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Some effects of ambiguity upon sentence comprehension
JOURNAL OF VERBAL LEARNING AND VERBAL BEHAVIOR 9, 6 9 % 7 0 6 (1970)
Some Effects of Ambiguity upon Sentence Comprehension z DONALD J. Foss University of Texas at Austin, Austin, Texas 78712
Three groups of 20 Ss were auditorily presented with 24 ambiguous and 24 unambiguous sentences. One group was asked to push a button whenever a particular phoneme occurred. Reaction time (RT) for this response was significantly longer in ambiguous sentences. A second group had to classifyeach sentenceas ambiguous or not after it was presented. Lexical ambiguities were discovered somewhat faster than underlying structure ambiguities. A third group performed both of these tasks. In this group, RT to monitor for the phoneme was longer for ambiguous than for unambiguous sentences only when S himself classified the sentence as ambiguous. The Ss in the third group were significantly slower on both tasks.
It is all but axiomatic that a competence model of linguistic knowledge such as that presented by Chomsky (1965) is not a performance model of language comprehension or production. The competence model does provide a rough specification of the theoretical entities that a comprehension model must recover from the input. It does not, however, necessarily describe the information processing procedures utilized in obtaining these entities. There are many demands upon an adequate comprehension model. It must, for example, be consistent with man's general information processing limitations. One particularly interesting interface between theories of linguistic competence and psycholinguistic performance models is provided by the existence of ambiguous sentences. A sentence is ambiguous when there is more than one way of representing its string of words in the competence model. It is clear, however, that many everyday sentences which in (linguistic) theory are ambiguous are not taken to be ambiguous by those who hear
them. This raises the question as to whether, in normal sentence processing, the comprehension mechanism recovers more than one grammatical interpretation of an ambiguous input sentence. This question is an important one. For if more than one interpretation of an ambiguous sentence usually results from the comprehension process, then we shall have to deal with questions such as why only one interpretation usually becomes conscious and how that interpretation was chosen. If, on the other hand, a single interpretation typically results from the comprehension process, then again we shall have to ask how that one was singled out. In either case, then, the question of how sentence context affects comprehension will eventually have to be faced. For the present, just knowing whether the comprehension process typically enters a state of multiple representations of ambiguous strings would seem to be a step forward in constraining models of comprehension. The studies which have addressed themselves to this issue have obtained seemingly 1 This study was supported by NSF-USDP grant contradictory results. In one study, Foss, GU-1598 to the University of Texas at Austin. The Bever, and Silver (1968) presented evidence author would like to thank Richard H. Lynch, Jr. and consistent with the hypothesis that Ss typically Charles Stewart for aid in gathering the data and David T. Hakes and Carlton T. James for valuable criticisms assign only one immediate interpretation to an ambiguous sentence. If that interpretation of the paper. 699
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was later found to be incorrect, Ss made a reinterpretation, a process which took time. Foss et al. showed Ss a picture immediately after each of a set of sentences was aurally presented and asked them to quickly verify whether the picture depicted the meaning of the sentence. It was found that the time to make such a judgment was no slower for ambiguous than for unambiguous sentences when the picture represented the expected meaning of the ambiguous sentence. (The "expected" interpretation was determined by a pretest with separate group of Ss.) When the picture represented the unexpected meaning, the average verification time increased significantly. The above result contrasts with results found by MacKay (1966). In the latter study, Ss were presented with incomplete sentence fragments (e.g., After taking the right turn at the intersection, I . . . ) and were asked to complete the sentences. When the fragment contained an ambiguity, the completion time was longer than when the fragment was unambiguous. This result suggests that Ss do treat ambiguous sentences in some special fashion. MacKay hypothesized that his results supported the "oblivion hypothesis . . . that neither meaning of an ambiguous word or set of words may be seen until the ambiguity is resolved on the basis of the non-ambiguous context of the sentence [p. 427]." (Italics in original.) These apparently contradictory results may simply be due to the fact that Foss, Bever, and Silver (1968) looked for effects of ambiguity after the sentences, while MacKay (1966) looked for such effects within sentence fragments. The present experiment was an attempt to clarify the processing strategies Ss use in dealing with ambiguity. I t utilized a technique that measured momentary processing difficulty within sentences. If ambiguity leads to additional processing on the part of S, then he should have less of his processing mechanisms available to deploy on an extraneous task. Hence, reaction time
(RT) to perform the extraneous task should be longer right after an ambiguity than after an unambiguous control. In the present study, S was asked to listen in each sentence for a word beginning with a particular letter or phoneme and to press a button when he heard that sound. This phoneme monitoring was the extraneous task. The RT in performing this task was taken as the index of sentence processing difficulty at the point where the target phoneme occurred. That is, it was assumed that RT to push a button in response to a particular phoneme would be positively correlated with sentence processing difficulty. Previous research has shown that phoneme monitoring RT does go up when the syntax of the sentence is difficult to process (Foss & Lynch, 1969; Hakes & Foss, in press), and when lexical items are difficult to process (Foss, 1969). The present experiment also measured the amount of time S took to decide whether each sentence was ambiguous or not. These times were collected for each of two types of ambiguity. MacKay and Bever (1967) found that Ss discovered certain types of ambiguities faster than other types and this finding was then used by MacKay (1966) to motivate the oblivion hypothesis discussed above. Although MacKay and Bever (1967) discussed three types of ambiguity, only two are relevant here: lexical ambiguities and underlying structure ambiguities. A lexical ambiguity occurs when the two meanings of a single sentence have the same syntactic structure, but the meaning of an ambiguous word, such as tank, is not resolved by the context, for example, The tank was too hot to touch. An underlying structure ambiguity occurs when the basic grammatical relations between the words of the sentence are different for its two meanings, for example, The elephant is ready to lift, can mean either that the elephant is ready to lift something or that the elephant is ready to be lifted by something. MacKay and Bever (1967) found that lexical ambiguities were discovered faster than underlying struc-
SENTENCE COMPREHENSION ture ambiguities. The present study provided a check on this finding us!ng aural rather than visual presentation of the sentences.
