal detection model uses therelationship between hit and false alarm rates to computeindependent measures of discrimination ability and crite-rion. The sensitivity (d 0) is a measure of how confusingthe stimuli are. In Experiment 1, all stimuli were unfamiliarand hence deciding which expression is meaningful (signal)and which is meaningless (noise) could be rather confusing.The larger the d 0, the easier it is to separate signal fromnoise. Sensitivity rate is computed from the hits and falsealarm rates (after they were converted to z-scores). Whenhits and false alarm rates are equal the NM can not be dis-tinguished from the UR (i.e., the sensitivity is zero). Such aresponse pattern would mean that when both types ofexpressions are unfamiliar, the hemisphere to which thestimulus was presented cannot discriminate meaninglessfrom meaningful expressions. However, if the hemispherethat gets the information can successfully distinguishbetween NM and UR, i.e., is sensitive to novel metaphoricrelations, than the hit rate will be higher than the falsealarm rate for any criterion. Criterion is measured by thetendency to respond ‘‘Yes’’, i.e., that the signal is present,in an ambiguous, unsure, situation. A higher criterion levelmeans that stronger evidence of a signal is required beforemaking a decision that the signal is present. When theresponse is unbiased, the value of beta is equal to one. Sub-jects (or each hemisphere) can adopt a liberal or a conser-vative response tendency. As the criterion becomes moreliberal (subjects are more likely to say ’Yes’, i.e., decide thatan expression is meaningful), both hits (i.e., deciding that

NM are meaningful) and false alarms (FA, i.e., decidingthat UR are meaningful) will increase. In this case, betavalue is smaller than one. A conservative response bias isa tendency to decide that the linguistic expression is mean-ingless. In this case, subjects are more likely to say ’No’,i.e., decide that an expression is meaningless and the betavalue is greater than one. Thus, the criterion providesinformation regarding the extent to which each hemisphereadopts a conservative or liberal response tendency in decid-ing whether two-word expressions are meaningful ormeaningless.

2. Methods

2.1. Stimuli

The stimuli in both experiments were two-word expres-sions. The first two pretests were common to both experi-ments hence described in this section. The aim of the firstpretest was to determine the type of each two-word expres-sion, (metaphoric, literal or unrelated word pairs). In orderto do so, 46 judges, who did not participate in the experi-ment, were presented with a list of two-word expressionsand asked to decide whether it is literally plausible, meta-phorically plausible or not plausible. Expressions that wererated by at least 75% of the judges as either metaphoricallyplausible, literally plausible or not plausible were selectedas metaphoric expressions, literal expressions or as unre-lated word pairs, respectively.

The second pretest tested word frequency. Forty-twoadditional judges, who participated neither in the formerpretests nor in the experiment, were presented with the listof all the words and asked to rate their degree of frequencyon a five point frequency scale ranging from 1 (highly non-frequent) to 5 (highly frequent). The average rates on thefrequency scale for the NM presented to LVF/RH, RVF/LH, LIT presented to LVF/RH, RVF/LH and UR pre-sented to LVF/RH, RVF/LH, were 3.64, 3.71, 3.64, 3.66,3.61, and 3.64, respectively. No significant difference wasfound between the six conditions (F < 1). Most (95%) ofthe target words, NM, LIT and the UR were abstractnouns that were also balanced between conditions accord-ing to syntactic structure.

2.2. Experiment 1

2.2.1. StimuliThe final stimulus pool consisted of 80 pairs of words,

all in Hebrew. Forty of these word pairs were novel meta-phoric, taken from poetry (e.g., conscience storm; imagina-

tion caves; mercy blanket) and 40 were unrelated word pairs(devastated weed; sofa stocks; opinion sky). Half of the stim-uli of each condition were presented to the RVF/LH andhalf were presented to the LVF/RH. The second word ineach word pair that served as the target word consistedof 3–7 letters and word length was counterbalanced acrossconditions (note that the order of words in Hebrew could

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be reversed). In addition to the previous pretests that weredescribed above, we performed another pretest in order toensure that the metaphorical expressions taken from poetrywere novel and unfamiliar. In this pretest, judges wereasked to rate the degree of familiarity of each metaphoricexpression on a 7 points familiarity scale ranging from 1(highly unfamiliar) to 7 (highly familiar). The 40 meta-phoric expressions that were rated less than 3 on the famil-iarity scale were selected for the study as novel metaphors(rating average 1.78).

