The Development of Memory for Ignored Speech

JOURNAL OF EXPERIMENTAL CHILD PSYCHOLOGY ARTICLE NO.

63, 239–261 (1996)

0049

The Development of Memory for Ignored Speech JOHN SCOTT SAULTS AND NELSON COWAN University of Missouri, Columbia Previous studies of the development of children’s memory have focused on attended stimuli. Here we examine short-term memory for spoken words that were ignored at the time of their presentation. In Experiment 1, a visual matching task was interrupted occasionally by a set of picture alternatives. The participant was to select the picture matching the most recently heard word, which had been presented 1, 5, or 10 s ago. An age difference in memory was found, but performance levels were quite high. Experiment 2 employed an alternative visual task to tie up phonological processing, bringing performance levels down to a more sensitive range. An age difference in the persistence of memory was obtained under these circumstances. We conclude that relatively attention-free properties of short-term memory may change with development in childhood. q 1996 Academic Press, Inc.

Language comprehension and recall often depend on the ability of the brain to store portions of the speech signal for a short time automatically (Cowan & Saults, 1995). Broadbent (1958) and subsequent cognitive researchers have viewed this memory for speech in an unattended channel as evidence of the large-capacity but short-lived automatic storage of unanalyzed sensory information in the brain. Although the capacity to analyze incoming information is limited, listeners can switch attention to a previously unattended channel and analyze whatever sensory information persists in memory. From this perspective, a change with age in the memory for ignored speech would indicate development of the automatic sensory storage capabilities underlying that memory. (For recent theoretically oriented reviews of automatic sensory storage processes, see Cowan, 1984, 1988; Cowan & Saults, 1995; Crowder,

This paper is based on Saults’ doctoral dissertation and was supported by NIH Grant HD21338. We thank Donald Kausler, John Mueller, Kenneth Sher, and James Koller for helpful comments. We also thank Jill Mosher, Paige Meinhart, Noelle Wood, Connie Keller, and Cara Saling for their assistance in recruiting participants and collecting data. Address reprints requests to Nelson Cowan, Department of Psychology, 210 McAlester Hall, University of Missouri, Columbia, MO 65211. E-mail: [email protected] or [email protected].

239 0022-0965/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.

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1983; Coltheart, 1984). In a popular account of working memory, Baddeley (1986) similarly viewed the persistence of phonological information in shortterm memory as automatic and fixed across ages. In contrast to this possibility, most previous research on memory development in childhood has focused on changes in strategies and knowledge factors (e.g., see Flavell, Miller, & Miller, 1993; Kail, 1990). Researchers apparently have assumed there are no changes in automatic storage capabilities with development (e.g., see Bjorklund, 1995, p. 104; Siegler, 1991, p. 206), though with little empirical justification. Some studies have looked for and failed to find developmental differences that could be attributed clearly to maturational changes in auditory sensory storage (e.g., Cowan & Kielbasa, 1986; Dempster & Rohwer, 1983; Engle, Fidler, & Reynolds, 1981; Frank & Rabinovitch, 1974), but those studies are incapable of ruling out such a developmental difference. Keller and Cowan (1994) collected developmental data on the duration of memory for tone pitch in children 6 to 12 years of age and in adults suggesting that the persistence of auditory sensory memory increases with age. Participants were to compare the exact pitch of two tones separated by a variable inter-tone interval, and the decline in performance with increasing intervals occurred faster in younger participants than in older ones. A second study included an interfering task during the inter-tone interval and indicated that tone comparison performance was not dependent on rehearsal, reinforcing the suggestion that a sensory storage difference underlies the developmental change. We do not know any comparable evidence for changes in memory for speech. The two-stimulus comparison procedure used by Keller and Cowan (1994) seems less appropriate for speech than for simple sound segments that can be adjusted for difficulty level without the worry of crossing category boundaries. One method to examine sensory memory of speech stimuli is to test recall of words that were ignored when they were presented. When attentive processes are not used for encoding the memory, storage should be ‘‘automatic’’ according to three criteria suggested by Posner and Snyder (1975): that it occurs without intention, that it is not readily available to introspection, and that it consumes few if any processing resources. Hasher and Zacks (1979) proposed further that automatic operations, as Posner and Snyder defined them, should change very little with development, but that hypothesis has not always been supported by the data (Kausler, 1990). The dramatic maturational changes taking place in the temporal lobes of the cerebral cortex throughout childhood (Rabinowicz, 1980), along with evidence that these areas are involved in auditory sensory storage (Colombo, D’Amato, Hillary, & Gross, 1990; Lu, Williamson, & Kaufman, 1992; and Sams, Hari, Rif, & Knuutila, 1993), provide good reasons to wonder if the automatic short-term storage of speech features does, in fact, change with maturation.