DESIGN AND METHOD Three groups of Ss were utilized in this study. The first (Mon Group) was told to monitor each sentence for a word starting with a particular phoneme, /b/, and to push a button when one occurred. These Ss were not informed that some of the sentences would be ambiguous; the instructions to them made no mention of ambiguity. (They were asked to repeat each sentence after they heard it to insure that they attended to the sentence.) The second (Amb Group) was asked to decide after each sentence whether or not it was ambiguous. This decision was timed. The third (Mon + A m b Group) was given both the phoneme monitoring and the ambiguity classification tasks. If ambiguity has an effect on spontaneous sentence processing, then longer phoneme monitoring times should be observed during ambiguous than during unambiguous sentences. This prediction is clearer for the M o n Group than for the Mort + Arab Group since S's in the former were naive about the existence of ambiguities in the experiment. If lexical ambiguities are more easily discovered, this should show up in both the Arab and M o n + Amb Groups. Further, under the assumption that overlapping processing mechanisms are used for these tasks, the M o n + Amb Group ought to be slower on each of its two tasks than the other two groups are on their respective single tasks. Subjects The Ss were 60 undergraduates (20 per group) from Introductory Psychology classes at the University of Texas at Austin. They participated in the 1-hr. experiment as part of a course requirement. Materials Forty-eight ambiguous sentences were constructed. These embodied a large number of syntactic constructions and ranged over a wide variety of topics. Twentyfour of these sentences were lexically ambiguous and 24 were ambiguous in underlying structure. One-half of each of these two sets of sentences was disambiguated for one subgroup of 10 Ss; the other half was disambiguated for the second subgroup. The unambiguous sentences were constructed by making a small number of substitutions into the ambiguous sentences. For example, The new men started to drill before they were ordered to do so (a lexical ambiguity) became The new men started to march before they were ordered to do so; and When ! purchased the hat for my friend, the businessman seemed very grateful (an underlying structure ambiguity) became When I purchased the hat with
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my friend, the businessman seemed very grateful. The sentences were assigned to each counterbalanced subgroup described above on a random basis. One-half of the sentences in each of the resulting subgroups contained the target phoneme /b/. The phoneme followed the point of ambiguity by a word or two (as in the examples above); the phoneme occurred in a corresponding position for the unambiguous sentences (again, as above). To summarize, for each S there were 24 ambiguous and 24 unambiguous sentences. Twelve of the ambiguities were lexical, 12 were in the underlying structure. There were also 12 sentences that could, for the sake of symmetry, be considered lexically unambiguous and 12 that could be considered unambiguous in underlying structure-even though the sentences in both these sets were, of course, completely unambiguous. Target phonemes occurred in half the sentences in the resulting four groups. There were two sets of materials counterbalanced for ambiguity. Thus, there were, overall, two between-S variables (three tasks, two sets of materials) and one within-S variable (two types of ambiguity). Procedure
The Ss in the Mon and the Mort + Arab Groups were told to push a button whenever they heard a word starting with [b/. The Ss in the Amb and M o n + A m b Groups were told to push a (different) button after each sentence when they had decided whether or not the sentence was ambiguous. Examples of ambiguity were given to insure that Ss had the proper concept of ambiguity. As soon as S pushed this button he was told to say either "Yes," indicating that he thought the sentence was ambiguous, or " N o , " indicating that he thought the sentence was not ambiguous. If he said "Yes" then he had to immediately state the two versions of the ambiguity. The S was seated behind a screen so that he could not see E during the experiment. The instructions and sentences were presented on tape over earphones. One timer was automatically started when a /hi occurred (for the relevant two groups) and a second timer was automatically started at the end of the sentence. The E recorded S's latencies and S's statements about the source of the ambiguity (for Ss in the relevant groups). There were approximately 15 sec. between sentences for these statements to be given and recorded.
RESULTS In order to reduce the variability in the latency data, the reaction times were converted into speed scores (1/RT). All analyses w e r e p e r f o r m e d o n t h e l a t t e r scores.
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Phoneme Monitoring Task The Ss in the M o n + Amb Group (who had two concurrent tasks) had some tendency to give slower responses than Ss in the Mon Group. Overall, the mean speed scores were 1.26 for the M o n + A m b Group and 1.48 for the Mon Group. This difference approached significance, F(1, 38)=3.50, p < . 0 7 . This effect did not significantly interact with a sentences' status as ambiguous or unambiguous. The speed scores for both ambiguous and unambiguous sentences considered alone were slower in the Mon + Amb Group than in the Mon Group. Confidence that the marginal difference between the Mon and Mon + Amb Groups was a real one is bolstered by the fact that Ss in the Mon Group pushed the button in response to the presence of the target phoneme in significantly more of the sentences (X = 10.50 per group of 12 sentences) than did Ss in the Mon + A m b Group (X=9.33), F(I, 38)= 5.04, p < .03. That is, Ss in the group with two tasks did not respond as often in the phoneme monitoring task as Ss in the group with only the monitoring task. Whether or not a sentence was ambiguous did have a significant effect upon speed scores for the Mon Group. The mean speed scores were 1.42 for the ambiguous sentences and 1.53 for the unambiguous sentences, F(1, 19) = 8.08, p < .02. Thus, phoneme monitoring was faster in unambiguous than in ambiguous sentences. The type of ambiguity (lexical, X = 1.46, vs. underlying, X = 1.50) had no significant effect on phoneme monitoring speeds, F(1, 19)= 1.14, p > .25. In addition, Ss pushed the response button significantly less often in ambiguous ( X = 5.12) than in unambiguous (X = 5.38) sentences, F(1, 19) = 4.13, p = .05. That is, the target phoneme was missed more often in ambiguous sentences. In contrast, whether or not a sentence was ambiguous had no significant effect upon speed scores in the M o n + Amb Group. The mean speed scores were 1.25 for the ambi-
guous sentences and 1.27 for the unambiguous sentences, F(1, 19)=0.11, p > . 2 5 . Again, type of ambiguity had no effect on phoneme monitoring speeds. The data on the number of sentences to which Ss responded paralleled those of the Mon Group. The Ss pushed the response button less often in ambiguous (X = 4.48) than in unambiguous (X = 4.85) sentences, F(1, 19) = 4.30, p < .05. This result contrasts sharply with the RT data since the latter measure was expected to be the more sensitive indicator of processing difficulty. It may be illuminated by the data reported below. When the data from the Mon + Amb Group were broken down in another way, significant effects of ambiguity on monitoring speeds were observed, however. The Ss in the Mon ÷ Arab Group had to state whether or not each sentence was ambiguous and their categorizations were often at variance with E's. It is possible, then, to look at the phoneme monitoring speed scores for the sentences which S thought were ambiguous or unambiguous. That is, a sentence's status as ambiguous or not can be left to S's judgment as well as to E's judgment (and theory). The sentences were therefore classified as ambiguous or unambiguous on the basis of S's judgments and on the basis of E's judgments, that is, a 2 x 2 classification was made: ambiguous or unambiguous according to S, ambiguous or unambiguous according to E. Again, the main effect of ambiguous versus unambiguous sentences, according to E's classification, had no effect on monitoring speeds. However, the average speed was 1.20 in the sentences Ss classified as ambiguous, and 1.32 'in the sentences Ss classified as unambiguous. This difference approached significance, F(1, 19) ~ 3.82, p < .07. (The number of sentences and the particular sentences classified as ambiguous naturally varied across Ss.) There was no interaction effect on monitoring speeds in this 2 x 2 classification (p = .60). When Ss self-categorized their responses in
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the above manner, the number of sentences in one of the four cells of the 2 × 2 matrix was for some Ss as small as one. In order to increase the reliability of the estimate of phoneme monitoring speeds, the six Ss who had at least one cell with just one observation were omitted. The analysis on the remaining 14 Ss showed the same pattern of results although the monitoring speeds were faster overall. In this analysis the effect of ambiguous (X = 1.31) versus unambiguous (X = 1.35) sentences, as classified by E, was still insignificant, F(1, 13) = .38, p > .25. On the other hand, the difference between monitoring speeds for ambiguous ( X = 1.27) and unambiguous ( X = 1.39) sentences, according to S's classification, did reach an acceptable level of significance, F(1, 13) = 6.70, p < .03.
