Continuous and Discontinuous Access in Spoken Word-Recognition · Continuous and Discontinuous Access in Spoken Word-Recognition: The Role of Derivational Prefixes LORRAINE KOMISARJEVSKY

JOURNAL OF MEMORY AND LANGUAGE 27,368-381 (1988)

Continuous and Discontinuous Access in Spoken Word-Recognition: The Role of Derivational Prefixes

LORRAINE KOMISARJEVSKY TYLER

MRC Language and Speech Group, University of Cambridge

WILLIAM MARSLEN-WILSON

Max-Planck Institute for Psycholinguistics, Nijmegen; and MRC Applied Psychology Unit, Cambridge

AND

JAMES RENTOUL AND PETER HANNEY

MRC Language and Speech Group, University of Cambridge

Theories of spoken word-recognition differ in the claims they make about continuous and discontinuous mapping of the speech input onto mental representations of lexical form. The research reported here contrasts the claims of a discontinuous, prefix-stripping model, and of a cohort-based continuous access model. In three experiments, using the gating, auditory lexical decision, and auditory naming tasks, we compared recognition-points for prefixed words (such as miscount) and their corresponding free stems (such as count). The results disconfirm the pretix-stripping model, showing that the recognition-point for a prefixed word was determined by the properties of the word as a complete, full form, and not, as a prefu-stripping model requires, by the properties of its stem. 6 1988 Academic PRSS, IIIC.

The mental representation of lexical form constitutes the search space within which the listener conducts the on-line process of lexical access. The research we report here is concerned with the properties of these mental representations and with the manner in which they are accessed over time. It contrasts, in particular, theories of continuous and of discontinuous access, and their associated claims about the representation of lexical form.

The theoretical framework for the current research is the “cohort” model of spoken word recognition (Marslen-Wilson, 1987; Marslen-Wilson & Tyler, 1980; Marslen-Wilson & Welsh, 1978). This model claims that spoken words are recognized by a process which involves the con-

Address correspondence and requests for reprints to Dr. Lorraine Komisajevsky Tyler, University of Cambridge, Department of Experimental Psychology, Downing Site, Cambridge CB2 3EB.

tinuous mapping of the sensory input onto representations of lexical form (Warren & Marslen-Wilson, 1987). Based on the prin- ciple of maximal processing efficiency, the model states that each word is recognized at that point, starting at word-onset, at which it becomes uniquely distinguishable from all other words in the language beginning with the same sound sequence.

During the recognition process a set of word-candidates is activated on the basis of the initial sensory input. This set includes all of the words in the language beginning with that initial sound sequence (the “word-initial cohort”). Only those word- candidates which continue to match the in- coming sensory input remain activated. This process continues until only a single word-candidate is left matching the input. This point in the word is called the “recognition point. ”

This model of sequential access and selection (Marslen-Wilson, 1987) places

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CONTINUOUS AND DISCONTINUOUS ACCESS 369

strong constraints on the properties of the recognition lexicon; that is, on the properties of the representations of lexical form onto which the speech signal is mapped in the initial stages of lexical access. Specifi- tally, the cohort model requires what But- terworth (1983) has labeled a full listing hypothesis: that for each word recognized there is a corresponding representation in the mental lexicon of the full acoustic-phonetic form of that word. Unless there is a full listing of lexical forms, the system will not be able to access the full word-initial cohort as the beginning of each new word is heard.

These views of representation and access contrast with theories which do not specify strictly sequential access, and where words are not necessarily accessed from their onset, but rather from some sub- lexical unit-for example, the word’s stem (Taft, Hambiy, and Kinoshita, 1986), or its initial strong syllable (Grosjean & Gee, 1987; Norris & Cutler, 1985). For polysyl- labic words, and for morphologically complex words (stems plus affixes), access to the lexicon can only begin when the access unit is encountered. This will lead to dis- continuities in lexical access when the stem or strong syllable is not the first syllable of the word.

These theories do not require full listing of word-forms in the recognition lexicon. The search space is organized in terms of the access units in question (such as stems or syllables), and the speech input has to be broken down into these units for the pur- pose of contacting the lexicon. We will refer to these as decomposition hypotheses of access and representation.

The competing claims of full-listing and decomposition hypotheses have been ex- haustively, though inconclusively, studied in the visual domain (cf. Butterworth, 1983; Henderson, 1985), in research looking at the perception of morphologically complex words. These studies, however, tell us little about the representation and access of lexical form during spoken word recognition.

Although there has also been some research on the role of morphological factors in the spoken domain (e.g., Fowler, Napps, & Feldman, 1985; Kempley & Morton, 1982; Tyler & Marslen-Wilson, 1986), almost all of this work has looked at the effects of inflectional and derivational suf- fixes. For these cases, where the affix follows the stem (in words likejumping and happiness), the predictions of continuous and discontinuous access models do not differ. The critical comparison is for prefixed words, where the &ix precedes the stem (in words like unhappy or displease), and where the full-listing hypothesis predicts immediate access and the decomposition hypothesis predicts delayed access.

