Interhemispheric Communication Is Via Direct Connections

BRAIN AND LANGUAGE 64, 28–52 (1998)ARTICLE NO. BL981954

Interhemispheric Communication Is Via Direct Connections

Marjorie Collins and Jeffrey Coney

Murdoch University

Two priming experiments, using normal university students as subjects, indepen-dently projected low imagery primes and concrete target words to the left or rightvisual fields (LVF or RVF) to examine the merits of three spreading activationmodels of interhemispheric communication: (i) callosal relay of a semantically en-coded prime; (ii) transfer of products activated as a result of the spread of activation;and (iii) direct connections between the hemispheres. The first experiment tempo-rally separated pairs by a stimulus onset asynchrony (SOA) of 250 ms and obtainedstrong support for the direct connections model. Priming effects were obtained onlywhen the prime was projected to the RVF and the target to the LVF. The patternof priming effects suggested that low imagery words projected to the left hemispherecan activate concrete associates in the right hemisphere via direct callosal connec-tions between the two. In the second experiment, the SOA was increased to 450ms. This time, RVF–RVF priming was obtained along with RVF–LVF priming.The findings are interpreted within a modification of Bleasdale’s (1987) framework,where abstract/low imagery words and concrete/high imagery words are representedin separate subsystems in the left hemisphere lexicon. Support was also found forthe view that the left hemisphere is comprised of a complex network of abstract andconcrete words, while the right hemisphere operates as a subsidiary word processor,subserving linguistic processing with a limited, special purpose lexicon comprisedof associative connections between concrete, imageable words (e.g., Zaidel, 1983a;Bradshaw, 1980). Interhemispheric communication in the priming procedure ap-pears to occur at the semantic level, via direct connections between the hemispheres. 1998 Academic Press

Although the functions of each cerebral hemisphere and the representationof information therein have been extensively investigated, less is currentlyknown about communication between the hemispheres. We have observedsome interesting patterns of interhemispheric priming in our investigationswhere prime and target words were projected to different cerebral hemi-

We thank three anonymous reviewers for their helpful and constructive criticism on anearlier draft of this paper.

Address correspondence and reprint requests to Dr. Marjorie Collins, Psychology Division,Murdoch University, Murdoch, Perth, Western Australia 6150, E-mail: [email protected].

280093-934X/98 $25.00Copyright 1998 by Academic PressAll rights of reproduction in any form reserved.

INTERHEMISPHERIC COMMUNICATION 29

spheres ([Collins]Abernethy & Coney, 1990, 1993, 1996; Collins-Aber-nethy, 1996). The present study will focus specifically upon the nature ofinterhemispheric communication in the priming procedure.

In considering interhemispheric communication, it is important to com-mence by examining how a prime selectively projected to one hemispheremay exert its influence on the opposite hemisphere and thereby influenceprocessing of a subsequent target. There are different stages of processingat which this influence may arise. One is during visual encoding of the prime,and prior to semantic encoding. Here, a prime presented in the LVF wouldbe transferred to the left hemisphere for linguistic processing, which mayresult in facilitation of responses to related targets subsequently projected tothe left hemisphere via the RVF. Alternatively, a prime may access the lexi-con in the hemisphere to which it is initially projected and exert its influenceon the opposite hemisphere after semantic encoding. For priming tasks em-ploying lexical decision, the bulk of the evidence is consistent with the lattersuggestion, which is commonly referred to as direct access. After a reviewof this evidence, Chiarello (1988) concluded that both hemispheres can makelexical decisions without the necessity for callosal relay of information, andwords are primarily encoded in the hemisphere to which they are initiallyprojected. Hardyck, Chiarello, Dronkers, and Simpson (1985) concur, in con-cluding that it is the actual lexical decision which is transmitted to the otherhemisphere and not the information required to make the decision. We havealso found evidence of direct access in a phonological priming task whereit appeared that the critical stage at which callosal transmission of primeinformation occurred was at the level of processing subsequent to semanticaccess and not at orthographic or phonological levels ([Collins]Abernethy &Coney, 1990; see Coltheart, 1980). Similarly, Zaidel, White, Sakurai, andBanks (1988) conclude that evidence from aphasics, commissurotomy pa-tients, and normal subjects all suggest that the right hemisphere is able tomake lexical decisions, and that both hemispheres have a lexicon. Of directrelevance in the present context is an experiment they described which inves-tigated semantic priming in each hemisphere and found evidence of directaccess, where primes were processed in the hemisphere to which they wereinitially projected. We have recently confirmed their finding in a study wherepriming for nonassociated category exemplars was confined to conditionswhere the prime was projected to the left hemisphere, irrespective of whethera related target was subsequently presented in the RVF or LVF ([Collins]Ab-ernethy & Coney, 1996).

It must be noted that the argument in favor of direct access for these tasksis based upon the assumption that the speed and degradation of semanticinformation transmitted from left to right hemisphere is symmetrical to thatfor information transmitted from right to left hemisphere. Recent studieshave raised the possibility that interhemispheric transmission times may beasymmetrical. However, the evidence on this issue is not yet clear (Berlucchi,

30 COLLINS AND CONEY

Aglioti, Marzi, & Tassinari, 1995; Hoptman & Davidson, 1994) with somestudies raising the possibility that the transmission of sensory informationmay be faster from right to left hemisphere than from left to right hemisphere(e.g., Braun, 1993; Braun & Daignneult, 1994; Marzi, Bisiacchi, & Nicoletti,1991), while others suggest faster transmission in the opposite direction (e.g.,Brown, Larsen, & Jeeves, 1994; Nowicka, Grabowska, & Fersten, 1996). Inview of evidence that the symmetry of interhemispheric transmission timedepends upon the stimulus (Nowicka et al., 1996), research into the symme-try of transmission times for lexical and semantic information is requiredbefore the viability of the direct access hypothesis for lexical decision taskscan be assessed in this light. To our knowledge, no such research has yetbeen conducted. Until then, we must accede to the current consensus thatfor priming tasks employing lexical decision, initial processing of the primeis performed in the stimulated hemisphere.

How then, does a prime exert its influence on the opposite hemisphere inthe priming procedure? One possibility is reflected in the views of Zaideland Coltheart, and may be conceptualized as ‘‘callosal relay of a semanti-cally encoded prime.’’ Zaidel (1983a) suggests that callosal relay of morecomplex aspects of a stimulus may occur subsequent to the initial directaccess. In a similar vein, Coltheart (1980) suggests that a word achievessemantic access in the hemisphere to which it is initially presented, and itis the resulting semantic code which is transferred to the opposite hemi-sphere, rather than a physical/visual representation of the word. This seman-tic code is likely to be in a form characteristic of the hemisphere in which itwas processed (Moscovitch, 1979). This model suggests specific predictionsabout priming effects for within- and cross-hemisphere presentations. Ac-cordingly, when prime–target pairs are presented to different hemispheres,activation would be expected to spread in the nonstimulated hemisphere afterthe semantically encoded prime has been relayed to that hemisphere. Conse-quently, the facilitation effects observed for cross-hemisphere presentationswould reflect the organizational format of the lexicon in the hemisphere towhich the prime is relayed, rather than that of the hemisphere to which itwas initially projected. To assess the viability of this model, as well as otherways in which a prime may exert its influence on the opposite hemisphere,it is necessary to consider priming studies which have included both within-and cross-hemisphere priming conditions in their design. Studies examiningwithin-hemisphere priming alone do not provide the necessary observationsfrom which to assess this and so will not be discussed here (e.g., Burgess &Simpson, 1988; Chiarello, Burgess, Richards, & Pollock, 1990; Chiarello &Richards, 1992; Chiarello, Senehi, & Nuding, 1987). Only a few studiesmeet this criterion ([Collins]Abernethy & Coney, 1990, 1993, 1996; Collins-Abernethy, 1996; Marcel & Patterson, 1978) and will be used as the basisupon which to examine cross-hemisphere communication in the priming pro-cedure.


