Mell/orr & Comition197J. Vol. I. No. J. 177-186

The interaction of perceptual processes and ambiguous sentences"

T. G. BEVERColumbia University, New York, New York /0027

M. F. GARRETTMassachusetts Institute of Technology, Cambridge, Massachusetts 02/39

and

R. HURTIGtNew York University, New York, New York /000]

(;cnnai pruuiplc-, of speech perception resolve <cvcral expcrimc ntal conflict-, about whether listeners interpret oneor all meanings of an ambiguous sentl'nc<' We argue that during an amhiguou x clause, both meanings are processed. butimmediately after the clause is over. it is rccodcd with only one meaning retained. This model resolves the apparentlyconflicting results of previous experiments: it also predicts that underlying structure ambiguity in incomplete clausesincreases comprehension time. In complete clauses, ambiguity docs not increase relative comprehension time; it mayreduce COin prehension time for ;lInbiguities whose interpretations arc perceptually distinct in those tasks where eithermeaning is appropriate. Two IlL'''· experiments offer preliminary confirmation of these predictions.

Many sentences could mean more than one thing, butin a normal context we usually notice only one meaning.This fact prcscnts a puzzle to the studcnt of languagebehavior: do listeners unconsciously decide between theinterprctations of an arnhiguous sentence or do theyanalyze the first meaning that comes to mind withoutanalyzing the other') Some recent experimental studieshave presented evidence that listeners unconsciouslyanalyze both meanings of an ambiguous sentence, eventhough they arc aware of only one meaning. Otherstudies have support cd the opposite claim. that listenersordinarily analyze only one meaning of an ambiguoussentence. In this paper. we review a theory of thenormal mechanisms of speech processing, usc this theoryto resolve the previous conflicting results, and presenttwo experimental results which support our view of thepercept ion of ambiguous sequences.

Recent research has suggested three principal featuresof speech percept ion: (I) the clause is a primaryperceptual unit. (~) within each clause direct mappingrules assign underlying structure relations among lexicalitems. and (3) after each clause is processed it is recededinto a rclatively abst ruct form, thereby leavingimmediate storage available for processing the nextclause.

"Research was supported by PHS Grant GM 16735 to theRockefeller University. Grant 5 RO I HD05168 from theNational Institute of Child Health and Human Development 10the Massachusetts Institute of Technology. and Office ofLdurat ion Grant 01(,-C-72-140J. Reprint requests should hesent to T. G. Ih'vL'I. Department of P~yd)()lo!!y. ColumbiaUniversity. New York. Nl'" York 10027. We are indebted toLdward Walker, David Hakes. and Robert Chiarello for advice onthe manu-cnpt.

+Now at Cnlumbia t 'nivcrvitv.

Responses to nonspcech stimuli presented duringsentences indicate that closure occurs at clauseboundaries. Clicks are rnislocated as occurring at suchpoints (Fodor & Bever, 1965: Garrett, 1965: Garrettct al, 1(66), but not at points of low transitionalprobability in general (Bever, Lackner, & Stolz, 1969),nor at all surface phrase structure breaks in general(Bever, Kirk, & Lackner, 1969). If speech is switchedfrom one ear to the other, the point of switching is mostaccurately located if it occurs at a clause boundary(Wingfield & Klein, 1(70). Reaction time to clicksbetween clauses is faster than to clicks not betweenclauses when Ss are motivated to emphasize sentenceperception (Holmes & Forster, 1972: Abrams, 1(73).These studies and others demonstrate the importance ofthe clause as a processing unit.'

The second principle is that within each perceptualunit, the underlying structure relations are assigned bydirect projection from the lexical sequences. A set ofperceptual mapping rules arc sensitive to specificfeatures of the surface sequence that signal particularunderlying structure relations. Sentences with explicitrelative clause markers (e.g., "who," "when") are lesscomplex than those without (Fodor & Garrett. 1967:Hakes & Cairns. 1(70). Similarly, explicit marking ofcomplement constructions (e.g., presence of "that") isfacilitating (Hakes, 1971). Sentences with verbs havingmore potential underlying structure roles are alsorelatively complex (Fodor, Garrett, & Bever. 1961':Hakes, 1971: Holmes & Forster, 1972). This fullowsfrom the fact that such verbs give less specific cues tnthe underlying structure of the sentences they arc in.

Both the clause segmentation and internal labelingprocesses place heavy emphasis on information inhcrcntto individual lexical items as well as surface propcnics l\f

sentences, On this view. sentence comprehension i, lhl'

278 BEVER. GARRETT AND Hl;R T ;

joint product of the projection of potentia! clauses andthe analysis of underlying grammatical relations withineach clause (for reviews. see Bever, 1970; Fodor, Bever,& Garrett. in press). On this view, the surfaceconstituents are directly marked for their primaryunderlying structure relations, e.g., "actor, action,object, modifier."