There are only two studies in the current literature looking at the perception of prefixed spoken words. One of them, the study by Jarvella and Meijers (1983) of inflectionally prefixed verb-forms in Dutch, does not provide a clear answer to the questions at issue here. More central to our concerns is the research by Taft et al. (1986) on derivationally prefixed English words, which transposes into the auditory domain the types of theory and experiment that Taft and his colleagues had previously developed for visual word-recognition (e.g., Taft, 1981, 1985; Taft & Forster, 1975).

For spoken as well as for written words, Taft et al. (1986) propose a strictly stem- based access theory, which requires the listener to strip off any prefixes in order to identify the word’s stem, which is then used to access representations in recognition lexicon. Taft et al. (1986) base this claim on the results of an auditory lexical decision experiment in which subjects heard nonwords consisting of various com- binations of real and nonexistent prefixes and stems. In the critical comparisons, real prefixes combined with real stems (as in dejoice) were contrasted with nonprefixes combined with real stems (as in tejoice), and real prefixes combined with nonstems (as in dejouse) were contrasted with non-

370 TYLER ET AL.

prefvtes combined with a nonstem (as in te- jouse). Taft et al. (1986) found that response latencies were slower when the nonword began with a real prefix as opposed to a nonprefix, and that this difference was larger when the stem was a real stem. This was interpreted as evidence that the listener strips the prefix off a prefixed word and attempts to access the lexicon on the basis of the stem. When the stem is a real stem, it takes the listener longer to de- cide that the sequence is a nonword than when the stem is not a real stem and therefore cannot access any lexical representation.

This result is inconsistent with the continuous access theory and its associated full listing hypothesis. It supports, instead, a discontinuous access theory, together with a view of lexical representation which claims that the recognition lexicon is organized around stems rather than full forms. Taft et al. (1986) argue, in fact, that even bound stems (they give the example of joice from rejoice) are represented in this lexicon, and can be activated as part of a word-initial cohort.

There are, however, a number of problems with the Taft et al. (1986) study. In particular, it is likely that the task of making nonword decisions about morphologically complex nonwords will induce processing strategies that are not part of normal listening. In our own research using nonword detection tasks (Marslen-Wilson, 1984), the only departure from strictly sequential processing came when subjects heard nonwords constructed by combining very common prefixes with nonsense stems. Response latencies to six items preceded by the prefix in were 150 ms longer than responses to items not preceded by a prefix. We argued (Marslen-Wilson, 1984) that the correct strategy for listeners to follow in these cases, given the produc- tivity of this prefix in the language, was to wait to see whether the sequence to which the in was attached formed a word or not, rather than to base their response on the

entire sequence, including the in. This, however, was clearly a secondary strategy, induced by the task and by the absence of a listing in the recognition lexicon of the [in + stem] sequence being heard. Hen- derson, Wallis, and Knight (1984) have suggested a similar task-induced account of lhft’s evidence for prefix-stripping in the visual domain.

In the research reported here, we avoid these problems by looking at the processing of derivationally prefixed real words. The cohort model, with its claims for continuous access and for the full listing of word-forms in the recognition lexicon, treats the recognition of derivationally prefixed words in the same way as the recognition of morphologically simple word-forms. The prefixed form is processed as an unde- composed sequence, which will be recognized at the point at which it diverges from all other candidates beginning with the same initial sound sequence. Derivationally prefixed words like remind or miscount will be identified at the point at which they di- verge from their closest competitors, such as remark or misconduct (as we noted above, this point of divergence is referred to as the recognition point).

A stem-based access theory, in contrast, requires that a prefixed word cannot be identified until the stem (the access unit) has been identified. This has two consequences. First, that the presence of the prefix will induce a temporal discontinuity in lexical access, and, second, that the recognition point for the word as a whole will depend on the recognition point for the stem on its own. A word like miscount will be identified, not when the entire sequence diverges from its competitors, but only after the stem (count) has diverged from ifs competitors (words like council). l

’ On the affii-stripping view, there will presumably be additional costs associated with the processes of search, etc., that need to take place after the stem has been identified. These should simply add a constant, on average, to recognition time.

In Experiment 1 we will use a gating task auditory stimulus for the prefix has been to determine directly whether the recogni- heard before access of the stem can begin. tion points for prefixed words are con- The cohort model predicts that there trolled by their stems, as a stem-based ac- should be no processing dependencies be- cess theory predicts, or whether, as the co- tween the two words during lexical access, hort model predicts, recognition point is since each should be recognized on the determined by the properties of the word basis of distinct representations in the rec- as a whole. In Experiments 2 and 3 we will ognition lexicon. The recognition-points for reexamine these and additional questions, the free morphemes in isolation have no using the auditory lexical decision and au- necessary implications for the recognition- ditory naming tasks. points of the prefixed words containing

these stems, since each is recognized on

EXPERIMENT I the basis of different word-initial cohorts. The recognition-point for count, for example, falls on the /t/, but for miscount it

This experiment will compare recogni- should fall on the /au/. Nor does the pres- tion-points for pairs of words such as count ence of a prefix mean that a prefixed word and miscount, where the prefixed member will always be recognized later than its cor- of the pair consists of a free (or unbound) responding isolated stem. Whether there is morpheme preceded by a prefix, and where a relative delay or not depends entirely on the other member of the pair is the same the properties of the word-initial cohorts free morpheme standing in isolation. for the two word-forms.

