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Different memory functions for consonants and vowels
COGNITIVE
Different
PSYCHOLOGY
Memory
4, 39-54 (1973)
Functions
for Consonants
and
Vowels’
RONALD A. COLE* Uniuersity
of Waterloo
Different memory functions were obtained for consonants (C) and vowels (V) in a serial recall task. In general, the most recently heard vowels in a sequence were easier to recall than the most recently heard consonants. This effect was observed for auditorily presented sequences of CV or VC syllables, but was not observed for visually presented stimuli. The results were explained in terms of a limited capacity acoustic storage in which vowels are preserved longer than consonants. Retrieval of the last vowels from this storage was presumed to cause the vowel recency effect.
There are several lines of evidence suggesting that consonants and vowels are perceived differently. Experiments at Haskins laboratories using synthetically produced speech have demonstrated that perception of certain consonants is categorical while vowel perception is continuous ( Liberman, Cooper, Shankweiler, & Studdert-Kennedy, 1967) .. For stop consonants, a listener perceives a range of acoustic stimuli as a single phoneme, and is unable to discriminate acoustic variations within this range. On the other hand, vowel perception is continuous in that the continuous change of an acoustic cue results in a corresponding perceptual change in the vowel sound. Other differences in consonant and vowel perception have been observed during dichotic listening tasks (Shankweiler & Studdert-Kennedy, 1966; Kimura, 1967). In general, when subjects are simultane’ously presented with a pair of consonants, one to each ear, there is a clear superiority in identifying those presented to the right ear (the effect is reversed for subjects with a dominant right hemisphere). Pairs of dichotically presented vowels, however, show a slight and usually nonsignificant advantage for those presented to the left ear. No experimental evidence has yet been produced that consonants and vowels are coded differently in short-term memory, despite the intensive ‘I gratefully acknowledge the assistance of Miss Marka Adams, Miss Michelle Harwayne, and especially Miss Andrea Torres for their help in data collection and analysis in Expts. I and II. *Send reprint requests to: Department of Psychology, University of Waterloo, Waterloo, Ontario, Canada. 39 Copyright @ 1973 by Academic Press, Inc. All rights of reproduction in any form reserved.
40
RONALD
A.
COLE
investigation of human information processing in recent years. In fact, such evidence as presently exists suggests that both consonants and vowels are coded in short-term memory as sets of binary distinctive features ( Wickelgren, 1965, 1966; Sales, Haber & Cole, 1968, 1969). Hdwever, these experiments do not rule out the possibility that consonants and vowels are processed differently in short-term memory since (1) only interphonemic intrusions were examined in these studies while over-all errors and serial position effects were ignored, and (2) recall performance was not compared for consonants and vowels presented under identical conditions. The present study was performed in order to determine whether consonant-vowel differences observed during phoneme recognition may be observed in short-term memory. EXPERIMENT
I
Method Subjects. One hundred and twenty students from the University of California at Riverside served as Ss. These Ss received credit toward their grade in a psychology course. Each S was pretested to ascertain whether he could discriminate among the stimuli. Six potential Ss were eliminated because they were unable to perform the /a/ /a/ distinction. Each S was tested individually in a small room with E. Design. Experimental Groups were defined by the number of syllables presented in a recall sequence (five, six, or seven) and the type of phonemic change that occurred within the sequence. In consonant groups each consonant was paired with /a/, so that only the consonant was heard to change between successive syllables. In vowel groups each vowel was paired with Id/, so that only the vowel was heard to change from syllable to syllable. Thus, the variables sequence-type and sequencelength were combined in a 2 x 3 factorial design to yield six experimental groups: five, six, or seven consonants followed by /al and five, six, or seven vowels preceded by Id/. Stimuli. Seven consonant and seven vowel phonemes were used as stimuli: Id/, IS/, lml, IdI, In/, lsl, lpl and Ii/, lel, Ire/, lul, la/, IO/, 131. Each consonant was followed by /a/ while each vowel was preced,ed by Id/, so that subjects heard the syllables da, sha, ma, Ba (as in think), na, sa, pa, or dee, day, daeh (as in dad), doo, dah, doe, daw. The seven consonants and the seven vowels were used to construct six sets of 98 sequences of five, six, or seven syllables per sequence. The 98 sequences in each group were constructed to meet the following rules of occurrence: ( 1) each phoneme was heard at most once per sequence, (2) each phoneme was presented an equal number of times in each
CONSONANT
AND
VOWEL
41
RECALL
serial position, and (3) each phoneme was presented an equal number of times in the same sequence with each other phoneme. The 98 recall sequences were randomly ordered for each group. These sequences were then recorded in a male voice at the rate of three syllables every 2 sec. Thus, each subject in a given group heard the recall sequences in the same order. Procedure. Twenty Ss were randomly assigned to each experimental group. Each S was tested individually in a session lasting approximately 1 hr. Subjects were presented with a series of sounds by means of a Wollensak tape recorder. After each trial, the tape recorder was stopped by means of a footswitch controlled by E. Subjects were told that, upon hearing the sound of the footswitch, they should verbally report the sounds in the order in which they heard them. If they were unable to recall a sound in a given position, they were allowed to guess. All responses were recorded by E after each trial, and each experimental session was tape recorded. After three practice trials, the experiment was begun. Scoring. For all experiments reported here, any sound not reported in its correct serial position was scored as an error. This included transpositions among sounds presented in the same sequence, errors in which S substituted sounds not heard in the sequence, and omission errors, where S simply responded “blank.” Results Figure 1 displays recall performance for consonants and vowels in each group as a function of serial position. Analyses of variance computed between the consonant and vowel groups at each sequence-length revealed that: (1) vowel recall was superior to consonant recall for sequences of five and six syllables ( p < .05), (2) recall varied as a function of serial position ( p < .Ol in all cases), and (3) there was a greater recency effect for vowels than for consonants (p < .Ol in all cases). This interaction between sequence-type and serial position is shown in Fig. Ia as the divergence of the two curves in the later serial positions.3 In order to directly compare the serial position effects for consonant and vowel sequences, the recall errors for each serial position were calculated for each S as a percentage of the total errors made by that S. This manipulation allows a direct comparison of serial position effects between consonant and vowel sequences when overall errors are equated for the two sequence-types ( McCrary & Hunter, 1953). The normalized ’ Robert Crowder, results of Expt. I.
working
at Yale University,
has independently
replicated
the
42
RONALD
A.
COLE
2
b i’?\
,.--.’
-..
7, I’ 12
4’
’
I----
I-.