The perceptual mapping operations do not applyhomogeneously during a sentence. Accordingly. changesin sentence processing activity are reflected in variationsin attention to nonspeech stimuli (e.g., the click locationand reaction time studies already referred to). Abramsand Bever (1969), Valian (1971), and Seitz (1972) haveargued that maximum processing load occurs at the endof clauses. Abrams and Bever interpret the phenomenain the following way: during a clause, listenersaccumulate information and hypotheses concerning itspotential underlying strucrurels): the end of the clause isthe point at which a particular complete underlyingstructure is determined. More recent research hassuggested -that once underlying structure of a clause isdetermined. its initial phonetic representation isremoved from immediate storage. While immediate recallof the meaning of the first clause of a two-clausesentence is virtually perfect, free recall of the exactwords in that clause is poor (Jarvella, 1970; Jarvella &Herman, 1972). Consistent with this is the fact thatlatencies to identify a probe word are faster if the wordis drawn from the terminal clause of a sentence than ifdrawn from an immediately preceding clause (Caplan,1972). (In all these studies, simple serial position effectswere controlled for.)

The following picture of speech perception emergesfrom these studies. During a clause a listener projectspotential organizations for the underlying semanticrelations in it. At the end of each clause a structure isassigned and the external form of the clause itself isdropped from immediate memory. This sets the scenefor the empirical question in the present paper: Howdoes such a perceptual mechanism for speech deal withsentence ambiguity? Specifically, do listeners .::omputeall the meanings of an ambiguous sequence or just one?We shall argue that within a clause, more than onepotential underlying structure may be simultaneouslyprocessed. but at the end of the clause, one structure ischosen and the others are dropped. If the plausiblestructural alternatives are analyzed during a clause, thenan ambiguous sequence should require extra processingspace during the clause containing it. After the clause isrecoded with a single interpretation, a subsequent taskshould not reflect the effect of an original ambiguity.This view explains the differences in the outcome ofprevious studies, although none were conductedspecifically to test it.

Consider the experimental support for the view thatboth meanings of an ambiguous sequence are availablewithin a clause. Foss (I970) found that discriminationreaction time is slower immediately following an

ambiguous portion of a sentence than following acorresponding nonarnbiguous portion. Lackner andGarrett (1972) showed that the interpretation of anambiguous sentence is affected by a simultaneouslypresented disambiguating context. They conclude fromthis that both interpretations of an ambiguous sentenceare immediately available during its processing. MacKay(1966) found that the time to complete an ambiguoussentence fragment [e.g., (1») is longer than the time tocomplete a corresponding fragment which is similar butnot ambiguous [e.g., (2)] .

(1) After the right turn was .(2) After the left turn was .

Finally, Mistler-Lachman (1972) reports that visuallypresented ambiguous stimulus sentences occasionrelatively long times to think up a following"appropriate" sentence. Her technique encouraged Ss tostart their responses as soon as possible, and herresponses (measured from stimulus onset) were veryshort (ca. 2 sec). It is likely that her S's started thinkingabout (and sometimes even uttering) their responsebefore they had finished interpreting the stimuli. Thus,her results are consistent with the view that ambiguitycomplicates tasks that are initiated before sentenceinterpretation is completed.

By contrast, a number of other experiments areconsistent with the view that postsentence tasks do notreveal any added complexity as a IUIldi0Il of anambiguity within the sentence. Foss, Bever, and Silver(1968) found evidence that after a sentenceinterpretation is completed, only one meaning isavailable to the listener. They followed an ambiguoussentence with a picture appropriate either to the morefrequent interpretation of the ambiguous sentence or tothe less frequent interpretation. S's latency to respondwas longer when the picture required the less frequentinterpretation of the ambiguous sentence. Foss et alconclude that their results "support a model of normalsentence comprehension which states that Ss typicallyassign only one immediate interpretation to anambiguous sentence. Only if that interpretation is foundto be incorrect does S reinterpret the sentence [Fosset ai, 1968. p. 306)."

Carey, Mehler, and Bever (1970) used the sameexperimental paradigm with sentences and pictures asdid Foss et a!. They preset Ss to expect a sentence withone of the two phrase structures illustrated in (3) and(4). Then S heard an ambiguous sentenc.e like (5)followed by a picture that could be appropriate or notto both interpretations.

(3) They are (smiling men)(4) They (are fixing) lunches(5) They are lecturing doctors

They found that the "unseen" meaning of the critical

INTERACTION OF PERCEPTUAL PROCESSES ANDSENTENCES 279

sentence affected S's reaction time only if Slaterreported having noticed the ambiguity at the time. Thatis, the two meanings of an ambiguous sentenceinteracted only if they were both noticed consciously. Ifthe ambiguity was unnoticed, the sentences wereresponded to as though unambiguous.

Though the two sets of experiments we have brieflydescribed lead to different conclusions about the effectsof ambiguity on sentence comprehension, the conflictingresults are interpretable in terms of differences in theexperimental paradigms used. Those experiments whichshow an effect of ambiguity used tasks initiated duringthe processing of the sentence, while those showing noeffect used tasks initiated after processing of thesentence was presumably complete. In the context ofthe perceptual theory discussed above, theend-of-sentence effects in these experiments may beviewed as a special case of clause boundary effects: tasksthat occur before such boundaries are complicated bythe presence of both interpretations of ambiguities,while subsequent tasks are not.

