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Transcript
MEMORY, 2000, 8 (3), 145–157
Phonological similarity and the irrelevant speech effect:
Implications for models of short-term verbal memory
Janet D. Larsen
John Carroll University, Cleveland, OH, USA
Alan Baddeley
University of Bristol, UK
Jackie Andrade
University of Sheffield, UK
Three experiments studied the interaction between irrelevant speech and phonological similarity within
both the remembered and the irrelevant auditory material. Phonological similarity within the remembered list impaired performance in both baseline and irrelevant speech conditions, whereas phonological
similarity between the remembered and ignored irrelevant items did not influence performance. Although
there was a tendency for similarity within the irrelevant items to reduce interference, this proved to be a
less robust finding. Implications for the theoretical interpretation of the irrelevant speech effect are
discussed.
Colle and Welsh (1976) were the first to describe a
phenomenon that they called the ‘‘acoustic
masking in primary memory’’ (p. 17), whereby the
immediate serial recall of a string of visually presented letters was impaired by the simultaneous
presentation of continuous spoken text in an
unfamiliar language, which the subject was
instructed to ignore. Colle (1980) proposed that
this effect must be a central process because it
occurred both for loud speech and for speech at
the level of a whisper. The irrelevant speech effect
occurs not only when the speech is in a language
that the subjects do not know (Baddeley &
Salame, 1986; Colle & Welsh, 1976; Salame &
Baddeley, 1986, 1989), but also when the speech is
nonsense syllables (Salame & Baddeley, 1982).
Furthermore, Salame and Baddeley (1982)
demonstrated that the effect did not operate at a
lexical level, as subjects who were remembering
visually presented digit sequences were no more
disrupted by streams of irrelevant digits than they
were by sequences of items comprising the same
phonemes but in a different order (e.g. TUN,
WOO rather than ONE, TWO). Colle and Welsh
referred to their results as a masking effect, but
they observed that a simple masking interpretation fails to account for two further results, namely
the failure of irrelevant Gaussian noise to impair
memory performance (Colle & Welsh, 1976),
together with the insensitivity of the irrelevant
speech effect to the intensity of the material that
was assumed to be masking the memory trace
(Colle, 1980).
While accepting that a simple masking interpretation was ruled out, Salame and Baddeley
(1982, 1987) suggested that something analogous
to a masking hypothesis could be incorporated
into the Working Memory model (Baddeley,
1986) by assuming that the phonological shortterm store allowed spoken material direct access,
Requests for reprints should be sent to Janet D. Larsen, Department of Psychology, John Carroll University, University Heights,
OH 44118, USA. Email [email protected]
Ó 2000 Psychology Press Ltd
http://www.tandf.co.uk/journals/pp/09658211.html
146
LARSEN, BADDELEY, ANDRADE
while visually presented material gained access to
the store only if subvocally articulated. They
proposed that the store was protected by an
auditory filter which prevented sounds that were
not speech-like from being registered. Provided
that the store is assumed to register information
regardless of auditory intensity, the lack of an
effect of loudness can readily be explained. Positive, if not particularly strong, evidence for
something analogous to masking came from the
observation that, although the memory for digits
was no more disrupted by the need to ignore digits
than to ignore other words made up from the same
phonemes, a third condition in which the irrelevant items were disyllables did reduce the magnitude of the irrelevant speech effect. Salame and
Baddeley (1982) suggested that this is because the
disyllabic irrelevant words were phonologically
dissimilar to the monosyllabic digits. As we shall
see, the robustness of this result has subsequently
been questioned (Jones & Macken, 1995b; Jones,
Madden, & Miles, 1992; Le Compte & Shaibe,
1997).
Salame and Baddeley (1986) attempted to test
this memory masking hypothesis more directly by
studying the effect of irrelevant speech on memory for letter sequences differing in degree of
intra-list phonological similarity and in the number of items in the list. Sequences of similar letters
were assumed to be harder to recall accurately
(the phonological similarity effect), because the
remembered items had fewer distinguishing phonological features, and hence were more vulnerable to trace decay. It was argued that if irrelevant
speech added noise to the memory trace, then
there should be an interaction between the effects
of irrelevant speech and phonological similarity,
such that similar items, having fewer distinguishing cues, should be more dramatically impaired.
There was no evidence of the predicted interaction at lists of intermediate length, although on
lists exceeding seven letters the influence of both
irrelevant speech and phonological similarity was
abolished. This result is broadly consistent with an
earlier report by Colle and Welsh (1976) who used
only lists of eight similar or dissimilar consonants,
observing that irrelevant speech abolished the
effect of phonological similarity. Abroadly similar
result was obtained by Jones and Macken (1995b).
On the other hand, Surprenant, Neath, and Le
Compte (1999) report that the phonological
similarity effect is not removed by irrelevant
speech, when the remembered items are presented auditorily. This pattern of results forms an
important component of the attempt by Neath (in
press) to test the feature model of the irrelevant
speech effect.