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The practical and theoretical importance of the development of memory for ignored speech, or for other ignored stimuli for that matter, has not been matched by a commensurate empirical research effort. Just as most research on memory development has focused on strategies and knowledge, most research on the development of selective attention has examined changes in the control of attentional processes rather than changes in what is stored without attention (for reviews of the development of selective attention, see Lane & Pearson, 1982; Small, 1990, pp. 257–260). Research on individual differences in children’s memory for speech under selective and divided attention conditions also has focused on the control of attention and on hemispheric differences, rather than on automatic memory processes (e.g., Dickstein & Tallal, 1987; Obrzut, Obrzut, Bryden, & Bartels, 1985; Pelham, 1979). A notable exception is an experiment conducted by Sipe and Engle (1986), who administered a selective listening task to good and poor readers and tested for recollection of items in the ignored channel after various poststimulus delays. In both groups, memory for unattended spoken items decreased as a function of the test delay, but the rate of forgetting was steeper for the poor readers. To our knowledge, similar data on age effects have not been obtained, but the use of a selective attention task to examine automatic memory processes seems promising. The specific selective attention task used by Sipe and Engle (1986), selective listening, is not ideal for determining the rate of forgetting an unattended auditory stimulus. Forgetting rates based on that type of task are influenced by interference from several potential sources: subsequent stimuli in the unattended channel; the subject’s own voice, if a vocal response to the attended stimuli is required; and, at the very least, contralateral interference from the attended channel. To observe forgetting of ignored sounds without acoustic interference, the attended task could be visual, and the unattended task, auditory. Eriksen and Johnson (1964) developed just such a selective attention procedure to examine adults’ memory for unattended sounds without acoustic interference. They presented occasional low-intensity tones while the participant read a novel. Some tones were targets as indicated by a memory response signal (one chamber light being turned off) at posttarget delays varying from 0 to 10.5 s. At the signal, the participant was to decide whether a tone recently had occurred. Tone detection declined markedly as a function of delay, presumably revealing a short-lived memory for unattended tones. Cowan, Lichty, and Grove (1990) modified Eriksen and Johnson’s (1964) procedure to examine adults’ memory for ignored speech. Instead of occasional tones, spoken syllables were presented at irregular intervals. The spoken stimuli included the nine consonant–vowel syllables that result when each of the consonants /b/, /d/, and /g/ are followed by each of the vowels /i/, /I/, and /E/ (as in beet, bit, and bet, respectively). Occasionally during the session

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(about nine times in an hour), a light was turned off to signal the participant to identify the most recently presented syllable. There was a silent delay of 1, 5, or 10 s between the last spoken syllable and this signal. Recognition accuracy was scored separately for the recall of consonants and vowels, to take advantage of findings from other types of procedure that auditory sensory memory is less useful for retaining stop consonants than it is for retaining acoustically simpler vowels (Cole, 1973; Crowder, 1971, 1973; Darwin & Baddeley, 1974; Pisoni, 1973). Results were similar to those of Eriksen and Johnson. Additionally, there was more forgetting for consonants than for vowels. This pattern of forgetting for ignored speech contrasted with results when the same participants attended to the speech sounds. In the attendedspeech trials, similar levels of performance were achieved for consonants and vowels. This suggests that memory for ignored speech consisted largely of the acoustic properties of the syllables, whereas memory for attended speech depended more upon categorical, perhaps phonemic, information that would not necessarily favor vowels over stop consonants. In the present study, the procedure of Cowan et al. (1990) was adapted for use in a developmental study with children. To this end, changes were made in the primary (visual) task, the auditory stimuli, and the auditory recognition procedure. In these developmental experiments, participants heard random sequences of the words bee, bow, tea, and toe (rather than nine nonsense syllables as in Cowan et al.) while simultaneously doing a visual matching task (rather than reading a novel as in Cowan et al.). The visual display was interrupted occasionally, at unpredictable intervals, and the participant was to report the most recently spoken word by selecting the corresponding picture on the computer screen (rather than by circling a syllable as in Cowan et al.). The visual matching tasks we used had several useful attributes. They were entertaining and could be accomplished by children of a wide range of ages; they could be carried out at the participant’s own pace, which could be measured over time as a way to monitor the allocation of attention; and they could not cause auditory interference. The timing of the auditory stimuli was independent of the visual task, which prevented participants from using the visual task to anticipate auditory stimuli. The small set of monosyllabic English words used as auditory stimuli allowed a response even by preliterate participants, and the responses could be scored for the accuracy of consonant and vowel recall separately. To increase the variability of the acoustic stimuli to approximate the situation of Cowan et al. (1990), three different tokens of each of the four spoken words were used. As in the adult studies upon which the present study was modeled, performance levels on both the auditory memory task and the visual matching task were observed under both single- and dual-task conditions, so that tradeoffs in the allocation of attention could be assessed. For the visual task, this was

