Lexical decision after left, right and bilateral presentation of function words, content words and non-words: Evidence for interhemispheric interaction

Pergamon

LEXICAL DECISION AFTER LEFT, RIGHT AND BILATERAL PRESENTATION OF FUNCTION WORDS, CONTENT WORDS AND NON-WORDS: EVIDENCE FOR INTERHEMISPHERIC INTERACTION BETTINA MoHR,*~ FREIDEMANN

PULVERM~;LLER:

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and ERAN ZAIDEL*

“UCLA. Department of Psychology. Los Angeles. CA 90024.1563. U.S.A.; tUni\ersity of Konstanr, Department of Psychology, 78464 Konstanz. Germany; ZUCLA. Department of Applied Linguistics, Los Angclzs. CA 90024-1543, U.S.A.: and #Untversity of Tiibingcn, Institute of Clinical Psychology and Behavioral Neurobiology, 72074 Tiibingen. Germany.

Abstract-Function words, content words and pronounceable non-words (pseudowords) were presented tachistoscopically either in the left or the right visual field or with identical copies flashed simultaneously to both visual half-fields. Consistent with earlier studies [IO]. function words were found to show a right visual field advantage. whereas for content words the right visual held advantage was absent. Compared to either of the unilateral modes of presentation, bilateral presentation oftdentical word stimuli improyed accuracy and latency significantly. The bilateral (BI) ad\,antage was largest for content words. and was also highly significant for function words in both latency and accuracy. The Bt gain was absent for non-words (srgnificant interaction of Wordness x Visual Field). These results indicate that the lexicons ofthe left and right hemisphere can “collaborate” rather than inhibit each other or act independently when processing the same linguistic stimuli. Our lindings at-e consistent with the view that the neuronal counterparts of words are Hcbbtan cell assemblies consisting of strongly connected excitatory neurons of both hemispheres. Since function words show a right visual ticld advantage in addition to their Bi gain, their assemblies are likely to hale most of their neurons located in the left hemisphere. Neuronal assemblies corresponding to content \\ords may be less strongly lateralized.

INTRODUCTION TM QUESTIOKof how language is processed and represented in the human brain is still far from being solved. However, there are some subquestions for which an answer seems to be within reach. Are real words and meaningless letter strings processed and represented differently in the human brain? Are there word classes for which cortical processing differs? Are there, for example. different types of neuronal networks corresponding to function (or closed-class) words. like “is”, .‘he”, or “since” and to content (or open-class) words, like “tea”. “go” or “green”‘? Such questions can be answered on the basis of current psycholinguistic, neurolinguistic and psychophysiological data. Evidence for processing differences distinguishing non-words. function words and content words comes from patients with focal brain damage. Whereas agrammatic aphasics have

‘Address for reprints: Vrrhaltensneurobiologie,

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difficulties in producing or recognizing function words. they usually have only mild deficits in processing content words 119. 361. In contrast. patients with anomia can have problems primarily with content words. These findings in aphasics nre paralleled by observations in patients with acquired dyslexia [ 181. Patients with deep dyslexia and phonological dyslexia read content words relatively well, but show strong impairments in reading function words, In surface dyslexics the opposite pattern applies, i.e. function words arc easier to read for these patients. Also the ability to process non-words is affected differentially in dyslexic syndromes. Further evidence in favor of the distinction between function and content words is provided by studies of event-related potentials (ERP) in healthy subjects. The N400. a negative component with a maximum at about 400 msec post stimulus onset. is larger for content words than for function words. The N400 of function and content words changcb differently with word context, and the latcralization of this component varies with word class [34,4.5]. Further distinct ER P components elicited by function and content words have been reported recently, namely a strongly lateralized N280 for function words and an almost symmetrical N350 for content words [38]. However, the nature and underlying cause of the differences between function and content words are still under discussion. Interesting findings concerning differences in processing content and function words come from behavioral studies using accuracy and latency measures. Using the lexical decision paradigm, BRADLEY [3] found that reaction times changed as a function of word frequency of content words. For function words, she did not obtain frequency sensitivity. In agrammatic aphasics, Bradley found frequency sensitivity for both words classes. However, in a series of experiments, GORDON and CARAMAZZA [20-231 showed that words of both classes are frequency sensitive in aphasics as well as in normals. Frequency sensitivity was absent only for function words with a frequency of occurrence of 400 per million or above. In summary, the differences in processing content and function words suggested by Bradley have not been confirmed. However, it might well be that very frequent words (with word frequency above 400/million) behave differently than words of lower frequency (below 400:‘miilion). Studies using tachistoscopic presentation of the stimuli in either of the visual half-fields suggested further differences between non-, content and function words. BRADLEY and GARRETT [4] reported the following accuracy data using a task in which subjects had to read aloud word stimuli flashed in the left (LVF) or right visual field (RVF). There was an overall right visual field advantage (RVFA) for words but not for non-words. In the RVF. words of both types were identified equally well. In the LVF. function words were identified less accurately than content words. A half-field study by CHIARELLO and NtiDINc; [lo] using a lexical decision paradigm led to similar results. Latency scores revealed an overall RVFA for words but not for non-words. In an ANOVA, an interaction of Visual Field x Word Class was obtained with subjects as the random variable. The RVFA was highly significant for function words but only marginally significant for content words. In the LVF, function words were recognized more slowly than content words; in the RVF. there was no difference. In summary, thcrc are language disorders after brain injury indicating distinct localizations for content words, function words and non-words. Studies using event related potentials suggest that the major word classes have also different psychophysiological properties. Finally, behavioral data provide further evidence for the word class distinction. In particular, there is (a) a larger RVFA for function words than for content words (d@renti~~I RVFA) and (b) an advantage of content words compared to function words presented in the LVF (corl~rrll u’& superiorit~~ iu t/w L I/%). We next discuss mode!s of