An alternative interpretation of this pattern of
results is to suggest that, although subjects preferentially use phonological coding for immediate
serial recall, when performance drops below some
critical level, they abandon phonological coding
and attempt to use other strategies such as
semantic coding in the case of words, or using
initials or association in the case of letters. Baddeley (1966a,b) showed that, whereas immediate
memory for sequences of five words relied on
phonological coding, with little influence of
semantic similarity, for sequences of ten words the
pattern was completely reversed. The tendency
for acoustic coding to be abandoned at longer list
lengths was at the root of a major controversy
concerning the relationship between immediate
memory and reading. Initial research by Mann,
Liberman, and Shankweiler (1980) noted that
children who were identified as poor readers
tended to show reduced evidence of phonological
coding in immediate recall of consonant sequences. It subsequently emerged, however, that their
result was dependent on using similar list lengths
for both good and poor readers; poor readers tend
to have shorter memory spans, and hence were
operating at a much higher error rate than the
good readers. When performance was studied
across a range of sequence lengths, it became clear
that both good and poor readers coded phonologically, provided the sequences were not too far
beyond their span (Hall et al., 1983; Johnston,
Rugg, & Scott, 1987); a similar interaction
between length and phonological coding was
shown for a group of children with specific language impairment by Gathercole and Baddeley
(1990). However, while we regard strategy
switching as a plausible interpretation of the
existing results, given that the study by Salame
and Baddeley (1986) is the only report of acoustic
similarity effects for visually presented items in
the presence of irrelevant speech, there is clearly a
need to investigate this issue further.
A second problem with the initial working
memory model stemmed from the observation
that immediate serial recall was disrupted not only
by speech, but also by music (Salame & Baddeley,
1989), and even by a noise stimulus, when that
stimulus fluctuated not only in intensity, but also
in pitch (Salame 1990). The auditory filter
hypothesis could still handle these results, given a
plausible further assumption, namely that a filter
PHONOLOGICAL SIMILARITY/IRRELEVANT SPEECH
that admitted speech would be unlikely to be a
perfect filter, and hence would also allow in other
sounds that were speech-like in nature.
Jones and his colleagues have extensively
investigated kinds of material other than speech
that may disrupt serial recall. Jones and Macken
(1993) have shown that even pure tones are
capable of disrupting performance, provided they
fluctuate in time, whereas babble, in which many
speakers talk simultaneously to produce a relatively homogeneous background, does not (Hellbruck & Kilcher, 1993; Jones & Macken, 1995a;
Kilcher & Hellbruck, 1993). Furthermore, they
showed that much less disruption is caused by a
single repeated item, such as the letter C, than by a
set of letters, such as C, H, J, U, repeated randomly (Jones et al., 1992). Jones et al. proposed
what they term the changing state hypothesis to
account for these results. The changing state
hypothesis proposes that memory will be disrupted by any fluctuation in the state of the
auditory irrelevant stimulus.
While the changing state hypothesis specifies
the type of irrelevant stimulus that will disrupt
serial recall, it does not imply a specific mechanism. In suggesting a possible mechanism, Jones
(1993) abandons the idea of a specific phonological store in favour of what he terms the ObjectOriented Episodic Record (O-OER) hypothesis.
This proposes a common memory store that contains the to-be-remembered items together with
clues about their order of presentation. In the case
of auditory material, the ‘‘objects’’ are spoken
sounds which presumably may be identified by the
listener as words or possibly longer prosodic units
such as phrases and sentences. When irrelevant
speech is present, a different set of items and their
associated order cues also enter the memory store.
Memory disruption is attributed to the confusion
among the order cues within the store, rather than
to any disruption of the items being stored. Furthermore Jones and Macken (1995b) suggest that
if the spoken disrupting words are similar in
sound, then they will tend to cohere into a single
auditory object, causing much less interference
than the many objects represented by different
sounding words. This prediction was tested by
Jones and Macken (1995b, Experiment 3) in a
study in which the to-be-remembered lists of letters were either highly confusable (B, C, D, G, T,
V, and P), or different (F, K, L, M, Q, R, and Y).
The irrelevant speech comprised lists of seven
words that either rhymed with the confusable
letters or each of which rhymed with one of the
147
different letters, rhymed with each other but not
with any of the letters, or did not rhyme with any
of the letters or with each other. They observed no
effect of similarity between the letters being
recalled and the words being ignored. This result
is inconsistent with Salame and Baddeley’s (1982)
finding of reduced disruption of digit recall from
dysyllabic spoken words, one of the few pieces of
evidence in favour of a masking hypothesis. In
contrast, Jones and Macken found that acoustic
similarity within the list of irrelevant spoken
words reduced the amount of interference, as the
O-OER hypothesis had predicted.
We are left, therefore, with three questions
concerning the relationship between phonological similarity and the irrelevant speech effect.
The first is the question of whether irrelevant
speech abolishes the phonological similarity
effect for visually presented letter sequences
(Jones & Macken, 1995b; Surprenant et al.,
1999), or whether, as the acoustic coding strategy
hypothesis suggests, the effect will be found provided sequence lengths are relatively short
(Salame & Baddeley, 1986). The second issue
concerns the question of whether the irrelevant
speech effect is greater when the items remembered and those ignored are phonologically
similar. The third issue concerns whether phonological similarity within the sequence of letters
to be ignored will reduce the capacity of such
letters to disrupt recall, as the O-OER hypothesis predicts (Jones, 1993).
Our first study, therefore, involved combining
the recall of letters varying in degree of within-list
phonological similarity with the effect of disruption by irrelevant sequences that are similar or
dissimilar to those being recalled.
EXPERIMENT 1
We used two types of phonologically similar
sequences: one comprised the letters C, D, G, P, T,
and V, a second comprised the set F, L, M, N, S,
and X, while the dissimilar set comprised the letters B, F, H, J, Q, and R. These were combined
with irrelevant spoken words that were phonologically either similar or dissimilar to the remembered letters. This design allowed us to ask three
questions, namely whether an acoustic similarity
effect occurred and survived the effect of irrelevant speech, whether the similarity between the
irrelevant and the remembered material was an
important variable, and finally whether similar
148
LARSEN, BADDELEY, ANDRADE
words in the irrelevant speech caused less interference than dissimilar words.