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done both at the beginning and at the end of the session to consider the effects of practice. Other concessions were made to accommodate younger participants. To minimize shifts of attention due to boredom and fatigue in young children, the overall procedure and the average time between memory probes both were shorter than in Cowan et al. (1990). Also, frequent feedback and reinforcement were added to the visual task to help maintain children’s attention and motivation. EXPERIMENT 1

In this experiment, memory for ignored speech was examined in first and fourth grade children. These groups bracket a rapid period of intellectual growth known to include a marked increase in the amount and complexity of strategic mnemonic behavior (for a review, see Kail, 1990). The ages were selected to determine if the persistence of automatic memory storage also changes within this important developmental period. Method

Participants Potential participants were identified from elementary school rolls and recruited by contacting parents by mail and phone. Thirty-six first-grade children began the experiment. Four chose to quit and two failed to finish because of equipment problems. The 30 first-grade participants who completed the experiment (13 females, 17 males) ranged in age from 5;9 to 7;6 at the time of testing (M Å 6;8). Thirty-four fourth-grade participants began the experiment. One chose to quit and three failed to finish because of equipment problems. The 30 fourth-grade participants who completed the experiment (14 females, 16 males) ranged in age from 8;9 to 10;5 (M Å 9;7). All participants were reported as having normal hearing and normal or correctedto-normal vision, including full color vision. Apparatus The study was conducted in a sound-attenuated chamber using an Apple Macintosh II computer with a high-resolution color monitor set for 8-bit color. Reaction times to visual shape arrays were measured to an accuracy of { 17 ms. Participants responded using a joystick with a button. The spoken words were natural speech sounds digitized at a sampling rate of 22.05 kHz, with an 8-bit dynamic range. Sounds were reproduced through comfortable, lightweight headphones driven by an amplifier connected to the computer’s built-in sound port. Auditory interstimulus intervals (ISIs) were controlled to an accuracy of {3 ms.

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Stimuli Sounds. The auditory stimuli were the words bee, bow, tea, and toe spoken in a male voice. Three tokens of each word were selected from a number of monotone utterances. They ranged from 240 to 275 ms in length (M Å 267.8, SD Å 11.5). The stimuli were presented at a moderately quiet intensity level of 50–55 dB(A) as measured with a sound level meter and accompanying earphone coupler. Visual stimuli. The visual task consisted of finding a picture that exactly matched one in the center of the screen. The stimuli for the visual matching task consisted of four different irregular hexagons filled by four different patterns (e.g., horizontal and vertical lines; unfilled circles). The patterns appeared in four different pairs of contrasting colors (one for the background and one for the pattern lines), which were selected for each stimulus from opposite sides of the color wheel in a manner that made the different color pairs very discriminable from one another. Thus, combining the dimensions of shape, pattern, and color yielded 64 different visual stimuli. The visual matching problems were arranged so that the target stimulus at the center of the computer screen was surrounded by four response choices. A variety of different matching trials were constructed, using balanced permutations of stimuli, in an attempt to make the task continuously demanding. Twelve different randomly ordered sequences of matching trials were constructed and each was paired with one of the 12 sequences of auditory stimuli. The memory probe pictures representing bee, bow, and toe were Apple Macintosh graphics created by Snodgrass and Corwin (1988) based on the set of pictures by Snodgrass and Vanderwart (1980). The picture representing the word tea was a combination of pictures of a kettle and a cup from the same source. Procedure Auditory-alone task. Children were tested one at a time in a sound-attenuated chamber. Before performing any tasks, each child was allowed to become comfortable with the experimenter, equipment, and surroundings and then told he or she would get to decorate an album with stickers to be awarded for excellent performance on a game. An auditory task practice served to familiarize children with the spoken words and their associated pictures, as well as with the response procedure. They first were shown the four memoryprobe pictures and were told their names. They also were instructed on the use of the joystick and practiced controlling the movement of the cursor. Then they heard the names of each of the memory probe pictures over the headphones, moved the cursor to the corresponding picture, and pressed the joystick button. This task was followed by four practice trials in which the pictures vanished from the screen while a list of the four words was heard,