According to a stem-based access To measure recognition-point for these theory, the recognition-points for the two stimulus pairs, we used the gating task words will be highly correlated, since both (Grosjean, 1980; Tyler & Marslen-Wilson, words are initially accessed via the same 1986; Tyler & Wessels, 1983, 1985). This is representation in the recognition lexicon: a task in which listeners hear successive the representation of the stem. In fact, the small increments of a word, and at each in- recognition-point for the prefixed member crement they are asked to state what they of the pair should fall at the same location think the word is, or is going to be. This in the stem as it does when the stem is gives a precise estimate of how much sen- heard in isolation. If, for example, the rec- sory information listeners need to hear be- ognition-point for the free morpheme count fore they can identify a word. These esti- falls on the /t/, then the prefixed word mis- mates correlate highly with performance in count will also be recognized on or after tasks such as word monitoring, which are the /t/. This means that recognition-points assumed to tap more directly the processes for prefixed words can be directly com- involved in normal on-line language pro- puted by adding the length of the prefix to cessing (Marslen-Wilson, 1984). the recognition-point calculated for the stem heard in isolation. Measuring from Method

word-onset, prefixed words will always be Materials. We selected 36 pairs of recognized later than their corresponding words, consisting of a free morpheme and a free stems, because of the delay induced by matched prefixed form which incorporated the presence of the prefix. this morpheme as its stem (as in frost-de-

Note that the nature and the length of frost or turn-return). The following criteria this delay necessarily follows from the se- constrained the selection of these stimuli: quential nature of speech. If word-forms 1. Derivational morphology: Since this are accessed from their stems, then the experiment was in part intended as a test of nrocessor simnlv has to wait until the full _ < the Taft et al. (1986) prefix-stripping hy-


372 TYLER ET AL.

pothesis, we followed their criteria for what constituted a prefixed item-namely that the word should be listed in the Shorter Oxford Dictionary as being derived from a prefix plus a stern.*

2. Theoretical recognition-points: So that the competing hypotheses would make distinct predictions, we needed to select word pairs whose recognition-points were the same on a prefix-stripping view but different on a cohort analysis. We therefore chose word pairs where the separation point of the prefixed form, on the full listing hypothesis, should occur one or two segments earlier in the stem than the separation point of the same stem when heard as a free morpheme. On average, this meant that the theoretical uniqueness points for the prefixed words fell about 90 ms earlier than the uniqueness points for the corresponding free stems. For example, the word rebuild becomes unique at the /I/

whereas its stem build becomes unique at the /d/. These points were determined by searching Chambers Twentieth Century Dictionary and Jones and Gimson’s (1984) pronouncing dictionary.

3. Word frequency: It was not possible to construct a stimulus set that met the other criteria and where the pairs were also fully matched in frequency. We chose, therefore, to construct a list where the frequency (Francis & Kucera, 1982) of the free morpheme was always higher (median frequency = 97) than the frequency of the prefixed form (median frequency = 7.5). The effect of this is to bias the results in favor of the stem-access theory. If frequency affects word-recognition, then higher frequency words should be recognized earlier than lower frequency words. This would have the effect of increasing the differences in the recognition-points of

* Note that on the prefix-stripping hypothesis, as transposed to the auditory domain, it does not matter whether the listener is hearing a true prefixed word or not. So long as the first syllable sounds like a prefix, then the system must strip it.

stems and prefixed words that the stem-access model predicts.3

4. Prefixes: We used a variety of common monosyllabic prefixes with the constraint that no more than six words in the total set contained the same prefix.

5. Stress: Several current theories claim that lexical access is driven by stressed, or strong, syllables and that unstressed, or weak, syllables are not used in initial access (Grosjean & Gee, 1987; Norris & Cutler, 1985). This raises the possibility of a confound with the effects of prefixing. If a prefix is unstressed, then this may be an additional reason-or even the sole reason-for a delay in lexical access. To control for this possibility, we varied the stress pattern of the prefixed words, such that 16 items began with an unstressed prefix (containing a reduced vowel), and 20 began with prefixes containing full vowels, carrying either primary or secondary stress.

Design and procedure. The 36 pairs of words were assigned to two lists, such that each list contained only one member of each pair, with an equal number overall of prefixed words and free morphemes. Each list contained a maximum of three words with the same prefix. The prefixed words and stems were pseudorandomly ordered within a list, with the same order of items maintained across lists. These lists, plus five practice items, were recorded by a fe- male speaker of British English.