Method
Participants. A total of 96 undergraduate
students, 44 men and 52 women, participated in
partial fulfilment of an Introductory Psychology
course requirement. Three sets of letters were
used as the to-be-remembered material. One set,
C, D, G, P, T, and V, all end with the long E sound.
Another set, F, L, M, N, S, and X, all start with the
short E sound. The final set, B, F, H, J, Q, and R,
do not have any sounds in common. The six letters
were each presented in the centre of the screen of
a personal computer for 600 ms, followed by a
blank screen for 240 ms. After the sixth letter
there was a delay of about 10 s, during which the
screen was blank. Then the word RECALL
appeared on the screen as a cue for the participants to write down the letters in the order they
had seen them. The computer created different
random orders of the letters for each trial for each
participant.
The irrelevant speech consisted of three sets of
six words. Words in one set, FEE, HE, KNEE,
LEE, ME, and SHE, end in a long E sound. A
second set of words, EBB, ECHO, EDGE, EGG,
ET, and ETCH, begin with a short E sound. The
third set of words, BAY, HOE, IT, ODD, SHY,
and UP do not have any sound in common and do
not have either a short E or long E sound. These
words were recorded in a female voice by computer in digitised sound files with 8-bit resolution
and a sampling rate of 11kHz. The files for all the
words were edited to be exactly the same length,
with silence being inserted at the end of a file
when necessary to make it the correct length. A
set of files of the same length containing no sound
was also created for use in the quiet condition.
Each file played for the 600 ms that the letter was
on the screen and there was no sound during the
240 ms delay between letters. The sound was
played at an average sound level of approximately
75 dB through a Labtec stereo computer speaker
system (Model CS-150) attached to the computer.
Each irrelevant speech condition and the quiet
condition was presented once in each block of four
trials, with the order within each block randomly
determined by the computer. For the trials with
irrelevant speech, the six words were presented
three times each, in a different random order on
each trial. As soon as each letter was displayed on
the screen, a sound file was played, making the
presentation of the letter and sound as close to
simultaneous as possible. The words continued to
be played at the same rate during the 10 s delay
period before the word RECALL appeared on the
screen as a cue to the participant to begin writing
down the letters.
Procedure. Participants were tested individually. They initiated each trial by pressing the
space bar. Each participant first did 12 practice
trials, followed by 80 experimental trials. Each
participant saw the same six letters throughout the
experiment so 32 participants studied each of the
three sets of letters. Participants were instructed
to try to ignore the words they heard and just
remember the order of the letters. As soon as the
word RECALL appeared on the screen, the participant wrote the six letters on an answer sheet.
The participant covered each response with a
masking paper before initiating the next trial. An
experimenter was present throughout the trials to
ensure that the participant did not begin to write
the letters prior to the RECALL cue and covered
the response to one trial before initiating the next
trial.
Results
The average number of letters per trial, out of six,
that participants wrote down in the correct serial
position with each of the different types of background, broken down by the set of letters participants studied, is shown in Table 1, along with
standard deviations.
The average number of letters recalled in the
TABLE 1
Experiment 1
Type of words
in irrelevant
speech
Long E
Short E
Unrelated
None
Sounds shared by letters to be
remembered
Long E
Short E
None
4.04 (1.16)
4.05 (1.17)
3.97 (1.09)
4.47 (1.03)
4.54 (1.04)
4.58 (1.08)
4.56 (1.15)
4.80 (1.01)
4.82 (0.90)
4.63 (1.06)
4.53 (0.96)
5.34 (0.65)
Average number (and standard deviations) of letters
remembered in the correct position in a list, out of six letters, in
Experiment 1.
N = 32 in each letter condition. All participants studied 20
lists in each background condition.
PHONOLOGICAL SIMILARITY/IRRELEVANT SPEECH
correct serial position was analysed with a 3 ´ 4
(letter set by irrelevant speech type) mixed factorial ANOVA with irrelevant speech type as the
within-subject factor. There was a main effect for
the letter set studied, F(2,93)= 4.324, p = .016.
Mauchly’s test indicated that sphericity could not
be assumed for irrelevant speech, so a multivariate
approach was used where irrelevant speech was
involved in the analysis. Pillai’s Trace showed that
there was a main effect for irrelevant speech,
F(3,91)= 22.144, p <.001, and an interaction
between letter set studied and irrelevant speech
type, F(6,184)= 2.467, p = .026.
In view of the significant interaction, a test of
simple main effects was conducted for each irrelevant speech condition to determine whether the
acoustic similarity effect was present. There was a
simple effect of the letter set studied in the long E,
unrelated, and quiet irrelevant speech conditions,
Fs(2,93)= 4.663, 3.084 and 7.384 with ps of .012,
.050 and .001 respectively. The simple effect of the
letter set studied in the short E irrelevant speech
condition was only marginally significant,
F(2,93)= 2.688, p = .073.