1234567 Position
FIG. la. Mean number of consonant and vowel errors at each serial position for sequences of five, six, and seven syllables. lb. Percentage of total number of consonant and vowel errors at each serial position for sequences of five, six, and seven syllables.
serial position curves shown in Fig. lb reveal that the greater recency effect for vowels is restricted to the final serial position of the sequence. Previous experiments have revealed that consonants and vowels are coded in STM as sets of distinctive features (Cole, 1970; Cole, 1971). If Ss in this experiment discriminated among phonemes in STM by their distinctive features as shown in Table 1, then intrusion errors should occur most frequently between phonemes having the same distinctive features. For example, in Table 1, /i/ and /e/ differ only by their feature value on the “grave” dimension. If this feature is forgotten while /i/ is coded in STM, S is likely to intrude /e/ for /i/ on the basis of the remaining feature information. In the present experiment, the greater number of consonant errors in the final serial positions of a sequence may reflect a greater amount of distinctive feature forgetting for these phonemes. If more consonant errors occur because of a greater loss of feature information, then distinctive feature theories should less accurately predict intrusion errors for consonant phonemes. This prediction is based on the reasoning that, when several (or all) features are forgotten for a given phoneme, S retains so little feature information about that phoneme that intrusion errors are randomly determined. Therefore, distinctive feature theories
CONSONANT
Distinctive
Feature
AND
VOWEL
43
RECALL
TABLE 1 Composition of Phonemes used in this Experiment Halle’s (1962) Distinctive Feature System
I based
on
Vowel phonemes /V
/e/
he/
/u/
/a/
/o/
b/
-
-
-
+ -
-
-
+ + +
+ -
+ -
+ + -
+
+
+
+
/s/
/P/
+ + -
Dimension Flat Compact Grave Diffuse
Consonant
phonemes
/d/
/8/
/m/
/el
/n/
-
-
+
-
-
-
+ -
+ -
+ -
+ -
+ -
+ +
+ -
+ +
+ + + -
Dimension Grave Diffuse Strident Nasal Contindant Voiced
+
+ -
should less accurately predict the rank order of intrusion errors for consonant phonemes than for vowel phonemes in the final serial positions of a sequence. In order to test this hypothesis, the rank order of presented phonemes in each experimental group was calculated for each intruded phoneme for each serial position. This manipulation yielded five, six, or seven separate confusion matrices for each group (C5, C6, C7, and V5, V6, V7) depending upon the number of phonemes presented for recall in that group. Halle’s distinctive feature system was then used to generate the expected rank order of intrusion errors for each phoneme. These predictions were based on the assumption that phonemes sharing the most identical distinctive features are most likely to be intruded for each other (see Sales, Haber, & Cole, 1968, for a more detailed description of this analysis). The ability of Halle’s distinctive feature system to predict the rank order of intrusion errors for consonant and vowel phonemes in each serial position is shown for each group in Fig. 2. This figure reveals that Halle’s distinctive feature system predicted the pattern of intrusion errors with greater than chance (50%) accuracy in all serial positions. However, intrusion errors were less accurately predicted for consonants than for
RONALD
A. COLE
-Vowels 0-
- l
1
Consonants
2
3 Serial
4
5
4
7
Position
FIG. 2. Accuracy with which Halle’s distinctive feature system predicts the frequency of intrusion errors at each serial position for consonants and vowels in sequences of five, six, and seven syllables.
vowels in the final serial positions in sequences of five and six syllables. In general, the analysis of intrusion errors supports the hypothesis that both consonants and vowels are coded in STM as sets of distinctive features at each serial position, and that more feature forgetting occurs for consonants than for vowels in the final serial positions of a recall sequence. These data do not reflect on the adequacy of Halle’s distinctive feature system, since subjects may remember phonemes by specific combinations of distinctive features (Sales, Haber & Cole, 1968). However,
CONSONANT
AND
VOWEL
RECALL
45
the analysis does reveal a tendency for Halle’s distinctive feature system to predict vowel intrusions in the later serial positions of a sequence more accurately than consonant intrusions, and this finding is suggestive, since it was predicted from the data presented in Fig. 1. EXPERIMENT
II
Because a clear difference was found between consonant and vowel recall in short-term memory, a second experiment was performed in order to test the effect of the syllable structure of a sequence on the recall of consonants and vowels. To this end, sequences were constructed where both the consonant and vowel phonemes were randomly varied from syllable to syllable within a sequence (e.g., /di pe mu 00 so/ ). TWO possibilities suggest themselves. If consonant and vowel phonemes are coded independently of their syllable context, then the final vowel in a sequence should be remembered better than the final consonant when both are presented in the same syllable. Thus, identical recall curves should be found for the consonants and vowels in Expts. I and II. On the other hand, if the syllable provides a strong cohesive or associative influence on its component phonemes, then recall performance should be similar for consonants and vowels in the same syllable. Method Subjects. Sixty students from the University of California at Riverside served as Ss. Each S was subjected to the identical pretest procedures as in Expt. I. Stimuli. The same consonant and vowel stimuli were used as in Expt. I. Lists of 98 trials were again constructed according to the rules of occurrence outlined previously. The additional requirement was made that each consonant and vowel phoneme was paired the same number of times in each serial position. Thus, each S heard each consonant-vowel combination twice in each serial position. Design. Twenty Ss were randomly assigned to each of three experimental groups that differed by the number of syllables in each sequence. The three groups were presented with 98 sequences of either five, six, or seven syllables (depending on the group) in which both the consonant and vowel phoneme were varied between each syllable. Procedure. For all three groups, the experimental procedure was identical for Expts. I and II. Results the The results of Expt. II are illustrated in Fig. 3a which &splays serial position curves for consonants and vowels that were paired in
46
RONALD
A.
COLE
*-Vowel --- Consonant b _-24. #’ ,’ ,.----*----mm 7’ n I’ d’ 20. .y.,-•;,” -I\
60- a
.*
w
40.
12.
60 40. 20.
‘12
~ ,F----.---------Lmmm :* , ------\ ;:: /v c’ 3
4
5
6
y-y 1
7
2
3
4
5.
6
7
Position FIG. 3a. Mean number of consonant and vowel errors at each serial position for sequences of five, six, or seven CV syllables. 3b. Percentage of total consonant and vowel errors at each serial position for sequences of five, six, or seven CV syllables.
syllables in the same sequence. Thus, Fig. 3 represents a within-subjects comparison, while Fig. 1 represents a between-subjects comparison. There was a significant effect of phoneme type (consonant vs vowel, p < .05), serial position ( p < .Ol), and a significant interaction between phoneme type and serial position (p < .Ol) for all three experimental groups. The normalized serial position curves shown in Fig. 3b reveal that the greater recency effect for vowels was again restricted to the last serial position in the sequence for strings of five, six, or seven different syllables. EXPERIMENT
III
The results of Expts. I and II reveal a clear superiority in the recall of vowel phonemes heard in the later serial positions of a sequence. Since only CV syllables were heard in these experiments, it is possible that superior recall for terminal vowel phonemes is due to the fact that a vowel phoneme is the most recently heard stimulus in each sequence. Experiment III tests this hypothesis by comparing recall performance for consonant and vowel phonemes in both CV and VC sequences.