The model of callosal relay of a semantically encoded prime predicts thata LVF prime would be semantically encoded in the right hemisphere andthen transferred to the left hemisphere. Given that the studies which haveincluded cross-hemisphere conditions in their design found RVF–RVF prim-ing for categorically ([Collins]Abernethy & Coney, 1996), associatively([Collins]Abernethy & Coney, 1993; Marcel & Patterson, 1978), and phono-logically related word pairs ([Collins]Abernethy & Coney, 1990), this modelpredicts that, in these studies, the subsequent presentation of a RVF targetwhich is related on any of these dimensions would facilitate responses rela-tive to a baseline condition (i.e., LVF–RVF facilitation). The priming effectsobtained do not fully support these predictions. LVF primes facilitated re-sponses to RVF targets when they were related by association ([Collins]Ab-ernethy & Coney, 1993; Marcel & Patterson, 1978), but not when categori-cally ([Collins]Abernethy & Coney, 1996) or phonologically related([Collins]Abernethy & Coney, 1990). The latter two observations are incon-sistent with the predictions of the callosal relay of a semantically encodedprime.

Although less likely, this model can also accommodate interhemisphericcommunication of linguistic information through the relay of a semanticallyencoded prime from left to right hemisphere. In this case, a prime directedto the RVF may be semantically encoded in the left hemisphere and thentransferred to the right hemisphere. Since the above-mentioned studies havefound LVF–LVF priming for associates ([Collins]Abernethy & Coney,1993; Collins-Abernethy, 1996; Marcel & Patterson, 1978) but not phono-logical ([Collins]Abernethy & Coney, 1990) or categorical relationships([Collins]Abernethy & Coney, 1990, 1996), the prime should have activatedits associates in the right hemisphere, but not categorical or phonologicalinformation. Therefore, this model predicts that, in these studies, a subse-quent LVF target should also have facilitated responses to associated targets,but not categorically or phonologically related targets. The findings were notentirely consistent with these predictions either. RVF primes did facilitateresponses to associated LVF targets ([Collins]Abernethy & Coney, 1993;Marcel & Patterson, 1978), and no such facilitation was observed for phono-logically similar LVF targets ([Collins]Abernethy & Coney, 1990). How-ever, two findings are inconsistent with the callosal relay model: RVF–LVF facilitation was found for categorical pairs that were not associated([Collins]Abernethy & Coney, 1996) but not for targets associated with thesubordinate meaning of a prime (Collins-Abernethy, 1996). Hence, the evi-dence for callosal transmission of a semantically encoded prime is equivocal.

[Collins]Abernethy and Coney (1990) suggested an alternative way inwhich a prime may exert its influence across the hemispheres: a prime isprocessed in the hemisphere to which it is initially projected, followingwhich interhemispheric communication occurs through the transfer of theproducts of the spread of activation rather than transfer of the prime itself.


These products may include associative, categorical, emotive, imaginal, orphonological aspects of the prime’s meaning. This will be referred to asthe ‘‘activated products model.’’ Another possibility is that direct callosalconnections exist between the prime in one hemisphere and its related con-cepts in the opposite hemisphere, with interhemispheric communication oc-curring through activation spreading along these direct connections. Both ofthese models assume, in line with the evidence, that the prime is processedin the hemisphere to which it is initially projected, and that the characteristicpattern of cross-callosal communication is determined by the organizationalformat of the hemisphere of initial reception, and not the hemisphere towhich the semantic code is transferred.

The two models can be distinguished experimentally by the different pre-dictions they make about the relationship between within- and cross-hemi-sphere priming effects. The activated products model assumes that relatedwords must be activated within the hemisphere receiving the prime beforetheir products can be transferred to the opposite hemisphere. Therefore,within-hemisphere priming facilitation must be concomitant with cross-hemisphere facilitation. The direct connections model, on the other hand,assumes that the presentation of a prime to one hemisphere may directlyactivate related words in the opposite hemisphere, without these words nec-essarily becoming activated within the hemisphere initially receiving theprime. Thus, priming effects for cross-hemisphere presentations are indepen-dent of any within-hemisphere effects.

These opposing predictions may also be evaluated in the context of theresearch which has examined both within- and cross-hemisphere priming.Firstly, with regard to left hemisphere priming, RVF–RVF facilitation hasbeen found for associates ([Collins]Abernethy & Coney, 1993; Marcel &Patterson, 1978), category exemplars ([Collins]Abernethy & Coney, 1996),category exemplars with some association ([Collins]Abernethy & Coney,1990) and phonologically similar pairs ([Collins]Abernethy & Coney, 1990).RVF-LVF facilitation accompanied within-hemisphere priming in each case,with the exception of phonologically similar pairs. Nevertheless, when cross-hemisphere facilitation occurred in these studies, it was always accompaniedby within-hemisphere facilitation. This is consistent with the activated prod-ucts model which assumes that these RVF primes activated associatively,categorically, and phonologically related words within the left hemisphere,and these products of the spread of activation were subsequently transferredto the right hemisphere. Therefore, a LVF target related on any one of thesedimensions should have facilitated responses. This was indeed the case, ex-cept for phonologically similar pairs. Only the latter finding is inconsistentwith the activated products model, and so does not constitute sufficient evi-dence to reject it. However, it must be noted that the pattern of priming inthese studies is also consistent with the predictions of the direct connectionsmodel, which assumes that the RVF primes activated associative and cate-


gorical information in the right hemisphere via activation spreading throughdirect connections between the hemispheres. Unlike the activated productsmodel, this model can accommodate the absence of cross-hemisphere facili-tation for phonology, by assuming that words in the left hemisphere are notdirectly connected with their phonological equivalents in the right hemi-sphere, as the latter does not have a phonological processor (Zaidel & Peters,1981). Clearly, left hemisphere priming effects do not provide sufficient evi-dence from which to choose between these two models.