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language representation and processing in the cortical hemispheres that may account for these differences. It is commonly believed that the neuronal counterparts of words arc located in the language cortices of the dominant hemisphere, the left hemisphere (LH) in most righthanced individuals. In this case. linguistic input to the RVF could reach the language machinery of the LH directly. Input to the LVF reaching the right hemisphere (RH) would have to be first relayed to the LH via the corpus callosum in order to activate the neuronal networks in the language regions. This T~n.~a/lo.s~l Rc~la!, H~pothrsis [49] can account for the RVFA usually obtained for words, since there is a time delay and loss of stimulus quality due o callosal relay. However. this model fails to account for both differential RVFA and content word superiority in the LVF. A faster relay mechanism through the corpus callosum for content words in comparison to function words is unlikely given that content and funcrion words are physically similar, i.e. equally long and visually complex. Findings from split-brain research provide further evidence against a callosal relay hypothesis. ZAIIXL. [46. SO] examined split-brain patients and subjects after left hemispherectomy and found ample evidence for receptive language abilities in the isolated right hemisphere. The vocabulary of the RH seems to be developed best for content words, especially those which are both short in length and concrete in meaning. Additional evidence for 2 lexicon in the right hemisphere comes from aphasics with large lesions destroying the entire perisylvian cortex of the LH. In such patients, repetition and \ierbal comprehension abilities have been demonstrated [I 1, 24, 431. It is possible to account for the summarized evidence by assuming that language m~ianisms are also present in the non-dominant right hetnisphere. In this view, each hemisphcrc has its own lexicon. For content words, the lexicons of both hemispheres are well dcvcloped. The function word lexicon may only be well developed in the left hemisphere, so that input to the RH would have to involve the LH lexicon as well. According to these assumptions. content words presented to either of the visual half-fields have direct access to one of the internal lexicons----either the left or the right (ni~c’t Auvss Model [49]). This would explain why the RVFA of content words is only marginally significant or even absent [IO:. In contrast, the function word lexicon could only be accessed directly through RVF-LH SIimulation, whereas some transcallosal relay would have to take place after function word presentation to the LVF-RH. Assuming direct access for content words but at least partial transcaliosal relay for function words. both of the reported findings can be explained, the larger RVFA of function words and the content word superiority in the LVF-RH. In atid,tion. the fact that split brain, hemispherectomy and aphasic patients show language abilities in rhe RH is compatible with this model. However, we still need to specify how the language machineries of the left and right hen-isphere interact in the intact brain. Does one hemisphere suppress the other one if both arc jtirnulated at the same time? Do they act independently. like in the split brain, so that the fastl:r hemisphere always wins and performs the task‘? Or do they collaborate cooperatively s::) that there is positive feedback between the left and right lexicons‘? A preliminary answer to these questions is possible on the basis of a neurobiological model of lexical processing and representation which is rooted in HERB’S cell tissemhly thror-y [26]. Hebb’s rule predicts and recent neurophysiological evidence [ 1, 251 clearly demonstrates that neurons of the cortex that are frequently active together at the same time strengthen their connections. There are strong connections between distant areas of the cortex [8,40] and, therefore, it seems appropriate to consider the cortex a large usso~iarice merno~~~ in which