CONSONANT
AND
VOWEL
RECALL
47
Method Subjects. Twenty students from the University of Waterloo served as Ss. These Ss received credit toward their grade in a psychology course. Each S was tested individually in a small room with E in a session lasting approximately a half hour. Stimuli. Six consonant and six vowel phonemes were used as stimuli: Id/, 19, h-4, InI, Is/, lpl, and lil, lel, lad, lul, loI, IDI. These phonemes were used to construct a set of 72 six-syllable CV sequences such that each phoneme was paired in a syllable with every other phoneme an equal number of times in each serial position. The 72 sequences were randomly ordered and recorded by E at the rate of three syllables every 2 sec. The set of VC sequences were then recorded at the same rate by reversing the order of the phonemes in each syllable. Procedure. Ten Ss were randomly assigned to one of two experimental groups which differed only in the order in which the consonant and vowel phonemes were heard in each syllable (CV or VC). Each S was tested for his ability to discriminate among the phonemes by tapping on a table immediately upon hearing a given phoneme in a sequence of syllables. Each S was tested on three successive sequences for each phoneme, and no perceptual errors were made in either the CV or VC condition. While each syllable was perfectly clear, it is interesting to note that consonants in VC syllables subjectively appear to be less distinct. Subjects were tested in a procedure identical to Expts. I and II, except for one modification. After each trial E manually stopped the tape by depressing the “pause” switch on the side of the recorder. This procedure was used to eliminate the slight possibility that the “click” produced by the mechanical footswitch was masking the final consonant sound in each sequence. Each S was seated next to the tape recorder, and was instructed to report back the sounds as soon as the tape recorder was stopped after each trial. Results Figure 4a displays the recall performance for consonants and vowels in each serial position in CV and VC sequences. Analysis of variance revealed no difference between the number of errors in CV and VC sequences. However, relatively more consonant errors occurred in VC than CV sequences, especially in the initial serial positions (phoneme X sequence-type X position, p < .05). Separate analyses of variance for the errors in each sequence-type revealed a main effect of serial position and a significant interaction between phoneme-type and serial position for both sequence-types. Thus, for both CV and VC sequences, vowels
RONALD
A.
COLE
a
b
40 .
30 -
ZO-
35-
,’ 25 -
I
.’
,*---
--0,
cv
,I’
\\
\
‘\
‘.
*..----7 /
P E !? d
/’
I’ I’
15
5’
e--. consonant -Vowel
/ 1
2
l ---
‘\
cv
‘b
I
7
I’
I,
3
4
5
6
1
2
3
4
5
6
Position FIG. 4a. Mean number of consonant and vowel errors at each serial position for CV and VC sequences. 4b. Percentage of total consonant and vowel errors at each serial position for CV and VC sequences.
are recalled more often than consonants and this effect is greater in the final serial positions of the sequence. Figure 4b displays the normalized serial position curves for consonant and vowel errors in both sequence types. This figure reveals that the relative superiority of vowel recall was once again limited to the last serial position in the sequence. EXPERIMENT
IV
It has long been known that consonant and vowel phonemes have different acoustic properties. Consonants are characterized by transient wave forms and high frequency energies, while vowels are characterized by low-frequency energy of relatively longer duration. Experiment IV was designed to test the hypothesis that the observed differences in consonant and vowel recall requires that S hears the stimuli. Method Subjects. Fifteen undergraduate Waterloo served as Ss.
students
from
the
University
of
CONSONANT
AND
VOWEL
49
RECALL
Stimuli. The consonant phonemes /cl/, Is/, /ml, In/, Is/, lpl and the vowel phonemes /iI, /e/, /,d, /u/, lo/, la/ were combined into consonant-vowel syllables. These syllables were printed on slides. Seventytwo sequences of six CV syllables were constructed so that no phoneme was repeated on a given trial, and so that each CV syllable occurred an equal number of times in each serial position. A typical sequence of slides was “dee mu pay sho neh sa.” Procedure. Each S was pretested to insure correct pronunciation of each of the syllables. Subjects were shown sequences of six slides via a Kodak Carousel slide projector that was timed to present the slides at the rate of l/set. After the last syllable, S was instructed to verbally report the syllables in the order in which he had seen them. Results Figure 5 displays the mean number of consonant and vowel errors at each serial position. Analysis of variance revealed no significant difference between the number of consonant and vowel errors. A significant effect of serial position was observed ( p < .Ol ) as well as a significant interaction of phoneme-type and serial position (p < .05). While the
Posit
ion
FIG. 5. Mean number of consonant and vowel sequences of visually presented CV syllables.
errors
at each serial
position
for
50
RONALD
A.
COLE
curves in Fig. 5 do show a crossover between positions three and four, there is a clear lack of divergence between the two curves in positions five and six. Thus, the previously observed differences in consonant and vowel recall are not observed when CV syllables are presented visually. This was observed most clearly from the normalized data. For visually presented syllables, there is no longer any vowel recency effect in the final serial position of the sequence. EXPERIMENT
V
One model that may be used to account for differences in the immediate recall of consonants and vowels assumes that acoustic stimuli are preserved in a physical form in a sensory (acoustic) storage while they are simultaneously rehearsed in verbal memory ( Neisser, 1967;. Crowder, 1969). Thus, as each stimulus is heard, it is first preserved in acoustic storage, and then converted to a “name” code for rehearsal. Just prior to reporting the final stimuli in a sequence, S retrieves these stimuli from acoustic storage. Recency effects in short-term memory for spoken items are thus due to the retrieval of the final stimuli in a sequence from acoustic storage. This model will account for the results of the present experiments if it is assumed that vowels are preserved more efficiently (longer) than consonants in acoustic storage. In this case, superior recall of vowel phonemes in the final serial positions of a sequence is assumed to reflect the decay of the consonant phoneme prior to its retrieval from acoustic storage. Experiment V was designed to test the hypothesis that vowels are preserved longer than consonants in acoustic storage. The paradigm used was developed by Coltheart and Allard ( 1970). These authors discovered that subjects could decide faster that the second of two successively spoken letters was the same as the first when the letters were spoken in the same voice rather than a different voice (e.g., a Female P followed by a Male P). This finding suggests that subjects are able to respond to a physical or sensory representation of a letter before they are able to respond to the name of the letter. If the superior recall for vowels heard at the end of a sequence is due to longer storage of vowels in acoustic storage, then the faster reaction times for physically identical stimuli should be observed for vowels but not for consonants. Method Subjects. Twenty-two right-handed undergraduate students from the University of Waterloo served as Ss. Each S was tested in a single session lasting approximately 75 min.