We report two experiments below in which weexamine this possibility and related issues. We use two"sentence completion paradigms," one applied to the"postsentence" case and one applied to the"within-sentence" case.

EXPERIMENT I

Foss et aI's and Carey et al's evidence support theclaim that one meaning for a sentence is chosen at itsconclusion. We ran a study comparing the time toprocess ambiguous and nonambiguous sentences with atechnique different from that used by Foss et al andCarey et al but still focused on the interval immediatelyfollowing the presentation of an ambiguous sentence.The dependent variable was the time taken to think upthe next sentence of a hypothetical story. This wassimilar to Mis t le r-Lachman's (1972) responserequirements. However. the materials were designed tohave only one-third of the stimulus sentences to beambiguous for each S (in her study, the proportion ofambiguous sentences was three-fourths): this was toreduce the likelihood that S would notice theambiguities. Also, S wasexplicitly instructed to interpreteach sentence before thinking up a response, and hisreading light was turned off when he started talking:these measures were to reduce the possibility that Swould start the response task before completing theinterpretation of the stimulus.

Method

Materials

Twenty-seven 7- to 9-word ambiguous sentences wereconstructed. Three types of ambiguity were used (nine sentencesof each type): lexical am higuities . in which there art' two distill"!meanings for one of the words in the <cntence. l'.g .. (6a): surta, ('

phrase structure ambiguities, in which there are two bracketingsof the words in the sentence, e.g., (7a); and underlying structureambiguities, in which the two readings of the sentences arereflected only in different underlying "logical" relations amongthe words, e.g., (8,') [see Mackay & Bever (1967) for adescription and further examples of these distinctions].Corresponding to each interpretation for an ambiguous sentencethere was a nonam biguous sentence which differed from it inone word and which presented unambiguously one or the otherof the structures of the ambiguous version [e.g .. the band cversions of (6), (7), and (8)].

6 (a) Be sure that you take the right turn(b) Be sure that you take the left turn(c) Be sure that you take the correct turn

7 (a) The paper presented carefully limited analyses of theproblem

(b) The paper presented today limited analyses of theproblem

(c) The paper presented very limited analyses of the problem

8 (a) The shooting of the Indians bothered the agent(b) The shooting from the Indians bothered the agent(c) The shooting at the Indians bothered the agent

These materials were arranged into three experimental groupsof 27 sentences, such that each group contained 3 ambiguoussentences of each type as well as 6 nonambiguous sentences ofeach type. In this way, each sentence that was ambiguous in onegroup had one of its nonambiguous versions in a second groupand its other nonambiguous version in the third group. Eachsentence was typed without punctuation on a 3 \ 5 in. card.

Subjects

The Ss were 18 male Harvard summer school students (allnative speakers of English) who volunteered for the experimentand were paid for their participation.

Procedure

Each S was tested individually with a screen separating himfrom E. The stimulus sentences were presented on cards one byone through a slot in the screen. E instructed S first to read eachsentence and then to think up "the nex t sentence of a storv inwhich the stimulus sentence might occur." 5 pressed a switch assoon as he started to respond. which turned off his reading light.A timer recorded the interval between the initial presentation ofa sentence and S's movement of the switch. All Ss were given thesame 6 practice sentences before the experimental s~t of 27sentences was presented. Each response was categorized ascorresponding to one of the two interpretations of the stimulusambiguity. There were two indicants of which interpretation theS intended: inspection of 5's responses by E. and posttestquestioning of S. To obtain the second measure. each S wasplayed his original tape-recorded responses and after eachsentence was asked if he had noticed the ambiguity of thestimulus sentence. Regardless of his answer to this question. thenature of the ambiguity was explained and he was asked whichof the two meanings he had in mind when he had produced hisresponse during the experimental session. There were nodisagreements between E's scoring and Ss' reports: when thestory continuation did not indicate clearly to I the meaning Shad in mind. S's posttest response was used to assign thecontinuation to one of the two interpretations. [In a f,'\~ ,'ases(less than Y; l. 5s claimed to have noticed the ambiau itv and tohave had both meaning, in mind or some third intcrprct.nionwhen thl'y produced t heir n..' ... ~'on...;\..': t hesc fl' ...p()n"'l'''; \\ ..'1"...'

ctimin.ucd from our :IIU" 'l".1

~80 BEVER, GARRETT AND HURTIG

Table 1Time (Seconds) to Start the Responses to Ambiguous and Unambiguous Sentences in Experiment I

Type of Ambiguity

Ambiguous sentencesCorresponding unambiguous sentencesMean difference of responses (ambiguous-unambiguous)Percentage of Ss responding faster to ambiguous versions

