JOURNAL
OF MEMORY
Continuous
AND
27,368-381
LANGUAGE
and Discontinuous Access in Spoken Word-Recognition:
The Role of Derivational Prefixes
LORRAINE
MRC
Language
Institute
KOMISARJEVSKY
and Speech
WILLIAM
Max-Planck
(1988)
for Psycholinguistics,
Group,
TYLER
University
of Cambridge
MARSLEN-WILSON
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.
tinuous mapping of the sensory input onto
representations
of lexical form (Warren &
Marslen-Wilson,
1987). Based on the principle 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 wordcandidates which continue to match the incoming 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
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 conAddress correspondence and requests for reprints
to Dr. Lorraine Komisajevsky Tyler, University of
Cambridge, Department of Experimental Psychology,
Downing Site, Cambridge CB2 3EB.
368
0749-5%x/88
Copyright
Ml rights
$3.00
0 1988 by Academic
Press, Inc.
of reproduction
in any form reserved.
CONTINUOUS
AND
DISCONTINUOUS
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. Specifitally, the cohort model requires what Butterworth (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 sublexical unit-for
example, the word’s stem
(Taft, Hambiy, and Kinoshita, 1986), or its
initial strong syllable (Grosjean & Gee,
1987; Norris & Cutler, 1985). For polysyllabic 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 discontinuities 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 purpose 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 exhaustively, 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.
ACCESS
369
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
suffixes. 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 stembased 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 combinations 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
prefvtes combined with a nonstem (as in tejouse). 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 accessthe lexicon on
the basis of the stem. When the stem is a
real stem, it takes the listener longer to decide 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 productivity 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
ET AL.
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. Henderson, 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 accessand 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 undecomposed 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 diverge 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.
CONTINUOUS
AND
DISCONTINUOUS
In Experiment 1 we will use a gating task
to determine directly whether the recognition points for prefixed words are controlled by their stems, as a stem-based access theory predicts, or whether, as the cohort model predicts, recognition point is
determined by the properties of the word
as a whole. In Experiments 2 and 3 we will
reexamine these and additional questions,
using the auditory lexical decision and auditory naming tasks.
EXPERIMENTI
This experiment will compare recognition-points for pairs of words such as count
and miscount, where the prefixed member
of the pair consists of a free (or unbound)
morpheme preceded by a prefix, and where
the other member of the pair is the same
free morpheme standing in isolation.
According to a stem-based access
theory, the recognition-points for the two
words will be highly correlated, since both
words are initially accessed via the same
representation in the recognition lexicon:
the representation of the stem. In fact, the
recognition-point for the prefixed member
of the pair should fall at the same location
in the stem as it does when the stem is
heard in isolation. If, for example, the recognition-point for the free morpheme count
falls on the /t/, then the prefixed word miscount will also be recognized on or after
the /t/. This means that recognition-points
for prefixed words can be directly computed by adding the length of the prefix to
the recognition-point calculated for the
stem heard in isolation. Measuring from
word-onset, prefixed words will always be
recognized later than their corresponding
free stems, because of the delay induced by
the presence of the prefix.
Note that the nature and the length of
this delay necessarily follows from the sequential nature of speech. If word-forms
are accessed from their stems, then the
nrocessor simnlv
_ < has to wait until the full
ACCESS
371
auditory stimulus for the prefix has been
heard before accessof the stem can begin.
The cohort model predicts that there
should be no processing dependencies between the two words during lexical access,
since each should be recognized on the
basis of distinct representations in the recognition lexicon. The recognition-points for
the free morphemes in isolation have no
necessary implications for the recognitionpoints of the prefixed words containing
these stems, since each is recognized on
the basis of different word-initial cohorts.
The recognition-point for count, for example, falls on the /t/, but for miscount it
should fall on the /au/. Nor does the presence of a prefix mean that a prefixed word
will always be recognized later than its corresponding isolated stem. Whether there is
a relative delay or not depends entirely on
the properties of the word-initial cohorts
for the two word-forms.