CONSONANT
AND
VOWEL
RECALL
51
Stimuli. The consonants D, P, T, C and the vowels A, E, 0, U (pronounced “00”) were used as stimuli. The consonants were chosen because each is composed of an initial consonant phoneme followed by the vowel phoneme /i/. Thus, S must attend to the consonant portion of the letter in order to make a same-different judgment between any two of these letters. Identical experimental tapes were made for consonants and vowels. Tapes were made by recording each of theCtimuli in a male and female voice, and storing these sounds on disc tape in an IBM 366/44 computer. Each stimulus was exactly 700 msec long. An experimental trial consisted of a 766msec warning tone followed 2 set later by the first letter. The second letter was presented either 34,2, or 8 set after the offset of the first letter. A tone recorded on a second channel at the onset of the second letter was used to start a Hunter 100 msec timer. On half of the trials S was presented with two different letters, while the other half consisted of the same letter repeated twice. On those trials where the same letter was repeated twice, the two letters were presented in the same voice 56% of the time (e.g., Male D followed by Male D) and in a different voice 56% of the time (e.g., Male D followed by Female D). The set of 144 trials was randomly presented to each S. All variables were balanced at each ISI. Procedure. All stimuli were presented binaurally to S by means of a Sony model TC 546 tape recorder via Koss Pro4A stereo headphones. Each S was seated with a two-way switch between the thumb and index finger of his right hand. The S was instructed to move the switch to the right if the second letter had the same name as the first, and to the left if the two letters did not have the same name. Subjects were told that the voice in which the letter was heard was irrelevant to the task. After each trial, E recorded S’s reaction time. Results Figure 6 displays the reaction time (RT) to pairs of consonants and vowels having the same name (“different” responses are not considered here). This figure reveals that subjects were faster deciding that two spoken letters are the same when both letters were spoken in the same voice. Analysis of variance revealed a main effect of voice (p < 661) and a significant interaction between voice and IS1 (p < .Ol) which reflects the convergence of the two sets of curves at 2 set ISI. It is clear from Fig. 6 that Ss are able to respond to physical characteristics of either a consonant or vowel for at least 8 set after the offset of the physical stimulus.
52
RONALD
A. COLE
Vowels
700.-,-
--
-‘--
0-o
__----
650 E .c .-; t z a
600-
Consonants
650 600.
;-----q,
550 e- --different U-come
500’
8
2
95 lntcrstimulus
Voice Voice
Interval
(sec.)
FIG. 6. Reaction time to the second of two consonants same or different voice at three different delays.
or vowels
heard
in the
DISCUSSION
When S hears a series of syllables for ordered recall, he is more likely to remember the vowels than the consonants. This difference is most evident for vowels and consonants heard in the final syllable of a sequence, due to a sharp decrease in errors for the most recently heard vowel phoneme. This “vowel recency” effect appears to depend upon the acoustic nature of the stimuli, since the effect was not observed for visually presented syllables. The difference between consonant and vowel recall was orderly and stable for sequences of five, six, or seven syllables. Similar differences were observed whether recall was compared for sequences of consonants paired with la/ and sequences of vowels paired with Id/, or whether recall was compared for consonants and vowels presented in the same sequences as members of a CV syllable. Since the number of consonant or vowel intrusions in a sequence is unaffected by the syllable structure of the sequence, it is likely that phonemes are remembered independently of the syllable in which they are heard. It was proposed above that the final vowel in a sequence is preserved
CONSONANT
AND
VOWEL
53
RECALL
longer than the final consonant in an acoustic storage, and that retrieval of information from this storage results in a strong recency effect for vowels. Experiment V showed that S is able to respond to the physical characteristics of a single consonant or vowel phoneme for at least 8 set after the phoneme is heard. Nevertheless, the just mentioned model should not be ruled out. It is quite possible that little or no decay occurs when only one item is maintained in acoustic storage (as in Expt. V) while decay is more rapid when S is presented with several auditory stimuli (Bryden, 1971). Thus, it is possible that the vowel recency effect is caused by the retrieval of vowels but not consonants from acoustic storage, and that this effect is only observed when several stimuli are presented to S. The analysis of the types of intrusion errors that occurred among phonemes showed that consonant and vowel phonemes were coded as distinctive features in all serial positions. Thus, if S retrieves the final vowel in a sequence from acoustic storage, it is likely that this vowel was represented in storage as a set of distinctive features prior to recall. When the vowel in acoustic storage has partially decayed S may retrieve a partial listing of distinctive features, so that he will intrude a vowel having similar distinctive features with the forgotten vowel. This situation is analogous to that reported by Miller and Nicely (1955), where S confuses phonemes heard in a background of noise with other phonemes sharing similar distinctive features. REFERENCES BRYDEN,
M. P. Attentional
strategies
and short-term
memory
in dichotic
listening.
Cognitiue Psychology, 1971, 2, 99-116. feature coding strategies COLE, R. A. Distinctive
in STM. Paper presented at the Psychonomic Society meeting, San Antonio, 1970. COLE, R. A. Phoneme independence in short-term memory. Unpublished Dbctoral Dissertation, University of California, Riverside, CA, 1971. COLTHEART, M., & ALLARD, F. Variations on a theme by Posner: Physical and name codes of heard letters. Paper presented at Tenth Annual Meeting, The Psychonomic Society, San Antonio, November, 1970. CROWDER, R. G. Precategorical acoustic storage (PAS), Perception and Psycho-
physics, 1969, 5, 365-373. HALLE, M. Phonology in a generative grammar. Word, 1962, 16, 54-72. KIMURA, D. Functional assymetry of the brain in dichotic listening. Cortex,
1967, 3, 163-178. LIBERMAN, A. M., COOPER, F. S., SHANKWEILER, D. P., & STUDDJZRT-KENNEDY, M. Perception of the speech code. Psychological Reuiew, 1967, 74, 431-461. MILLER, N., Br NICELY, P. An analysis of perceptual confusions among some English consonants. 10urnd of the Acoustical Society of America, 1955, 27, 33S-352. MCCRARY, J. W., JR., Br HUNTER, W. S. Serial Position curves in verbal learning. Science, 1953, 117, 3032, 131-134.
54
RONALD
A. COLE
NEISSER, U. Cognitive psychology. New York: Appleton-Century-Crofts, 1967. SALES, B. D., HABER, R. N., & COLE, R. A. Mechanisms of aural encoding: III. Distinctive features for vowels. Perception and Psychophysics, 1968, 4, 321-327. SALES, B. D., HAFIER, R. N., & COLE, R. A. Mechanisms of aural encoding: IV. Hearsee, say-write interactions for vowels. Perception and Psychophysics, 1969, 6, 385-390. SHANKWEILLER, D., & STUDDERT-KENNEDY, M. Lateral differences in perception of dichotically presented synthetic consonant-vowel syllables and steady-state vowels. Paper given at the Seventy-first Meeting of the Acoustical Society of America, Boston, June, 1966. WICKELGREN, W. A. Short-term memory for phonemically similar lists. The American Journal of Psychology, 1965, 78, 567574. WICKELGREN, W. A. Distinctive features and errors in short-term memory for English vowels. J~~rnul of the Acoustical Society of America, 1965, 38, 583588. WICKELGREN, W. A. Distinctive features and errors in short-term memory for English consonants. Jourd of the Acoustical Society of America, 1966, 39, 38%398. (Accepted
March
13, 1972)
Different
PSYCHOLOGY
Memory
4, 39-54 (1973)
Functions
for Consonants
and
Vowels’
RONALD A. COLE* Uniuersity
of Waterloo
Different memory functions were obtained for consonants (C) and vowels (V) in a serial recall task. In general, the most recently heard vowels in a sequence were easier to recall than the most recently heard consonants. This effect was observed for auditorily presented sequences of CV or VC syllables, but was not observed for visually presented stimuli. The results were explained in terms of a limited capacity acoustic storage in which vowels are preserved longer than consonants. Retrieval of the last vowels from this storage was presumed to cause the vowel recency effect.