The expressions in each condition were either in theform of noun–noun (70%) or noun–adjective (30%). Allnoun target words, both at the NM and the UR condition,were abstract except for one target word that was a con-crete noun (i.e., 5%).

2.2.2. Participants

Thirty five undergraduate students, aged 20–25, partici-pated in the study. All were native Hebrew speakers andyielded a laterality quotient of at least +90 on the Edin-burgh Inventory, indicating right handedness (Oldfield,1971). All participants had normal or corrected to normalvision and were given course credits for their participation.

2.2.3. Procedure

Each participant was presented with all the 80 wordpairs. Cell means were thus based on 19–22 trials per con-dition per participant. Stimulus presentation and responseswere controlled and recorded by a PC Pentium 4.3 com-puter using SuperLab software (version 2.0; http://www.cedrus.com).

The first word of each pair was centrally presented fol-lowed by the target stimulus displayed 2.6 degrees to theright or to the left of a centrally presented ‘‘+’’. Targetssubtended, on average, 1.90 degrees of horizontal visualangle (0.7 degrees vertical) at a viewing distance of55 cm. The participant placed his/her right index fingerbetween two keys on the computer keyboard and waitedfor a focusing signal (200 ms duration), which appearedon the center of the screen and indicated the onset of atrial. Immediately following the disappearance of thefocusing signal, the first word appeared for 900 ms. Partic-ipants were instructed to read the word silently. Next, thefocusing signal reappeared and remained on the screenfor 200 ms, until the end of the target stimulus presentationto ensure full fixation. The participant was instructed at thebeginning of the session to focus on the central ‘‘+’’ andnot to move his/her eye while it was present. The targetstimulus was presented randomly to the RVF or LVF for180 ms (SOA = 1100 ms) and participants were instructedto indicate as rapidly and accurately as possible whetherthe target stimulus formed a meaningful expression withthe preceding prime word by lifting and moving the rightindex finger from the middle position to the right or leftkeys. Assignment of the keys to ‘‘related’’ (meaningfulexpression) or ‘‘unrelated’’ (meaningless expression)responses was counterbalanced over participants. The next

trial began with a fixation point 2500 ms after target wordoffset. The session began with a practice list, consisting ofsix word pairs not used in the experimental lists.

2.3. Experiment 2

2.3.1. ParticipantsTwenty-nine undergraduate students, aged 20–25, par-

ticipated in the study. All were native Hebrew speakersand yielded a laterality quotient of at least +90 on theEdinburgh Inventory, indicating right handedness (Old-field, 1971). All participants had normal or corrected tonormal vision and were given course credits for theirparticipation.

2.3.2. StimuliThe stimulus pool consisted of 40 literal expressions and

40 unrelated word pairs (same as in Experiment 1), all inHebrew. Half of the stimuli in each condition were pre-sented to the RVF/LH and half the LVF/RH. The secondword in each word pair that served as the target word con-sisted of 3–7 letters and word length was counterbalancedacross conditions.

3. Results

3.1. Experiment 1

We calculated the sensitivity (d 0) and log of the criterion(b) based on the hit (NM judged as meaningful) and FA(UR judged as meaningful) rates1. In addition, we per-formed repeated measures ANOVA for the reaction times(RT). Six out of 35 participants responded correctly eitherto all the novel metaphoric expression or to all the unre-lated word pairs. When either the hit rate is one (100%)or the FA rate is zero, d 0 and b have an unlimited numericvalue and hence can not be included in the analysis. Inorder to overcome this problem, the estimates of d 0 and bhave been converted as recommended by Macmillan andCreelman (1991). Thus, proportions of one were convertedto 1 � 1/(2N) and proportions of zero were converted to 1/(2N), with N referring to number of trials contributing tothe proportion.