The test words were then digitized at a sampling rate of 20 kHz. The wave-form for each word was displayed on a CRT screen and 50-ms segments were marked off starting from the beginning of the word. The word was then output from the com- puter in a sequence of fragments, each of

3 It is, in fact, not quite clear what a stem-base access theory predicts here. If initial access is via the same stem representation for both the prefixed form and for the free stem, then it may be the frequency of the free stem that determines the frequency effect. I f so, then the differences in frequency between the prefixed full-forms and the free stem would be irrelevant.


which increased by 50 ms in duration. The first fragment consisted of the first 50 ms of the word, the second consisted of the first 100 ms, and so on until the whole word had been output. A smoothing function was applied at the offset of each fragment to re- duce splicing artifacts. The number of fragments required for each word varied according to the total duration of the word, ranging from 7 to 17 gates. There was a 5-s interval between each fragment to give subjects time to write down their response.

In preparing the stimuli we measured the length of each word, and, for the prefixed words, the length of their component prefixes and stems. These measurements were made directly from the wave-forms, using auditory information where necessary. We also established, for each stimulus, where the theoretical uniqueness points fell, measured in milliseconds from the word-onset. These theoretical uniqueness points were based on dictionary counts, and the mea- surement-point was defined as the onset of the segment at which the word became unique.

Subjects were tested in groups of four, each seated in a separate carrel. They heard the stimuli over closed-ear headphones and were instructed to write down what word they thought they were hearing after each fragment, together with a rating of their confidence in this judgement. The rating scale ran from 1 to 10, with 1 being labeled as “Pure Guess” and 10 as “Com- pletely Sure.” Each testing session lasted 1 V2 hr.

Subjects. Thirty subjects (15 on each of the two lists of items) were tested. They were taken from the MRC Language and Speech subject panel and were paid for their participation.

Results and Discussion

Following the procedures used in other gating studies (e.g., Grosjean, 1980; Tyler & Wessels, 1983, 1985) we calculated recognition-points for each item. The recognition-point is defined as the mean gate at

which subjects correctly identify the word they are hearing, and assign a confidence rating of 80% or more to their response. For seven items, the subjects’ responses failed to meet the recognition-point crite- rion, and these items, together with the other member of the pair, were discarded from the analysis .4 All of the analyses we report are based on the remaining 29 pairs of items.5 These items are listed in Ap- pendix A.

A stem-based access theory makes three main predictions here: that the presence of a prefix will necessarily cause a delay in the recognition of a prefixed word as compared to its stem heard in isolation; that the recognition-point for the prefixed word can be simply calculated by adding together the duration of the prefix and the recognition- point for the stem in isolation; and that there will be a strong correlation between recognition-points for stems heard in isolation and recognition-point for the same stems heard as part of a prefixed word.

All three of these predictions are discon- firmed by the results. First, there is no difference in the mean recognition-points for the prefixed words and for their corresponding free stems. The mean recognition-point for the prefixed forms is 434 ms as opposed to 444 ms for the isolated stems. This difference does not approach significance (F2 < 1). The exact relationship in length between a given set of prefixed and free stems is of course arbitrary, reflecting as it does the independent rela- tionships of the free stems and the prefixed

4 The discarded items were primarily cases where the speech editing process had created “click” artifacts at the onset of the stimulus, leading to difficulties in identifying the initial segment of the item. This does not reflect any general degradation of the speech signal by the digitizing and playback procedures used here.

J The analyses of variance were carried out on item means only. Mean recognition-points could not be meaningfully computed for subjects, since the critical comparisons involved contrasts between matched pairs of items.

374 TYLER ET AL.

forms to their respective-and quite different-word-initial cohorts. What is im- portant about the result is that it shows that there is no necessary increase in recognition-time associated with the addition of a prefix.

Second, as this first result implies, recognition-point for the prefixed words is not the sum of the length of the prefix and the recognition-point for the stem. The mean length of the prefixes was 183 ms. Sub- tracting this from the total recognition- point for the prefixed items, we can estimate that the mean recognition point for these items fell 251 ms after the onset of the stem. This is nearly 200 ms earlier than the mean recognition-point (444 ms) for the isolated stems. This difference cannot be explained in terms of durational differences between the stems in the prefixed words and the same stems in isolation, since they only differ in length by 28 ms. The mean duration of the isolated stems is 548 ms, as opposed to 520 ms for the stems in prefixed words.

Third, there is no relationship between recognition-point for stems heard as free morphemes and stems heard in prefixed words. To establish this, we first subtracted the length of each prefix from the overall gating recognition-point for that word. On the stem-based access theory, access only begins after the prefix has been heard, so that variations in the length of the prefix are simply noise as far as the question at issue is concerned. These corrected recognition-points for the prefixed words were then correlated with the recognition-points for the corresponding free stems. The result was a nonsignificant correlation of + .019 (df = 27), indicating that the two sets of recognition-points have essentially no variance in common.