There are several lines of evidence suggesting that consonants and vowels are perceived differently. Experiments at Haskins laboratories using synthetically produced speech have demonstrated that perception of certain consonants is categorical while vowel perception is continuous ( Liberman, Cooper, Shankweiler, & Studdert-Kennedy, 1967) .. For stop consonants, a listener perceives a range of acoustic stimuli as a single phoneme, and is unable to discriminate acoustic variations within this range. On the other hand, vowel perception is continuous in that the continuous change of an acoustic cue results in a corresponding perceptual change in the vowel sound. Other differences in consonant and vowel perception have been observed during dichotic listening tasks (Shankweiler & Studdert-Kennedy, 1966; Kimura, 1967). In general, when subjects are simultane’ously presented with a pair of consonants, one to each ear, there is a clear superiority in identifying those presented to the right ear (the effect is reversed for subjects with a dominant right hemisphere). Pairs of dichotically presented vowels, however, show a slight and usually nonsignificant advantage for those presented to the left ear. No experimental evidence has yet been produced that consonants and vowels are coded differently in short-term memory, despite the intensive ‘I gratefully acknowledge the assistance of Miss Marka Adams, Miss Michelle Harwayne, and especially Miss Andrea Torres for their help in data collection and analysis in Expts. I and II. *Send reprint requests to: Department of Psychology, University of Waterloo, Waterloo, Ontario, Canada. 39 Copyright @ 1973 by Academic Press, Inc. All rights of reproduction in any form reserved.
40
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investigation of human information processing in recent years. In fact, such evidence as presently exists suggests that both consonants and vowels are coded in short-term memory as sets of binary distinctive features ( Wickelgren, 1965, 1966; Sales, Haber & Cole, 1968, 1969). Hdwever, these experiments do not rule out the possibility that consonants and vowels are processed differently in short-term memory since (1) only interphonemic intrusions were examined in these studies while over-all errors and serial position effects were ignored, and (2) recall performance was not compared for consonants and vowels presented under identical conditions. The present study was performed in order to determine whether consonant-vowel differences observed during phoneme recognition may be observed in short-term memory. EXPERIMENT
I
Method Subjects. One hundred and twenty students from the University of California at Riverside served as Ss. These Ss received credit toward their grade in a psychology course. Each S was pretested to ascertain whether he could discriminate among the stimuli. Six potential Ss were eliminated because they were unable to perform the /a/ /a/ distinction. Each S was tested individually in a small room with E. Design. Experimental Groups were defined by the number of syllables presented in a recall sequence (five, six, or seven) and the type of phonemic change that occurred within the sequence. In consonant groups each consonant was paired with /a/, so that only the consonant was heard to change between successive syllables. In vowel groups each vowel was paired with Id/, so that only the vowel was heard to change from syllable to syllable. Thus, the variables sequence-type and sequencelength were combined in a 2 x 3 factorial design to yield six experimental groups: five, six, or seven consonants followed by /al and five, six, or seven vowels preceded by Id/. Stimuli. Seven consonant and seven vowel phonemes were used as stimuli: Id/, IS/, lml, IdI, In/, lsl, lpl and Ii/, lel, Ire/, lul, la/, IO/, 131. Each consonant was followed by /a/ while each vowel was preced,ed by Id/, so that subjects heard the syllables da, sha, ma, Ba (as in think), na, sa, pa, or dee, day, daeh (as in dad), doo, dah, doe, daw. The seven consonants and the seven vowels were used to construct six sets of 98 sequences of five, six, or seven syllables per sequence. The 98 sequences in each group were constructed to meet the following rules of occurrence: ( 1) each phoneme was heard at most once per sequence, (2) each phoneme was presented an equal number of times in each
CONSONANT
AND
VOWEL
41
RECALL
serial position, and (3) each phoneme was presented an equal number of times in the same sequence with each other phoneme. The 98 recall sequences were randomly ordered for each group. These sequences were then recorded in a male voice at the rate of three syllables every 2 sec. Thus, each subject in a given group heard the recall sequences in the same order. Procedure. Twenty Ss were randomly assigned to each experimental group. Each S was tested individually in a session lasting approximately 1 hr. Subjects were presented with a series of sounds by means of a Wollensak tape recorder. After each trial, the tape recorder was stopped by means of a footswitch controlled by E. Subjects were told that, upon hearing the sound of the footswitch, they should verbally report the sounds in the order in which they heard them. If they were unable to recall a sound in a given position, they were allowed to guess. All responses were recorded by E after each trial, and each experimental session was tape recorded. After three practice trials, the experiment was begun. Scoring. For all experiments reported here, any sound not reported in its correct serial position was scored as an error. This included transpositions among sounds presented in the same sequence, errors in which S substituted sounds not heard in the sequence, and omission errors, where S simply responded “blank.” Results Figure 1 displays recall performance for consonants and vowels in each group as a function of serial position. Analyses of variance computed between the consonant and vowel groups at each sequence-length revealed that: (1) vowel recall was superior to consonant recall for sequences of five and six syllables ( p < .05), (2) recall varied as a function of serial position ( p < .Ol in all cases), and (3) there was a greater recency effect for vowels than for consonants (p < .Ol in all cases). This interaction between sequence-type and serial position is shown in Fig. Ia as the divergence of the two curves in the later serial positions.3 In order to directly compare the serial position effects for consonant and vowel sequences, the recall errors for each serial position were calculated for each S as a percentage of the total errors made by that S. This manipulation allows a direct comparison of serial position effects between consonant and vowel sequences when overall errors are equated for the two sequence-types ( McCrary & Hunter, 1953). The normalized ’ Robert Crowder, results of Expt. I.
working
at Yale University,
has independently
replicated
the
42
RONALD
A.
COLE
2
b i’?\
,.--.’
-..
7, I’ 12
4’
’
I----
I-.