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neurons ofdistant cortical areas get more strongly associated after simultaneous activity 191. Briefly, during early language acquisition neurons in the motor system (that cause an articulation) and neurons in the auditory system (that are stimulated by the auditory signal produced by the articulation) get active frequently at the same time and develop into an assembly. If a word is produced repeatedly, a cell assembly may form thut ir~c1ude.smwms qf both the motor und rhe auditory c.or.tic.u/ .s~~.stetns. Such assemblies would be the cortical equivalent of individual word forms [6, 12, 26, 42, 441. In most right-handers, these early developing assemblies appear to be lateralized to the LH, i.e. most oftheir mwv~.~ are located in the language areas of the LH. This may be due to neuroanatomical asymmetries of the language regions [37, 411. How would the neurobiological framework account for differences in lateralization of lexical categories‘! Early developing lateralized assemblies can represent function words and the syntactic rules they have to obey if they establish connections to other cell assemblies in the language regions. Therefore, these ,jim~tiot~ ~~~ssett~h/ie.sremain strongly lateralized. Assemblies representing content words (content as.senzhlie.s)are likely to be organized differently. Consider, for example, the word “cat”. During language learning, it occurs frequently together with certain visual. auditory, or somatosensory stimuli causing neuronal activity in both hemispheres. Thus, according to Hebb’s rule simultaneously active neurons outside the language cortex and the neurons of the word form assembly will strengthen their connections. What results is a cell assembly including neurons scattered over the entire cortex corresponding to content words. Ofcourse, assemblies ofthis kind will be less strongly lateralized than function assemblies. These assumptions predict that content assemblies can be activated equally well by input to the left and right hemispheres and that the strongly 1ateraliLed function assemblies are much harder to activate by stimulation of the right. Thus. differences in the degree of lateralization of assemblies can explain the stronger RVFA for function words as well as the content word superiority in the LVF. With regard to interhemispheric interaction, the neurobiological framework suggests the following: Since production and perception of any word are usually related to brain activity in both hemispheres, twmm of’hoth hrtni.splww.s must hr port of’wll aset~zh1ie.s inwlwd irt lexical pvcrssit~y. Within-assembly connections can be provided only by far-reaching axons of spiny cells which are known to be excitatory [S]. Thus, interhemispheric cooperation of both hemispheres during lexical processing must take place. However, it has been claimed that simultaneous processing of linguistic material in both hemispheres leads to interhemispheric inhibition (see Refs [49] and [Sl]). There is also a third possibility, namely that the two hemispheres act independently when processing words. Finally, the interaction between both hemispheres could be far more complicated than the alternatives mentioned. Interhemispheric interaction can be investigated by experiments in which individual stimuli are not only presented either to the left or the right visual field, but also simultaneously to both visual half-fields. HELIJGE and coworkers [27-331 used these presentation modes extensively and found evidence for “metacontrol” of one of the hemispheres in various linguistic and visual tasks. In a lrrter cotnput~isotz task 1311, the performance pattern after RVF presentation was similar to the bilateral condition, suggesting that the LH exercises control in this task. Reaction times indicate that the response pattern after bilateral presentation of C1/C .s~/lnhles (consisting of one consonant followed by a vowel and again by a consonant) was similar to that of the RH, leading the authors to conclude that the RH exercises “metacontrol”and, so to speak, dominates the LH in this task 1331. However, when error rates were compared. the bilateral condition

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sometimes tended to improve performance compared to both unilateral conditions. In the light of these findings, processing of letters and CVC syllables has been assumed to be controlled either by the left or by the right hemisphere, suggesting a complex mechanism of interhemispheric inhibition and activation during processing of these linguistic stimuli. The bilateral condition may also provide information about interhemispheric interaction during kxicui processing. If two copies of the same word are presented in addition to unilateral stimulation the following three outcomes appear to be likely. (i) bilateral presentation leads to slower and less accurate responses compared to unilateral presentation. This result could be explained by irzterhemisphe~ic inhihitiorz [49, 511; if activity in one hemisphere (after a short latency) causes inhibition ofthe other, bilateral presentation would mdke both hemispheres inhibit each other at the same time, leading to slower and less accurate processing. (ii) If the bilateral pattern mirrors the performance of one of the hemispheres, this hemisphere may be assumed to control information processing during the bilateral task (monohemispheui~ control). Given left hemisphere dominance for language, similar response patterns after RVF and bilateral presentation of words (and non-words) could be expected. (iii) If fastest response times and lowest error scores occur in the bilateral condition, intrrl7emispl~rri~ collaboration may be assumed. The cell assembly model predicting word representations distributed over both hemispheres would be compatible with this result but incompatible with the former two. M’ith these hypotheses and predictions in mind, we conducted two experiments. Experiment 1 was designed to replicate the findings of CHIARELL~ and NUDING [IO] and to investigate whether word frequency and certain stimulus parameters influence the results. Experiment 2 was designed to explore the effects of bilateral presentation of content and function words, as well as pronounceable non-words.

EXPERIMENT

1

With Experiment 1, we intended to confirm that there is a right visual field advantage (RVFA) for words which is largest for function words, but that there is no RVFA for nonwords. A lexical decision task was used in which subjects had to decide as fast as possible whether a string of letters, presented either in the left or right visual field, is a word or not (yes/no responses). Reaction times and error scores were measured. This experiment is similar to Experiment 1 in CHIARELLOand NUDINC; [lo]. However, our design differed in the following ways. (i) word stimuli consisted of high frequency words and “very high” frequency words with a word frequency above 400/‘million. The two lists were constructed in order to test whether high and very high frequency words lead to differing response patterns. (ii) Word stimuli were subject to a number of strict criteria. Words with initial stop consonant were excluded, since short words starting with a stop consonant (like ci~t or hat) seem to be particularly difficult to process in the right hemisphere [47,49]. Note that, in contrast to content words, English function words very rarely start with a stop. Also, prepositions and quantifiers were excluded, since they are hard to classify either as function or as content words, since they can be used in both ways (see Refs [ 161 and [17]). (iii) subjects were asked to express their word/non-word decision bimanually with two fingers, one ofeach hand. Slrhirc,ts.Eleven male and eleven female undergraduate UCLA students (average age: 19.5years) participated in this experiment in order to fulfill an introductory psychology course requirement. All subjects were strongly right-