The final possibility we investigated was whether the overall effects concealed an interaction with the stress value of the prefix, If syllabic stress does play a role in lexical access, then it is possible that prefixes carrying stress behave differently to unstressed prefixes. We therefore separated

out the 29 pairs according to the stress value of the prefix. Twelve of the prefixed words began with unstressed syllables containing reduced vowels and 17 began with stressed syllables containing full vowels. The mean recognition-point for items containing unstressed prefixes was earlier (401 ms) than for items containing prefixes with either secondary or primary stress (457 ms).‘j Note that the direction of this effect is the opposite to what is predicted by theories which claim that lexical access is driven by stressed or strong syllables (Grosjean & Gee, 1987; Norris & Cutler, 1985). These theories predict that words containing a stressed prefix should be recognized earlier than those containing an unstressed prefix, because of the delay in contacting the lexicon induced by the unstressed prefix.

The difference we found here between stressed and unstressed prefixed words in fact has little to do with the value of stressed or unstressed syllables as access units. Instead, it corresponds almost ex- actly to the difference in average length (62 ms) between stressed and unstressed prefixes. The stimulus set contained, for example, six items beginning with the prefix re-: the unstressed cases averaged 139 ms in duration, while the stressed cases averaged 201 ms. This increase is primarily due to an increase in the length of the vowel. From the perspective of lexical access and selection, this has the effect of delaying the point in the signal, measuring from word- onset, at which the listener starts to get information about the next segment. This, in turn, has the consequence of delaying the point at which, eventually, the listener re- ceives the discriminatory acoustic-phonetic information necessary to identify the word.

To correct for these differences, we sub-

6 On a separate analysis, including only the prefuted items, there was a significant main effect of stress (F2 (2,28) = 4.691). On an overall analysis, including the free stems as well. There was no overall effect of stress, nor a significant interaction between stress and word-type.


tracted prefix length from recognition-point for each item (overall mean duration was 213 ms for stressed prefixes, and 141 ms for unstressed). This yielded a mean recognition-point for the stressed prefixes of 244 ms from the onset of the stem, and of 260 ms for the unstressed prefixes. This difference did not approach significance (F2 < 1). There is no evidence here to suggest that the stress value of a prefix has any consequences for its status in the access process.

In contrast to the problems these results pose for stem-based access theories, they fit very well with a continuous access, full- listing hypothesis of the cohort type. The prefixed forms and their matched free stems begin with different speech sounds, and are recognized, therefore, on the basis of completely different word-initial cohorts. This is why the recognition-points for the two kinds of items have no variance in common, and why there is no necessity for the recognition-point for a prefixed word to occur later in the word, measuring from word-onset, than the recognition- point for its stem heard in isolation. The information provided by the prefix need be no less effective in constraining the left-to- right on-line decision-space than the information provided by any other type of word-initial syllable. And any variations in the length of this first syllable-whether a prefix or not- will have the effect of increasing or decreasing average recognition time.

Additional support for the continuous access hypothesis comes from the relationship between the recognition-points we obtained in the gating task and the theoretical recognition-points we calculated on the basis of dictionary counts. If the cohort model is correct in claiming that the recognition points for both free stems and prefixed words are determined by the properties of their respective word-initial cohorts, then there should be a significant correlation between the observed gating recognition points and the recognition-points calculated on theoretical grounds. This is

what we found. The correlation between the gating recognition-points and theoretical recognition-points was +0.797 (df = 27, p < .OOl) for the free stems, and + 0.562 (df = 27, p < .OOl) for the prefixed words.

We now turn to a second experiment in which we test the same stimuli in an auditory lexical decision task.

EXPERIMENT 2

Although the results of the gating study were unequivocal in rejecting the prefix- stripping hypothesis, we decided to run an additional test, using the auditory lexical decision task with the same stimuli. The previous experiment differs from Taft et al. (1986) not only in the type of stimulus it used (real words as opposed to nonwords), but also in the kind of experimental paradigm (gating as opposed to lexical decision). The goal of this second experiment, therefore, was to establish the role of these two different tasks in generating such different outcomes. In particular, we wanted to rule out the possibility that the gating task itself, with its strictly sequential and incremental mode of presentation, might be artificially inducing a continuous left-to- right access strategy.

Method

Materials. We used the acoustic tokens of the 36 test pairs recorded for the gating study. In addition, we selected 96 nonwords and 60 filler real-words. Of the nonwords, 60 were monosyllabic, 26 were prefixed bi- or trisyllabic strings, and 10 were unprefixed bisyllables. The real-word tillers consisted of 40 monosyllables and a further 20 monomorphemic two or three syllable words. An additional practice list, of 50 real words and nonwords, was also constructed. These materials were then recorded by the same speaker who had recorded the original gating test pairs. There were no detectable differences between the earlier and the later recording.

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mo test versions were constructed by cross-recording from master tapes. Each version contained half the prefixed items and half the free stems. These test items were pseudorandomly interspersed with filler items. Timing pulses were then placed (to within 10 ms accuracy) on the non- speech channel of the tape at the onset of each test word. These pulses served to trigger a timing device which was stopped when the subject pressed a response key.