Ambiguity Identification Task The times for Ss to decide whether or not a sentence was ambiguous were, of course, much longer than the phoneme monitoring times. Hence the speed scores are much smaller. The Ss in the M o n + Amb Group had significantly slower speeds than the Ss in the Amb Group. The average overall speeds were .10 for the M o n ÷ Arab Group and .16 for the Amb Group, F(1, 36) = 10.43, p < .01. Thus, the Ss with two tasks took longer to identify the ambiguities than the Ss with that single task. The two sets of materials did not lead to different identification times, F(1, 36) = 0.38, p > .25. The Group x Materials interaction effect approached significance, F(1, 36)= 3.96, p < .06, indicating that the difference between the Amb and M o n + Amb Groups was more pronounced for one set of materials. The effect was in the same direction for both sets of materials, however. The materials variable did not interact with any other variable. In contrast to the results observed in the phoneme monitoring task, the type of ambiguity had a marginally significant overall effect on ambiguity identification speeds. The mean 26
speed score for lexical ambiguities (X = .14) was faster than the mean speed score for underlying ambiguities ( X = .12), F(1, 3 6 ) = 3.78, p < .06. This variable did not interact significantly with any other variable. Decisions concerning the sentences that were in fact ambiguous (i.e., E considered them to be ambiguous) were made significantly faster than decisions concerning unambiguous sentences. The means were. 16 a n d . 10, respectively, F(I, 36) = 27.81, p < .01. As in the phoneme monitoring task, the ambiguity identification data (for both groups) were cross-classified both according to whether E said the sentence was ambiguous or not and whether S said the sentence was ambiguous or not. The mean speed for Ss to say that a sentence was ambiguous was. 17; this figure dropped to .08 for those sentences S said were unambiguous, F(1, 3 6 ) = 59.44, p < .01. That is, S decided to say that a sentence was ambiguous much faster than he decided to say that a sentence was unambiguous. These variables--ambiguous versus unambiguous according to E, ambiguous versus unambiguous according to S--interacted significantly, F(1, 36)= 23.96, p < .01. The mean speed scores are shown in Table 1. TABLE
1
MEAN SPEED SCORES: AMBIGUITY IDENTIFICATION TASK
Ss' Categorizations E's Categorizations Ambiguous Unambiguous
Ambiguous
Unambiguous
.23 .11
.09 .08
It can be seen from Table 1 that the fastest decisions occurred for those sentences which were classified as ambiguous by both S and E. When S said the sentence was ambiguous, but E had classified it as unambiguous, the decision speed dropped appreciably. It also appears that the slowest decision times were
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for those sentences which S and E agreed were unambiguous. The pattern of the data in Table 1 is the same for both the Amb and the M o n + A m b Groups. The Table 1 variables did significantly interact with groups, however, F(1, 36)= 7.23, p < .02. This interaction reflects the fact that the decisions concerning the sentences represented by the upper left cell of Table 1, that is, where S and E agreed that the sentences were ambiguous, were made much more quickly (X = .31) in the Amb Group than in the Mon + A m b Group ( X = . 1 5 ) . The remaining cells of Tables 1 are quite representative of the data for both the Amb and the Mon + Amb Groups, except that the figures for the former group average about .02 faster. This says that the "real" ambiguous sentences were found quickly; they were found even faster by the Ss who only had the ambiguity search task. No other interactions proved to be significant. DISCUSSION The fact that the Mon + Amb Group was slower on the phoneme monitoring task than the Mon Group, and slower on the ambiguity identification task than the Amb Group, is evidence that overlapping processing mechanisms are used in the two tasks. This, in turn, supports the reasoning used to motivate phoneme monitoring as a technique for measuring momentary sentence processing difficulty. Since lexical ambiguities were identified somewhat faster than underlying structure ambiguities, the results of MacKay and Bever (1967) receive additional support, z The finding that Ss decided in the affirmative about ambiguity faster than they decided in the negative is not surprising. Presumably, the search for ambiguity terminates as soon as one is found. When one is not found, S can persist in his search for as long as his patience holds up (or the 15-sec. intersentence interval lasts). 2 T. G. Bever (personal communication) has also independently verified this aspect of MacKay and Bever (1967) using auditorily presented materials.