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separated by randomly assigned ISIs of 1, 5, and 10 s. Immediately after each set of four words, the pictures reappeared and the child was to select the one representing the most recently presented word. If the participant failed to select the correct picture, the procedure was repeated until four consecutive correct responses occurred. The next block was similar to the prior presentation of the lists of four words, except that the last word of each list was followed by delays of 1, 5, or 10 s before the memory-probe pictures appeared. Each word was tested once at each delay. This procedure provided control measurements of participants’ performances when they were allowed to attend fully to the auditory stimuli during and after their presentation. Visual-alone task. Next, each child practiced the visual matching task. The experimenter instructed and assisted the child on the first trial. Additional assistance was provided until the child was able to make four consecutive correct choices without the experimenter’s intervention. As in all of the following procedures, including both single- and dual-task versions, the child was given feedback on every matching trial by the appearance of a star in place of the chosen shape when the response was correct, and by the addition of a small star to the cumulative set of stars awarded for correct matches and displayed in columns along the right side of the screen. After the practice, baseline control data were collected for the visual-alone task. The child was told to find the correct match as quickly as possible to see how many correct answers (and how many stars) he or she could gain in one minute. At the end of this trial, the shapes vanished and animated feedback showed how many colored stickers the child’s performance merited. Dual task. Children were next asked to continue trying to match the shapes as quickly as possible while they heard the words bee, bow, tea, and toe. They were told to try to ignore the words so that they would not interfere with the matching game. However, as soon as the child saw the pictures of the bee, bow, tea, and toe on the computer screen, the picture representing the word heard last was to be selected. Children were also told that it was possible to do well on this task without attending to the words. After the last word of each sequence and a subsequent delay of 1, 5, or 10 s, the probe pictures suddenly appeared on the computer screen where the matching stimuli had been. When a probe picture was selected, the child saw the animated feedback and then put reward stickers in their album. No feedback was provided on the accuracy of the response to the memory probe. For each of 12 test sequences, a different sequence of the four spoken words was created and was presented with inter-word silent intervals of 1, 5, and 10 s randomly intermixed. These sequences varied in length from about 0.5 to 1.5 min. Each test sequence ended with a different auditory stimulus, one of the three tokens of the four words. The stimuli and the ISIs were in the same pseudorandom order for every participant, with one

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important exception. The critical final delays between the test target word and the cue to recall were counterbalanced between participants by using three distinct assignments of memory test delays to trials. For each participant, each word was probed once at each delay, and each of the 12 tokens (three per word) was used once. Across participants, each trial position and each word token was probed an equal number of times at test delays of 1, 5, and 10 s. Visual-alone follow-up task. The final task was another one-minute trial of the visual matching task performed without any auditory stimuli. This provided another sample of single-task performance after the lengthy practice of the previous trials. The entire session lasted approximately 45–50 min. Results

Auditory Memory Performance Each word identification response was scored separately for correctness of the consonant and of the vowel. These scores were averaged across trials to yield proportions correct for consonants and vowels at each delay, for each participant in each condition. To gain a more complete picture of the memory results, recognition accuracy by word also was analyzed. In this scoring method the response was counted as correct only if both phonemes were correctly identified. Auditory-alone task. In the auditory-alone practice session, performance was nearly perfect at all test delays. Across test delays, first-grade children correctly identified .97 of the consonants and .98 of the vowels, and fourthgrade children correctly identified .99 of the consonants and over .99 of the vowels. Auditory memory within the dual task. In the dual-task experimental conditions, performance was slightly worse than the single-task auditory practice at the 1-s delay and declined across the longer delays. This decline was more pronounced for the consonants than for the vowels. A similar pattern of performance occurred for both age groups, as shown in Table 1. Memory performance was examined statistically in a 2 1 3 1 2 mixed analysis of variance (ANOVA) of the proportion correct with phoneme type (consonants versus vowels) and test delay (1, 5, and 10 s) as within-participant factors and age (first or fourth grade) as a between-participant factor. There were significant main effects of phoneme type, F(1,58) Å 5.49, MSe Å .01, p õ .05, and of test delay, F(2,116) Å 15.32, MSe Å .02, p õ .001. Additionally, the Phoneme Type 1 Test Delay interaction reached significance, F(2,116) Å 5.12, MSe Å .02, p õ .01. Specifically, the pattern of results observed in adults by Cowan et al. (1990, Experiment 1), increasing vowel superiority across test delays, was replicated here (see Table 1). The age main effect also was significant, F(1,58) Å 12.48, MSe Å .05 p õ .001. The younger