Procedure. Twenty-five subjects (from the MRC Language and Speech subject pool) were tested in groups of four, each seated in separate carrels. They heard the stimuli over closed ear headphones. Given the problems associated with making No responses in this context (as we pointed out in the Introduction), the subjects were required to make Yes responses only. They were asked to press the response key in front of them as rapidly as possible when- ever the sequence they heard was a real word. Subjects were paid for the 30-min testing session.


The data from three subjects were discarded because their responses were slow and erratic (their average latencies were over 1 s and they had high error rates). This left 11 subjects for each of the two experimental versions. Extreme outliers (defined as RTs over 1500 ms) and missing values were omitted from the data analysis. These totaled 2.3% of the total. We then computed midmean RTs for each item and each subject, including only the data for the 29 pairs used in the gating analyses. These were then entered into two sets of analyses of variance, on subjects and on items, with the factors of Word Type (Prefixed vs. Free Stem) and of Stress (primary and secondary stress vs. unstressed).

As in our discussion of the gating results, we begin by examining the three main predictions made by a stem-access, prefix- stripping hypothesis. The first two of these concern the length of the RTs to the pre-

fixed words as opposed to the free stems. There is now an overall difference between the two sets of 45 ms, with mean response times of 795 ms for free stems as opposed to 840 ms for prefixed words. This difference is significant on the subject (Fl (1,21) = 11.328, p < .Ol) and the item (I?2 (1,27) = 5.053, p < .05) analyses, but not on the Min F’ statistic (Min F’ (1,45) = 3.494, p < .lO).

In so far as there is a systematic effect here, it seems to be entirely due to the prefixed items with primary or secondary stress. The unstressed prefixes (mean RT of 806 ms) do not differ from their matched free stems (mean of 795 ms). Where there is a difference is between the stressed prefixes (mean RT of 864 ms) and their matched free stems (mean of 794 ms).7 The difference between stressed and unstressed prefixed words is numerically almost identical to the differences in mean recognition- point for these two classes of prefixed items in the gating study, and undoubtedly derives from the same source-namely, the 62-ms difference in mean duration between stressed and unstressed prefixes.

The overall difference in lexical decision time between free stems and prefixes- whatever its source-is in any case far smaller than any prefix-stripping account could predict. Given the mean length of the prefixes (183 ms), response-time to the prefixed words should have been of the order of 980 ms, rather than 840.

The other main prediction of the stem- access theory concerned the relationship between recognition-times in prefixed words and in the matched free stems. Fol- lowing the same procedure as in Experi- ment 1, we subtracted prefix duration from the mean RT for each prefixed word, and correlated these corrected RTs with the

’ On the overall analysis, these effects were only significant on the subjects analysis, where there was a main effect of Stress (Fl (1.21) = 4.876, p < .05) and an interaction with Word Type (Fl (1,21) = 4.775, p < .05). Neither of these was significant on the items analysis or on the Min F’ statistic (Min F’ < 1).


RTs for the free stems. As in the gating study, the outcome (r = +.017, df = 27) suggests that the two sets of RTs have no variance in common.

This dissociation is confirmed by the frequency effects for the two groups. Overall, the correlation between RT and word-frequency is - .358 (df = 27)*. This also holds within the prefixed group alone, with a correlation between the frequency of the full form and RT of - .360 (& = 27, p = .05). What we do not find, however, is a significant correlation between prefixed word RT and the frequency of the free stem (df = 27, r = - .153). On a stem-access hypothesis, where morphologically complex forms are initially accessed via their stems, one might expect the frequency of the stem to contribute to the frequency effect at least as much as the frequency of the form as a whole.

Given these results, we can exclude the possibility that the failure of a discontinuous access model in Experiment 1 was in any way due to the special properties of the gating task. Beyond this, the results confirm that the recognition points computed in the gating task accurately reflect the point at which listeners can recognize prefixed words and free stems in ordinary listening. If the recognition points computed in Experiment 1 are correlated with the mean RTs for the corresponding items in Experiment 2, we obtain a correlation of + .81 for the free stems and + .69 for the prefixed forms. The gating and the auditory lexical decision tasks provide very similar estimates of the basic timing of spoken word-recognition processes.

We now turn to a third experiment, testing the same stimuli in an auditory naming task.

EXPERIMENT 3

Despite the agreement between the

s This is another potential source for slower responses to prefixed items, since these were much lower in frequency than the free stems.

gating and lexical decision tasks, there are two potential criticisms that could be levied against lexical decision as a test of the prefix-stripping hypothesis. First, there is the now quite widely held belief (e.g., Sei- denberg, Waters, Sanders, & Langer, 1984) that the task has a strong postaccess component, so that it is not a pure reflection of the immediate recognition process. Second, there is the possibility that the task encourages a strategy where subjects wait until the end of the word to respond, independent of when they actually recognize it, in order to make sure that is does not become a nonword. If these criticisms are correct, so that the lexical decision response is primarily determined by a late, postaccess representation of the input, then this might make it insensitive to the early processes of decomposition and stem-based search predicted by a discontinuous access hypothesis of the prefix- stripping type.