handed (assessed by a abort version (live ~tcms) from the Edinburgh Handedness Inventory [39]) and had no left-banders or amhldcxtcrs in their f~m~l). All wcrc native speakers of English with normal or corrected-to-normal vision in both eyes. .~ppurtrtuv. Subjects wcrc scated approximately 56 cm from an Amdck VIDEO-3 IOA CRT monitor ofan IBM AT compatihic computer. During the cxpcriment subjects had their chin\ in a chinrest with a forehead restraint centcrcd relative to the viewing \cI-ccn. Four keys on the computer keyhoard wc~-c used to collect subjects’ responses. Proc~~dr~~. Fvcry subject participated in one cxpcrimcntal session of approximately 25 min duration Subjects were instructed lo dccidc whcthcr they consider a certain letter string to he a real English word or a non-word in English. This decision was made by pu\hlng two out of four buttons on the computer keyboard at the same time. Subject5 were instructed to be ax fast and accurate as possible. Bcforc starting with the experimentdi blocka. subjects had to participate in a practice s&on 01‘24 trials. The experiment itsclfcon\i\tcd of two blocks. each containing all 160 items (X0 words and 80 non-wol-ds). The block order was counterbalanced between subjects. For each subject the aequcncc ofwords and non-words WXL;random&d in each hiock. Every item (word or non-word) appeared once in each visual half-field. Each block was di\ldcd into tno parts with at lenst 5 min rats in between. Subjects were instructed to lixatc their eyes on a lisallon cross appearing 111the middle of the computer screen. After the presentation of the central fixation cro&\ for X00 mscc, there was a warning tone lasting 200 msec, indicating that a stimulus would follow Then. a stimulus wa> shown for 100 m~c in cithcr the RVF or the LVF. Subjects‘ eye movements were ohrcrvcd carefully and continuously by the expcrlmcntccr in order to ascertain that they did not move their eyes. Subjects had to indicate their word,:non-word decision by pressing ANY>response buttons aimultancously within a 2.5 set interval before the next stimulus was presented. Only. the fastest button press was used for latency analyses. Finger responses for word and non-word decisions using Index or middle fingers wcrc counterbalanced between subjects. Stimuli ucrc presented horizontally in lowercase letters either in the LVF or RVF, their innermost edge I.23 from !ixation. The stnnuli subtended hctwcen 2.4 and 3.9 ofhorizontal and 0.6 of vertical visual angle. Stimulus n~a/~rirrl\. A total of X0 \bords and X0 non-words wcrc chosen. These items are presented in Appendix A. Words consisted of 40 function (closed-class) and 40 content (open-&b.\) words matched for frequency of occurrcncc according to FKA~IS and K~.uK,\ [ 151. Sincc GORI)ON and CAKAMAI~A’S [?I] results indicate that word5 O~I.PI.J high frcqucncy (> 4OO’Mio.) and ofhigh frcyuency ( i 400:Mio.) could hehavc diffcrcntly. we made up tw’o frequency groups of 40 iiems each (30 function .lnd 20 content words). Half of the word sample had a high frequency of occurrcncc of 70 420 per mi!lion. the other half consisted of bery high frcqucncq words having a frequency :Ihovc 420 pcl- million. In addition. all words w?re matched for wordlcngth counted in graphemes (2 7 letters.wol-ds: meall-4.37: S.D. = 1.24). wordlengh in phonemes (2 6 phonemes: mean=3.lY!: S.D. =0.94). number olayllahlcs (60 one syllable and 70 two \gll:thlc wol-ds) and regulnl-lty ofspelhng. Regularity ofspelling ~a% determined for each word hy examming whcthcr- the rhqmc of i!s syllable(s) was in most I‘LI’IC~pronounced BS in jhc word (as for “\vay”: cf. “raq “. ‘.maq’“. “\a!“) or diffcrcntly (as for “\+ iii.“; d. “far”. ‘jar” , .’star”). In Appendix R, wcgi\c some details about the functlon.contcnt word matching. Content uords were; concrete nouns. verbs and adjcctiveb The function word sample included articles. pronouns. auxihary \erhs. complementizcrs, conjunctions. and non-l) adverbs. It did not Include prcpohitions or quantlficrs. Words heginning vath an initial stop consonant were excluded. If a word of one class had a homophonoua word ofthe other class. it was only included If the frequcncq 01 occurrence of the w,vd of the first class outnumbcrcd the homophonous sister hq a factor of 5 or more. For each word, a pronounccahle pseudoword (“non-woi-d”l was conhtructed cithcr by permutating sonic of its leitcrs or hq exchanging some of 11sletters with Icttera of other words of the ‘ramc arcgory. No~v~ords wcrc also matched for length (counted in graphemes and phonemes) and number of aqllnbles. All non-v
Since preliminary analyses ruled out any reliable cfkts of Sex, Regularity of Spelling, and Number of Syllables, these variables were collapsed in subsequent analyses. Percent correct scores and median reaction times were analyzed usins mixed design analyses of variance. In the word vs non-word analyses the between-subject variables were Wordness and Visual Field (VF). In the function vs content word analyses, the between-subject variables were Word Class, Word Frequency and Visual Field. Subjects wcrc used as the random variable. WOIY/.SES r~~-\vo~tis. Reaction time analysis revealed significant main effects of wordness [F(l, 20)=21.62, P
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Fig. 2. Results of Experiment I Adequacy scores (a) and response times (b) for function and content words presented in the LVF or RVF. The RVF advantage is 4gnificant for function words but not for content words. The Word Class x Visual Field interaction was marginally sigmficant in the reaction time analysis.