The most salient finding of this experiment was that phoneme monitoring was faster in unambiguous sentences than in ambiguous sentences in the Mon Group. This indicates that Ss enter into a special processing state immediately subsequent to hearing a (potential) ambiguity. Questions as to the nature of this state, for example, whether S has calculated both interpretations, or neither, will be returned to below. The present result seems to run counter to that reported in Foss, Bever, and Silver (1968). In the latter study it was found that Ss could verify expected interpretations of ambiguous sentences as fast as they could verify unambiguous sentences. The conflict between the two studies may be due to the difference in point of measurement. The present experiment measured ambiguity effects soon after the point of ambiguity while Foss et al. took their measures at the end of each sentence. This difference is itself of some interest as will be noted below. Alternatively, the difference between the two studies may be due to the differing task demands made upon the Ss. That is, the kind or depth of comprehension required of a sentence is, perhaps, a function of what Ss are asked to do. Whereas interpreting the phoneme monitoring results of the Mon Group poses many problems, at least the results themselves seem quite straightforward. The same is not the case with the Mon + Amb Group. In order to account for the fact that Ss' own categorizations of the sentences as to ambiguous or unambiguous predicted the speed scores, while E's categorizations did not, one might hypothesize that there are individual differences in the grammars (competences) of the Ss and that the speed data simply reflect these differences. That is, Ss were slower for the sentences that their own grammars would classify as ambiguous. This hypothesis is doubtful at best, however. For if such individual differences among grammars existed for the M o n + Amb Group, there is every reason to suppose that similar differences would exist among Ss in
SENTENCE COMPREHENSION
the Mon Group. And if such differences in grammars did exist in the latter group, then the phoneme monitoring speed data should have been equal for the (E-defined) ambiguous and unambiguous sentences, just as was the case for the E-defined ambiguous and unambiguous sentences in the Mort + Amb Group. Since the E-defined ambiguous and unambiguous sentences were different in the Mon Group, the "different grammars" hypothesis cannot be correct. In addition, this hypothesis is not consistent with the interaction observed in Table 1. Since the Ss in the Mon + Arab Group were almost certainly not engaged in normal sentence processing procedures, it is tempting to say that the monitoring results for that group are not worth considering in detail. But since the results were systematic, some further explanation seems called for. One seemingly plausible notion is this: Since Ss in the M o n + A m b Group were searching for ambiguity (and thus not engaged in normal sentence processing) they may have often been caught by a potentially ambiguous word or phrase that was disambiguated by earlier or later material in the sentence. When they noticed the "ambiguity" they had, simply as a result of noticing it, less of their processing mechanisms available for phoneme monitoring. If the phoneme then occurred, slower monitoring speeds would result. Later, S would attempt to justify the ambiguity he had considered. Of course, this justification would be easier if other material had not disambiguated the sentence. Thus, ambiguity justification (identification) would be faster where S and E agreed that the sentence was ambiguous. That is, identification speeds would be fastest where S said the sentence was ambiguous and, in fact, it was. The ambiguity identification data strongly support this interpretation. Identification was much faster when S and E agreed that a sentence was ambiguous than when only S said it was ambiguous (see Table 1). This line of reasoning assumes that search26*
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ing for an ambiguity without finding one takes less processing than is required after a putative ambiguity is discovered in the input string, an assumption that seems reasonable. It also points out the concrete reason--the search process--why overall speeds were slower in the Mon + Amb Group than in the Mon Group. A tighter reconciliation of the present results with those of Foss, Bever, and Silver (1968) would, of course, be desirable. In addition, reconciling the present results with our everyday experience of ambiguity is needed. It is relevant to note that in the normal course of conversations we rarely notice ambiguous sentences and we rarely make incorrect interpretations of the ambiguities we hear. The latter fact is obviously due, in some way, to the effects of prior context on comprehension. Context determines interpretations, "sets the switches," so that correct interpretations of ambiguities almost always result. It is possible to ask whether prior context has its effect upon input automatically, in much the way that throwing a switch determines the flow of current without the expenditure of any energy at the "choice point," or whether potentially ambiguous messages must be checked against the switch settings. That is, we can ask whether a portion of the analyzing mechanisms must be devoted to discovering the correct interpretation at the point of ambiguity in the input. It is clear that the data from the present experiment favor the latter interpretation over the former. Thus, we infer that the processor must check for the effects of prior information at the point of ambiguity and this activity itself utilizes some of S's limited analyzing mechanisms. Presumably some checking for contextual cues for the resolution of an ambiguity will always occur. If the context is neutral, then the interpretation of the ambiguity may be determined by some rule of frequency of interpretation or simplicity of resulting structure. The above suggestion is testable. If Ss must check for contextual determination, whether
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or not context has biased the interpretation of the ambiguity, then the same pattern of results as obtained in the present study should occur whether or not prior context has determined the interpretation of the ambiguity. It would probably be the case that the differences between ambiguous and potentially ambiguous segments (the latter disambiguated by prior context) would be much attenuated as compared to the present results, but they should still be present. One more point. If prior context is neutral, the ambiguity may still be resolved by later context. But the processes involved probably differ depending upon how much later the resolving context occurs. On the rare occasions when we give an incorrect interpretation to an ambiguous sentence the disambiguating material usually follows the ambiguous segment by some distance as in (1). (1) The shooting of the prince dumbfounded his subjects and his friends since they all thought he was an excellent marksman. An interpretation is probably assigned to
the shooting of the prince in (1) on the basis of some rule such as, "Surface structure order will have the deep structure object following the verb." The S does not wait indefinitely for disambiguating context. It may be that delaying the assignment of an interpretation means that S has to store a great deal more material than after an interpretation has been made. He can store unanalyzed material for a short while, but as more and more material enters, his memory and processing mechanisms get overtaxed and he must make a decision or lose the material altogether and hence not comprehend the sentence. Note that in sentences
like (2) the listener may not get fooled as often as he does in (1). (2) The shooting of the prince who was an excellent marksman dumbfounded his subjects and his friends. This anecdotal evidence indicates that decisions may be briefly delayed while either (a) S checks to see whether prior context can aid in resolving the ambiguity, or (b) S waits for soon-to-come information that will resolve the ambiguity. Normally, of course, there is more than enough prior context so that (a) describes what happens best. In any event, we have evidence here that Ss must deal with ambiguous stretches of input in some way that taxes his processing mechanisms more than an unambiguous input does. This fact alone is a constraint on models of comprehension. REFERENCES CHOMSKY, N. Aspects of the theory of sytnax. Cambridge: M.I.T. Press, 1965. Foss, D. J. Decision processes during sentence comprehension: Effects of lexical item difficulty and position upon decision times. Journal of Verbal Learning and Verbal Behavior, 1969, 8, 457-462. Foss, D. J., BEVER,T. G., & SILVER, M. The comprehension and verification of ambiguous sentences. Perception andPsychophysics, 1968, 4, 304-306. Foss, D. J., & LYNCH, R. H. JR. Decision processes during sentence comprehension: Effects of surface structure on decision times. Perception and Psychophysics, 1969, 5, 145-148. HAKES, D. T., & FOSS, D. J. Decision processes during sentence comprehension: Effects of surface structure reconsidered. Perception and Psychophysics, (in press). MACKAY, D. G. To end ambiguous sentences. Perception andPsychophysics, 1966, 1, 426-436. MACKAY, D. G., & BEVER, T. G. In search of ambiguity. Perception and Psychophysics, 1967, 2, 193-200. (Received June 29, 1970)
Some Effects of Ambiguity upon Sentence Comprehension z DONALD J. Foss University of Texas at Austin, Austin, Texas 78712
Three groups of 20 Ss were auditorily presented with 24 ambiguous and 24 unambiguous sentences. One group was asked to push a button whenever a particular phoneme occurred. Reaction time (RT) for this response was significantly longer in ambiguous sentences. A second group had to classifyeach sentenceas ambiguous or not after it was presented. Lexical ambiguities were discovered somewhat faster than underlying structure ambiguities. A third group performed both of these tasks. In this group, RT to monitor for the phoneme was longer for ambiguous than for unambiguous sentences only when S himself classified the sentence as ambiguous. The Ss in the third group were significantly slower on both tasks.