Lexical

5.284.93

+0.3534'7c

SurfaceStructure

9.039.40

-0.2756'/(

UnderlyingStructure

8.839.72

-0.8982'7c

Results

The time to start the response sequence was about8 sec (note that this includes reading time for thestimuli). Overall, lexical sentences (both ambiguous andunambiguous) were responded to more quickly thanwere sentences of other types (p < .01 by two-tailed signtest across Ss for ambiguous and nonambiguoussentences separately). Other differences between typesof sentences were not significant. The results in Table 1show an interaction of the effect of ambiguity withambiguity type. For lexical and surface structureambiguities, there was no significant difference betweenambiguous sentences and their nonambiguous controls(tested by sign test across Ss and across sentences); forunde rlying structure ambiguities, the mean forambiguous sentences was significantly shorter than themean for the control sentences (p < .03, two-tailed signtest across Ss; p < .05 across sentences). The overallpattern that underlying Structure ambiguities facrlitatedlatencies compared with their controls more than theother ambiguity types was significant (p < .01 bytwo-tailed sign test across Ss): the fact that the lexicalstructure ambiguities slowed latencies compared withtheir controls more than the other ambiguity types wasalso significant (p < .01 by two-tailed sign test acrossso.

This relative speed of underlying structure ambiguitiesappears to be due almost entirely to a decrease in thetime Ss take to arrive at a meaning corresponding to thatof the slower of the two' unambiguous versions. Wecompared separately the mean response time for eachinterpretation of an underlying structure ambiguity withits corresponding unambiguous control. The responsetimes of ambiguous interpretations corresponding to theslower of the two controls were significantly faster thancontrols (p < .05 by two-tailed sign test acrosssentences): the response times of the ambiguousinterpretations corresponding to the faster of the twocontrols were not significantly faster than their controls.That is, the relative speed of processing the underlyingstructure ambiguities is due primarily to dropping someof the responses to the slower interpretation rather thanto increasing the speed of the responses to the fasterinterpretation.

Discussion

The finding that ambiguous and unambiguous

"lexical" sentences are responded to faster than areother types may be due to the relative simplicity of theconstructions used in the lexical sentences: the potentialpresence of structural ambiguity necessitates a relativelycomplex construction even in the nonambiguousversions [cf. the difference between (6b) and (6c), (7b)and (7c), and (8b) and (8c)].

However, our careful matching of ambiguous andunambiguous stimuli allowed us to examine the resultsfor the effects of ambiguity, despite the overalldifferences. This comparison shows that there is noincrease in story-continuation time as a consequence ofambiguity in a stimulus sentence (numerically it is, infact, a decrease). This is compatible with those previousexperiments that showed that ambiguity does notcomplicate processing once a stimulus sentence isfinished. That is, when S is encouraged to complete theinterpretation of a sentence before thinking up aresponse, there is no interference from the "unselected"meaning.

An unexpected result was the fact that sentences withunderlying structure ambiguities were responded tofaster than their unambiguous controls. This result is,however, entirely compatible with the view that Sswerecomputing both interpretations independently duringthe stimulus presentation. Since either interpretation wasacceptable in the experimental situation, "savings" inresponse times could result [given that the firstinterpretation to be consciously perceived stops allfurther sentence processing (e.g., Smith, 1968)].Suppose that we view the perception of each meaning ofan underlying structure ambiguity as an independentevent with a range of times for its completion. When thestructures are combined within an ambiguous stimulussentence, the probability that at least one of themeanings is perceived by a particular time is, in general,greater than the probability for either of the interpre­tations in isolation, viz, (9).

PI = probability that mean­ing 1 is perceived bya particular time, ti

P2 = probability that mean­ing ::! is perceived bya particular time. ti

Il':TERACTION 01=' Pl:.RCEPTUA '_PROCESSES AND SENfENCES 2g I

Pll or 2) is higher than PI or P2 alone if both arebetween I and O.

A physical analogue to the comprehension ofambiguous sentences can clarify the application of Eq. 9to the results in Experiment I. Suppose we presented ablindfolded S with a box containing one tennis ball. Thebox has a hole in the top of it large enough to allow theS to bring out the ball. The S is asked to reach into thebox and bring out a ball as quickly as he can. Clearly, ifthere are in fact two tennis balls in the box, tennis balldiscovery time would be shorter than if the box containsonly one ball, if it does not matter which of the two theS comes up with. Equation 9 expresses exactly the"savings" that would result (in this case, "perceptiontime" corresponds to "tennis ball discovery time").

The times to find each ball alone would have adistribution range. Suppose that the slowest response toone of the balls was intrinsically faster than to the other,as could happen if one were easier to grasp. This wouldhave the effect that the slow responses to the harder ballthat occur when It is alone would never occur when it istogether with the faster ball. This is because even theslowest response to the easier ball would occur beforesome of the responses to the harder one. Finally, if thetwo balls are not independent of each other (e.g., if theyare attached), this would reduce the effect of Eq.9,since finding one ball would be tantamount to findingthe other.

It would be simplistic to apply this physical modeldirectly to sentence perception. However, it doesexemplify the possible interactions between themeanings of an ambiguity. In the ambiguous version, themean time to perceive the structure corresponding tothat of the slower control sentence should be affectedmore than the time to perceive the structurecorresponding to the faster control sentence. Thisfollows from the fact that the probability of the fasterinterpretation reaches I before the probability of theslower meaning (by definition. if the responsedistributions to the two meanings have the same shape).Accordingly, the mean time of those responsesassociated with the slower structure which do occur willtend to be from the faster part of its distribution. Thiswas exactly the result found. (Notice that there may bean interaction between the probability of a meaning andits relative time of computation; but we have noindependent assessment of the probabilities for theseparate meanings in our stimulus sentences.)