These simple main effects were further examined, using the Bonferroni correction to adjust for
multiple comparisons. In the presence of irrelevant speech consisting of words ending with the
long E sound, people who studied the unrelated
letters remembered more letters than those
studying the long E letters, p = .010, but the scores
of those studying the short E letters were not
different from either of these conditions. With
unrelatedwords, the differencein the recall of long
E letters and short E letters was marginally significant (p = .089). In the quiet condition, Dunnett’s C was used because there was not
homogeneity of variances in this condition. In
quiet, people who studied the unrelated letters
remembered more letters than those studying the
long E letters, p = .001, and marginally more than
those studying the short E letters, p = .063, but
those studying the short E letters were not different from the long E letter condition, p = .440. A
follow-up test for the marginally significant main
effect for letter set within short E irrelevant
speech, showed that the letter sets with the
greatest difference, the long E and the unrelated
letters, were not significantly different (p = .115).
Thus the acoustic similarity effect was not the
same across all of the irrelevant speech conditions.
In the long E, and quiet conditions, the unrelated
letters were remembered better than the long E
letters, and the short E letters were not different
149
from either letter set. In the other two irrelevant
speech conditions, the phonological similarity
effect was only marginally significant but there was
a trend in the data for better recall of the unrelated
letters than the long E letters here as well.
Next, the question of whether the effects of the
different types of irrelevant speech were similar
within each letter set was examined. There was a
simple effect for irrelevant speech conditions
within the long E letter set, F(3,93)= 10.539,
p <.001, and within the unrelated letter condition,
F(3,93)= 26.76 p <.001. Mauchly’s test indicated
that sphericity could not be assumed for the short
E letter set. Pillai’s Trace showed that, for short E
letters, there was not a simple effect for irrelevant
speech, F(3,29)= 1.674, p = .194.
When studying the long E letters, people
remembered better in the quiet condition than
with any of the kinds of irrelevant speech (long E,
p = .005, short E, p = .001, and unrelated words,
p <.001). None of the irrelevant speech conditions
differed from each other, providing no evidence
that similar sounding words will cohere into a
single auditory object. When studying unrelated
letters, people remembered the letters significantly better in the quiet condition than with
the long E, short E, and unrelated words irrelevant speech, all ps <.001. They also remembered
them better with long E words than with unrelated
words, p = .033, but there was no difference
between the long E and the short E, p = .290, or
the short E and the unrelated words, p = 1.00. This
does provide some evidence of auditory streaming, as predicted by the changing state hypothesis.
The prediction that the long E irrelevant
speech would interfere more with recall of the
long E letters and short E irrelevant speech would
interfere more with recall of the short E letters
was not confirmed. When only these two levels of
irrelevant speech and letter sets were examined,
the interaction was not significant, F(1,62) <1.
Discussion
The effects of both phonological similarity and
irrelevant speech were different, depending on the
type of letters people were trying to recall. Surprisingly, when people studied the letters starting
with the short E sound, there was no irrelevant
speech effect in any condition.
The prediction that long E irrelevant speech
would interfere more with recall of the long E
letters and that short E irrelevant speech would
150
LARSEN, BADDELEY, ANDRADE
interfere more with recall of the short E letters
was not confirmed.
The prediction of the O-OER hypothesis that
people will be better able to ignore irrelevant
speech if the words have a sound in common than
if they are all different was supported only when
people studied unrelated letters. Even then, this
difference was found only between words ending
in long E and unrelated words. Words starting
with short E did not show any tendency to cohere
into a unitary auditory object. There was also no
difference between any of the sets of words when
people studied the long E or the short E letters.
EXPERIMENT 2
Comments from participants indicated that they
used various mnemonic strategies to remember
the order of the letters. Certain letter combinations, such as TV and FM are meaningful because
they are common abbreviations in the language.
Other letter combinations were of personal significance to individual participants, such as the
initials of a friend. The logic of this experiment as
a test of the phonological masking hypothesis is
compromised if people were not remembering the
letters with a phonological code, but as the concepts for which letter combinations might stand. It
is possible that the degree of recoding may have
differed across letter sets, resulting in the absence
of phonological similarity effect for the letters
with a common initial sound, a result at variance
with earlier studies (Conrad & Hull, 1964). As the
presence of a standard phonological similarity
effect in the baseline control condition is necessary for a satisfactory test of our hypothesis, we
opted in Experiment 2 to concentrate on the long
E letter set, and to encourage our subjects to
encode phonologically.
One way to induce people to rely on a phonological code is to have people recite the letters
aloud while they are waiting for the recall signal.
Preliminary work suggested that people use one of
two rehearsal strategies. Some people read each
letter as it appeared and began to recite the string
of six letters in order only after they have seen and
read the sixth letter. Others rehearsed cumulatively, saying the first letter, then the first and
second letter as the second letter appeared, and so
forth. To keep the rehearsal strategy consistent,
we instructed participants to use the cumulative
strategy, providing training during the practice
trials.
To be sure that the participant’s own voice did
not mask the irrelevant speech, participants heard
the irrelevant speech through headphones. In this
study we manipulated similarity within subjects,
using the long E and dissimilar letter sets. There
were three irrelevant speech conditions, words
with the long E sound, words with no sounds in
common, and silence.
Method
Participants. A total of 22 undergraduate
students at John Carroll University, 13 men and 9
women, participated to partially fulfil a course
requirement.
Materials. The stimulus materials were the
same as those used in the long E and unrelated
letter conditions and the long E, unrelated and
quiet irrelevant speech conditions in Experiment
1. There were 14 trials for each letter in each
irrelevant speech condition. Participants heard the
irrelevant speech over Sony stereo headphones
(Model MDR-V200). For each participant the
computer determined a different random order of
trials of irrelevant speech conditions and letter set,
in blocks of six trials. The order of the letters
within each trial was also randomly determined
for each participant.