There was no further interaction of Word Frequency with any other factor, so that essentially the same pattern of results was obtained for high and for very high frequency words.

Experiment 1 revealed a marginally significant interaction of Word Class and Visual Field with a significant right visual field advantage (RVFA) for function words, but no VFA for content words or for non-words. Words led to faster and more accurate responses than nonwords. Higher frequencies of word stimuli improved error scores and shortened reaction times, but did not affect the RVFA. The word frequency effect appeared to be the same for content and function words. Our data confirm the general pattern of results O~CHIARELLO and NUIIING [lo]. However, our results differed from their in two ways. Whereas we only observed a marginally significant interaction of Visual Field x Word Class (P < O.OS), Chiarello and Nuding report a significant interaction (PcO.05). Furthermore, we did not obtain a RVFA for content words, whereas Chiarello and Nuding observed a marginally significant RVFA for content

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words. Both differences could be explained either by the choice of stimuli or by a slightly larger variance in our data due to the fact that we counterbalanced finger responses for word and non-word decisions (using index and middle fingers) between subjects. However note that for content words, a trend towards a RVFA is present in our data as well (cf. Fig. 2). Thus, the more stringent criteria we applied for stimulus selection did not affect the overall results of the experiment. The variation of the RVFA with word class (nonsignificant RVFA for content words vs significant RVFA for function words) has theoretical implications. In particular, it is consistent with the assumption that each hemisphere has its own content word lexicon, so that fast direct access is possible not only through the RVF, but also through the LVF. For function words, lexical input to the LVF-RH may be relayed to the LH, causing delay and a higher error rate which shows up as a clear RVFA on both dependent measures. Alternatively. function words and content words could both have interhemispheric cell assemblies that differ in their degree of lateralization (function assemblies strongly lateralized, content assemblies only weakly lateralized).

EXPERIMENT

2

Given that each hemisphere is equipped with its own lexicon, how do these lexicons interact? Experiment 2 was designed to answer this question. As detailed in the Introduction, one could expect interhemispheric inhibition, independence, cooperation, or a complex pattern of inhibition and excitation. In a lexical decision task similar to Experiment 1, we presented the same list of words and non-words either unilaterally in the left visual field (LVF) or right visual field (RVF), or in both visual fields (BVF) with two copies of the same word form.

A lexical decision task was performed with stimuli presented either in one of the visual fields or in both visual fields at the same time. Presentation conditions were intermixed in each block. Suhjrcts. Twelve male and nine female undergraduate UCLA students participated in this experiment (average age 20.9 years). Characteristics and selection criteria were the same as in Experiment I. 4pp~t~1fu.s. See Experiment I. PI.o~P~~E. Subjects participated in one experimental session of approximately 40 min duration. Subjects were instructed to decide whether they consider a target letter string appearing in either visual field or in both visual fields simultaneously a real English word or not. In this experiment, there was a practice block of 24 trials and three experimental blocks each consisting of 160 items (80 words and 80 non-words). Every item was shown once in each presentation condition (LVF. RVF and BVF). There was at least a 5 min rest in between the blocks. All furthe! instruclions and procedures were the same as in Experiment 1. .Sfirnulusmtrtrrirrls.Stimuli were the same as in Experiment I.

Re.sults

Preliminary analyses of variance indicated that two within-subject variables Word Frequency and VF, interacted with the between-subject variable Sex of Subjects in the error score analysis. Furthermore, an effect of th$ factor Number of Syllables was obvious in the word vs non-word analyses. Therefore, we report the results of two 4-way ANOVAs. In the word vs non-word analyses the within-subject factors were Wordness, Number of Syllables and Visual Field (VF), and the between subject factor was Sex. In the function vs content word analyses, the within-subject fa.ctors were Word Class, Word Frequency and VF, and the between-subject factor was again Sex. These ANOVAs were carried out for both error score and reaction time data. For the variable Visual Field with the three levels LVF, RVF

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and BVF, multivariate tests (Wald Test, Wilks’ Lambda) were used to assess significance. These are reported as F-values. Words usnon-wds. In the reaction time analysis significant main effects of Wordness [F(l, 58)=52.91, P
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Fig. 3. Results of Experiment 2. Adequacy scores (a) and response times (b) for word and non-word stimuli presented in the LVF, RVF, or both visual fields (BVF). There is a highly significant Bi gain for word stimuli only. The interaction Wordness x Visual Field was highly significant in the error analysis.

significant differences between words and non-words in all three Visual Field conditions: in the LVF [F(l, 5X)= 11.47, P
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RVFA for function words but not for content words. For content words there was a significant difference between the LVF and BVF [F (1, 58)= 38.72, P< O.OOOl], as well as between the RVF and BVF [F (1, 58) = 24.6, P
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117