It is all but axiomatic that a competence model of linguistic knowledge such as that presented by Chomsky (1965) is not a performance model of language comprehension or production. The competence model does provide a rough specification of the theoretical entities that a comprehension model must recover from the input. It does not, however, necessarily describe the information processing procedures utilized in obtaining these entities. There are many demands upon an adequate comprehension model. It must, for example, be consistent with man's general information processing limitations. One particularly interesting interface between theories of linguistic competence and psycholinguistic performance models is provided by the existence of ambiguous sentences. A sentence is ambiguous when there is more than one way of representing its string of words in the competence model. It is clear, however, that many everyday sentences which in (linguistic) theory are ambiguous are not taken to be ambiguous by those who hear
them. This raises the question as to whether, in normal sentence processing, the comprehension mechanism recovers more than one grammatical interpretation of an ambiguous input sentence. This question is an important one. For if more than one interpretation of an ambiguous sentence usually results from the comprehension process, then we shall have to deal with questions such as why only one interpretation usually becomes conscious and how that interpretation was chosen. If, on the other hand, a single interpretation typically results from the comprehension process, then again we shall have to ask how that one was singled out. In either case, then, the question of how sentence context affects comprehension will eventually have to be faced. For the present, just knowing whether the comprehension process typically enters a state of multiple representations of ambiguous strings would seem to be a step forward in constraining models of comprehension. The studies which have addressed themselves to this issue have obtained seemingly 1 This study was supported by NSF-USDP grant contradictory results. In one study, Foss, GU-1598 to the University of Texas at Austin. The Bever, and Silver (1968) presented evidence author would like to thank Richard H. Lynch, Jr. and consistent with the hypothesis that Ss typically Charles Stewart for aid in gathering the data and David T. Hakes and Carlton T. James for valuable criticisms assign only one immediate interpretation to an ambiguous sentence. If that interpretation of the paper. 699
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was later found to be incorrect, Ss made a reinterpretation, a process which took time. Foss et al. showed Ss a picture immediately after each of a set of sentences was aurally presented and asked them to quickly verify whether the picture depicted the meaning of the sentence. It was found that the time to make such a judgment was no slower for ambiguous than for unambiguous sentences when the picture represented the expected meaning of the ambiguous sentence. (The "expected" interpretation was determined by a pretest with separate group of Ss.) When the picture represented the unexpected meaning, the average verification time increased significantly. The above result contrasts with results found by MacKay (1966). In the latter study, Ss were presented with incomplete sentence fragments (e.g., After taking the right turn at the intersection, I . . . ) and were asked to complete the sentences. When the fragment contained an ambiguity, the completion time was longer than when the fragment was unambiguous. This result suggests that Ss do treat ambiguous sentences in some special fashion. MacKay hypothesized that his results supported the "oblivion hypothesis . . . that neither meaning of an ambiguous word or set of words may be seen until the ambiguity is resolved on the basis of the non-ambiguous context of the sentence [p. 427]." (Italics in original.) These apparently contradictory results may simply be due to the fact that Foss, Bever, and Silver (1968) looked for effects of ambiguity after the sentences, while MacKay (1966) looked for such effects within sentence fragments. The present experiment was an attempt to clarify the processing strategies Ss use in dealing with ambiguity. I t utilized a technique that measured momentary processing difficulty within sentences. If ambiguity leads to additional processing on the part of S, then he should have less of his processing mechanisms available to deploy on an extraneous task. Hence, reaction time
(RT) to perform the extraneous task should be longer right after an ambiguity than after an unambiguous control. In the present study, S was asked to listen in each sentence for a word beginning with a particular letter or phoneme and to press a button when he heard that sound. This phoneme monitoring was the extraneous task. The RT in performing this task was taken as the index of sentence processing difficulty at the point where the target phoneme occurred. That is, it was assumed that RT to push a button in response to a particular phoneme would be positively correlated with sentence processing difficulty. Previous research has shown that phoneme monitoring RT does go up when the syntax of the sentence is difficult to process (Foss & Lynch, 1969; Hakes & Foss, in press), and when lexical items are difficult to process (Foss, 1969). The present experiment also measured the amount of time S took to decide whether each sentence was ambiguous or not. These times were collected for each of two types of ambiguity. MacKay and Bever (1967) found that Ss discovered certain types of ambiguities faster than other types and this finding was then used by MacKay (1966) to motivate the oblivion hypothesis discussed above. Although MacKay and Bever (1967) discussed three types of ambiguity, only two are relevant here: lexical ambiguities and underlying structure ambiguities. A lexical ambiguity occurs when the two meanings of a single sentence have the same syntactic structure, but the meaning of an ambiguous word, such as tank, is not resolved by the context, for example, The tank was too hot to touch. An underlying structure ambiguity occurs when the basic grammatical relations between the words of the sentence are different for its two meanings, for example, The elephant is ready to lift, can mean either that the elephant is ready to lift something or that the elephant is ready to be lifted by something. MacKay and Bever (1967) found that lexical ambiguities were discovered faster than underlying struc-
SENTENCE COMPREHENSION ture ambiguities. The present study provided a check on this finding us!ng aural rather than visual presentation of the sentences.