Procedure. Participants were instructed to
say each letter aloud as they saw it on the screen
and, after the first letter, to recite the previously
seen letters while seeing each new letter. During
the 12 practice trials participants were corrected if
they failed to follow this procedure. By the time
the fifth letter was being displayed, some subjects
could not speak fast enough to say all the letters
before the sixth letter appeared on the screen so
they were permitted to just add the sixth letter at
that point and to continue reciting the six letters in
order while waiting for the recall cue. Participants
were permitted to whisper the letters rather than
speak them loudly if they preferred, as long as the
experimenter could hear them reciting the letters.
Following 12 practice trials, each participant
completed 84 experimental trials.
Results and discussion
The results are shown in Table 2. A 2 ´ 3 (letter
set by irrelevant speech type) within-subject
ANOVA showed that there was a main effect for
PHONOLOGICAL SIMILARITY/IRRELEVANT SPEECH
TABL E 2
Experiment 2
Type of words
in irrelevant
speech
Sounds shared by letters to be
remembered
Long E
None
Overall
Long E
Unrelated
None
3.42 (1.00)
3.27 (0.86)
3.69 (0.87)
4.63 (0.76)
4.28 (0.89)
4.95 (0.46)
4.03 (0.73)
3.78 (0.81)
4.33 (0.55)
Overall
3.46 (0.86)
4.62 (0.65)
Average number (and standard deviations) of letters
remembered in the correct position in a list, out of six letters, in
Experiment 2.
N = 22 studied 14 lists in each background condition.
letter set F(1,21)= 54.342, p <.001. The expected
phonological similarity effect was found. Participants recalled more letters in the correct position
for the letters with no sounds in common than for
long E letters. There was a main effect for irrelevant speech condition, F(2,42)= 18.28.
Using the Bonferroni correction for multiple
comparisons to examine the irrelevant speech
conditions, participants recalled more letters in
quiet than in the long E, p = .002, or unrelated
word conditions, p <.001 and significantly more in
the long E than in the unrelated word condition,
p = .049. On this occasion, therefore, there was
support for the prediction of the O-OER
hypothesis that similar items would cohere into a
single auditory ‘‘object’’, hence causing less disruption of recall.
There was no significant interaction between
letter type and irrelevant speech conditions
F(2,42)= 1.41, p = .256. This absence of an interaction has two separate theoretical implications.
First of all, there is no evidence that similarity
between remembered items and irrelevant speech
influences performance, as the simple mnemonic
masking hypothesis predicted. Second, the failure
of the irrelevant speech effect to remove the effect
of phonological similarity, which is similar to the
finding of Surprenant et al. (1999) that the
phonological similarity effect is not abolished by
irrelevant speech when participants say the letters
as they see them, suggests that the relation of
irrelevant speech to articulatory suppression must
be more complicated than simply being the same
phenomenon.
Experiments 1 and 2 are consistent in showing
first of all that the effect of phonological similarity withstands the influence of irrelevant speech,
at least when sequence lengths are relatively
151
short. Second, we found no effect of phonological similarity between the items remembered
and those ignored, a result that replicates other
findings in the literature (Jones & Macken,
1995b; Le Compte & Shaibe, 1997). The question of whether phonologically similar streams
interfere less than dissimilar ones shows some
inconsistency between the two studies. Experiment 1 found a significant effect of similarity
within the unattended material for only one of
the three conditions, namely that involving recall
of unrelated letters, whereas an effect was found
for both types of remembered letters used in
Experiment 2. In an attempt to resolve the discrepancy we conducted a third experiment using
a new set of words that either shared a common
initial speech sound, had a common rhyme, or
were dissimilar. Each was combined with the
requirement to remember sequences of dissimilar consonants.
EXPERIMENT 3
The O-OER hypothesis predicts that strings of
phonologically related words are likely to cohere
into a single object, hence causing less interference. In Experiment 3, in order to optimise the
chance of producing an effect, we used two types
of similar words, one list having a common rhyme
such as LAY and DAY, and another list having a
common onset such as ALE and AID. It has been
shown by Conrad using consonants, that similarity
of both onset and rhyme influences both perceptual errors of auditorily presented letters and
immediate recall of letter sequences (Conrad,
1964; Conrad & Hull, 1964). We chose words
made up by re-ordering approximately the same
phonemes as occurred in each of the words on the
other list, so that differences other than the onset–
rhyme difference were minimal. In addition to
these two sets of similar irrelevant material, there
were also irrelevant sequences that had no sounds
in common, and a quiet control condition. The
O-OER hypothesis would presumably predict
that both of the similar irrelevant sequences
would interfere less than the sequence comprising
many different sounds.
Method
Participants. A total of 39 undergraduate
students at John Carroll University participated in
partial fulfilment of a course requirement.
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LARSEN, BADDELEY, ANDRADE
Materials. The irrelevant speech was recorded in the manner described for Experiment 1.
The rhyming words were DAY, JAY, LAY,
MAY, PAY, and SAY, the corresponding words
with similar onset were AID, AGE, ALE, AIM,
APE, and ACE, while the dissimilar words, with
minimal phonemic overlap, were HOE, IT,
KNEE, ODD, SHY, and UP. The material to be
remembered comprised the letters B, F, H, K, Q,
and R.