Right hemisphere priming effects are more informative in this context.Priming within the right hemisphere has been found for associates ([Collins]-Abernethy & Coney, 1993; Collins-Abernethy, 1996; Marcel & Patterson,1978). LVF–RVF facilitation accompanied LVF–LVF facilitation in eachcase. However, LVF–RVF facilitation has been found independently ofLVF–LVF facilitation for associates separated by an SOA of 250 ms ([Col-lins]Abernethy & Coney, 1993) and category exemplars confounded by asso-ciation ([Collins]Abernethy & Coney, 1990). Hence, in two studies we haveobtained a profile of priming effects that is inconsistent with the activatedproducts model. The direct connections model receives more support fromthis research, as the observed pattern of within- and cross-hemisphere prim-ing effects correspond exactly with its predictions.

The model of callosal relay of a semantically encoded prime receives lim-ited support from priming studies. Although more support has been obtainedfor the direct connections model of interhemispheric communication thanthe activated products model, this evidence is less than compelling. Only twostudies have found cross-hemisphere facilitation independently of within-hemisphere priming. In both cases, LVF–RVF facilitation was obtained in-dependently of LVF–LVF facilitation. In light of this, the present study aimsto examine the viability of the three above-mentioned models of interhemi-spheric communication in a design specifically aimed at investigatingwhether a complementary profile of within- and cross-hemisphere primingeffects can also occur. That is, whether RVF–LVF priming can occur inde-pendently of RVF–RVF priming. If so, this would provide more compellingevidence in favor of the direct connections model.

EXPERIMENT 1

To simultaneously examine the viability of these models, a task was re-quired which could activate information in both hemispheres, and where itwas possible for callosal transfer of information to occur from left to righthemisphere and vice versa. Since both within- and cross-hemisphere priminghas been obtained for associates with a priming procedure ([Collins]Aber-nethy & Coney, 1993; Marcel & Patterson, 1978), it was selected for thepresent inquiry. It was also necessary to use a task involving a responsewhere the performance of both hemispheres was as closely matched as possi-


ble. Both hemispheres can make lexical decisions (Chiarello, 1988) and canperform equally well in making nonword responses, although a RVF advan-tage has been noted for ‘‘word’’ decisions (Chiarello, 1988). Even so, thisRVF advantage is often absent or reduced for concrete, imageable nouns oradjectives (e.g., Day, 1977, 1979). It seems the right hemisphere can recog-nize imageable words (Boles, 1989) and direct lexical access in the righthemisphere for high frequency, concrete, or imageable words has been noted(Bradshaw, 1980; Zaidel et al., 1988; Chiarello, 1988). Studies with clinicaland commissurotomy patients also demonstrate the right hemisphere’s abil-ity to comprehend single words, mainly concrete nouns (Ely, Graves, & Pot-ter, 1989). Hence, concrete imageable nouns and adjectives were selectedas word targets in order to provide a relatively equal opportunity for bothhemispheres to process the targets and a response for which the two hemi-spheres are comparatively well matched.

Last and most important, primes were selected so that the spread of activa-tion could be restricted to one hemisphere only. After presentation of theprime, if activation spreads in one hemisphere and not the other, then isola-tion of the source of interhemispheric communication is possible. Underthese conditions, the pattern of cross-hemisphere facilitation in relation towithin-hemisphere facilitation effects will allow specific conclusions to bedrawn pertaining to whether the prime is transferred or in some other wayactivates associates in the opposite hemisphere. The right hemisphere hasbeen shown to perform particularly poorly when processing words whichare abstract or low in imageability (Ely et al., 1989; Bradshaw, 1980; Boles,1989) and processing verbs, irrespective of their imageability (Day, 1979). ARVF advantage has even been noted for verbs of high frequency and mediumimagery (Hernandez, Nieto, & Barroso, 1992; Rastatter & Loren, 1988).Hence, low imagery words were selected as primes. Due to restrictions im-posed upon us by the necessity of selecting a sufficient number of suitablelow imagery primes from word association norms, we were compelled tosupplement the prime set with a very small number of verbs. This is unlikelyto compromise the outcome of the current study, which aims to assess theviability of three models of interhemispheric communication rather thanpriming for verbs and low imagery words per se. For convenience, all primeswill be referred to as low-imagery words. These primes should preferentiallyactivate the left hemisphere. The right hemisphere’s difficulty in processingthese primes would be expected to hamper any spread of activation. Thus,activation is likely to spread within the left, but not the right hemisphere,even though both hemispheres are sensitive to associations. Consequently,presentation of this type of prime to the right hemisphere is not expected tofacilitate responses to an associated concrete target subsequently presentedto the right hemisphere (i.e., no LVF–LVF priming).

The hypothesized priming effect for the presentation of a low imageryprime to the left hemisphere, followed by presentation of an associated con-


crete target to the same hemisphere, is somewhat more complicated. Bleas-dale’s (1987) proposal that concrete and abstract words are represented intwo separate representational systems within the lexicon, and Kroll andMerves (1986) finding of a slowing of response when switching from abstractto concrete words, must be borne in mind. These findings suggest that foran automatic-processing task presenting paired stimuli in quick succession,left hemisphere facilitation would only occur for associates which are homo-geneous in concreteness and not for abstract–concrete or concrete–abstractassociates. The prime–target pairs to be used in the present experiment areheterogeneous in imageability, and since a high correlation exists betweenconcreteness and imageability (Paivio, Yuille, & Madigan, 1968; Toglia &Battig, 1978), these pairs are also likely to be heterogeneous in concreteness.Marcel and Patterson (1978) report that imageability but not concretenessinfluences word recognition and that imageability interacts with hemisphericpresentation, where the right but not the left hemisphere is selectively disad-vantaged in processing low imagery words. Day (1979) and Boles (1989)have expressed similar views. Consequently, it is more likely that Bleas-dale’s findings of separate lexical representations relate to high and low im-agery words, rather than concrete and abstract words per se. If so, high andlow imagery words may be organized into separate representational systemsin the left hemisphere. Therefore, in a priming task, left hemisphere facilita-tion would be expected to arise earlier for associates which are homogeneousin imageability than for heterogeneous pairs. After presentation of a lowimagery prime to the RVF, activation within the left hemisphere shouldspread initially to other low imagery words and only later to concrete, im-ageable associates. Hence, it is unlikely that left hemisphere facilitation forheterogeneous pairs will occur when there is a very short temporal intervalbetween the presentation of a prime and target (called SOA). The first experi-ment incorporated this information into the design to allow for the possibilityof cross-hemisphere priming for RVF–LVF presentations without concomi-tant within-hemisphere priming for RVF–RVF presentations. Heterogeneousimagery pairs were used and separated in time by a short SOA of 250 msspecifically for this purpose. Hence, it is possible that the low imagery primesin the present study will not have sufficient time to facilitate responses toassociated concrete targets presented to the left hemisphere a mere 250 mslater.