DESIGN AND METHOD Three groups of Ss were utilized in this study. The first (Mon Group) was told to monitor each sentence for a word starting with a particular phoneme, /b/, and to push a button when one occurred. These Ss were not informed that some of the sentences would be ambiguous; the instructions to them made no mention of ambiguity. (They were asked to repeat each sentence after they heard it to insure that they attended to the sentence.) The second (Amb Group) was asked to decide after each sentence whether or not it was ambiguous. This decision was timed. The third (Mon + A m b Group) was given both the phoneme monitoring and the ambiguity classification tasks. If ambiguity has an effect on spontaneous sentence processing, then longer phoneme monitoring times should be observed during ambiguous than during unambiguous sentences. This prediction is clearer for the M o n Group than for the Mort + Arab Group since S's in the former were naive about the existence of ambiguities in the experiment. If lexical ambiguities are more easily discovered, this should show up in both the Arab and M o n + Amb Groups. Further, under the assumption that overlapping processing mechanisms are used for these tasks, the M o n + Amb Group ought to be slower on each of its two tasks than the other two groups are on their respective single tasks. Subjects The Ss were 60 undergraduates (20 per group) from Introductory Psychology classes at the University of Texas at Austin. They participated in the 1-hr. experiment as part of a course requirement. Materials Forty-eight ambiguous sentences were constructed. These embodied a large number of syntactic constructions and ranged over a wide variety of topics. Twentyfour of these sentences were lexically ambiguous and 24 were ambiguous in underlying structure. One-half of each of these two sets of sentences was disambiguated for one subgroup of 10 Ss; the other half was disambiguated for the second subgroup. The unambiguous sentences were constructed by making a small number of substitutions into the ambiguous sentences. For example, The new men started to drill before they were ordered to do so (a lexical ambiguity) became The new men started to march before they were ordered to do so; and When ! purchased the hat for my friend, the businessman seemed very grateful (an underlying structure ambiguity) became When I purchased the hat with
701
my friend, the businessman seemed very grateful. The sentences were assigned to each counterbalanced subgroup described above on a random basis. One-half of the sentences in each of the resulting subgroups contained the target phoneme /b/. The phoneme followed the point of ambiguity by a word or two (as in the examples above); the phoneme occurred in a corresponding position for the unambiguous sentences (again, as above). To summarize, for each S there were 24 ambiguous and 24 unambiguous sentences. Twelve of the ambiguities were lexical, 12 were in the underlying structure. There were also 12 sentences that could, for the sake of symmetry, be considered lexically unambiguous and 12 that could be considered unambiguous in underlying structure-even though the sentences in both these sets were, of course, completely unambiguous. Target phonemes occurred in half the sentences in the resulting four groups. There were two sets of materials counterbalanced for ambiguity. Thus, there were, overall, two between-S variables (three tasks, two sets of materials) and one within-S variable (two types of ambiguity). Procedure
The Ss in the Mon and the Mort + Arab Groups were told to push a button whenever they heard a word starting with [b/. The Ss in the Amb and M o n + A m b Groups were told to push a (different) button after each sentence when they had decided whether or not the sentence was ambiguous. Examples of ambiguity were given to insure that Ss had the proper concept of ambiguity. As soon as S pushed this button he was told to say either "Yes," indicating that he thought the sentence was ambiguous, or " N o , " indicating that he thought the sentence was not ambiguous. If he said "Yes" then he had to immediately state the two versions of the ambiguity. The S was seated behind a screen so that he could not see E during the experiment. The instructions and sentences were presented on tape over earphones. One timer was automatically started when a /hi occurred (for the relevant two groups) and a second timer was automatically started at the end of the sentence. The E recorded S's latencies and S's statements about the source of the ambiguity (for Ss in the relevant groups). There were approximately 15 sec. between sentences for these statements to be given and recorded.
RESULTS In order to reduce the variability in the latency data, the reaction times were converted into speed scores (1/RT). All analyses w e r e p e r f o r m e d o n t h e l a t t e r scores.
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Phoneme Monitoring Task The Ss in the M o n + Amb Group (who had two concurrent tasks) had some tendency to give slower responses than Ss in the Mon Group. Overall, the mean speed scores were 1.26 for the M o n + A m b Group and 1.48 for the Mon Group. This difference approached significance, F(1, 38)=3.50, p < . 0 7 . This effect did not significantly interact with a sentences' status as ambiguous or unambiguous. The speed scores for both ambiguous and unambiguous sentences considered alone were slower in the Mon + Amb Group than in the Mon Group. Confidence that the marginal difference between the Mon and Mon + Amb Groups was a real one is bolstered by the fact that Ss in the Mon Group pushed the button in response to the presence of the target phoneme in significantly more of the sentences (X = 10.50 per group of 12 sentences) than did Ss in the Mon + A m b Group (X=9.33), F(I, 38)= 5.04, p < .03. That is, Ss in the group with two tasks did not respond as often in the phoneme monitoring task as Ss in the group with only the monitoring task. Whether or not a sentence was ambiguous did have a significant effect upon speed scores for the Mon Group. The mean speed scores were 1.42 for the ambiguous sentences and 1.53 for the unambiguous sentences, F(1, 19) = 8.08, p < .02. Thus, phoneme monitoring was faster in unambiguous than in ambiguous sentences. The type of ambiguity (lexical, X = 1.46, vs. underlying, X = 1.50) had no significant effect on phoneme monitoring speeds, F(1, 19)= 1.14, p > .25. In addition, Ss pushed the response button significantly less often in ambiguous ( X = 5.12) than in unambiguous (X = 5.38) sentences, F(1, 19) = 4.13, p = .05. That is, the target phoneme was missed more often in ambiguous sentences. In contrast, whether or not a sentence was ambiguous had no significant effect upon speed scores in the M o n + Amb Group. The mean speed scores were 1.25 for the ambi-
guous sentences and 1.27 for the unambiguous sentences, F(1, 19)=0.11, p > . 2 5 . Again, type of ambiguity had no effect on phoneme monitoring speeds. The data on the number of sentences to which Ss responded paralleled those of the Mon Group. The Ss pushed the response button less often in ambiguous (X = 4.48) than in unambiguous (X = 4.85) sentences, F(1, 19) = 4.30, p < .05. This result contrasts sharply with the RT data since the latter measure was expected to be the more sensitive indicator of processing difficulty. It may be illuminated by the data reported below. When the data from the Mon + Amb Group were broken down in another way, significant effects of ambiguity on monitoring speeds were observed, however. The Ss in the Mon ÷ Arab Group had to state whether or not each sentence was ambiguous and their categorizations were often at variance with E's. It is possible, then, to look at the phoneme monitoring speed scores for the sentences which S thought were ambiguous or unambiguous. That is, a sentence's status as ambiguous or not can be left to S's judgment as well as to E's judgment (and theory). The sentences were therefore classified as ambiguous or unambiguous on the basis of S's judgments and on the basis of E's judgments, that is, a 2 x 2 classification was made: ambiguous or unambiguous according to S, ambiguous or unambiguous according to E. Again, the main effect of ambiguous versus unambiguous sentences, according to E's classification, had no effect on monitoring speeds. However, the average speed was 1.20 in the sentences Ss classified as ambiguous, and 1.32 'in the sentences Ss classified as unambiguous. This difference approached significance, F(1, 19) ~ 3.82, p < .07. (The number of sentences and the particular sentences classified as ambiguous naturally varied across Ss.) There was no interaction effect on monitoring speeds in this 2 x 2 classification (p = .60). When Ss self-categorized their responses in
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SENTENCE COMPREHENSION
the above manner, the number of sentences in one of the four cells of the 2 × 2 matrix was for some Ss as small as one. In order to increase the reliability of the estimate of phoneme monitoring speeds, the six Ss who had at least one cell with just one observation were omitted. The analysis on the remaining 14 Ss showed the same pattern of results although the monitoring speeds were faster overall. In this analysis the effect of ambiguous (X = 1.31) versus unambiguous (X = 1.35) sentences, as classified by E, was still insignificant, F(1, 13) = .38, p > .25. On the other hand, the difference between monitoring speeds for ambiguous ( X = 1.27) and unambiguous ( X = 1.39) sentences, according to S's classification, did reach an acceptable level of significance, F(1, 13) = 6.70, p < .03.