The left sides of Figs. 1 and 2 represent the sensitivity(d 0) and criterion (b), respectively, for detecting NM inthe two hemispheres. Two-tailed T-test for dependent vari-ables revealed greater sensitivity (d 0) in the LVF/RH(M = 2.10; SD = 0.67) than in the RVF/LH (1.50; 0.66),

1 For each subject hit and false alarm rates were calculated and thenconverted to z-scores (z(H) and z(FA), respectively). Then sensitivity (d 0)and criterion (b) were calculated as follow: d 0 = z(H) � z(FA);b = e�0.5 * d0 * (z(H) + z(FA)). Thus, when there is no sensitivity d0 = 0 andwhen the response is unbiased b = 1. b > 1 reflects a ‘‘conservative’’response bias and the hemisphere’s tendency to response ‘‘No’’ and viceversa, b < 1 reflects a ‘‘liberal’’ response bias and the hemisphere’stendency to response ‘‘Yes’’.

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t(34) = 6.03, p < .0001. However, for the criterion level, nosignificant difference was found between the two hemi-spheres (b = 1.23 and 1.15, in the RH and in the LH,respectively), p > .1. The statistical analysis of b was per-formed on the natural logarithm of b since the distributionof b is positively skewed.

3.1.1. RT analysis

RT and accuracy are presented in Table 1. Two-wayRepeated measures ANOVA with visual field (LVF/RH,RVF/LH) and stimulus type (NM, UR) as within subjectsfactors for RT revealed a main effect of visual field(F(1,34) = 25.78; p < .001). Target words presented to theLVF/RH (M = 1077.36 ms; SD = 265.19) were processedfaster than target words presented to the RVF/LH(mean = 1122.67 ms; SD = 248.89). A main effect of stimu-lus type was also significant, F(1, 34) = 7.79; p < .01.Responses to NM were faster (M = 1034.13 ms;SD = 237.11) than to unrelated word pairs(M = 1165.90 ms; SD = 256.38). The interaction was notsignificant (F > 1). However, pairwise t-test showed fasterRTs to NM when presented to the LVF/RH than whenpresented to the RVF/LH, t(34) = 2.98; p < .001 (Table 1).

3.2. Experiment 2

Eleven out of the 29 participants correctly identifiedeither all the literal expressions (LIT) or the unrelated wordpairs (UR). When either the hit rate is one (100%) or theFA rate is zero, d 0 and b have an unlimited numeric valueand hence can not be included in the analysis. In order toovercome this problem, the estimates of d 0 and b have been

converted as recommended by Macmillan and Creelman(1991) (See explanation in Experiment 1).

The right sides of Figs. 1 and 2 represent the sensitivity(d 0) and criterion (b), respectively, for the LIT in the twohemispheres. Two-tailed T-test for dependent variablesrevealed no significant difference in sensitivity betweenthe LVF/RH (M = 2.93; SD = 0.65) and the RVF/LH(M = 2.99; SD = 0.69) (p > .1). However, a two-tailed T-test for dependent variables revealed higher criterion level(b) in the LVF/RH (M = 1.51) than in the RVF/LH(M = 1.01), t(28) = 3.07, p < 0.01. The statistical analysisof b was performed on the natural logarithm of b sincethe distribution of b is positively skewed.

3.2.1. RT analysis

RT and accuracy are presented in Table 2. Two-wayRepeated measures ANOVA with visual field (LVF/RH,RVF/LH) and stimulus type (LIT, UR) as withinsubjects factors for RT revealed a main effect of stimulustype (F(1, 28) = 42.34; p < .0001). Responses to LITwere faster (M = 796.66 ms; SD = 198.00) than to UR(M = 937.52 ms; SD = 236.35). The interaction was alsosignificant, F(1,28) = 4.42; p < .05. Pairwise t-test revealedthat LIT presented to the RVF/LH (M = 775.19 ms;SD = 191.02) were processed faster than LIT presentedto the LVF/RH (M = 818.14 ms; SD = 205.82),t(28) = 2.68; p < .05.

3.3. Combined results from Experiments 1 and 2

In order to directly test the differences in results betweenExperiments 1 and 2, we conducted two additional sensitiv-ity (d 0) and criterion (b) one-way ANOVAs. In each anal-ysis, Experiment (Experiment 1 vs. Experiment 2) servedas a between-subject variable.