Procedure. The letters were presented
visually as in Experiments 1 and 2, and the irrelevant speech was presented over headphones as
in Experiment 2. After 12 practice trials, participants did 80 trials with the three types of irrelevant speech and silence, randomised within blocks
of four trials.
Results and Discussion
Table 3 shows the mean number of letters correct
out of six for each of the four conditions, along
with the standard deviations. A one-way withinsubject ANOVA indicated a significant effect of
conditions, F(3,114)= 27.80, p <.001. Follow-up
tests with the Bonferroni correction for multiple
comparisons showed that more letters were
remembered in the quiet condition than in each
irrelevant speech condition, all ps <.001, but that
there was no difference in the number of letters
recalled in the three irrelevant speech conditions.
Failure to observe an effect of similarity of either
onset or rhyme on the magnitude of the effect of
irrelevant speech failed to support the O-OER
hypothesis.
TABLE 3
Experiment 3
Type of words in
irrelevant speech
Similar onset
Rhyming
Dissimilar
None
Mean
SD
4.80
4.78
4.69
5.37
0.74
0.75
0.80
0.51
Average number (and standard deviation) of
letters remembered in the correct position in a
list, out of six unrelated letters, in Experiment 3.
The 39 participants studied 20 lists in each
background condition.
GENERAL DISCUSSION
Experiments 1 and 2 corroborate the results of
Jones and Macken (1995b) and Le Compte and
Shaibe (1997) in finding no influence on the
magnitude of the irrelevant speech effect of the
degree of phonological similarity between the
material to be remembered and the items being
ignored. In the present study, similarity was
maximised within and between both remembered
and ignored sets. It therefore seems unlikely that
the previous failures of Jones and Macken to find
an effect could be attributable to a lower degree of
similarity obtainable using a design in which
similarity within lists is minimised. Finally, the
failure by Le Compte and Shaibe (1997) to replicate the observation by Salame and Baddeley
(1982) of reduced disruption in memory for digit
sequences when irrelevant material was disyllabic
removes even this rather slender support for the
masking hypothesis. The mnemonic masking
model was introduced largely because it was one
of the apparently few detailed explanations possible within the limited specification of the phonological loop model. As the model contained no
explanation of how serial order was retained, it
was clearly not in a position to give a detailed
interpretation of the irrelevant speech effect in
terms of the serial order component of the process, unlike the approach of Jones which emphasises serial order as a primary feature. However,
the nature of the influence of phonological similarity was itself largely unspecified, and paradoxically, a rival interpretation presented by
Neath (in press) offers a way in which the
mnemonic masking hypothesis could in fact be
preserved, despite the clear lack of any effect of
similarity between the remembered and ignored
material.
Neath (in press) has used the feature model
(Nairne, 1990; Neath & Nairne, 1995) to produce
simulated data that are similar to the effects of
irrelevant speech on serial recall. The feature
model assumes that the memory trace of an item is
carried out by a number of features, making up the
cue in primary memory. Forgetting occurs
through interference when the cue’s features are
re-set by disrupting material, which might result
from either overwriting by subsequent items or
feature adaptation from irrelevant speech. At first
sight this seems very similar to the mnemonic
masking hypothesis proposed by Salame and
Baddeley (1982), but it differs in one crucial way.
The degree of disruption is assumed to be deter-
PHONOLOGICAL SIMILARITY/IRRELEVANT SPEECH
mined by the overlap between the specific
features in the item to be remembered, and those
in the irrelevant sound. Note that when the
remembered and irrelevant items are acoustically
similar, the maximal overlap, and hence maximal
interference, will occur on the features that are
common, in the present case, the vowel sounds.
Note also that these are highly redundant. Thus,
similar irrelevant speech will differ from dissimilar
irrelevant speech only in its capacity to disrupt the
common, and hence totally redundant, vowel
sound. Those components of the memory trace
that are not dependent on the common vowel will
presumably be disrupted just the same amount by
similar and dissimilar words. The overall disruption is thus likely to be approximately the same in
the similar and dissimilar irrelevant speech conditions.
The crucial difference therefore between
model used by Neath (in press) and the unspecified interference process assumed by Salame and
Baddeley (1982) is thus in the detailed assumptions as to the way in which the mnemonic
masking might occur. Salame and Baddeley made
the apparently plausible assumption that similar
items would mask more than dissimilar, whereas
Neath and Nairne’s model defines similarity much
more precisely. There is, of course, no reason why
Salame and Baddeley could not simply accept
Neath’s interpretation of the effect of similarity,
and hence continue to maintain a modified
mnemonic masking hypothesis.
Although such a step might preserve the
mnemonic masking hypothesis, it emphasises the
danger of opting for simple verbal models of rich
and complex tasks such as serial memory span. A
more constructive step for a phonological loop
model might be to attempt to incorporate the data
on similarity within a more complete and more
precisely specified model of serial recall—one that
at the very least has the capacity to explain how
serial order is maintained, a problem that has
never been adequately tackled within the original
phonological loop hypothesis. We will return to
this issue after considering what other constraints
the evidence from our three experiments might
place on theories of the subsequent irrelevant
speech effect.