Previous experiments have brought out a variety ofbehavioral differences in response to the differentstructural types of ambiguity. For example, in thesentence completion experiment (MacKay, 1966), it wasfound that ambiguity at the underlying structure levelhad the largest retarding effect on completion time forsentence fragments. Yet it is these ambiguities inExperiment I which were responded to significantly[aster than their corresponding controls. This apparentinconsistency can be resolved in terms of the perceptualmodel we outlined above.

/5,,:-::--imperative NP .vP---- /''j\ 5~

V Ad] I NP VPI I I I'

be su'e thaI yOu V NP<,- - -- I /\, .......

toke A N S- I I I"the turn "

NP VP/'N ~ )djI I I{.I }the turn ~ ~

left

S/~

Imperative NP VP--- - /'VP 5~/' 1"----·V Adj NP VP

I I I I "be sure that you V NP-- -- I r-:-:

toke A N 5-\ I I"the turn "

NP VP"I' I"

A N V AdjI I I .1 1the turn is right

correct

Fig. I. Lexical structure ambiguity.

S//~

~p VP___..

\ / "'---~A ~ V ADV ------ NP S

'I /j -<>.ore poper ;Jre~nIS icor~fully" ~. / NP /VP"

-'--' / I -,~ ~ V /\ ~ ~ cample~n!

analyses NP vp I I / '\-- I \\analYSes ere /at theprablem

N V Adj

I \ \analyses ~ limited

Fig. 2, Surface structure ambiguity.

282 BEVER. GARRETT AND HURTIG

~NP VPI .r<:5 V NP

~ "'-.. i / -,NP \PiA N

r<: I~ I IA N NP ~c'~ereQ the agent1 I "-.... ----

the indians shoot\ N,-~ I, shoot, lomeane

5

~NP VPI /"-....S V NP

»<>«: ! />.~p ~~ I f I

N V NP bothered the agentI I / <, ----

someone rshoot } A N~- I Il shoot at the Indians

Fig. 3. Deepstructure ambiguity.

Consider first the relative independence of theperceptual mapping rules used to analyze the twostructural interpretations in each kind of ambiguity.Underlying structures are shown in Figs. 1·3 for thesample ambiguities (6). (7), and (8). The twointerpretations of lexical ambiguities have identicalunderlying structures; the structures for the twointerpretations of surface structure ambiguities have aminor difference, characteristically in the placement of amodifier: and finally, the structures for the two interpre­tations of underlying structure ambiguities are mostdifferent. characteristically in the assignment of severalof the basic grammatical relations, "agent, action,object, modifier." In the perceptual model outlinedabove, direct perceptual operations map surfacesequences onto underlying relations. Thus, the types ofambiguities differ in the extent to which the perceptualmapping operations used to perceive one interpretationare identical with those used to perceive the otherinterpretation. In lexical ambiguities, the perceptualrules for the two meanings are virtually identical; insurface structure ambiguities, they may differ slightly;and in underlying structure ambiguities they differconsiderably. Initial experimental support for the viewthat the meanings of underlying structure ambiguitiesare relatively independent comes from the findings ofMacKay and Bever (1967). They reported that the timeto discover an underlying structure ambiguity is strictly

an inverse function of the relative probability of thesecond meaning that S perceives: this is just what wouldbeexpeeted if the meanings are perceptuallyindependent. The relation found for the other twoambiguity types was not of this form, suggesting that thetwo meanings in those ambiguity types are notperceptually independent (cf. MacKay & Bever, 1967,p. 195, Fig. 2).

The experimental differences between the differentkinds of ambiguities found in tasks involving sentencecomprehension may be explained by the relativeperceptual independence of the interpretations in eachambiguity type. In a task like Experiment I in whicheither interpretation is appropriate and the response taskcomes after the sentence boundary, independence of thetwo meanings in an ambiguity is an advantage: it leads tofaster responses as predicted in (9). Accordingly, theclassification of ambiguities according to the relativeindependence of their interpretations predicts thatunderlying structure ambiguities would show greatestfacilitation over unambiguous controls, with surfacestructure ambiguities showing some relative facilitationand lexical ambiguities showing relatively little: this wasthe obtained result.

This perceptual differentiation of ambiguity typesmay account for the contrary effects of underlyingstructure ambiguities when incomplete sentences are thestimuli (MacKay, 1966): in these paradigms, thepresence of an underlying structure ambiguity retardedthe response compared with 1tS unambiguous control.According to our perceptual theory, S is carrying outtwo distinct perceptual operations during presentationof a sentence fragment with an underlying structureambiguity which yield two potential underlyingstructures for the lexical sequence. If called upon tocomplete the sentence fragment before the clauseboundary, S must choose prematurely between the twoincomplete and independent interpretations of thesentence in order to find a semantic basis for acompletion. It is this active, premature choice whichincreases the time taken to complete an underlyingstructure ambiguous sentence fragment. Surfacestructure ambiguities have lesseffect on completion timebecause the perceptual operations involved in the twointerpretations are similar to each other, therebyreducing the effect of choosing one or the other. Lexicalambiguities have the least effect in the sentencefragment completion paradigm because theirin t erpretations use identical perceptual mappingoperations and no active choice of competing perceptualoperations is required.