1234567 Position
FIG. la. Mean number of consonant and vowel errors at each serial position for sequences of five, six, and seven syllables. lb. Percentage of total number of consonant and vowel errors at each serial position for sequences of five, six, and seven syllables.
serial position curves shown in Fig. lb reveal that the greater recency effect for vowels is restricted to the final serial position of the sequence. Previous experiments have revealed that consonants and vowels are coded in STM as sets of distinctive features (Cole, 1970; Cole, 1971). If Ss in this experiment discriminated among phonemes in STM by their distinctive features as shown in Table 1, then intrusion errors should occur most frequently between phonemes having the same distinctive features. For example, in Table 1, /i/ and /e/ differ only by their feature value on the “grave” dimension. If this feature is forgotten while /i/ is coded in STM, S is likely to intrude /e/ for /i/ on the basis of the remaining feature information. In the present experiment, the greater number of consonant errors in the final serial positions of a sequence may reflect a greater amount of distinctive feature forgetting for these phonemes. If more consonant errors occur because of a greater loss of feature information, then distinctive feature theories should less accurately predict intrusion errors for consonant phonemes. This prediction is based on the reasoning that, when several (or all) features are forgotten for a given phoneme, S retains so little feature information about that phoneme that intrusion errors are randomly determined. Therefore, distinctive feature theories
CONSONANT
Distinctive
Feature
AND
VOWEL
43
RECALL
TABLE 1 Composition of Phonemes used in this Experiment Halle’s (1962) Distinctive Feature System
I based
on
Vowel phonemes /V
/e/
he/
/u/
/a/
/o/
b/
-
-
-
+ -
-
-
+ + +
+ -
+ -
+ + -
+
+
+
+
/s/
/P/
+ + -
Dimension Flat Compact Grave Diffuse
Consonant
phonemes
/d/
/8/
/m/
/el
/n/
-
-
+
-
-
-
+ -
+ -
+ -
+ -
+ -
+ +
+ -
+ +
+ + + -
Dimension Grave Diffuse Strident Nasal Contindant Voiced
+
+ -
should less accurately predict the rank order of intrusion errors for consonant phonemes than for vowel phonemes in the final serial positions of a sequence. In order to test this hypothesis, the rank order of presented phonemes in each experimental group was calculated for each intruded phoneme for each serial position. This manipulation yielded five, six, or seven separate confusion matrices for each group (C5, C6, C7, and V5, V6, V7) depending upon the number of phonemes presented for recall in that group. Halle’s distinctive feature system was then used to generate the expected rank order of intrusion errors for each phoneme. These predictions were based on the assumption that phonemes sharing the most identical distinctive features are most likely to be intruded for each other (see Sales, Haber, & Cole, 1968, for a more detailed description of this analysis). The ability of Halle’s distinctive feature system to predict the rank order of intrusion errors for consonant and vowel phonemes in each serial position is shown for each group in Fig. 2. This figure reveals that Halle’s distinctive feature system predicted the pattern of intrusion errors with greater than chance (50%) accuracy in all serial positions. However, intrusion errors were less accurately predicted for consonants than for
RONALD
A. COLE
-Vowels 0-
- l
1
Consonants
2
3 Serial
4
5
4
7
Position
FIG. 2. Accuracy with which Halle’s distinctive feature system predicts the frequency of intrusion errors at each serial position for consonants and vowels in sequences of five, six, and seven syllables.
vowels in the final serial positions in sequences of five and six syllables. In general, the analysis of intrusion errors supports the hypothesis that both consonants and vowels are coded in STM as sets of distinctive features at each serial position, and that more feature forgetting occurs for consonants than for vowels in the final serial positions of a recall sequence. These data do not reflect on the adequacy of Halle’s distinctive feature system, since subjects may remember phonemes by specific combinations of distinctive features (Sales, Haber & Cole, 1968). However,
CONSONANT
AND
VOWEL
RECALL
45
the analysis does reveal a tendency for Halle’s distinctive feature system to predict vowel intrusions in the later serial positions of a sequence more accurately than consonant intrusions, and this finding is suggestive, since it was predicted from the data presented in Fig. 1. EXPERIMENT
II
Because a clear difference was found between consonant and vowel recall in short-term memory, a second experiment was performed in order to test the effect of the syllable structure of a sequence on the recall of consonants and vowels. To this end, sequences were constructed where both the consonant and vowel phonemes were randomly varied from syllable to syllable within a sequence (e.g., /di pe mu 00 so/ ). TWO possibilities suggest themselves. If consonant and vowel phonemes are coded independently of their syllable context, then the final vowel in a sequence should be remembered better than the final consonant when both are presented in the same syllable. Thus, identical recall curves should be found for the consonants and vowels in Expts. I and II. On the other hand, if the syllable provides a strong cohesive or associative influence on its component phonemes, then recall performance should be similar for consonants and vowels in the same syllable. Method Subjects. Sixty students from the University of California at Riverside served as Ss. Each S was subjected to the identical pretest procedures as in Expt. I. Stimuli. The same consonant and vowel stimuli were used as in Expt. I. Lists of 98 trials were again constructed according to the rules of occurrence outlined previously. The additional requirement was made that each consonant and vowel phoneme was paired the same number of times in each serial position. Thus, each S heard each consonant-vowel combination twice in each serial position. Design. Twenty Ss were randomly assigned to each of three experimental groups that differed by the number of syllables in each sequence. The three groups were presented with 98 sequences of either five, six, or seven syllables (depending on the group) in which both the consonant and vowel phoneme were varied between each syllable. Procedure. For all three groups, the experimental procedure was identical for Expts. I and II. Results the The results of Expt. II are illustrated in Fig. 3a which &splays serial position curves for consonants and vowels that were paired in
46
RONALD
A.
COLE
*-Vowel --- Consonant b _-24. #’ ,’ ,.----*----mm 7’ n I’ d’ 20. .y.,-•;,” -I\
60- a
.*
w
40.
12.
60 40. 20.
‘12
~ ,F----.---------Lmmm :* , ------\ ;:: /v c’ 3
4
5
6
y-y 1
7
2
3
4
5.
6
7
Position FIG. 3a. Mean number of consonant and vowel errors at each serial position for sequences of five, six, or seven CV syllables. 3b. Percentage of total consonant and vowel errors at each serial position for sequences of five, six, or seven CV syllables.
syllables in the same sequence. Thus, Fig. 3 represents a within-subjects comparison, while Fig. 1 represents a between-subjects comparison. There was a significant effect of phoneme type (consonant vs vowel, p < .05), serial position ( p < .Ol), and a significant interaction between phoneme type and serial position (p < .Ol) for all three experimental groups. The normalized serial position curves shown in Fig. 3b reveal that the greater recency effect for vowels was again restricted to the last serial position in the sequence for strings of five, six, or seven different syllables. EXPERIMENT
III
The results of Expts. I and II reveal a clear superiority in the recall of vowel phonemes heard in the later serial positions of a sequence. Since only CV syllables were heard in these experiments, it is possible that superior recall for terminal vowel phonemes is due to the fact that a vowel phoneme is the most recently heard stimulus in each sequence. Experiment III tests this hypothesis by comparing recall performance for consonant and vowel phonemes in both CV and VC sequences.