To meet these possible objections, we ran the same stimuli on a third task (auditory naming). This is a task which-at least for the naming of visual stimuli- seems to be relatively less sensitive to postaccess strategic processes (e.g., Balota & Chumley, 1984, 1985), and where there is no necessity for subjects to wait until the end of the word to respond. They do not hear any nonwords, and can respond as soon as they have identified the word they are hearing (Marslen-Wilson, 1985).

Method

Materials. For this experiment we cross- recorded the prefixed words, the free stems, and the associated real word tillers used in the lexical decision study (together with their timing pulses) onto two new tapes, omitting the nonwords, to create two new experimental versions. This meant that we could use the identical acoustic tokens as in the first two experiments. The timing pulses (as before, placed at word onset) activated a timer which was con- nected to a voice-key. The timer stopped when the subject named the word.

378 TYLER ET AL.

Procedure. Twenty-two subjects were tested individually in a quiet room. Sub- jects heard the stimuli over closed ear headphones and were asked to say each word out loud as soon as they recognized it. Each experimental session lasted 20 min and subjects were paid for their participation.


Out of the 11 subjects tested on each of the two versions of the stimuli, two from each group were omitted from the data analyses because their responses were slow and erratic. The data from the remaining 18 subjects were anlyzed after missing and extreme values (Cl%) had been removed. Midmeans for each item and each subject were computed and entered into two two-way ANOVAs (on Word-Type and Stress) which formed the basis for the Min F’ statistic.

The pattern of results was almost identical to the lexical decision results, except that overall response time was consider- ably faster (650 as opposed to 817 ms)-indicating that listeners were responding much earlier, and in some cases well before they reached the end of the word. There was again an overall difference between prefixed words and free stems, with mean latencies, respectively, of 672 and 628 ms. This difference was significant on both subject (Fl (1,17) = 11.934, p < .Ol) and item (F2 (1,27) = 5.808, p < .05) analyses, but not on the Min F’ statistic (Min F’ (1,43) = 4.907, p < .lO). The size of this effect is identical to what we observed for the same items in the lexical decision study, and is again entirely due to the stressed prefixed items, with naming latencies of 702 ms, as opposed to the 630 ms mean latencies for

9 On the overall analysis there is no main effect of Stress (Min F’ < l), but there is a marginally significant interaction between Stress and Word Type for the subject (Fl (1,17) = 6.473, p < .05) and for the item (F2 (1,27) = 3.760, p < .lO) analyses, although not on the Min F’ statistic.

TABLE 1 PERFORMANCE IN EXPERIMENTS 1-3 (MS)

Gating recog-

Free

stems Unstressed

prefixes Stressed prefures

nition-points 444 401 458 Lexical decision

reaction-times 794 806 864 Naming latency 629 630 702

the unstressed items.9 This seems to be the same effect that we saw in the gating and the lexical decision studies, reflecting the slight increase in duration of stressed prefixes (see Table 1)

There is no evidence here either to support a prefix-stripping account, or to suggest that the gating and the lexical decision tasks give an inaccurate picture of the properties of the spoken word-recognition process. In fact, as Table 1 illustrates, the overall pattern is very similar across the three tasks, despite the superficially very different kinds of response that each task requires.

CONCLUSIONS

The research we reported here was de- signed to contrast the predictions of continuous and discontinuous theories of lexical access, and the differing implications of these claims for the representation of lexical form. The results, for a stimulus set tested in three quite different experimental situations, provide no evidence here for any discontinuity in the access process, whether associated with derivational morphology, or with the stress value of the initial syllable. The presence of a prefix, whether stressed or unstressed, does not

to Other research (Hall, 1987; Marsien-Wilson, Tyler, Hall, & Warren, unpublished) shows, in a similar set of experiments, that the prefix induces no delays in the access process, irrespective of the relationship between the prefix and its stem. Cases like rebuild, where the relationship between stem and prefix is transparent, behave in the same way as cases like delight, where the relationship is opaque.


introduce any necessary delay into the access process, lo and there was nothing in the results to support the claim that lexical access is delayed until the stem can be identified. Words are simply recognized too early for the system to be waiting that long.

The results, instead, are much more compatible with the type of continuous access process suggested by the cohort model. Prefixed and nonprefixed words ap- pear to be recognized in the same way, by continuously mapping the sensory input onto representations of lexical form and identifying the word at the point at which it becomes unique, relative to its word-initial cohort. The significant correlations between theoretical recognition-points and gating recognition, for both free stems and for prefixed words, reflect this dependence of the on-line recognition process on the decision-space defined by the full-forms of the words involved. l1

We turn now to the implications of this result for the representation of lexical form. We should stress immediately that we are concerned here only with what we have called the recognition lexicon: those representations of lexical form that are the direct targets of the lexical access process, and onto which the listener maps the in- coming speech signal during the on-line process of lexical choice. We have nothing to say here about higher levels of lexical representation, and in particular about their semantic properties. Any claims we make for or against decomposition in the access process do not necessarily apply to the subsequent process of syntactic and semantic interpretation.