However, this non-significant interaction indicated a similar VF pattern as in Experiment 1: the RVFA appeared to be present for function words but not for content words (accuracy). In addition to the usual effects of Word Frequency and Visual Field condition, we obtained some evidence that the number of syllables of items could be relevant for word processing. Finally, some data suggest that the Sex of subjects may have some influence on word processing strategies. Clearly, words are processed faster and more accurately when they are presented bilaterally. At first glance, the Bi gain could be due to some strategy of our subjects to focus their gaze on, or shift their attention to, one of the visual fields. However careful and continuous monitoring of eye movements during the experiment ascertained that no gaze shifts occurred. Further, shifting of attention should have led to better scores in the bilateral condition, regardless of the nature of stimuli. Instead, the Bi gain was not observed for nonwords; non-words elicited similar responses when they were presented uni- or bilaterally. This argues that the Bi gain was not the result of a nonspecific attentional strategy. Assuming distinct lexicons in the LH and in the RH, the Bi gain gives evidence for cooperation between the lexicons of the hemispheres. It is also possible that there are not two distinct lexicons but only one lexicon with word representations (cell assemblies) distributed over both hemispheres. Models of interhemispheric interaction that postulate independent processing of the two hemispheres or inhibition between them during word processing cannot account for the obtained gain in the bilateral condition. While there was a marginally significant interaction of Word Class x Visual Field in Experiment 1 and a significant interaction in an earlier study [lo], this interaction did not reach significance in Experiment 2. Note, however, that the planned comparison carried out on error scores makes it likely that function words were still processed more accurately in the right visual field condition than in the left, whereas there was no difference for content words. The stimulus material in this experiment was made up of high and very high frequency words and we expected interactions of Word Frequency with various other variables. For example, the original Bradley-hypothesis (content words are frequency-sensitive, function words are not, see Ref. [3]) predicts an interaction of Word Class and Frequency. Even GORDON and CARAMAZZA'S [21] results make it likely that very high frequency words show word class specific differences, whereas other words do not. In general, these expectations have not been confirmed by our data (see Ref. [S]). Only the marginally significant Word Class x Word Frequency interaction obtained in Experiment 2 is compatible with the view that the two word classes differ in their frequency sensitivity. However, since this result was obtained only in Experiment 2, but not in Experiment 1 or in other similar studies, it cannot provide compelling support for the Bradley hypothesis. Nevertheless, a more detailed item analysis could still uncover differences between frequency effects for function and content words. The main effect of Number of Syllables per word indicated that higher numbers of syllables of words and non-words may slow down processing speed. In addition, the interaction of Wordness x Number of Syllables in the latency analysis was significant and the planned comparison revealed that only responses to words were influenced by the number of syllables (one-syllable words were faster than two-syllable words, but there was no difference for nonwords). This suggests that the number of syllables of a word affects cortical processing of elements of the lexicon. However, we cannot decide on the basis of our data whether the number of syllables of word stimuli itself was the critical parameter or the covarying parameter of word length, counted either in phonemes or graphemes per word. An influence of word

length (counted in graphemes/word) has been reported before by others (e.g. Ref. 1141). These authors concluded that Word Length effects can occur for both words and non-words in both visual fields. In this context, it is noteworthy that in our data the Wordness x Number of Syllables effect was also present in all Visual Fields. There are several reasons to interpret our results on Sex differences with caution. First, our subjects included 12 males and only 9 females, so that the data for females are based on a relatively small number of subjects. Second, the observed main effects in latency and accuracy point in opposite directions: whereas females were faster than males, they made more errors. This could be explained by a tendency of our females to emphasize speed, whereas our malts could have emphasized accuracy. (Note that we asked the subjects to be as fast atzd as accurate as possible). Third, an effect of sex has not been obtained either in our first experiment or in other experiments in which we used the same stimuli and experimental procedures. Previous experiments using similar paradigms (e.g. Refs 131, [IO] and 1301) did not tind any sex differences as well. Since the Sex x Word Frequency and Sex x Visual Field interactions obtained in the error score analysis comparing content and function words may be subject to one or more of the above objections, we do not wish to give them an extensive discussion here.

GENERAL

DlSClJSSION

Our results. as well as previous studies, revealed consistent main effects of VF, Wordness and Word Frequency. In addition, the interaction of Wordness x VF was clearly significant in Experiment 2 and close to significance in Experiment 1. The factors Word Class and Visual Field interacted when exclusively unilateral stimulation was used (our Experiment 1; [lo]). This consistency of results across experiments suggests that lexical decision experiments with hemiheld presentation provide reliable data on the processing of pronounceable non-words, functions words, and content words in the two cerebral hemispheres. In what follows, we will discuss explanations of the word-.spec[fic Bi @II which was not observed for non-words and the,fiutictiotr KYUY~ spectfic RVFA which was not present either for content words or for non-words. In doing so, we will refer to brain-theoretical, cognitive, and computational models. The neurobiological model allows for an explanation of both the RVFA for function words and the Bi gain for both kinds of words by relating these phenomena to physiological processes. If a cell assembly has many of its neurons located in a particular hemisphere, stimulation of that hemisphere is likely to reach many assembly neurons. Fewer assembly neurons are reached if only a small number is located in the stimulated hemisphere. Bilateral input reaches more neurons of interhemispheric assemblies than unilateral stimulation. The more assembly neurons receive stimulation. the faster the assembly ignites and the lower becomes the probability that random activity (noise) will ignite another assembly instead (false responses). This may be caused by spatial surtmatiot~, a well-known neurophysiological process where two excitatory post synaptic potentials (EPSPs) sum up so that the neuron will fire earlier (and the process is less likely to be disturbed by ‘noise”, e.g. an interfering inhibitory PSP). Because a cell assembly is a strongly connected neuronal network, spatial summation can occur in many of its neurons at the same time. This results in faster ignition if larger numbers of assembly neurons have been stimulated initially. Since function word assemblies have only few neurons located in the RH but many of them in the LH, stimulation of the RVF-LH leads to faster ignition than stimulation of the LVF-RH (RVFA for function