In the first analysis, a significant difference in sensitivitywas found between the two experiments (Wilks’ Lamb-ada = .43, F(2, 61) = 38.92, p < .001). Scheffe post hoc

Fig. 2. Mean decision criterion (b) of Experiment 1 (novel metaphoricexpressions vs. unrelated word pairs; left bars) and of Experiment 2 (literalexpressions vs. unrelated word pairs; right bars) in the LVF and RVFincluding standard errors of the mean. Star indicates significant difference.

Table 1RTs (SD) and accuracy (below) of novel metaphoric expressions (NM)and unrelated word pairs (UR) of Experiment 1

NM UR

RVF/LH 1067.51 (203.39) 1177.84 (232.30)75.26% (16.84) 74.49% (13.01)

LVF/RH 1000.76 (204.45) 1153.96 (281.64)84.57% (10.55) 82.32% (11.34)

Table 2RTs (SD) and accuracy (below) of literal expressions (LIT) and unrelatedword pairs (UR) of Experiment 2

LIT UR

RVF/LH 775.19 (191.02) 942.05 (254.03)92.93% (7.49) 91.55% (7.08)

LVF/RH 818.14 (205.82) 932.99 (221.69)90.17% (7.13) 93.44% (6.84)

Fig. 1. Mean sensitivity (d 0), including standard errors of the mean, fordetecting novel metaphoric expressions in Experiment 1 (left bars) andliteral expressions in Experiment 2 (right bars) in the LVF/RH and RVF/LH. Star indicates significant difference.

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analysis revealed a main effect of experiment (F(1, 62) =56.57, p < .001) and visual field (F(1, 62) = 15.91,p < .001). Thus, sensitivity was higher in Experiment 2(mean = 2.96) than in Experiment 1 (1.80) and higher inthe RH (2.41) than in the LH (2.18). The interaction wasalso significant (F(1, 62) = 24.30, p < .001). Scheffe posthoc analysis revealed higher RH sensitivity in Experiment2 (M = 2.93; SD = 0.65) as compared to Experiment 1(M = 2.10; SD = 0.67), i.e., better ability to discriminatebetween literal and unrelated expressions than to discrimi-nate between novel metaphoric and unrelated expressionsin the RH. This difference was also significant and largerin the RVF/LH. Scheffe post hoc analysis revealed higherLH sensitivity in Experiment 2 (M = 2.99; SD = 0.69) ascompared to in Experiment 1 (M = 1.5; SD = 0.66), i.e.,better ability to discriminate between literal and unrelatedexpressions than to discriminate between novel metaphoricand unrelated expressions in the LH. Thus, although bothhemispheres showed higher sensitivity in Experiment 2than in Experiment 1, the difference was larger in theLH, that was significantly less sensitive than the RH tothe novel semantic relation in Experiment 1.

In the second analysis, a significant differencebetween the two experiments was found for the criterionmeasure (Wilks’ Lambda = .83, F(2, 61) = 5.72, p < .01).Scheffe post hoc analysis revealed a main effect of visualfield (F(1,62) = 5.92, p < .05). Thus, the criterion washigher in the RH (1.35) than in the LH (0.75). Theinteraction was also significant (F(1, 62) = 4.57, p < .05).Scheffe post hoc analysis revealed more conservativeresponse criterion for LVF/RH presented targets inExperiment 2 (M = 1.51) as compared to Experiment 1(M = 1.23) (p < .05), i.e., higher RH tendency to decidethat the linguistic stimuli are meaningless when the sig-nal was represented by literal expressions as comparedto when it was represented by novel metaphoric expres-sions. The difference in the LH between response crite-rion for RVF/LH presented targets in Experiment 1(M = 1.15) as compared to Experiment 2 (M = 1.01)was in the opposite direction. Thus, the LH showed ahigher tendency to decide that the expression is mean-ingless when it was a novel metaphoric than when itwas a literal expression although the difference wasnot significant.