All three experiments showed effects of both
irrelevant speech and of phonological similarity in
the material to be recalled, although the detailed
patterns were not always straightforward. In
Experiment 1, for example, while the long vowel
sound items consistently produced a phonological
153
similarity effect, the short vowel sounds were
much less clear, both in the effect of phonological
similarity and in their interaction with other
variables. We suspect that this might be because
the particular letters (F, L, M, N, S, and X) could
have lent themselves to semantic coding of the
type mentioned by some of the subjects in this
experiment, to a greater extent than the letters
with long E sounds, although it is also possible that
they may simply have been less mutually phonologically similar. In the case of the long E letters
however, a clear and significant difference from
dissimilar letters occurred in the silent control
conditions for both Experiments 1 and 2.
A second question concerns the influence of
irrelevant speech on this difference. It may be
recalled that Colle and Welsh (1976) found no
acoustic similarity effect in the presence of irrelevant speech, an effect also reported by Jones and
Macken (1995b), leading Surprenant et al. (1999)
to conclude that irrelevant speech obliterates the
phonological similarity effect for visually presented items. In contrast, Salame and Baddeley
(1986) report a slightly more complex picture,
whereby the effects of both irrelevant speech and
acoustic similarity interact with list length. For
short lists, neither is present because of ceiling
effects. When lists become long enough for errors
to appear, then clear and additive effects of similarity and irrelevant speech are found, both disappearing when list length increases. It is
suggested that this interaction with list length
reflects a tendency for subjects to abandon
phonological coding when their performance
drops beyond a critical point, an effect that has
been observed in a number of other comparable
situations (Gathercole & Baddeley, 1990; Hall et
al., 1983). The presence of a clear effect of
phonological similarity across irrelevant speech
conditions in both Experiments 1 and 2 lends
further weight to the conclusion that, at moderate
list lengths, effects of both phonological similarity
and irrelevant speech are detectable.
A third issue concerns the effect of phonological similarity among the irrelevant spoken items.
The phonological masking hypothesis makes no
prediction on this point, whereas the O-OER
hypothesis predicts that similar items will be
combined into a smaller number of auditory
objects, hence leading to less disruption. Our
evidence on this point is somewhat mixed. In
Experiment 1, the only statistically reliable effect
of similarity in the unattended words occurred
when subjects were studying dissimilar letters,
154
LARSEN, BADDELEY, ANDRADE
when subjects recalled significantly more items
when ignoring the long E words than when
ignoring dissimilar words (p = .033). Four of the
other five potential comparisons were in the right
direction, suggesting the possibility of a genuine
but rather weak effect. A statistically significant
effect was observed in Experiment 2, but was
notably absent from Experiment 3, which focused
on this issue. The most likely interpretation of this
rather mixed pattern of results would seem to be
that an effect probably does exist but is far from
robust. If, as Jones plausibly argues, the effect
stems from a tendency for subjects to incorporate
similar letters into a unitary auditory percept, then
it is not implausible to assume that this may be a
relatively fragile effect which requires more
careful control than simply specifying the nature
of the irrelevant material. It clearly merits more
detailed investigation.
What are the broader theoretical implications
of our results? One should perhaps begin by noting
the comparative dearth of carefully worked out
models of the irrelevant speech effect. This is most
clearly illustrated in the case of the phonological
loop interpretation, which had virtually no
detailed specification at the modelling level. As we
have seen, even the tentative suggestion of some
form of mnemonic masking (Salame & Baddeley,
1982), which was subsequently discarded (Salame
& Baddeley, 1986), does not give rise to clear
predictions in the absence of a specific model of
the way in which similarity influences performance. Hence the model is consistent with either
an interaction, as suggested by Salame and Baddeley (1982), or with the absence of such an
interaction, in the case of a feature-based interpretation of similarity such as that proposed by
Neath (in press). Clearly, the phonological loop
hypothesis is in need of much more precise
specification. Fortunately, clearly specifiedmodels
are now beginning to appear (Burgess & Hitch,
1996; Henson, 1998; Page & Norris, 1998a,b), and
it is surely only a matter of time before they are
applied to the irrelevant sound effect.
The O-OER hypothesis proposed by Jones
(1993) meets with mixed success in accounting for
the results obtained. Experiment 2 clearly supported the prediction that similar irrelevant items
would disrupt performance less than dissimilar,
but only mixed support was provided by the first
experiment, while Experiment 3 clearly did not
support the hypothesis. Furthermore, although
the O-OER hypothesis explicitly refers to serial
order, the exact manner in which it operates
remains poorly specified, making it difficult to test
directly. The model does, however, have two
assumptions that explicitly differentiate it from
the phonological loop approach. The first concerns the assumption that the effects of irrelevant
speech and of articulatory suppression have a
common source, an assumption that is also made
by Macken and Jones (1995) and also Neath (in
press). The second concerns the assumption that
both visual and verbal memory have a similar
basis (Jones, Farrand, Stuart, & Morris, 1995).
Although we take issue with both of these
assumptions, discussion is beyond the limits of the
present study.
A third theoretical approach is that recently
outlined by Neath (in press). As this implementation of the feature model expressly predicts
that the effects of phonological similarity on recall
of visually presented sequences will be abolished
by irrelevant speech, our finding of a phonological
similarity effect in some of the irrelevant speech
conditions in Experiment 1 appears to be inconsistent with this model.
Although our data do not offer resounding
support for either a masking hypothesis or the
O-OER hypothesis, the clear absence of an
interaction between the similarity within the
remembered and irrelevant lists clearly favours
the latter hypothesis over the Salame and Baddeley (1982) version of mnemonic masking.