w-ords). Content word assemblies are less strongly lateralized and many assembly neurons are reached by stimulation through either ofthe visual half-fields. Thus, no significant RVFA occurs. Both kinds of words benefit from bilateral presentation, since bilateral input increases the number of assembly neurons that receive activity and, thus, the amount of the loosely connected neurons neuronal activity within the assembly. In contrast, corresponding to non-words do not allow for an ignition. The negative lexical decision can take place if no word assembly has ignited and, therefore. activity has ceased. Since nonwords have no strongly connected interhemispheric assemblies, simultaneous stimulation through both visual fields does not lead to a pronounced summation effect. Thus. the theory predicts that non-words should benefit less from bilateral presentation than real words. In this study non-words did not show any bilateral gain. Function word specific RVFA and word specific Bi gain could also be obtained in a neuronal network model (like the models proposed in [13, 351) if the network meets the following conditions: (i) distinct layers for content and function word processing are introduced, and (ii) each layer consists of artificial “neurons” that represent real neurons in horh hcwi.splwes. However, compared to the cell assembly model such a network would have one important disadvantage. Its structure, i.e. its layers and connections, would be hard to relate to the neuroanatomical structure ofthe real cerebral cortex. If the layers were supposed to stand for cortical regions, it would be unrealistic to assume anything else but very strongly excitatory within-layer connections and topographic and reciprocal between-layer projections. This would amount to changing the paradigmatic poly-layer perceptron in a fundamental way. We feel that. at this point, networks related to the neuroanatomical structure of the cortex are more realistic paradigms for modelling higher cortical mechanisms (for further discussion, see Refs [7] and 1421). The RVFA specific for function words (see Ref. [7]) is also in agreement with current theories of hemispheric specialization. The absence of any RVFA for content words is compatible with the assumption that not only the LH but also the RH plays a crucial role in processing these words. This has been postulated in the Direct Access Model [49]: both cortical hemispheres are equipped with their own content word lexicon and direct lexical access can take place in the LH as well as in the RH. Since there is a RVFA for function words. the RH is likley to play only a minor role in processing these words. Therefore, in order to perceive function words the RH has to relay information to the LH. Alternatively, the RH could be equipped with a processing device that handles function word only slowly and inaccurately. If there are lexical representations for all kinds of words in both cortical hemispheres, the Bi gain specific for words could demonstrate that these lexicons cooperate rather than compete or act independently. This calls for a modification of the Direct Access Model to make it applicable to our results. For example, the two lexical representations of a particular word could be assumed to have reciprocal excitatory connections between each other. Within the cell assembly framework, further well-known results obtained in many reaction time tasks can be explained by referring to the neuronal substrate. For example, ~vord superiority (faster and more accurate processing of words compared to non-words) can be explained in the following way. Cell Assemblies that correspond to words consist ofstrongly interconnected neurons which become active when words are presented. Due to the strong positive connections within an assembly, the whole neuron population ignites easily and quickly after stimulation, allowing for fast and accurate lexical decision. The neuronal counterparts of non-words can be conceptualized as neuron populations with weak internal connections in which activity spreads more slowly and no explosive activation process

120

B.

MOHK.

F. PULVLKM~~LLEK and E. ZAIULL

(ignition) can take place. The non-word decision takes place if, after a certain time interval, no assembly ignition has occurred. Presentation of non-words leads to more slowly spreading cortical activity (slower reaction times) which is also more sensitive to noise (higher error scores). Therefore, the negative lexical decision can take place only after a longer delay and is more likely to be false. The bvordfrequency
121

I?;TLKHt~lISPHFKICINTEKACTION

ultimately more fruitful to explain behavioral and theory.

data by interfacing

it with neurobiological

data

.Ic,r:,lo~c/rtlUer,lunrs~We wish to thank Jan Rayman for various discussions and extensive help in designing the reported experiments. For comments on earlier versions of the manuscript, we are grateful to Jan Rayman. Joseph Hellige. Katrin Mittelstaedt, Ron Mucha and John Schumann. We also thank two anonymous referees for their helpful comments and suggestions. This research was supported by grant Pu 97;1-I of the Deutschc I‘orschungsgemeinschaft to the second author and by NH1 grants NSZOI 87 and NIMH RSA MHOOI 79 to the third author.

REFERENCES I. AHISSAK. E.. VAAUIA, E.. AHISSAK, M., BEKG~IAN, H.. AKILL.I, A. and ABELLS, M. Dependence 2. 3. 4. 5, 6.

7. 8. 9. IO. II. 12. 13. 14. 15. 16. 17. IX. 19. 20. ?I. 22. 23. 24.