4. Discussion

The signal detection theory is a general frameworkdesigned to quantify an estimate of sensitivity (d 0), whichis independent of the participant’s response criterion, i.e.,the tendency to response ‘‘signal’’ or ‘‘noise’’ (Green &Swets, 1966). In the present study, participants had todecide whether each linguistic expression is meaningful(signal), or not (noise). In Experiment 1, half of the stimuliwere novel metaphoric expressions taken from poetry(NM) and half were unrelated word-pairs (UR). In Exper-iment 2, half of the stimuli were literal expressions (LIT)

and half were unrelated word-pairs (UR). The ‘‘noise’’was represented by the UR and was the same in bothexperiments whereas the ‘‘signal’’ changed: in Experiment1 it was represented by the NM while in Experiment 2 itwas represented by the LIT. The signals in the two experi-ments differed in their familiarity level. Thus, whereas thenovel metaphoric expressions of the first experiment wereunfamiliar (mercy blanket) to the participants, the literalexpressions of the second experiment were familiar (loyal

friend). In both experiments, the ‘‘noise’’, i.e., the unrelatedword pairs, was unfamiliar. It was anticipated that makingthe decision that NM are meaningful expressions would bemore difficult than making this decision for LIT since in thecase of the NM, the unfamiliar signal was embedded in anunfamiliar noise.

Results of Experiment 1 were in line with the predictionthat the RH will demonstrate higher sensitivity, i.e., higherability to discriminate unfamiliar meaningful from unfamil-iar meaningless expressions than the LH. The RH showedhigher sensitivity than the LH to novel metaphoric expres-sions, i.e., greater ability to differentiate between meaningfuland meaningless unfamiliar linguistic expressions. Thishemispheric difference was beyond the subjects’ general ten-dency to decide that unfamiliar linguistic stimuli are mean-ingful. Thus, no difference between the hemispheres wasfound in b. When subjects had to decide that the presentedexpression was a ‘‘signal’’ (i.e., the linguistic stimulus ismeaningful) the same decision criteria were applied by bothhemispheres. Results from the RT analysis revealed no sig-nificant interaction between visual field and stimulus type.However, consistent with the findings of a previous studythat used the same NM and UR stimuli and the same exper-imental task (Faust & Mashal, 2006), pairwise t-test showedfaster RT to NM when presented to the LVF/RH than whenpresented to the RVF/LH. Thus, although this previousstudy used the conventional measures of percent of correctresponses and RT whereas the present study used signaldetection measures and RT, both the previous and the pres-ent studies’ results showed RH advantage for NM expres-sions in accuracy and RT. The findings of both studiessuggest that the RH is better, i.e., more accurate and faster,than the LH in successfully discriminating between unfamil-iar meaningful and meaningless linguistic stimuli. These con-sistent findings strongly support the claim that the RH maybe critically involved in the integration of the individualmeanings of two seemingly unrelated concepts into a mean-ingful metaphoric expression.

In contrast, the results of the second experiment did notshow RH advantage in discriminating between familiarmeaningful and unfamiliar meaningless linguistic expres-sions. When novel metaphoric expressions were replacedby literal expressions, both hemispheres showed equal sen-sitivity in discriminating between the LIT and the UR.These findings suggest that RH advantage over the LHto semantic relations is unique to unfamiliar, distant mean-ings and does not extend to familiar semantic relations.However, with regard to response criterion, results of

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Experiment 2 revealed that the LH tended to be more lib-eral, i.e., to decide that LIT are meaningful, than the RH.Furthermore, the interaction between visual field and stim-ulus type showed that correct responses to LIT were signif-icantly faster in the LH than in the RH. Taken together,the findings of Experiment 2 show LH advantage in pro-cessing familiar literal expressions. Thus, as compared tothe RH, the LH showed a higher tendency to decide thata familiar literal expression is meaningful and these deci-sions were faster.

Based on the findings of the two experiments, the mainconclusion of the present research is that, as expected, theRH was more sensitive than the LH to the semantic differ-ences between two unfamiliar linguistic expressions, NMand UR, both representing nonsalient meanings. ThisRH advantage was found to be specific to novel meta-phoric, nonsalient meanings, since the findings of Experi-ment 2 showed that discriminating between salient andnonsalient meanings, i.e., between LIT and UR showedthe same sensitivity level for both hemispheres. These find-ings suggest that RH advantage for the comprehension ofunfamiliar metaphoric expressions reflects unique sensitiv-ity to distant, nonsalient semantic relations.