Having ascertained that the first experiment is unlikely to find LVF–LVFfacilitation, and the possibility that RVF–RVF facilitation may not occureither, we turn to the question of the cross-hemisphere effects predicted byeach of the three models of interhemispheric communication. According tothe callosal relay model, a low imagery prime presented to the right hemi-sphere will be transferred to the left hemisphere. Subsequent presentation ofan associated concrete target to the left hemisphere will result in facilitation(i.e., LVF–RVF facilitation). This can occur only if the prime is transferred


to the left hemisphere for processing, as no facilitation should be present forprime–target presentations to the right hemisphere (i.e., LVF–LVF). Fur-thermore, it is extremely unlikely that these low imagery primes would betransferred from the left to the right hemisphere, so presentation of the primeto the left hemisphere would not be expected to facilitate responses to targetssubsequently presented to the right hemisphere (i.e., no RVF–LVF facilita-tion). If RVF–LVF facilitation does occur, this will provide strong evidenceagainst the model of callosal relay of a semantically encoded prime and willindicate that the prime has activated the opposite hemisphere by some othermechanism. Moreover, the case against callosal transfer of the prime willbe strengthened by the presence of RVF–LVF facilitation accompanied bythe absence of facilitation for both LVF–LVF and LVF–RVF presentations.

The activated products model assumes that those associates which are acti-vated as a result of the spread of activation within the left hemisphere maythen be transferred to the right hemisphere, resulting in facilitation for RVF–LVF presentations. Associates must be activated within the left hemispherebefore they can be transferred to the right hemisphere. Therefore, supportfor this model is found only if cross-hemisphere facilitation accompanieswithin-hemisphere facilitation. The primes to be used in the present task arenot expected to activate associated concrete word targets in the left hemi-sphere within an SOA of 250 ms and so should not be available for transferto the right hemisphere. Therefore, no RVF–LVF facilitation would be ex-pected. Neither LVF–LVF or LVF–RVF facilitation are expected either, asactivation should not spread from low imagery primes to associated targetsin the right hemisphere.

In contrast, the direct connections model assumes that cross-hemispherefacilitation is independent of within-hemisphere facilitation. In order to testthis assumption, stimulus pairs were selected where it was possible for facili-tation to occur in the former condition but not the latter. Word pairs heteroge-neous in imagery were selected for this purpose for the following reasons.There is good evidence that the right hemisphere represents concrete, im-ageable words within the lexicon but not abstract, low imagery words orverbs (Zaidel, 1978a, 1978b; 1983a; Day, 1979) and they are organized inan associative network (Coltheart, 1980). So, in Bleasdale’s (1987) terms,the right hemisphere has a concrete, but not an abstract, representationalsystem. Consequently, a low imagery prime presented to either hemispherewould not be expected to activate low imagery associates in the right hemi-sphere. Further, a low imagery prime presented in the RVF would initiallyactivate the abstract/low imagery representational system in the left hemi-sphere and would fail to activate concrete associates within the left hemi-sphere at this stage. However, since the right hemisphere does not have anabstract representational system, the same RVF prime may immediately ac-cess the concrete representational system in the right hemisphere, producingRVF–LVF facilitation. Hence, the heterogeneous nature of these prime–tar-


get pairs is expected to influence the spread of activation within the lefthemisphere, by initially routing the activation to abstract/low imagery words,but it should not stall the activation of concrete associates in the right hemi-sphere. Consequently, at a short SOA of 250 ms, it should be possible toobtain RVF–LVF facilitation without RVF–RVF facilitation. If this patternof priming is obtained, it would provide strong support for the direct connec-tions model which assumes that there are direct connections between a wordrepresented in the left hemisphere and its associates in the right hemisphere.So it predicts that the presentation of a low imagery prime to the left hemi-sphere will activate concrete associates in the right hemisphere via directcallosal connections from left to right hemisphere. This will result in facilita-tion for RVF–LVF presentations. RVF–RVF facilitation need not accom-pany this effect. Neither LVF–LVF or LVF–RVF facilitation is expected,since the right hemisphere is selectively disadvantaged in processing theprimes used here.

Method

Subjects. Forty-one undergraduate psychology students were selected as subjects. Data forthree of these subjects were discarded, as their error rates exceeded 30% in either the wordor nonword condition. Nineteen female and 19 male subjects remained, their mean age being23.4 years. All had normal or corrected-to-normal vision, were right-handed by self report,and English was their first language.

Apparatus. Subjects were tested in a well-lit cubicle room containing a modified CBM 4032microcomputer system which controlled trial sequencing, stimulus presentation, timing, anddata collection. The onset and offset of all stimuli was controlled with a circuit that allowedthe screen to be written while blank and then ‘‘flashed’’ on (or off) within a single rasterscan. Screen intensity was diminished to the minimum level of the factory capability in orderto decrease phosphor persistence. Reaction time (RT) was measured to a resolution of 1 msvia a centrally positioned microswitch response box connected to the user port of the micro-computer. The stimuli were printed in green capital letters on a high-bandwidth monitor. Eachletter was 3 mm wide and 4 mm high, with 1 mm spacing. In light of evidence that specialtechniques for monitoring eye movements in laterality research is unwarranted (e.g., Jones &Santi, 1978) and simply watching subject’s eyes is ‘‘remarkably accurate’’ (Young, 1982,p. 18), we monitored eye movements via a Sanyo video camera connected to a video monitor,which provided a magnified image of the subject’s eyes. No formal recordings of eye move-ments were taken. A chin rest was used to stabilize the subject’s head in the correct positionrelative to the screen. EMH12 High Performance earmuffs were used to minimize possiblenoise interference.

Design. RT was chosen as the principal dependent variable as it is a more sensitive anddirect measure of hemispheric processing than accuracy (Santee & Egeth, 1982; Young, 1982).Errors were also recorded and analyzed. A lexical decision task was used which requiredsubjects to discriminate words from nonwords using a GO–NOGO response procedure. Thefirst experimental variable was stimulus pair, incorporating 4 prime–target conditions. Therelated word condition consisted of 128 associated pairs selected from word association norms(Shapiro & Palermo, 1968; Keppel & Strand, 1970; Thomson, Meredith, & Browning, 1976).Most were primary associates, although all were at least fifth highest associates. Of these pairs,116 were comprised of a low imagery noun or adjective and an associated concrete, imageablenoun or adjective. These were used as primes and targets, respectively (e.g., ADAGE–OLD;


SERIES–BOOK). Imageability of primes was determined after collection of normative dataon 270 potential stimulus words from the above-mentioned word association norms where atotal of 238 undergraduate psychology students rated these words on an imagery scale rangingfrom 1–7, with 1 being low imagery and 7 high imagery ([Collins]Abernethy, 1991; Collins-Abernethy, 1996). Only words rated 4.0 or less for imagery on these norms were used as primesin this study. After exhaustion of words fitting these criteria, it was necessary to complete therelated set with 12 verbs which were associated with a concrete, imageable noun or adjective.These were used as primes and targets respectively (e.g., DEVELOP–FILM; WRITE–LET-TER). Verbs were used, as Day (1979) has shown that the right hemisphere has difficultyprocessing verbs, irrespective of their imageability, whereas the left hemisphere performs wellin recognizing such words. This fulfils the requirements for the stimulus set in the presentstudy. These additional pairs comprise a very small proportion of the related set.1 A further128 pairs formed the baseline condition. This condition consisted of the prime ‘‘BLANK’’paired with each of the word targets from the related set (e.g., BLANK–OLD).