The principle of recoding at clause boundariesoutlined above resolves another apparent contradictionin previous studies of ambiguity. While MacKay (1966)found that lexicalambiguitieshave the least effect on sen­tence completion. Foss et al (1969) and Cairns (in press)found that such ambiguities are more difficult for S torecode into a second inte~pretation after havingassigned

INTERACTION OF PERCEPTUAL PROCESSES ANDSENTENCES 283

Table 2Mean Completion Time (Seconds) as a Function of Clause

Structure of Fragment to Ss*

Incomplete CompleteType Clause] Clausej r

Ambiguous6.53 6.30Fragments

Nonambiguous6.21 6.24Fragments

Difference 0.32 0.06

*Original data taken from MacKay (1966), Appendix A,excluding "multiple ambiguities."tN = 20, lexical = 7, surface = 7, deep = 6

tt» = 22, lexical = 7, surface = 7, deep = 8

it one interpretation. That is, it is apparently easier toreinterpret a sentence with a deep structure ambiguitythan a sentence with a lexical ambiguity. Theassumption that lexical material is dismissed fromimmediate storage after a clause boundary offers onereason why this should be so: just after an ambiguoussentence is completed, the listener has dismissed thelexical material but still has available an underlyinginterpretation of the major phrases and their semanticrelations. Reorganizing these relations may be easierthan recovering the discarded lexical material in order tofind a different interpretation for a particular word.

EXPERIMENT II

We have suggested that the point in a sentence atwhich S starts to organize the response governs theeffect of stimulus ambiguity on performance: if S mustrespond at a point before perceptual closure, thenambiguity increases processing time; if the response taskoccurs after perceptual closure, then the ambiguity hasno retarding effect unless the experimental situationdemands aninterpretation different from the one S hasalready selected. In a situation like Experiment I inwhich either interpretation of a sentence is acceptable,ambiguity in a complete unit can result in reducedprocessing time if the two meanings are perceptuallydistinct.

This interpretation of Experiment I depends on theassumption that Ss followed our instructions and did notstart preparing their responses until after reading (andcomprehending) each stimulus sentence. The responsetimes were longer than those of Mistler-Lachman (1972),suggesting that our instructions were successful.However, a stronger test of the hypothesis thatambiguity increases complexity before perceptual clauseclosure and may decrease it thereafter would be to varythe stimuli themselves rather than the instructions: thatis, a more critical test would be a direct comparison ofthe effect of ambiguity in response to speech stimulithat are themselves complete or incomplete perceptualunits. MacKay (1966) presents part of his completion

dat a by individual sentence fragment, allowingcomparison of responses to stimuli that were completeperceptual units with those that were not. We categoriedhis stimulus sentence fragments into two types, thosewhich could be complete clauses at the underlyingstructure level [e.g., (10) and (11)] and those that wereincomplete [e.g., (12) and (13)]. Table 2 presents thecompletion times for the different types of ambiguousfragments and their unambiguous controls [fromMacKay (1966), appendix].

(10) Although knew the new position hadadvantages ...

(11) Brahms, knowing how great symphonies sound,played ...

(12) After her injury that summer she couldn't bear ...(13) Even if I did laugh at the church, I ...

The fragments which were complete clauses inthemselves show no significant effect of the ambiguity:fragments that could not be interpreted as completeclauses do show such an effect. Thus, MacKay's data alsooffer confirmation of our general view of the relationbetween clausecompletion and sentence ambiguity.

With a modification of the sentence completionparadigm, we examined further the hypothesis thatambiguity interacts with perceptual closure. In thisexperiment, Ss responded to incomplete sentencefragments which were varied so as to be either completeclauses or incomplete clauses. The results show thatambiguity of a sentence fragment increases complexityof processing primarily when the fragment is itself anincomplete clause.

Method

Materials

Six incomplete sentence fragments 5 to 12 words in length ofeach type of ambiguity were adapted from MacKay (1966) [e.g.,'(l14a-c), (l Sa-c), -and (16a-c)]. Each fragment had threeambiguous versions and three nonambiguous versions: in oneambiguous version the fragment was an incomplete clause (a), ina second version it was a complete clause (b), and in the thirdversion it was a complete clause with the addition of the firstword of a second clause (c). There were corresponding versionsfor the unambiguous control sentences (d, e, f). Each sentencewas typed individually without punctuation on a 3 x 5 in. card.On this study we only used one of the possiblenonambiguous interpretations as the control for each ambiguoussentence and three of each structural type for the surfacestructure and deep structure ambiguities. The decision as towhich meaning of each lexical ambiguity to use as a control wasarbitrary.)