CONSONANT
AND
VOWEL
RECALL
47
Method Subjects. Twenty students from the University of Waterloo served as Ss. These Ss received credit toward their grade in a psychology course. Each S was tested individually in a small room with E in a session lasting approximately a half hour. Stimuli. Six consonant and six vowel phonemes were used as stimuli: Id/, 19, h-4, InI, Is/, lpl, and lil, lel, lad, lul, loI, IDI. These phonemes were used to construct a set of 72 six-syllable CV sequences such that each phoneme was paired in a syllable with every other phoneme an equal number of times in each serial position. The 72 sequences were randomly ordered and recorded by E at the rate of three syllables every 2 sec. The set of VC sequences were then recorded at the same rate by reversing the order of the phonemes in each syllable. Procedure. Ten Ss were randomly assigned to one of two experimental groups which differed only in the order in which the consonant and vowel phonemes were heard in each syllable (CV or VC). Each S was tested for his ability to discriminate among the phonemes by tapping on a table immediately upon hearing a given phoneme in a sequence of syllables. Each S was tested on three successive sequences for each phoneme, and no perceptual errors were made in either the CV or VC condition. While each syllable was perfectly clear, it is interesting to note that consonants in VC syllables subjectively appear to be less distinct. Subjects were tested in a procedure identical to Expts. I and II, except for one modification. After each trial E manually stopped the tape by depressing the “pause” switch on the side of the recorder. This procedure was used to eliminate the slight possibility that the “click” produced by the mechanical footswitch was masking the final consonant sound in each sequence. Each S was seated next to the tape recorder, and was instructed to report back the sounds as soon as the tape recorder was stopped after each trial. Results Figure 4a displays the recall performance for consonants and vowels in each serial position in CV and VC sequences. Analysis of variance revealed no difference between the number of errors in CV and VC sequences. However, relatively more consonant errors occurred in VC than CV sequences, especially in the initial serial positions (phoneme X sequence-type X position, p < .05). Separate analyses of variance for the errors in each sequence-type revealed a main effect of serial position and a significant interaction between phoneme-type and serial position for both sequence-types. Thus, for both CV and VC sequences, vowels
RONALD
A.
COLE
a
b
40 .
30 -
ZO-
35-
,’ 25 -
I
.’
,*---
--0,
cv
,I’
\\
\
‘\
‘.
*..----7 /
P E !? d
/’
I’ I’
15
5’
e--. consonant -Vowel
/ 1
2
l ---
‘\
cv
‘b
I
7
I’
I,
3
4
5
6
1
2
3
4
5
6
Position FIG. 4a. Mean number of consonant and vowel errors at each serial position for CV and VC sequences. 4b. Percentage of total consonant and vowel errors at each serial position for CV and VC sequences.
are recalled more often than consonants and this effect is greater in the final serial positions of the sequence. Figure 4b displays the normalized serial position curves for consonant and vowel errors in both sequence types. This figure reveals that the relative superiority of vowel recall was once again limited to the last serial position in the sequence. EXPERIMENT
IV
It has long been known that consonant and vowel phonemes have different acoustic properties. Consonants are characterized by transient wave forms and high frequency energies, while vowels are characterized by low-frequency energy of relatively longer duration. Experiment IV was designed to test the hypothesis that the observed differences in consonant and vowel recall requires that S hears the stimuli. Method Subjects. Fifteen undergraduate Waterloo served as Ss.
students
from
the
University
of
CONSONANT
AND
VOWEL
49
RECALL
Stimuli. The consonant phonemes /cl/, Is/, /ml, In/, Is/, lpl and the vowel phonemes /iI, /e/, /,d, /u/, lo/, la/ were combined into consonant-vowel syllables. These syllables were printed on slides. Seventytwo sequences of six CV syllables were constructed so that no phoneme was repeated on a given trial, and so that each CV syllable occurred an equal number of times in each serial position. A typical sequence of slides was “dee mu pay sho neh sa.” Procedure. Each S was pretested to insure correct pronunciation of each of the syllables. Subjects were shown sequences of six slides via a Kodak Carousel slide projector that was timed to present the slides at the rate of l/set. After the last syllable, S was instructed to verbally report the syllables in the order in which he had seen them. Results Figure 5 displays the mean number of consonant and vowel errors at each serial position. Analysis of variance revealed no significant difference between the number of consonant and vowel errors. A significant effect of serial position was observed ( p < .Ol ) as well as a significant interaction of phoneme-type and serial position (p < .05). While the
Posit
ion
FIG. 5. Mean number of consonant and vowel sequences of visually presented CV syllables.
errors
at each serial
position
for
50
RONALD
A.
COLE
curves in Fig. 5 do show a crossover between positions three and four, there is a clear lack of divergence between the two curves in positions five and six. Thus, the previously observed differences in consonant and vowel recall are not observed when CV syllables are presented visually. This was observed most clearly from the normalized data. For visually presented syllables, there is no longer any vowel recency effect in the final serial position of the sequence. EXPERIMENT
V
One model that may be used to account for differences in the immediate recall of consonants and vowels assumes that acoustic stimuli are preserved in a physical form in a sensory (acoustic) storage while they are simultaneously rehearsed in verbal memory ( Neisser, 1967;. Crowder, 1969). Thus, as each stimulus is heard, it is first preserved in acoustic storage, and then converted to a “name” code for rehearsal. Just prior to reporting the final stimuli in a sequence, S retrieves these stimuli from acoustic storage. Recency effects in short-term memory for spoken items are thus due to the retrieval of the final stimuli in a sequence from acoustic storage. This model will account for the results of the present experiments if it is assumed that vowels are preserved more efficiently (longer) than consonants in acoustic storage. In this case, superior recall of vowel phonemes in the final serial positions of a sequence is assumed to reflect the decay of the consonant phoneme prior to its retrieval from acoustic storage. Experiment V was designed to test the hypothesis that vowels are preserved longer than consonants in acoustic storage. The paradigm used was developed by Coltheart and Allard ( 1970). These authors discovered that subjects could decide faster that the second of two successively spoken letters was the same as the first when the letters were spoken in the same voice rather than a different voice (e.g., a Female P followed by a Male P). This finding suggests that subjects are able to respond to a physical or sensory representation of a letter before they are able to respond to the name of the letter. If the superior recall for vowels heard at the end of a sequence is due to longer storage of vowels in acoustic storage, then the faster reaction times for physically identical stimuli should be observed for vowels but not for consonants. Method Subjects. Twenty-two right-handed undergraduate students from the University of Waterloo served as Ss. Each S was tested in a single session lasting approximately 75 min.