To measure recognition-point for these
stimulus pairs, we used the gating task
(Grosjean, 1980; Tyler & Marslen-Wilson,
1986; Tyler & Wessels, 1983, 1985). This is
a task in which listeners hear successive
small increments of a word, and at each increment they are asked to state what they
think the word is, or is going to be. This
gives a precise estimate of how much sensory information listeners need to hear before they can identify a word. These estimates correlate highly with performance in
tasks such as word monitoring, which are
assumed to tap more directly the processes
involved in normal on-line language processing (Marslen-Wilson, 1984).
Method
Materials.
We selected 36 pairs of
words, consisting of a free morpheme and a
matched prefixed form which incorporated
this morpheme as its stem (as in frost-defrost or turn-return).
The following criteria
constrained the selection of these stimuli:
1. Derivational morphology: Since this
experiment was in part intended as a test of
the Taft et al. (1986) prefix-stripping hy-
372
TYLER
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.
ET AL.
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 female 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 computer 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. If
so, then the differences in frequency between the prefixed full-forms and the free stem would be irrelevant.
CONTINUOUS
AND DISCONTINUOUS
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 reduce 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 measurement-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 “Completely Sure.” Each testing session lasted
1 V2hr.
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
ACCESS
373
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
criterion, 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 Appendix 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 recognitionpoint 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 disconfirmed 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 relationships 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
forms to their respective-and
quite different-word-initial
cohorts. What is important 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. Subtracting this from the total recognitionpoint 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
ET AL.
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 exactly 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 wordonset, 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 receives 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.
CONTINUOUS
AND
DISCONTINUOUS
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, fulllisting 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 recognitionpoint for its stem heard in isolation. The information provided by the prefix need be
no less effective in constraining the left-toright 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
375
ACCESS
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 prefixstripping 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-toright 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.
376
TYLER
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 nonspeech 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 whenever the sequence they heard was a real
word. Subjects were paid for the 30-min
testing session.
Results and Discussion
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, prefixstripping hypothesis. The first two of these
concern the length of the RTs to the pre-
ET AL.
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 recognitionpoint 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 prefixeswhatever 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 stemaccess theory concerned the relationship
between recognition-times
in prefixed
words and in the matched free stems. Following the same procedure as in Experiment 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).
CONTINUOUS
AND
DISCONTINUOUS
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
Despite
the agreement
3
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.
ACCESS
377
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., Seidenberg, 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 prefixstripping 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 stimuliseems 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 crossrecorded
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 connected to a voice-key. The timer stopped
when the subject named the word.
378
TYLER
Twenty-two subjects were
tested individually in a quiet room. Subjects 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.
ET AL.
TABLE
Procedure.
PERFORMANCE
1-3
(MS)
stems
Unstressed
prefixes
Stressed
prefures
444
401
458
794
629
806
630
864
702
Free
Gating recognition-points
Lexical decision
reaction-times
Naming latency
1
IN EXPERIMENTS
Results and Discussion
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 considerably faster (650 as opposed to 817 ms)-indicating that listeners were responding
much earlier, and in some caseswell 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.
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 designed 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.
CONTINUOUS
AND DISCONTINUOUS
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 appear 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 incoming 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 representational 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).
ACCESS
379
claimed that these required afull 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 anything like a cohort model is correct, then
continuous access would be possible
without full listing for caseswhere 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; Stemberger & 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 prefixation, 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 (MarslenWilson, 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 structural 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 Rentom and Hanney as an undergraduate research project
in the Department of Experimental Psychology, University 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 andMarslen-Wilson.
APPENDIX A
The following items were used in Experiment
with their recognition-points (in ms).
Prefixed words
Stems
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
1,
439
368
390
454
479
446
388
425
414
423
518
545
450
467
454
477
467
450
383
483
438
487
408
508
355
336
458
346
333
RP
Moral
Light
Sleep
Large
Mind
Bound
Count
Drift
Charge
Vision
Text
Class
Form
Build
Fortune
Number
Think
File
Lead
Frost
Card
Grace
Arrange
Fund
B.tm
Claim
Judge
Fart
Head
404
383
421
558
550
477
405
362
533
358
500
529
511
379
500
362
354
535
446
465
533
409
446
555
417
427
383
362
323
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