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DEVELOPMENT OF MEMORY TABLE 1 Recognition Accuracy of Auditory Stimuli by Age Group and Test Delay in the Dual-Task Condition for Children in Experiment 1 Test delay (s) Scoring unit

1

5

10

0.93 0.11

0.88 0.17

0.75 0.22

0.93 0.16

0.88 0.16

0.84 0.19

0.98 0.06

0.95 0.12

0.88 0.17

0.99 0.05

0.98 0.10

0.93 0.15

First-grade children Consonants M SD Vowels M SD

Fourth-grade children Consonants M SD Vowels M SD

children accurately identified .87 of the phonemes, as compared with the older children’s accuracy of .95. However, none of the interactions between age and other factors approached significance. Whole word-recognition scores are plotted in the left panel of Fig. 1. An ANOVA yielded results very similar to the phoneme recognition data, with significant effects for test delay, F(2,116) Å 22.99, MSe Å .02, p õ .001, and age, F(1,58) Å 15.70, MSe Å .05, p õ .001, but no reliable Age 1 Test Delay interaction. Visual task performance. Performance on the visual matching task was measured in terms of reaction time (RT) and accuracy. Mean accuracy and RTs were calculated for each participant for each of the 12 experimental blocks and for the two single-task control blocks. To maintain consistency across trial blocks of differing lengths, only responses made during the last 30 s of each trial block were included in these calculations. Incorrect responses were excluded from calculations of mean RTs. Reaction time and accuracy means for all trial blocks are displayed in Fig. 2. Accuracy scores were consistently high (above .90). Overall means for speed and accuracy were calculated by averaging the two control blocks and averaging the 12 experimental blocks. The first-grade children had mean accuracies of .95 and .95 and mean RTs of 3.52 and

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FIG. 1. Mean proportion correct identification of words at each test delay in the dual-task condition for participants in each of the two age groups used in Experiment 1 (left panel) and each of the three age groups used in Experiment 2 (right panel).

3.53 s, in the single- and dual-task conditions, respectively. The fourth-grade children had mean accuracies .96 and .97 and mean RTs of 2.40 and 2.21 s, in the single- and dual-task conditions, respectively. These accuracy and speed measures were analyzed in separate 2 1 2 ANOVAs with age as a betweensubject factor and task type (single or dual task) as a within-subject factor. The analysis of accuracy yielded no reliable effects for age, task type, or their interaction, but undoubtedly these data were limited by ceiling effects. Analysis of RTs yielded a significant main effect for age, F(1,58) Å 53.67, MSe Å 0.83, p õ .0001; the older group was faster. There was no statistically significant effect for Task Type, F(1,58) Å 1.10, nor for the interaction of Task Type 1 Age, F(1,58) Å 1.33, with p ú .05 in both cases. Most importantly, these visual task data provide no support for the possibility that tradeoffs between the visual and auditory tasks occurred (i.e., no evidence of a single-task advantage). In the younger children, the visual task accuracy and RTs were nearly identical for the single- and dual-task situations. In the older children, performance actually was a little more accurate and faster in the dual-task situation (although not significantly so), probably because more improvement with practice occurred during the early trials (see Fig. 2). The figure illustrates that there was very little actual discrepancy between single- and dual-task performance levels, supporting the assumption that the ignored speech did not require substantial processing resources that would have to be drawn away from the visual matching task. Discussion Recognition accuracy declined over 10 s, with recognition of consonants decreasing more rapidly across delays than recognition of vowels, replicating

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FIG. 2. Mean accuracy (top) and reaction time (bottom) in the visual matching task for each trial block in single-task (unfilled shapes) and dual-task (filled, connected shapes) conditions for first grade children (triangles) and fourth grade children (circles) in Experiment 1.