For the negative set, 128 additional primes were each paired with an orthographically legal,pronounceable nonword target which was matched in length with the targets in the positiveset (e.g., CRYPTIC–ELD; GRADE–MUNET). These primes were matched with the primesin the related-word condition with respect to word length, grammatical category, and fre-quency. The nonword targets were derived by changing between one to three letters in targetsfrom the positive set but never the first letter, unless it was the same as the prime with whichit was to be paired. The nonword targets were also paired with the word BLANK to form128 pairs which mirrored the neutral condition in the positive set (e.g., BLANK–ELD). Noneof the nonwords were homophonic with real English words.

All pairs were phonemically dissimilar, and none began with the same phoneme. Ortho-graphic similarity was minimized and no pairs started with the same first letter. All primeswere within a range of 3 to 8 letters in length, while targets were 3 to 7 letters. The meanfrequency of primes in the positive and negative sets was 78.3 and 75.99 words per million,respectively (Kucera & Francis, 1967).

Visual field of presentation was also manipulated and comprised four levels: (i) prime andtarget to the RVF (henceforth referred to as RVF–RVF); (ii) prime to the LVF and target tothe RVF (LVF–RVF); (iii) prime and target to the left visual field (LVF–LVF); (iv) primeto the RVF and target to the LVF (RVF–LVF). Each stimulus pair was presented in only onevisual field condition. Selection of visual field for each pair was randomly determined, as wasorder of presentation of word pairs in each condition. Thus, each subject was exposed to adifferent distribution of pairs in each visual field and a different sequence of prime–targetconditions. Each stimulus pair was presented only once.

Procedure. Subjects were seated in front of the display monitor with their heads positionedby a chin-rest directly in front of, and 45 cm distant from, the center of the screen. Taskinstructions emphasized the necessity of maintaining fixation on the central fixation crossduring trial presentation. Subjects were given 64 practice trials with the same structure as theexperimental trials. No stimuli from the experimental set were used. Feedback relating toaccuracy was provided after each practice trial. If the video monitor revealed any deviationof eyes from fixation during the practice trials, subjects were reminded of the importance ofmaintaining fixation at all times. If visible eye movements occurred during experimental trials,the subject’s entire data set was discarded.

1 Despite the evidence of a RVF advantage for processing both verbs and low imagerywords, it is conceivable that these two types of words engender different priming effects ineach hemisphere. Unfortunately, the very small number of verbs in the current stimulus set,combined with randomization of visual field and order of presentation of each stimulus pairfor each subject, precludes any separate analysis of the two types of words in the currentstudy. However, this is an interesting avenue for future research.


Each trial began with a central fixation cross which remained on throughout the trial. After750 ms, a prime word was displayed in the LVF or RVF for 150 ms. Two hundred and fiftyms after the prime appeared, the target was presented to the LVF or RVF for 160 ms. Thetarget appeared on the horizontal meridian, but 5 mm beneath the display location of theprime, to minimize masking effects. The fixation cross then disappeared and the entire screenremained blank for 1500 ms, during which the subject signalled a response. All stimuli werepresented 2 degrees of visual angle from the central fixation and subtended a horizontal visualangle of between 2 and 6 degrees. Randomization of trials ensured that subjects were unableto predict the visual field of presentation of either prime or target.

Subjects responded in accordance with a GO–NOGO procedure. Consistent with the neces-sity to control for asymmetric mediation of responses in divided visual field tasks (Hellige &Sergent, 1986) when the target was a word, subjects were required to respond by simulta-neously depressing two centrally positioned microswitches with both index fingers, the fasterof the two responses being taken as RT for that trial. Subjects were permitted 1500 ms afterthe disappearance of the target to respond. Failure to respond within 1500 ms was treated asa NOGO response. Following trials in which an incorrect response was made, the word ‘‘ER-ROR’’ appeared directly above the fixation. Subjects were permitted to rest after each blockof 64 trials and were given feedback regarding their accuracy and overall speed for the preced-ing block. Sessions required approximately 25 minutes to complete, excluding rest periods.

Results

Statistical analyses were carried out on mean correct reaction time (RT)for the positive set. A three-way analysis of variance, 2 3 (2 3 4), treatingall variables as fixed effects, was computed on sex, stimulus pair relationship,and visual field of presentation. Sex was the only between-subjects variable.There was no main effect for sex (F(1, 36) 5 1.4; p 5 .24) and sex did notinteract with stimulus pair relationship or visual field of presentation. Thisis consistent with our previous research, which has consistently failed tofind any sex differences on tasks investigating hemispheric processing at thesemantic level ([Collins]Abernethy & Coney, 1990, 1993, 1996; Collins-Abernethy, 1996). All remaining effects were significant. The main effectfor stimulus pair relationship (F(1, 36) 5 23.12; p 5 .0001) was due to anoverall facilitation in response latency of 16 ms for related pairs relative tobaseline pairs. The main effect for visual field (F(3, 108) 5 18.16; p 5.0001) reflected the consistently faster responses to RVF targets (see Fig.1). This is consistent with previous findings of a left hemisphere advantagefor ‘‘word’’ decisions (e.g., [Collins]Abernethy & Coney, 1993 & 1996;Chiarello, 1988).

The interaction between stimulus pair relationship and visual field (F(3,108) 5 20.33; p , .0001) is of particular interest, as it reflects primingdifferences in the hemispheres. Four related samples t tests were used tocarry out planned comparisons between each related condition and its base-line. Only one of these comparisons reached significance. RT to LVF targetswas 58 ms faster when they were preceded by an associated low imageryprime in the RVF (t (37) 5 9.115; p , .001). In contrast, the 2 ms advantagefavoring the related condition for each of the remaining conditions was not


FIG. 1. Mean Correct RT for associates and neutral word pairs as a function of VisualField, at SOA 250 ms.

significant (RVF–RVF presentations: t (37) 5 .375; p . .05; LVF–RVF pre-sentations: t (37) 5 .53; p . .05; and LVF–LVF presentations: t (37) 5 .306;p . .05). Hence, no priming facilitation was evident for either of the within-hemisphere conditions, with projection of associated prime–target pairs toeither the RVF or LVF failing to differ from the baseline. Only one cross-hemisphere condition resulted in facilitation, the RVF–LVF condition. Thiseffect was very large, at 58 ms, and exceeds the magnitude of priming wehave obtained in any other study.