Furthermore, although the effects are relatively
weak, the tendency in several conditions for
similar irrelevant items to disrupt recall less also
offers some support to Jones’s position. However,
while in the absence of well worked-out alternative explanations of the irrelevant speech effect
the O-OER hypothesis is clearly the front-runner,
if one moves beyond this phenomenon, there are a
number of reasons for being reluctant to abandon
the phonological loop model in favour of the
O-OER hypothesis. First of all, the phonological
loop hypothesis gives a good account of a range of
effects including those of phonological similarity,
word length, articulatory suppression, and the
interaction of these variables. It is not clear that
the O-OER hypothesis can give a good account of
this rich array of robust results. For example, in
attempting to extend the generality of the O-OER
hypothesis, Macken and Jones (1995) have argued
for the functional equivalence of articulatory
suppression and the irrelevant speech effect.
However, Gupta and MacWhinney (1995) have
presented evidence for a separation of these two
effects. Furthermore, if as Jones (1993) proposes,
PHONOLOGICAL SIMILARITY/IRRELEVANT SPEECH
repetition of a single unattended item has virtually
no effect on concurrent memory performance,
then one might expect that articulatory suppression would be ineffective when a single item is
repeated. However, this is one of the most common methods of suppression, with frequent
repetition of a single word such as ‘‘the’’ causing
marked disruption (e.g., Murray, 1968).
A further extension of the O-OER hypothesis
has been to suggest that visual and verbal memory
may have a similar basis, with both showing evidence of disruption from concurrent activity
(Jones et al., 1995). However, such a view is quite
inconsistent with the neuropsychological evidence, which indicates that digit span may be
disrupted while spatial span is preserved (Basso,
Spinnler, Vallar, & Zanobio, 1982), and performance on the spatial Corsi block span test may be
impaired while digit span is well preserved
(Hanley, Young, & Pearson, 1991). Further evidence for a dissociation between verbal and
spatial span comes from the recent work contrasting performance on visual and spatial
immediate memory of people with Downs Syndrome, for whom verbal span is particularly poor
relative to spatial span, and those with Williams
Syndrome who show the opposite pattern (Wang
& Bellugi, 1994). Finally, PET studies suggest that
quite different areas of the brain are involved in
visual and verbal immediate memory performance (see Smith, Jonides, & Koeppe, 1996 for a
review).
The failure of the O-OER hypothesis to
account for the neuropsychological and other
interference data may, of course, reflect incompleteness rather than inadequacy. While there is
abundant evidence for separate visual and auditory memory systems, there is also evidence for a
degree of interaction, such that serial verbal recall
of visually presented material may be influenced
by such characteristics as visual complexity
(Chincotta & Underwood, 1997) and visual similarity (Della Sala et al., 1999). Similarly, it is clear
that verbal serial recall can be influenced by
semantic (Baddeley, 1970; Baddeley & Levy,
1971) and lexical factors (Hulme, Maughan, &
Brown, 1991). In so far as working memory is
capable of storing and manipulating multimodal
information, then there will be a need for a level of
storage that extends beyond the specific inputs.
As discussed earlier, our data are still compatible with a modified mnemonic masking
hypothesis, but even if this in turn were disproved,
it would not make it necessary to abandon the
155
whole phonological loop model of verbal recall.
One could concur with Jones’s proposal that the
irrelevant speech effect operates through disruption of order cues, and still maintain the remainder of the model. The simple verbal model of
working memory, presented by Baddeley (1986),
does not contain a mechanism capable of reproducing serial order, and this omission has stimulated a number of recent attempts to provide a
more detailed specification of the phonological
loop.
The first such model to be formulated was that
of Burgess and Hitch (1992). This model explains
serial order by separating out the phonological
store from the series of contextual cues that
specify the order in which the items are retrieved.
As a result of criticism of some of its assumptions,
the model has been modified (Burgess & Hitch,
1996), but both models separate the storage of
item information from that of order, a process that
relies on contextual cues. It is entirely plausible to
assume that irrelevant speech might disrupt this
process of contextual order cueing. It remains to
be seen whether this model can give an account of
the full range of irrelevant speech data.
Another attempt to give an account of serial
verbal recall is provided by the primacy model of
Page and Norris (1998a,b). This also involves two
separate stages, one in which the incoming items
are associated with a cue representing the beginning of the list, while the second is a readout
mechanism that selects the strongest trace,
reproduces it, and subsequently inhibits it to prevent its reappearance as an intrusion. Logically,
the irrelevant speech effect could occur at either
the item store or the read-out stage. It should be
possible to use the model to explain irrelevant
speech effects, once there is general agreement
about the phenomena that must be explained.
What are these phenomena? Unfortunately the
area has so far suffered from being studied by a
sequence of different investigators. First Colle,
then Salame and Baddeley, more recently Jones,
and subsequently Le Compte have each tended to
concentrate on a somewhat different question,
and to use their own favoured techniques. Consequently, while there is no doubt about the
robustness of the basic phenomena, there has
simply not yet been sufficient cross-laboratory
replication of some of the subtler effects. The
present study suggests that there is clearly a need
for a concerted effort to establish which effects are
sufficiently robust to be used to test the computational models, and which should be addressed
156
LARSEN, BADDELEY, ANDRADE
when we have a better understanding of the
empirical phenomena. Fortunately, there are signs
that the irrelevant speech or sound effect is being
studied in an increasingly wide range of laboratories, suggesting that it should not be long before
we achieve a sufficiently rich and detailed data
base to allow the various computational models to
be adequately tested.
Manuscript received 10 June 1998
Manuscript accepted 3 November 1999
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