14 (a) After taking the right turn at the

284 BEVER. GARRETT AND HURTIG

Procedure

Subjects

A stack of cards was presented to S face down; S wasinstructed to turn over a card. read the sentence fragmentsilently. and then, as soon as possible, to read the fragment and a

The Ss were 24 M.I.T. summer school students and 24Columbia, Hunter, and New York University students, nativespeakers of English who volunteered for paid participation.

Results

completion aloud. S was instructed to make the completion aswell formed and relevant to the sentence fragment as possible.After the experimental session. [ and S went over S's responsesto the fragments. S indicated on a S-point scale an evaluation ofthe appropriateness of the original response to the sentencefragment. S was also asked, for each ambiguous fragment,whether he or she noticed its ambiguity at the time when he orshe first saw it. The few responses for which S reported havingnoticed the ambiguity were excluded from further consideration.The entire experimental session was recorded. Turning over thestimulus card triggered a tone (inaudible to S) which wasrecorded on the response tape. The tape was later played backusing a voice switch to determine the interval from stimuluspresentation to onset of response.

Analyses were performed on scores for the intervalbetween stimulus presentation and onset ofverbalization. The mean time to think up a completionwas about 5 sec. The mean response times for eachcondition are given in Table 3. For each of theambiguity types (lexical, surface, and underlying), threeorthogonal contrasts were computed within a FragmentType by Ambiguity analysis of variance (Hayes, 1963).The three contrasts compared response times forambiguous vs unambiguous versions in each of the threefragment types.! Table 3 presents the results of thecontrasts for lexical, surface, and underlying ambiguities.None of the contrasts is significant for the lexical orsurface structure ambiguities. For the underlyingstructure ambiguities, however, ambiguous fragments inthe incomplete clause condition have significantly longerthinking times than do their control fragments (p <.025). For ambiguous fragments in the complete clausecondition, the results are reversed, but not significantly,such that ambiguous fragments have shorter times thando their controls (p < .10). Note (see means in Table 3)that the same pattern of reversal of ambiguity effectfrom incomplete to complete fragment types may alsobe present for surface structure ambiguities, althoughthe differences do not reach significance. (Reanalysisusing speed scores yielded no change in the significanceof the contrasts reported.)

These results might be affected by the fact that theyinclude sentence completion responses that were notappropriate to the stimulus sentence-that is, onoccasion, S's response was clearly inappropriate to eithermeaning of the stimulus sentence fragment. This wasoperationally checked by having Ss rate the relativeappropriateness of their own responses on a 5-pointscale. There was no difference in this subjective ratingacross the ambiguity types. When we consider only thetwo best-rated categories of responses, the pattern ofresults is the same, as shown in Table 4.

When Ss are performing most appropriately to thetask in their own subsequent opinion, they show theeffect of clause segmentation and ambiguity. It remainsthe case that the effects were due primarily tounderlying structure ambiguities. (Because of thedifference in cells, orthogonal contrasts were not

StartNext

ClauseEnd ofClause

Fragment Type

InClause

*df= 1,47

LexicalAmbiguous 5.44 4.03 4.30Unambiguous 5.20 4.20 4.61

F<l F<l F < 1p> .05 p> .05 p> .05

Surface StructureAmbiguous 5.34 4.29 4.32Unambiguous 4.25 4.70 5.07

F =3.17 F < 1 F =1.52p < .10 p> .05 p> .05

Underlying StructureAmbiguous 6.07 4.93 5.22Unambiguous 5.03 5.65 4.76

F =5.88 F =3.40 F<lP < .025 P < .10 p> .05

Table 3Mean Response Times (Seconds) for Ambiguous and Unam­biguous Versions of Each Fragment Type in Lexical, Surface,

and Underlying Structure Ambiguities(All Data from Experiment D)*

Six experimental groups were arranged from these sentencessuch that each group contained one sentence of each type fromthe 18 different types of sentences (ambiguous vs unambiguouscontrol by three types of ambiguity by three fragment lengths =18 sentence types). A group of 11 similar sentences was used forwarm-up and one at the end to reduce end-of-session effects.

(b) After taking the right turn at the intersection(c) After taking the right turn at the intersection I(d) After taking the left turn at the(e) After taking the left turn at the intersection(0 After taking the left turn at the intersection I

15 (a) After the paper presented carefully limited(b) After the paper presented carefully limited analyses(c) After the paper presented carefully limited analyses they(d) After the paper presented very limited(e) After the paper presented very limited analyses(0 After the paper presented very limited analyses they

16 (a) After the agent claimed the shooting of the Indians was(b) After the agent claimed the shooting of the Indians was

dreadful~~. • sc the agent claimed the shooting of the Indians was

..~ Iful he(d) After the agent claimed the shooting at the Indians was(e) After the agent claimed the shooting at the Indians was

dreadful(0 After the agent claimed the shooting at the Indians was

dreadful he

INTERACTION OF PERCEPTUAL PROCESSES AND SENTENCES 285

advisable; a by-S Wilcoxon matched-pairs signed-rankstest showed that the in-clause break result wassignificant at p < .025 (one-tailed) for underlyingstructure ambiguities, while the other differences werenot significant.