Error rates for both the positive and negative sets were analyzed by meansof a three-way analysis of variance, 2 3 (4 3 4), on sex, stimulus pair, and


TABLE 1Percent Error for Word and Nonword Pairs in Each Visual Field at SOA 250 ms

Visual Field of Presentation

RVF–RVF LVF–RVF LVF–LVF RVF–LVF

Stimulus pairRelated word 8.22 9.62 18.75 17.02Neutral word 7.97 9.62 16.69 24.09Word/Nonword 14.22 16.77 17.59 20.87Neutral/Nonword 10.19 15.46 18.09 20.23Mean error 10.15 12.86 17.78 20.55

visual field of presentation. Again, there was no main effect for sex (1,36)5 .37; p 5 .55) and sex did not enter into any interactions. All other effectswere significant. The main effect for stimulus pair (F(3, 108) 5 4.75; p 5.0038) reflected the greater accuracy for the word target conditions (at 13.4%errors for the related word condition and 14.6% for the neutral word condi-tion, relative to 17.4% for the word nonword condition, and 15.99% errorsfor the neutral nonword condition). The main effect for visual field (F(3,108) 5 51.11; p , .0001) is consistent with the RT data, where projectionof the target to the RVF resulted in better performance than projection tothe LVF (see Table 1). An interaction between visual field and stimulus pairwas also present (F(9, 324) 5 4.91; p , .0001). The RVF advantage wasgreater for word decisions than for nonword decisions, which is consistentwith previous research (e.g., [Collins]Abernethy & Coney, 1996; Chiarello,1988). No speed–accuracy tradeoffs were evident.

Discussion

The present results provide clear support for the direct connections modelof interhemispheric communication. This support relies on the absence inthe present results of any facilitation effects for the within-hemisphere condi-tions. Consistent with the hypothesis, primes projected to the LVF did notfacilitate responses to associated concrete targets subsequently presented tothe LVF. As expected, there was no facilitation for the complementarywithin-hemisphere condition either, where these associates were both pro-jected to the RVF. The only facilitation effect occurred when a prime wasdirected to the RVF, followed by an associated target in the LVF.

Turning to the question of the means by which interhemispheric communi-cation occurs, there is clear evidence against the callosal relay model of asemantically encoded prime. The facilitation found for RVF–LVF presenta-tions provides the strongest evidence against it. The magnitude of this effectwas very large, at 58 ms, and in sharp contrast to the complete absence ofany facilitation effects in the other three visual field conditions. Here, a prime


directed to the left hemisphere facilitated responses to associatively relatedconcrete targets subsequently presented to the LVF. Evidently, a verb orlow imagery prime presented to the left hemisphere activated its concreteassociates in the right hemisphere. Considering the particular difficulty theright hemisphere displays in processing such words, it is unlikely that thisfacilitation arose through callosal transfer of the prime from left to righthemisphere. This is attested to by the absence of facilitation for LVF–LVFpresentations.

Further evidence against the callosal relay model is found in the completeabsence of facilitation for LVF–RVF presentations. Presentation of the primeto the right hemisphere did not facilitate responses to associated concretetargets subsequently presented to the left hemisphere. Apparently, the primewas not transferred from right to left hemisphere, even though the right hemi-sphere is selectively disadvantaged in the processing of these primes (seeLVF–LVF in Fig. 1) and transmission is faster from the nonspecialized to thespecialized hemisphere (Nowicka et al., 1996). Evidently cross-hemispherecommunication of associative information was not achieved by callosaltransfer of the prime itself. Interhemispheric communication of semantic in-formation was achieved by some other means.

How do the two remaining models fare in accounting for the pattern ofresults? The independence of within-hemisphere and cross-hemisphere facil-itation effects for RVF–LVF and RVF–RVF presentations provides clearsupport for the direct connections model in preference to the activated prod-ucts model. A RVF prime did not activate its concrete associates within theleft hemisphere, although it successfully activated these associates in theright hemisphere. The 58 ms facilitation for RVF–LVF presentations standsin striking contrast to the absence of priming in the other three presentationconditions. Evidently, activation of associates within the left hemisphere isnot a prerequisite for a left hemisphere prime to activate its associates in theright hemisphere. This is consistent with the prediction of the direct connec-tions model that a prime directed to the left hemisphere activates associatesin the right hemisphere via direct callosal connections from words in the lefthemisphere to their associates in the right hemisphere. This in turn resultsin facilitation to associated targets subsequently presented to the right hemi-sphere.

The independence of RVF–LVF and RVF–RVF facilitation in the presentstudy is complementary to two earlier studies which found LVF–RVF facili-tation without LVF–LVF facilitation ([Collins]Abernethy & Coney, 1990,1993). This corroborative evidence further strengthens the argument in sup-port of the direct connections model. Although no firm conclusions can bemade on the nature of cross-hemisphere activation in the earlier study, asthe stimulus set was confounded by associations between pairs, it is possiblethat the concrete primes projected to the right hemisphere activated associ-ates in the left hemisphere rather than category exemplars per se. The absence


of LVF–RVF facilitation when using nonassociated category pairs ([Col-lins]Abernethy & Coney, 1996) and its presence when using associates([Collins] Abernethy & Coney, 1993), strengthens this argument. This com-bined evidence is consistent with the conclusion that low imagery words inthe left hemisphere are directly connected to their concrete associates in theright hemisphere, while concrete/imageable words in the right hemisphereare directly connected with their concrete associates in the left hemisphere.

EXPERIMENT 2

Although the first experiment has provided evidence in support of thedirect connections model, the predicted profile of priming in each hemispherewas based upon theoretical speculations derived from Bleasdale’s model(1987). A second experiment was designed to examine whether these specu-lations were reasonable. This is important to confirm the conclusions drawnfrom Experiment 1. Consequently, a second experiment was conducted toexamine further the suggestion derived from Bleasdale’s model, that the lefthemisphere represents abstract/low imagery words and concrete/high imag-ery words in separate lexical subsystems, where activation within the lefthemisphere takes time to spread from the abstract to the concrete representa-tional system and vice versa. Some evidence has been obtained which isconsistent with this proposal. Left hemisphere facilitation has been obtainedfor associates which are homogeneous in imagery ([Collins]Abernethy &Coney, 1993; Chiarello et al., 1987; Zaidel et al., 1988) while Experiment1 failed to find left hemisphere priming for pairs heterogeneous in imagery.It remains to be seen whether activation spreads from one of these subsys-tems to the other. The second experiment investigated this question by exam-ining hemispheric priming when associates heterogeneous in imagery areseparated by a longer temporal interval than that used in the first experiment.An SOA of 450 ms was chosen for this purpose, as research has indicatedthat hemispheric activation patterns for associates changes considerably be-tween 250 and 450 ms ([Collins]Abernethy & Coney, 1993).

Method

Thirty undergraduate psychology students acted as subjects. Data for two subjects werediscarded, as their error rates exceeded 30% in either the word or nonword condition. Fourteenmale and 14 female subjects remained, their mean age being 21.8 years. The selection criteriaused in the first experiment were also applied in this experiment. Apart from the SOA of 450ms employed to separate prime and target presentation, this experiment was identical in allother respects to the first.

Results

A three-way analysis of variance, 2 3 (2 3 4), was computed on meancorrect RT for sex, stimulus pair relationship, and visual field of presentation.


FIG. 2. Mean Correct RT for associates and neutral word pairs as a function of VisualField, at SOA 450 ms.