CONSONANT
AND
VOWEL
RECALL
51
Stimuli. The consonants D, P, T, C and the vowels A, E, 0, U (pronounced “00”) were used as stimuli. The consonants were chosen because each is composed of an initial consonant phoneme followed by the vowel phoneme /i/. Thus, S must attend to the consonant portion of the letter in order to make a same-different judgment between any two of these letters. Identical experimental tapes were made for consonants and vowels. Tapes were made by recording each of theCtimuli in a male and female voice, and storing these sounds on disc tape in an IBM 366/44 computer. Each stimulus was exactly 700 msec long. An experimental trial consisted of a 766msec warning tone followed 2 set later by the first letter. The second letter was presented either 34,2, or 8 set after the offset of the first letter. A tone recorded on a second channel at the onset of the second letter was used to start a Hunter 100 msec timer. On half of the trials S was presented with two different letters, while the other half consisted of the same letter repeated twice. On those trials where the same letter was repeated twice, the two letters were presented in the same voice 56% of the time (e.g., Male D followed by Male D) and in a different voice 56% of the time (e.g., Male D followed by Female D). The set of 144 trials was randomly presented to each S. All variables were balanced at each ISI. Procedure. All stimuli were presented binaurally to S by means of a Sony model TC 546 tape recorder via Koss Pro4A stereo headphones. Each S was seated with a two-way switch between the thumb and index finger of his right hand. The S was instructed to move the switch to the right if the second letter had the same name as the first, and to the left if the two letters did not have the same name. Subjects were told that the voice in which the letter was heard was irrelevant to the task. After each trial, E recorded S’s reaction time. Results Figure 6 displays the reaction time (RT) to pairs of consonants and vowels having the same name (“different” responses are not considered here). This figure reveals that subjects were faster deciding that two spoken letters are the same when both letters were spoken in the same voice. Analysis of variance revealed a main effect of voice (p < 661) and a significant interaction between voice and IS1 (p < .Ol) which reflects the convergence of the two sets of curves at 2 set ISI. It is clear from Fig. 6 that Ss are able to respond to physical characteristics of either a consonant or vowel for at least 8 set after the offset of the physical stimulus.
52
RONALD
A. COLE
Vowels
700.-,-
--
-‘--
0-o
__----
650 E .c .-; t z a
600-
Consonants
650 600.
;-----q,
550 e- --different U-come
500’
8
2
95 lntcrstimulus
Voice Voice
Interval
(sec.)
FIG. 6. Reaction time to the second of two consonants same or different voice at three different delays.
or vowels
heard
in the
DISCUSSION
When S hears a series of syllables for ordered recall, he is more likely to remember the vowels than the consonants. This difference is most evident for vowels and consonants heard in the final syllable of a sequence, due to a sharp decrease in errors for the most recently heard vowel phoneme. This “vowel recency” effect appears to depend upon the acoustic nature of the stimuli, since the effect was not observed for visually presented syllables. The difference between consonant and vowel recall was orderly and stable for sequences of five, six, or seven syllables. Similar differences were observed whether recall was compared for sequences of consonants paired with la/ and sequences of vowels paired with Id/, or whether recall was compared for consonants and vowels presented in the same sequences as members of a CV syllable. Since the number of consonant or vowel intrusions in a sequence is unaffected by the syllable structure of the sequence, it is likely that phonemes are remembered independently of the syllable in which they are heard. It was proposed above that the final vowel in a sequence is preserved
CONSONANT
AND
VOWEL
53
RECALL
longer than the final consonant in an acoustic storage, and that retrieval of information from this storage results in a strong recency effect for vowels. Experiment V showed that S is able to respond to the physical characteristics of a single consonant or vowel phoneme for at least 8 set after the phoneme is heard. Nevertheless, the just mentioned model should not be ruled out. It is quite possible that little or no decay occurs when only one item is maintained in acoustic storage (as in Expt. V) while decay is more rapid when S is presented with several auditory stimuli (Bryden, 1971). Thus, it is possible that the vowel recency effect is caused by the retrieval of vowels but not consonants from acoustic storage, and that this effect is only observed when several stimuli are presented to S. The analysis of the types of intrusion errors that occurred among phonemes showed that consonant and vowel phonemes were coded as distinctive features in all serial positions. Thus, if S retrieves the final vowel in a sequence from acoustic storage, it is likely that this vowel was represented in storage as a set of distinctive features prior to recall. When the vowel in acoustic storage has partially decayed S may retrieve a partial listing of distinctive features, so that he will intrude a vowel having similar distinctive features with the forgotten vowel. This situation is analogous to that reported by Miller and Nicely (1955), where S confuses phonemes heard in a background of noise with other phonemes sharing similar distinctive features. REFERENCES BRYDEN,
M. P. Attentional
strategies
and short-term
memory
in dichotic
listening.
Cognitiue Psychology, 1971, 2, 99-116. feature coding strategies COLE, R. A. Distinctive
in STM. Paper presented at the Psychonomic Society meeting, San Antonio, 1970. COLE, R. A. Phoneme independence in short-term memory. Unpublished Dbctoral Dissertation, University of California, Riverside, CA, 1971. COLTHEART, M., & ALLARD, F. Variations on a theme by Posner: Physical and name codes of heard letters. Paper presented at Tenth Annual Meeting, The Psychonomic Society, San Antonio, November, 1970. CROWDER, R. G. Precategorical acoustic storage (PAS), Perception and Psycho-
physics, 1969, 5, 365-373. HALLE, M. Phonology in a generative grammar. Word, 1962, 16, 54-72. KIMURA, D. Functional assymetry of the brain in dichotic listening. Cortex,
1967, 3, 163-178. LIBERMAN, A. M., COOPER, F. S., SHANKWEILER, D. P., & STUDDJZRT-KENNEDY, M. Perception of the speech code. Psychological Reuiew, 1967, 74, 431-461. MILLER, N., Br NICELY, P. An analysis of perceptual confusions among some English consonants. 10urnd of the Acoustical Society of America, 1955, 27, 33S-352. MCCRARY, J. W., JR., Br HUNTER, W. S. Serial Position curves in verbal learning. Science, 1953, 117, 3032, 131-134.
54
RONALD
A. COLE
NEISSER, U. Cognitive psychology. New York: Appleton-Century-Crofts, 1967. SALES, B. D., HABER, R. N., & COLE, R. A. Mechanisms of aural encoding: III. Distinctive features for vowels. Perception and Psychophysics, 1968, 4, 321-327. SALES, B. D., HAFIER, R. N., & COLE, R. A. Mechanisms of aural encoding: IV. Hearsee, say-write interactions for vowels. Perception and Psychophysics, 1969, 6, 385-390. SHANKWEILLER, D., & STUDDERT-KENNEDY, M. Lateral differences in perception of dichotically presented synthetic consonant-vowel syllables and steady-state vowels. Paper given at the Seventy-first Meeting of the Acoustical Society of America, Boston, June, 1966. WICKELGREN, W. A. Short-term memory for phonemically similar lists. The American Journal of Psychology, 1965, 78, 567574. WICKELGREN, W. A. Distinctive features and errors in short-term memory for English vowels. J~~rnul of the Acoustical Society of America, 1965, 38, 583588. WICKELGREN, W. A. Distinctive features and errors in short-term memory for English consonants. Jourd of the Acoustical Society of America, 1966, 39, 38%398. (Accepted
March
13, 1972)