Results of the common analysis revealed that the sensi-tivity of both hemispheres was higher in the second exper-iment (mean = 2.96) than in the first experiment(mean = 1.8). As expected, these findings show that itwas more difficult to discriminate between signal and noisewhen both were unfamiliar as compared to when the signalwas familiar and the noise was unfamiliar. In addition, thecommon analysis showed that in the RH, d 0 for novel met-aphors is 2.10, and for literal pairs it is 2.93, and that the d 0

for these conditions is significantly different by 0.83. In theLH, d 0 for novel metaphors is the lowest—1.5, and for lit-eral pairs it is twice that, i.e., 2.99. These findings indicatethat although both hemispheres are better at literal/famil-iar relations, the LH is selectively worse at novel meta-phoric relations. That is, the RH is showing highsensitivity to both familiar, close semantic relationships,and to unfamiliar weak semantic relationships. RH advan-tage over the LH in processing semantic relations thusseems to be specific to unfamiliar, distant meanings anddoes not extend to familiar semantic relations.

Results of the RT analysis also support the conclusionthat it is more difficult to discriminate between signal andnoise when both are unfamiliar. Although the UR werethe same in both experiments, RT for UR was faster inthe second experiment (mean = 937.52 ms) than in the firstexperiment (1165.90 ms). The longer RT for the UR in thefirst experiment can be attributed to the fact that the taskwas more confusing when both the signal and the noisewere unfamiliar. Thus, when the difference between signaland noise becomes more obvious, as in the case of LITvs. UR, the effort invested in differentiating between themdecreases, hence the shorter RT for the UR in the secondexperiment compared with the first experiment and thelower FA rates for the UR in the second (7.5%) compared

to the first (22%) experiment. These results indicate thatwhen the signal and noise share common properties it isharder to discriminate between the two and consequentlysensitivity level decreases (from 2.96 of Experiment 2 to1.8 of Experiment 1). Thus, the decision that the UR aremeaningless was easier when the signal could be clearly dif-ferentiated from the noise. An alternative explanation forthe observed sensitivity of the RH as compared with theLH to novel metaphoric relations could result from taskdemand. It has been suggested that when task difficultyarises, additional RH brain regions are recruited (e.g.,Ferstl, Rinck, & von Cramon, 2005; Xu, Kemeny, Park,Frattali, & Braun, 2005). In a recent fMRI study (Ferstlet al., 2005) subjects listened to short stories, half of whichcontained (chronologically or emotionally) an inconsistentword. Increased activation was observed in the right ante-rior temporal lobe when subjects detected the inconsistentword. The authors noted that when demand level increasesthe RH becomes engaged. This idea was also supported byXu et al. (2005) who reported that when brain’s responsesto random words, unconnected sentences, and narrativeswere compared, the RH was increasingly active as contex-tual complexity increased. The most robust activation ofthe RH was found at the narrative level.

The criterion decision (b) adopted in Experiment 2 wasdifferent than that in Experiment 1. Results of Experiment2 showed higher b values for target words presented to theLVF/RH than presented to the RVF/LH. A higher deci-sion criterion means that more information and/or cer-tainty are required for making the decision that ameaningful linguistic stimulus was presented. These resultsindicate that when readers have to discriminate betweenmeaningful and meaningless linguistic stimuli, the LH ismore liberal than the RH when familiar, literal expressionsare involved. In Experiment 2, the RH seems to miss moreliteral expressions than the LH (10.17% and 7.84% in theRH and LH, respectively) and showed less FA comparedto LH (7.16% and 8.88% in the RH and LH, respectively).This means that when both the signal and the noise wereunfamiliar and hard to discriminate, both hemispheresapplied the same response criterion (Experiment 1), butwhen it was relatively easy to discriminate between the sig-nal and the noise (Experiment 2) the two hemispheresapplied different strategies. Thus, in the case of familiar, lit-eral expressions, a lower level of certainty and/or less infor-mation might be required by the LH for making thedecision that the linguistic stimulus is a meaningful expres-sion whereas the RH may require a higher certainty leveland/or more information in order to decide that a familiarlinguistic stimulus is meaningful.