Again, there was no main effect for sex (F(1, 26) 5 1.43; p 5 .24) and sexdid not interact with stimulus pair or visual field. All remaining effects weresignificant. The main effect for stimulus pair relationship (F(1, 26) 5 25.99;p , .0001) revealed an overall facilitation of 23 ms for related pairs relativeto baseline pairs. The main effect for visual field (F(3, 78) 5 15.64; p ,.0001) reflected, as in the first experiment, the consistently faster responsesto RVF targets (refer to Fig. 2). The interaction between stimulus pair andvisual field (F(3, 78) 5 8.09; p , .0001) was examined in greater detail withfour related-samples t tests between each related condition and its baseline.Significant priming facilitation of 47 ms was found for RVF–LVF presenta-tions (t(28) 5 6.98; p , .001), and 28 ms facilitation was obtained for RVF–RVF presentations (t (28) 5 4.28; p , .001). However, for LVF–RVF pre-sentations, the 15 ms advantage favoring the related condition was not sig-nificant (t (28) 5 1.92; p . .05) nor was the 1 ms advantage for related pairsin the LVF–LVF condition (t(28) 5 0.17; p . .05). Thus, as in the firstexperiment, a large facilitation was found for RVF–LVF presentations. Fur-


TABLE 2Percent Error for Word and Nonword Pairs in Each Visual Field at SOA 450 ms

Visual Field of Presentation

RVF–RVF LVF–RVF LVF–LVF RVF–LVF

Stimulus pairRelated word 5.91 5.91 15.95 11.16Neutral word 6.58 5.24 15.51 15.4Word/Nonword 13.39 17.84 17.18 16.62Neutral/Nonword 10.82 14.95 18.19 17.96Mean error 9.17 10.98 24.57 15.28

thermore, unlike the previous experiment, the projection of both the primeand target to the RVF facilitated responses. A related samples t test compar-ing the magnitude of priming for these two conditions indicates that primingmagnitude was significantly smaller for RVF–RVF presentations than forRVF–LVF presentations (t (27) 5 2.56; p 5 .016).

Error rates were also analyzed by means of a three-way analysis of vari-ance, 2 3 (4 3 4). There was no main effect for sex (F(1, 26) 5 .14; p 5.71) and sex did not interact with the remaining variables. All other effectswere significant. The main effect for stimulus pair (F(3, 78) 5 8.72; p ,.0001) reflects the greater accuracy for the word target conditions (at 9.73%errors for the related word condition, 10.7% for the neutral word condition,16.3% for word/nonword, and 15.5% for the neutral nonword condition).As in Experiment 1, and consistent with the RT data, the main effect forvisual field (F(3, 78) 5 18.13; p , .0001) reflected greater accuracy whenthe target was presented in the RVF (see Table 2). The interaction betweenstimulus pair and visual field (F(9, 234) 5 4.28; p , .0001) again reflectedthe larger RVF advantage for word decisions than for nonword decisions. Forboth RVF–RVF and RVF–LVF presentations, accuracy was higher when thetarget was preceded by a related prime relative to responses when precededby the neutral prime. These data are consistent with the RT data, and nospeed–accuracy tradeoffs were evident.

GENERAL DISCUSSION

When a prime was presented in the RVF, there was no facilitation of re-sponses to concrete associates presented in the RVF just 250 ms later. How-ever, when the temporal interval separating prime and target was extendedby 200 ms, significant facilitation was observed for this condition. Evidently,a low imagery word in the RVF can activate associated concrete words inthe left hemisphere, although it takes some time to do so. This is consistentwith findings of a slowing of response when switching from abstract to con-crete words (Kroll & Merves, 1986) and differences in patterns of hemi-


spheric activation of associates at SOAs of 250 and 450 ms ([Collins]Aber-nethy & Coney, 1993).

Left hemisphere priming for associates which are homogeneous in con-creteness has previously been found when pairs are separated by an intervalof 250 ms ([Collins]Abernethy & Coney, 1993). In contrast, the present studydemonstrates that associates which are heterogeneous in imagery fail to pro-duce facilitation at such a short interval. Facilitation for these pairs did notbecome evident until the longer SOA of 450 ms. These observations can beusefully interpreted within the framework of Bleasdale’s model (1987) asfollows: activation takes time to spread between ‘‘abstract’’ and ‘‘concrete’’representational systems in the left hemisphere, where the presentation of alow imagery word to this hemisphere initially activates other low imageryassociates therein (so associative priming for low imagery associates wouldbe expected to emerge at this point. This has recently been confirmed in thefirst author’s laboratory, in an as yet unpublished study, using associatedlow imagery primes and targets which were separated by an SOA of 250ms.) Concrete/imageable associates are not activated at this point. Hence,no priming is evident in the left hemisphere when a low imagery word ispresented to the RVF, followed just 250 ms later by an associated concretetarget in the same visual field. Some time later, activation spreads from the‘‘abstract’’ representational system to the ‘‘concrete’’ system in the lefthemisphere, activating concrete words associated with the low imageryprime. Hence, responses to associated concrete targets are facilitated whenpresented some 450 ms after a low imagery prime was projected to the lefthemisphere. These observations are consistent with the suggestion that theleft hemisphere represents abstract/low imagery and concrete/imageablewords in separate subsystems in the lexicon.

Perhaps of greatest import in the present study is the RVF–LVF primingobtained for associates which are heterogeneous in imageability. Low imag-ery primes presented to the left hemisphere activated concrete associateswhich were presented in the right hemisphere 250 and also 450 ms later.The magnitude of facilitation for this condition was considerably larger thanthat for projection to the other three visual field conditions. Moreover, eventhough RVF–RVF facilitation accompanied this RVF–LVF facilitation inthe second experiment, the magnitude of priming was significantly largerfor the latter condition (47 ms cf. 28 ms). This suggests that the primingeffects for the within- and cross-hemisphere conditions were independent inthe second experiment, just as they were in the first. This independence isconsistent with the predictions of the direct connections model and confirmsour complementary but reverse patterns of priming effects, where LVF–RVFpriming was obtained without concomitant LVF–LVF priming ([Collins]Ab-ernethy & Coney, 1990, 1993).

The absence of priming in the remaining two visual field conditions ofthe present study indicates that presentation of a low imagery prime to the


LVF does not serve to activate its associates in either the left or right hemi-sphere. Moreover, there is no evidence that the prime itself was semanticallyencoded and then transferred from the right to the left hemisphere. If thishad been the case, some LVF–RVF priming would be expected in the secondexperiment, since facilitation was found for RVF–RVF presentations ofthese associates. Certainly, no RVF–LVF priming should have been ob-tained. This conclusion is bolstered by Nowicka et al.’s (1996) suggestionthat transmission is faster from the nonspecialized to the specialized hemi-sphere. Together, this provides further strong evidence against the callosalrelay of a semantically encoded prime.

The absence of LVF–LVF priming for these associates in the present studyserves to qualify our earlier conclusion that the right hemisphere is sensitiveto associations ([Collins] Abernethy & Coney, 1993). It seems that this hemi-sphere is sensitive to associations between concrete words, but not low imag-ery words and their concrete associates. Even when taking into account theretardation of activation within the right hemisphere relative to the left ([Col-lins] Abernethy & Coney, 1993), the complete absence of priming in thepresent study when low imagery primes and their concrete associates areboth directed to the right hemisphere leads to such a conclusion. This isconsistent with previous findings of a special purpose lexicon in the righthemisphere comprised primarily of concrete, imageable nouns (Zaidel,1983a; Bradshaw, 1980; Coltheart, 1980, 1983).