Ambiguity Identification Task The times for Ss to decide whether or not a sentence was ambiguous were, of course, much longer than the phoneme monitoring times. Hence the speed scores are much smaller. The Ss in the M o n + Amb Group had significantly slower speeds than the Ss in the Amb Group. The average overall speeds were .10 for the M o n ÷ Arab Group and .16 for the Amb Group, F(1, 36) = 10.43, p < .01. Thus, the Ss with two tasks took longer to identify the ambiguities than the Ss with that single task. The two sets of materials did not lead to different identification times, F(1, 36) = 0.38, p > .25. The Group x Materials interaction effect approached significance, F(1, 36)= 3.96, p < .06, indicating that the difference between the Amb and M o n + Amb Groups was more pronounced for one set of materials. The effect was in the same direction for both sets of materials, however. The materials variable did not interact with any other variable. In contrast to the results observed in the phoneme monitoring task, the type of ambiguity had a marginally significant overall effect on ambiguity identification speeds. The mean 26
speed score for lexical ambiguities (X = .14) was faster than the mean speed score for underlying ambiguities ( X = .12), F(1, 3 6 ) = 3.78, p < .06. This variable did not interact significantly with any other variable. Decisions concerning the sentences that were in fact ambiguous (i.e., E considered them to be ambiguous) were made significantly faster than decisions concerning unambiguous sentences. The means were. 16 a n d . 10, respectively, F(I, 36) = 27.81, p < .01. As in the phoneme monitoring task, the ambiguity identification data (for both groups) were cross-classified both according to whether E said the sentence was ambiguous or not and whether S said the sentence was ambiguous or not. The mean speed for Ss to say that a sentence was ambiguous was. 17; this figure dropped to .08 for those sentences S said were unambiguous, F(1, 3 6 ) = 59.44, p < .01. That is, S decided to say that a sentence was ambiguous much faster than he decided to say that a sentence was unambiguous. These variables--ambiguous versus unambiguous according to E, ambiguous versus unambiguous according to S--interacted significantly, F(1, 36)= 23.96, p < .01. The mean speed scores are shown in Table 1. TABLE
1
MEAN SPEED SCORES: AMBIGUITY IDENTIFICATION TASK
Ss' Categorizations E's Categorizations Ambiguous Unambiguous
Ambiguous
Unambiguous
.23 .11
.09 .08
It can be seen from Table 1 that the fastest decisions occurred for those sentences which were classified as ambiguous by both S and E. When S said the sentence was ambiguous, but E had classified it as unambiguous, the decision speed dropped appreciably. It also appears that the slowest decision times were
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for those sentences which S and E agreed were unambiguous. The pattern of the data in Table 1 is the same for both the Amb and the M o n + A m b Groups. The Table 1 variables did significantly interact with groups, however, F(1, 36)= 7.23, p < .02. This interaction reflects the fact that the decisions concerning the sentences represented by the upper left cell of Table 1, that is, where S and E agreed that the sentences were ambiguous, were made much more quickly (X = .31) in the Amb Group than in the Mon + A m b Group ( X = . 1 5 ) . The remaining cells of Tables 1 are quite representative of the data for both the Amb and the Mon + Amb Groups, except that the figures for the former group average about .02 faster. This says that the "real" ambiguous sentences were found quickly; they were found even faster by the Ss who only had the ambiguity search task. No other interactions proved to be significant. DISCUSSION The fact that the Mon + Amb Group was slower on the phoneme monitoring task than the Mon Group, and slower on the ambiguity identification task than the Amb Group, is evidence that overlapping processing mechanisms are used in the two tasks. This, in turn, supports the reasoning used to motivate phoneme monitoring as a technique for measuring momentary sentence processing difficulty. Since lexical ambiguities were identified somewhat faster than underlying structure ambiguities, the results of MacKay and Bever (1967) receive additional support, z The finding that Ss decided in the affirmative about ambiguity faster than they decided in the negative is not surprising. Presumably, the search for ambiguity terminates as soon as one is found. When one is not found, S can persist in his search for as long as his patience holds up (or the 15-sec. intersentence interval lasts). 2 T. G. Bever (personal communication) has also independently verified this aspect of MacKay and Bever (1967) using auditorily presented materials.