25.

of cortical plasticity on correlated activity of single neurons and on behavior context. Science 257, 1412 1415. 1997. BANICH. M. T. and KAROL, D. L. ElTects of information redundancy on bihemisphcric processing. (Forthcoming.) BKA~L~Y. D. C. Computtrtimol Distincrio,ls o/ C’oc~~hulrrr~ Tlpr. Indiana Universtly Linguistics Club. Bloomington. IN. 1983. BKAIILEY. D. C. and GAKKk.rT. M. F. Hemisphere ditferences in the recognition ofclosed and open class words. N~~rropv~rhoio~~itr 21, 155-l 59, 1983. BKAIT&~.BEKG.V. In Theoretictrl Apprm”hes to Camp/r\- Sy.~rerns. (Lecture notes in biomathematics, Vol. 71 ), R. HEIM and G. PALM (Editors). pp. 171 188. BKAIT~YIIEKG.V. In L’~umfutnento fn~udrsciplinrrrr a//o Srudio de/ Lingqutry~qio,G. BKAGA, V. BKAITFNBI-KG. C. CIpot LI, E. COSFKIL, S. CKESPI-REGHIZZI. J. M~HL~K and R. TITON~ (Editors). pp. 96 10X. Franc0 Angeli Editore, Mllano. 1980. BKAITENB~KG. V. and PLL.VEKMCLLI-K,F. Entwurf einer neurologischen Theorie dcr Sprache. !VuturG\~rrr-.schc$er~ 79, 103-l 17, 1992. BKAITE~BLKG, V. and SCHULZ. A. .4nc~to~n~ o/‘r/lp C‘ortr~x. S!ntistics md Grorwfry. Spring, Berlin. 1991. BKAITTNBFKC;.V. and SCHI:Z, A. In Lurlyucc
122

26 27 28 29. 30 31 32 33. 34. 35. 36. 37 3x. 39. 40. II. 41. 43. 44. 45. 46. 47. 48. 49.

so. 51.

B. MOHK. F. PI,LVIKMOLLLK and E. ZAII~~I hippocampus using dcpolarillng current pulses a the conditioning stimulus IO single volley synaptic potentials. J. /Vruro\ci. 7, 774 780, 19X7. HI-1313,D. 0. 7% Oryar~iru~iort ,>/ Brlztrt-ior. /I !V~,trrt,p\,‘c.ho/r,r/ic,~// %wr~,. John Wiley, New York, 1949. Ht.1 I.I(;I:. J. B In Ducdily cd C tzif~ 4 r/w Brtr~n. D. 01 IOSOU (Editor). pp. 454 465. MacMtllxn Preth. Hampshire, 1987. HI.LLI~;L. J. B. Hemispheric asymmetry. Awr. Rrr. P\JYM. 41, 55 80, 1990. H~LLIG, J. B.. COWIN, E. L.. Ijv;, T. and St.tcGt’Nl~, V. Perceptual reference frames and visual field asymmetries for verbal processing. Nrwop.\~,< /w/o. xnd MAITSOZ, R. In Hrrwpltrric~ f~~rrrtwrio~~. F. KI~TIXLI (Editor). Lawrence Erlhaum. Hillsdale. NJ.

INTERHEMISPHERIC

INTERACTlOX

APPENDIX

FUNCTION

WORDS

SUBGROUP

FWlA

(one syllable;

123

A

high frequency)

yet, why, thus, shall, whose, am, nor, whom, none, ought SUBGROUP rather,

FWlB

(two syllables;

often, ever, whether,

SUBGROUP

high frequency)

either, further,

FW2 (one syllable;

except, neither,

herself, unless

very high frequency)

then, now, such. our, me, must, your, much, where, just, those, how, here, though, while, might, us, since, once, less

CONTENT SUBGROUP

WORDS CWlA

(one syllable;

high frequency)

foot, arm, fire, road, way, hard, red, view, low, wish, SUBGROUP remain,

CWlB

office, mother,

SUBGROUP

(two syllables;

high frequency)

money, letter, father, music, occur, agree, machine.

CW2 (one syllable;

very high frequency)

say, man, see, walk, find, look, world, hand, long, house, child, ask, eye, head, face, run, high, war, room, light

NON-WORDS loyt, stuld. thyll, fasp, wem, roog, noom, rhas, nelt, oune, erthar, reitoon,

ruhert, xepty, heinert,

wod, shiw, reinam,

netor, veret, herew,

sherfel, seluns, mook, fraf, vad, yod, roak, hanv, rel, sar,

iffoce, rethom,

rassar, rertel, tharef, cimus, rucco, rega, nachime,

hem. wogh, heb. surm, rez, sumt, moop, rach, xn, jars, hosk, splog, heerf, mool, how, thorn, stum, neesh, sonc, leb, yearn, namp, ress. wak, noif, echo, quab, Iuw, haar, shaf, ilch, swaz, yees, vex,

heaf, wad,

naz, reen. nase, grif

B. MOHR. F. PULVERMCLLERand E. ZAIDEL

124

APPENDIX Means are given for all item groups;

FWl (high)

CWI (high)

FW2 (very high)

CW2 (very high)

standard

deviations

B

are presented

in brackets

2.33

4.95

3.50

1.5

(.I@

(1.25)

(.95)

(.5)

2.35

4.85

3.50

1.5

(.11)

(.97)

(.75)

(.5)

2.94

4.00

2.85

1.0

(.11)

(.83)

(.56)

(.O)

2.89

3.80

2.95

1.0

(.17)

(.64)

(.45)

(.O)