Discussion

These results indicate that for the sentencecompletion task, underlying structure ambiguity doesincrease processing difficulty if the stimulus fragment isitself an incomplete clause. This finding was predictedby the assumption that the clause is. the main perceptualuni t of speech, that listeners assign a singleinterpretation to each such unit, and that the meaningsof underlying structure ambiguities are more distinctfrom each other perceptually than are the meanings inother ambiguity types. When an ambiguous fragment isbroken off before the completion of either underlyingstructure, then the two partial interpretations are bothpresent and involve added processing complexity if atask is interposed at that point. If the fragment is brokenoff after completion of the perceptual unit, theperceptual unit is recoded with one interpretation andthe fact that the unit was originally ambiguous does notaffect the complexity revealed in later processing. Whenboth interpretations are possible responses to the task,the processing time should be relatively fast inunderlying structure ambiguities, since the two meaningsutilize relatively independent perceptual rules. Thispattern of resultswas found in Experiments I and II.

A number of issues remain concerning the generalperceptual model. First, our stimuli did not criticallytest whether the perceptual unit relevant for our effectsis the surface structure clause or underlying structuresentoid: in our materials, the complete units weresimultaneously surface and underlying structure clauses[cf. (14b), (1Sb), and (16b)]. To test the role ofunderlying structure organization as the unit relevant toambiguity, one would have to study the effects offragments that are complete and incomplete underlyingstructure units without any surface structure clausemarkers. For example, the ambiguity in neither (17a)nor (17b) is in a complete surface structure clause, but(17a) completes an underlying structure unit [see Beveret al (1969) for a description of the relevance of suchunits in perceptual segmentation, using unambiguoussentence pairs like (17a) and (17b)]. If the underlyingstructure unit alone is sufficient for closure, theambiguity in (17b) but not in (17a) should involverelative processing complexity.

17 (a) The English teacher's wife desired .(b) The English teacher's wife defied .

Previous research has tended to refer to the structuraldefinition of the ambiguity type as the main variableper se. However, our interpretation of the previously and

Table 4Mean Response Times (Seconds) for Ambiguous and Unam­biguous Versions of Each Fragment Type in Lexical, Surface,and Underlying Structure Ambiguity (Data from Best Two

SUbjective Rating Categories from Experiment II)

Fragment Type

StartIn End of Next

Clause Clause Clause

LexicalAmbiguous 4.73 3.71 4.17Unambiguous 5.02 3.80 4.38

Surface StructureAmbiguous 4.37 4.51 3.72Unambiguous 4.38 3.71 4.52

Underlying StructureAmbiguous 5.79 4.50 4.53Unambiguous 4.42 4.75 4.59

presently observed results classifies ambiguities in termsof the independence of the perceptual mapping rulesused in the comprehension of the two interpretations.The hypothesis that it is the perceptual independence ofinterpretations that governs behavioral differences inresponse to ambiguities could be tested by usingambiguities of the same structural "type" that vary therelative differences of the perceptual rules used ininterpreting each of the meanings. For example, (18a)and (ISb) are both classified as "surface structureambiguities," but the two underlying structures for(18b) differ more than those in (l8a).

18 (a) John looked up the street(b) They fed her dog biscuits

Similarly, (19a) and (19b) are both "underlyingst ructure ambiguities," but the two underlyingstructures for (19b) differ more than those of (I9a).

19 (a) The natives dislike sailing in the harbor(b) The duck is ready to eat

The relative complexity of underlying structures awaitsdevelopment of a serious formal metric. Finally, theparadigms we have used are not ideal tests of perceptualsystems. The outcome is confounded with otherresponse systems; the long response times attest to this.

Within these limits, the experimental facts reported inthis paper support our theoretical resolution of theapparent contradictions among previous findings onsentential ambiguity. Both the previous and presentlyreported results support the general theory of sentenceperception outlined in the introduction whichemphasizes the processes of clause segmentation,recoding clause by clause, and the use of direct mappingrules to assign internal relations to surface sequences.

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286 BEVER. GARRETT AND HURTIG

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NOTES

1. In this discussion we are not concerned with the distinctionbetween underlying structure sentoids and surface structureclauses. The data are unclear as to the effect of underlyingstructure segmentation alone on click location (cf. Bever, Kirk,& Lackner, 1969; Chapin et al, 1972).

2. Note that there is no uniform prediction for either effect ofambiguity or fragment type; the test of our hypothesis resides inthe interaction and hence, the orthogonal contrasts. TheAmbiguity by Fragment Type interactions had F ratios of .29(df = 2,94) for lexical, 2.78 (df = 2,94) for surface, and 4.54 (df= 2,94) for underlying ambiguities. None of the F ratios forfragment or ambiguity effects reached significance except forthat of fragment type in the lexical ambiguities (F = 4.23, df =2,47). That effect was attributable to slower response times forall the incomplete lexical fragments.

(Received for publication October 28, 1972;revision received February 6,1973.)