In our earlier discussion of the represen- tational requirements of the cohort model, and of a continuous access theory, we

I1 Note that we also obtained significant correlations between theoretical uniqueness-points and response latencies in the two response-time studies. For free stems and for prefixed words, the correlations were, respectively, + 26 and + 20 in the lexical decision experiment, and + .37 and + 40 in the naming experiment (df = 27 throughout).

claimed that these required a full listing hypothesis: that to construct the word-initial cohort, the system in some sense requires all the possible candidates to be already listed. In the discussion here, we explore some of the limits on this notion, beginning with the question of the abstractness of the representation we are postulating.

We have talked so far as if “full-listing” meant the storage in the recognition lexicon of a specific acoustic-phonetic realization of a given word-form. This cannot be literally true. The specific realization of a word-form will vary radically as a function of differences in speakers, in speech rate, and in the phonological environment in which the item occurs. The representation in the recognition lexicon must abstract away from these sources of variation. It is an urgent question for the study of spoken word-recognition in general to determine just what the nature of this abstract representation might be.

Second, even if full-listing applies to derivationally prefixed words, do we want to claim that it also applies to derivationally or inflectionally suffixed words in English, and to inflectionally prefixed words in languages like Dutch or German? On general processing grounds, there seems to be less benefit for the processing system to insist on full-listing for suffixed forms. If any- thing like a cohort model is correct, then continuous access would be possible without full listing for cases where the stem is encountered first. Functional arguments in this domain can be quite misleading, however.

In fact, as far as English is concerned, the evidence currently available in the spoken domain tends to support full listing of derivationally suffixed words, as well as of the more frequent inflectionally suffixed words (e.g., Fowler et al,, 1985; Stem- berger & MacWhinney, 1986). There is very little work on languages other than English in the spoken domain. One recent exception, the research by Jarvella and Meijers (1983) looking at inflectionally pre-

380 TYLERETAL.

fixed words in Dutch, presents a complex pattern of results for which it is difficult to find a unitary interpretation. On a priori grounds, however, it is for inflectional pre- fixation, if anywhere, that one might expect the full-listing hypothesis to fail for prefixed words, and this is clearly an area which requires more research.

Finally, we should make it clear that a full listing hypothesis for the purposes of lexical access is by no means incompatible with a decomposition hypothesis for the purposes of lexical integration (Marslen- Wilson, 1987)-that is, for the purposes of mapping lexical semantic and syntactic information onto higher levels of interpretation. As we have argued elsewhere (Tyler and Marslen-Wilson, 1986), it is likely that there is on-line decomposition of inflectionally suffixed words as they are heard in an utterance context. In this earlier research we were able to demonstrate the differential sensitivity of stems and inflectional aflixes to the properties of the struc- tural semantic and syntactic environment in which they were being heard. By the same token, it is also possible that at least some derivationally prefixed words-for example, relatively transparent forms like rebuild or unhappy-may be decomposed as they are heard for the purposes of semantic interpretation. But if this kind of decomposition does occur, our evidence here shows that it occurs after access, and not as part of the access process itself.

ACKNOWLEDGMENTS

Experiment 1 in this report was carried out by Ren- tom and Hanney as an undergraduate research project in the Department of Experimental Psychology, Uni- versity of Cambridge, under the supervision of the first two authors. We thank Vera Kamphuis and Marie Jefsioutine for canying out Experiments 2 and 3, and Howard Cobb, Chris H&l, and Paul Warren for their help with many aspects of this study. The research was supported in part by an MRC program grant to Tyler and Marslen-Wilson.

APPENDIX A The following items were used in Experiment 1,

with their recognition-points (in ms).

Prefixed words Stems

RP RP

Amoral Delight Asleep Enlarge Remind Rebound Discount Adrift Discharge Provision Pretext Outclass Deform Rebuild Misfortune Outnumber Rethink Profile Mislead Defrost Discard Disgrace Prearrange Refund Return Proclaim Prejudge Depart Ahead

439 Moral 404 368 Light 383 390 Sleep 421 454 Large 558 479 Mind 550 446 Bound 477 388 Count 405 425 Drift 362 414 Charge 533 423 Vision 358 518 Text 500 545 Class 529 450 Form 511 467 Build 379 454 Fortune 500 477 Number 362 467 Think 354 450 File 535 383 Lead 446 483 Frost 465 438 Card 533 487 Grace 409 408 Arrange 446 508 Fund 555 355 B.tm 417 336 Claim 427 458 Judge 383 346 Fart 362 333 Head 323

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(Received May 13, 1987) (Revision received February 1, 1988)

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Continuous and Discontinuous Access in Spoken Word-Recognition · Continuous and Discontinuous Access in Spoken Word-Recognition: The Role of Derivational Prefixes LORRAINE KOMISARJEVSKY