The results of the common analysis also support thisnotion. The RH was more liberal in Experiment 1 (NMand UR) than in Experiment 2 (Lit and UR) while theLH tended to be more conservative in Experiment 1 thanin Experiment 2. It might be concluded that the RH adoptsa more liberal response bias for unfamiliar metaphoricthan for familiar literal expressions whereas the LH shows

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the opposite response tendency. This difference in responsebias may be one aspect of LH well-known superiority forliteral language.

The findings of the two experiments thus support theo-ries suggesting that the RH and the LH process languagein a qualitatively different manner. The coarse vs. finesemantic coding model (Beeman, 1998; Jung-Beeman,2005) postulates that the RH activates and maintains abroad range of word meanings including distantly relatedconcepts and peripheral semantic features, whereas theLH activates and maintains the most closely related con-cepts and central semantic features (for review see Kacinik& Chiarello, 2007). In addition, the present results alsosupport the GSH (Giora, 2003). According to this model,RH advantage is predicted for the processing of linguisticexpressions that contain nonsalient interpretations, i.e.,for unfamiliar meaning which are not coded in the mentallexicon. Understanding novel metaphoric expressionstaken from poetry requires the processing of distant, non-salient semantic relations in order to form novel combina-tions between seemingly unrelated words. Thus, the presentstudy confirms that the RH is better than the LH in pro-cessing unfamiliar metaphoric expressions and, further-more, that this RH advantage reflects higher sensitivity todistant, nonsalient semantic relations and not a generalresponse tendency with regard to linguistic expressions.

The results of the present research are also consistentwith several neuroimaging (Mashal et al., 2005; Jung-Bee-man et al., 2004; Bottini et al., 1994) and behavioral studies(e.g., Schmidt et al., 2007; Anaki et al., 1998) showingunique RH involvement in processing linguistic expressionscontaining distant semantic relations. RH superiority fornovel metaphor comprehension is supported by the find-ings of a recent fMRI study (Mashal et al., 2005) that usedthe same stimuli and a similar experimental task and foundthat the right homologue of Wernicke’s area (right PSTS)was activated during the processing of the novel meta-phoric expressions. However, this brain area was not partof the neural network recruited for the processing of salientmeanings, i.e., literal expressions or conventional meta-phors. Also, in accordance with the fMRI coordinates ofthe right PSTS, a recent TMS experiment (Pobric et al.,submitted) that used the same stimuli and task, showed aselective increase in response times for the novel meta-phoric expressions following magnetic stimulation of theright PSTS. In contrast, responses to the literal and theconventional metaphoric expressions were not affected byrTMS in right PSTS. Furthermore, the results are consis-tent with the findings of ERP study that also used the samestimuli and experimental task and applied source localiza-tion analyses to the N400 data (Arzouan et al., submitted).

To sum up, the present study applied the signal detec-tion theory to test the hypothesis that the unique RHinvolvement in novel metaphor comprehension found inprevious research reflects higher sensitivity to distant,nonsalient semantic relations and that this selective RHadvantage for the processing of distant semantic relations

does not extend to familiar meaningful expressions suchas literal expressions. Our results also suggest that theLH is selectively worse than the RH in discriminatingbetween meaningful and meaningless linguistic expres-sions containing distant semantic relations. Since RHsensitivity to novel metaphoric semantic combinationsmay require the activation of wider and less differenti-ated semantic fields including nonsalient meanings, thefindings support both the coarse semantic coding theory(Beeman, 1998) and the GSH (Giora, 1997, 2003). Fur-thermore, these hemispheric differences in sensitivity fornovel metaphoric as compared to literal linguistic expres-sions support the claims that the RH and LH may con-stitute two different semantic systems. Thus, the findingscould serve as an important example of how LH lan-guage system is supported by the very different kind ofsemantic processes subserved by the RH and how theavailability of both systems enables a wide variety of lin-guistic functions to be exploited as needed (Chiarello,2003). Future research should look further at the impli-cations of the different in sensitivity between the LHand RH for linguistic forms that are characterized byunusual semantic relations. Specifically, RH unique sensi-tivity for distant meanings may be critical for under-standing the more complex uses of figurative languagein poetry, fiction and other forms of creative language.

Acknowledgments

Partial support for this work was provided by the US-Israel Binational Science Foundation (BSF) Grant No.2003317. We thank Inna Schneiderman for her useful assis-tance in conducting the first experiment.

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