The most salient finding of this experiment was that phoneme monitoring was faster in unambiguous sentences than in ambiguous sentences in the Mon Group. This indicates that Ss enter into a special processing state immediately subsequent to hearing a (potential) ambiguity. Questions as to the nature of this state, for example, whether S has calculated both interpretations, or neither, will be returned to below. The present result seems to run counter to that reported in Foss, Bever, and Silver (1968). In the latter study it was found that Ss could verify expected interpretations of ambiguous sentences as fast as they could verify unambiguous sentences. The conflict between the two studies may be due to the difference in point of measurement. The present experiment measured ambiguity effects soon after the point of ambiguity while Foss et al. took their measures at the end of each sentence. This difference is itself of some interest as will be noted below. Alternatively, the difference between the two studies may be due to the differing task demands made upon the Ss. That is, the kind or depth of comprehension required of a sentence is, perhaps, a function of what Ss are asked to do. Whereas interpreting the phoneme monitoring results of the Mon Group poses many problems, at least the results themselves seem quite straightforward. The same is not the case with the Mon + Amb Group. In order to account for the fact that Ss' own categorizations of the sentences as to ambiguous or unambiguous predicted the speed scores, while E's categorizations did not, one might hypothesize that there are individual differences in the grammars (competences) of the Ss and that the speed data simply reflect these differences. That is, Ss were slower for the sentences that their own grammars would classify as ambiguous. This hypothesis is doubtful at best, however. For if such individual differences among grammars existed for the M o n + Amb Group, there is every reason to suppose that similar differences would exist among Ss in
SENTENCE COMPREHENSION
the Mon Group. And if such differences in grammars did exist in the latter group, then the phoneme monitoring speed data should have been equal for the (E-defined) ambiguous and unambiguous sentences, just as was the case for the E-defined ambiguous and unambiguous sentences in the Mort + Amb Group. Since the E-defined ambiguous and unambiguous sentences were different in the Mon Group, the "different grammars" hypothesis cannot be correct. In addition, this hypothesis is not consistent with the interaction observed in Table 1. Since the Ss in the Mon + Arab Group were almost certainly not engaged in normal sentence processing procedures, it is tempting to say that the monitoring results for that group are not worth considering in detail. But since the results were systematic, some further explanation seems called for. One seemingly plausible notion is this: Since Ss in the M o n + A m b Group were searching for ambiguity (and thus not engaged in normal sentence processing) they may have often been caught by a potentially ambiguous word or phrase that was disambiguated by earlier or later material in the sentence. When they noticed the "ambiguity" they had, simply as a result of noticing it, less of their processing mechanisms available for phoneme monitoring. If the phoneme then occurred, slower monitoring speeds would result. Later, S would attempt to justify the ambiguity he had considered. Of course, this justification would be easier if other material had not disambiguated the sentence. Thus, ambiguity justification (identification) would be faster where S and E agreed that the sentence was ambiguous. That is, identification speeds would be fastest where S said the sentence was ambiguous and, in fact, it was. The ambiguity identification data strongly support this interpretation. Identification was much faster when S and E agreed that a sentence was ambiguous than when only S said it was ambiguous (see Table 1). This line of reasoning assumes that search26*
705
ing for an ambiguity without finding one takes less processing than is required after a putative ambiguity is discovered in the input string, an assumption that seems reasonable. It also points out the concrete reason--the search process--why overall speeds were slower in the Mon + Amb Group than in the Mon Group. A tighter reconciliation of the present results with those of Foss, Bever, and Silver (1968) would, of course, be desirable. In addition, reconciling the present results with our everyday experience of ambiguity is needed. It is relevant to note that in the normal course of conversations we rarely notice ambiguous sentences and we rarely make incorrect interpretations of the ambiguities we hear. The latter fact is obviously due, in some way, to the effects of prior context on comprehension. Context determines interpretations, "sets the switches," so that correct interpretations of ambiguities almost always result. It is possible to ask whether prior context has its effect upon input automatically, in much the way that throwing a switch determines the flow of current without the expenditure of any energy at the "choice point," or whether potentially ambiguous messages must be checked against the switch settings. That is, we can ask whether a portion of the analyzing mechanisms must be devoted to discovering the correct interpretation at the point of ambiguity in the input. It is clear that the data from the present experiment favor the latter interpretation over the former. Thus, we infer that the processor must check for the effects of prior information at the point of ambiguity and this activity itself utilizes some of S's limited analyzing mechanisms. Presumably some checking for contextual cues for the resolution of an ambiguity will always occur. If the context is neutral, then the interpretation of the ambiguity may be determined by some rule of frequency of interpretation or simplicity of resulting structure. The above suggestion is testable. If Ss must check for contextual determination, whether
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or not context has biased the interpretation of the ambiguity, then the same pattern of results as obtained in the present study should occur whether or not prior context has determined the interpretation of the ambiguity. It would probably be the case that the differences between ambiguous and potentially ambiguous segments (the latter disambiguated by prior context) would be much attenuated as compared to the present results, but they should still be present. One more point. If prior context is neutral, the ambiguity may still be resolved by later context. But the processes involved probably differ depending upon how much later the resolving context occurs. On the rare occasions when we give an incorrect interpretation to an ambiguous sentence the disambiguating material usually follows the ambiguous segment by some distance as in (1). (1) The shooting of the prince dumbfounded his subjects and his friends since they all thought he was an excellent marksman. An interpretation is probably assigned to
the shooting of the prince in (1) on the basis of some rule such as, "Surface structure order will have the deep structure object following the verb." The S does not wait indefinitely for disambiguating context. It may be that delaying the assignment of an interpretation means that S has to store a great deal more material than after an interpretation has been made. He can store unanalyzed material for a short while, but as more and more material enters, his memory and processing mechanisms get overtaxed and he must make a decision or lose the material altogether and hence not comprehend the sentence. Note that in sentences
like (2) the listener may not get fooled as often as he does in (1). (2) The shooting of the prince who was an excellent marksman dumbfounded his subjects and his friends. This anecdotal evidence indicates that decisions may be briefly delayed while either (a) S checks to see whether prior context can aid in resolving the ambiguity, or (b) S waits for soon-to-come information that will resolve the ambiguity. Normally, of course, there is more than enough prior context so that (a) describes what happens best. In any event, we have evidence here that Ss must deal with ambiguous stretches of input in some way that taxes his processing mechanisms more than an unambiguous input does. This fact alone is a constraint on models of comprehension. REFERENCES CHOMSKY, N. Aspects of the theory of sytnax. Cambridge: M.I.T. Press, 1965. Foss, D. J. Decision processes during sentence comprehension: Effects of lexical item difficulty and position upon decision times. Journal of Verbal Learning and Verbal Behavior, 1969, 8, 457-462. Foss, D. J., BEVER,T. G., & SILVER, M. The comprehension and verification of ambiguous sentences. Perception andPsychophysics, 1968, 4, 304-306. Foss, D. J., & LYNCH, R. H. JR. Decision processes during sentence comprehension: Effects of surface structure on decision times. Perception and Psychophysics, 1969, 5, 145-148. HAKES, D. T., & FOSS, D. J. Decision processes during sentence comprehension: Effects of surface structure reconsidered. Perception and Psychophysics, (in press). MACKAY, D. G. To end ambiguous sentences. Perception andPsychophysics, 1966, 1, 426-436. MACKAY, D. G., & BEVER, T. G. In search of ambiguity. Perception and Psychophysics, 1967, 2, 193-200. (Received June 29, 1970)