The present findings are consistent with those obtained by Day (1977),who found a left hemisphere advantage for categorizing abstract–concreteword pairs. However, they are somewhat inconsistent with those of Marceland Patterson (1978), who found symmetrical priming within each hemi-sphere in a lexical decision task with low and high imagery primes and aminimum SOA of 500 ms. They found facilitation of equal magnitude forwithin- and cross-hemisphere conditions. It must be noted though, that Saf-fran, Bogyo, Schwartz, and Marin (1980) failed to replicate Marcel and Pat-terson’s findings, using a similar procedure. Moreover, it is not clear whethertheir targets were high or low in imageability, and primes were masked intheir study but not in the present one. The present results also appear todiffer from those of Chiarello et al. (1987) who found symmetrical automaticpriming for associatively related abstract–concrete and concrete–concreteword pairs separated by an SOA of 500 ms. They concluded that activationis equal in both hemispheres for such stimuli. Even though the SOA usedin our second experiment was only 50 ms shorter than that used by Chiarelloet al., we found asymmetrical priming effects for low imagery primes andassociated concrete targets. Differences in methodology may account for thisdiscrepancy. In the present study, both prime and target were selectivelyprojected to one hemisphere, while Chiarello et al. presented the prime cen-trally. The method of prime presentation is likely to be the important differ-ence here, as Zaidel et al. (1988) note that central presentation of primes in


hemispheric priming experiments is problematic because the left hemispheremay ‘‘take over in some individuals’’ (p. 87). If so, presenting a prime cen-trally may be equivalent to presenting a prime in the RVF. Hence, conditionscomprising central primes followed by RVF targets would be equivalent tothose conditions comprising RVF–RVF presentations of prime and targetpairs, while presentations involving a central prime and LVF target wouldbe equivalent to RVF–LVF presentations. If this is the case, the findings ofthe second experiment are in accord with those of Chiarello et al. They foundfacilitation when centrally presented abstract primes were followed 500 mslater by concrete associates projected to either the RVF or LVF. Similarly,in the current study low imagery primes projected to the RVF facilitatedresponses to concrete associates projected 450 ms later to either the LVF orRVF. Hence, method of prime presentation appears to be an important factorto consider in interpreting the pattern of priming in each hemisphere. Thissuggestion is consistent with Chiarello et al.’s (1990) finding that primingfor lexical decisions was symmetrical in each hemisphere when primes werepresented centrally, but hemispheric differences emerged when both primesand targets were lateralized.

Although the current results are consistent with the direct connectionsmodel, the peculiar pattern of priming for the RVF–LVF and RVF–RVFconditions at the two SOAs may also be accommodated by the activatedproducts model. Consistent with Bleasdale’s model (1987), we have arguedthat a RVF prime initially activates associates which are homogeneous inimageability within the left hemisphere, with a delay occurring before hetero-geneous associates are activated. Hence, in the current study, RVF–RVFpriming was absent when the heterogeneous associates were separated by atemporal interval of 250 ms, yet present when the target was delayed by afurther 200 ms. In essence, this means that associates which are heteroge-neous in imageability produce priming within the left hemisphere only whenthere is a sufficient delay between the prime and its associate. In view ofthe right hemisphere’s relative inefficiency in processing linguistic material,it is possible that the delay in processing incurred by projecting a target tothe LVF serves the same purpose as delaying a RVF target by 450 ms. Itmay be this delay which provides the necessary conditions for RVF–RVFpriming to occur later than RVF–LVF priming, in the following manner:after a low imagery prime is presented to the RVF, there is sufficient timefor priming to take place across the ‘‘abstract’’ and ‘‘concrete’’ subsystemswithin the left hemisphere, followed by the transfer of products activatedtherein to the right hemisphere, with this transfer occurring before a LVFtarget has been fully processed in the right hemisphere. In other words, prod-ucts activated within the left hemisphere may be transferred to the right hemi-sphere in time for a LVF target to benefit from an associated RVF prime.The veracity of this notion can be assessed by considering RTs for RVF andLVF presentations in conjunction with estimates of interhemispheric trans-


mission time. If the delay in right hemisphere processing in conjunction withinterhemispheric transmission time is around 200 ms, then there is consider-able merit in this notion. In the current study, RTs for RVF presentationswere an average of 36 ms shorter than RTs for LVF presentations. Moreover,RTs for RVF–RVF presentations were 47 ms faster than for LVF–LVF pre-sentations in the first experiment and 44 ms faster in the second. Since esti-mates of interhemispheric transmission time range around 2–26 ms (Rugg,Lines, & Milner, 1984; Hoptman & Davidson, 1994), this amounts to amaximum of around 49–73 ms delay imposed upon the processing of tar-gets projected to the LVF. However, this delay is too short to have producedthe same sort of result as that derived from an extra 200 ms in the secondexperiment. This conclusion is bolstered when one considers that the RVF–LVF condition produced a 47 ms priming effect when pairs were separatedby 450 ms, while the priming effect for the RVF–RVF condition was sig-nificantly smaller, at 28 ms. Even if the possibility of asymmetric inter-hemispheric transmission times is taken into account, the spread of acti-vation model does not adequately account for the pattern of priming in thecurrent study. Afterall, Nowicka et al. (1996) found that interhemispherictransmission time depends on the stimulus, with transmission being fasterfrom the nonspecialized hemisphere to the specialized hemisphere. Giventhat words were used as stimuli in the current study, and the primeswere words which are particularly difficult for the right hemisphere to pro-cess, we would thereby expect faster transmission from right to left hemi-sphere. Instead, we found priming for left to right hemisphere conditions.Hence, the activated products model cannot adequately account for the cur-rent results.

In conclusion, this study has elucidated the mechanism of cross-hemi-sphere communication in the priming procedure, providing evidence consis-tent with the direct connections model. A low imagery word projected tothe left hemisphere appears to directly activate its concrete associates in theright hemisphere. This parallels previous research which indicates that a con-crete word in the right hemisphere can also directly activate its concreteassociates in the left hemisphere ([Collins]Abernethy & Coney, 1990, 1993).Consistent with Bleasdale’s (1987) distinction, the present study also sug-gests that the left hemisphere lexicon is complex in organization, havingseparate representational subsystems for abstract/low imagery and concrete/high imagery words, where activation takes some time to spread betweenthe two. In general, our research is consistent with the view that the lefthemisphere lexicon is comprised of a complex network of abstract, low im-agery, and concrete words, while the right hemisphere operates as a subsid-iary word processor, subserving linguistic processing with a limited, specialpurpose lexicon comprised of associative connections between concrete, im-ageable words (e.g., Zaidel, 1978a, 1978b; 1983a, 1983b; Bradshaw, 1980;Coltheart, 1980, 1983).


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Interhemispheric Communication Is Via Direct Connections

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