the finding of Experiment 1a of Cowan et al. (1990) with adult participants. The pattern here was similar for the two age groups, although the older participants performed better than the younger participants overall. In this experiment and Experiment 1a of Cowan et al. (1990), phoneme type is confounded with the order of phonemes within the syllable, which might also explain the vowel advantage. However, in Experiment 2a of Cowan et al., vowel–consonant (VC) syllables were used and no advantage for consonants (as predicted by an order account) emerged. Instead, vowels and consonants were remembered equally well, suggesting that the acoustic simplicity of phonemes and the order within the syllable both influence the relative level of recall of consonants and vowels. Memory performance for the children in the present experiment was, however, better than that reported for adults in Cowan et al. (1990). This level difference might be due to fewer response choices in this experiment (four compared to nine), to the use of real words rather than nonsense syllables,

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to differences in the processing requirements of the primary task, or to other factors. The most interesting possibility is that this experiment used a nonverbal primary task (visual matching) instead of silent reading, thus minimizing verbal (e.g., articulatory and phonetic) interference. Unattended speech may be encoded and retained much better, even by children, when there is no such verbal interference. There was no evidence that participants were dividing attention between the auditory stimuli and the visual matching task. Participants performed the visual task just as well whether or not they were hearing the auditory stimuli. This experiment did not, however, reveal any age differences that are clearly attributable to memory storage. The superior memory task performance for ignored speech in the older children across all three test delays may indicate better memory, but it also may indicate better encoding of the ignored words. A developmental difference in memory storage might be established by (a) changing the demands of the attended task to make it more likely that an Age 1 Test Delay interaction can be observed if there is a difference in memory persistence and (b) increasing the age range to increase the amount of developmental change under observation. The second experiment incorporated both of these design modifications. EXPERIMENT 2

Even while the children in Experiment 1 were apparently absorbed in a moderately complex discrimination task, they were able to encode and remember irrelevant spoken words accurately. Why did the visual task have so little effect on memory for the ignored words? Perhaps substantial forgetting of speech information can be observed only if the primary task engages not only attention but also phonemic encoding processes that require little or no attention (Baddeley, 1986). In a review of research on memory for speech, Cowan and Saults (1995) suggested that there are two types of information about speech sounds that are formed automatically in the brain: auditory sensory information and a more abstract phonological code that can be constructed for read or imagined words as well as for spoken words (see also Baddeley, 1986). It was suggested that the duration of the auditory sensory information in memory can be shortened by acoustic interference, whereas the duration of the phonological information can be shortened by subsequent phonologically coded material (see also Cowan et al., 1990; Penney, 1989). The visual matching task of Experiment 1 would have interfered with neither type of information, and the availability of both auditory and phonological information could account for the very high level of memory for ignored speech in that experiment. In the present experiment, a rhyming task was used instead, in hopes that it would engage the phonological coding mechanism and cause phonological interference, thereby lowering the levels of performance and allowing the interaction

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between age and test delay to be observed in a more sensitive range. The task, matching names that rhyme, is little more complicated that the prior visual matching task, and children were able to perform it consistently and accurately. The participants included adults in addition to two child groups similar in age to the first experiment. Method

Participants Potential child participants were identified from elementary school rolls and recruited by contacting parents by mail and phone. Twenty-seven firstgrade children began the experiment. Two chose to quit and one failed to finish because of equipment problems. The 24 first-grade participants who completed the experiment, including 13 females and 11 males, ranged in age from 6;6 to 7;8 at the time of testing (M Å 7;2). Twenty-five third-grade participants began the experiment. One failed to finish because of equipment problems. The 24 third-grade participants who completed the experiment, including 12 females and 12 males, ranged in age from 8;7 to 9;9 (M Å 9;1). None of the children had participated in Experiment 1. Twenty-seven adults participated for credit in their introductory psychology class. Two adult participants failed to finish the experiment because of equipment problems and the data of another were unusable because of a storage error. The other 24 adults, including 14 females and 10 males, had an average age of 19;4. Participants had normal hearing and vision according to the same description as in Experiment 1. Apparatus and Stimuli The apparatus and the auditory stimuli were the same as in the first experiment. The sound intensity was measured at 52–57 dB(A). The visual stimuli consisted of 14 sets of pictures. Each set was composed of five pictures with names that rhymed (e.g., mail, nail, snail, tail, and pail). The pictures were color drawings based on illustrations from several different children’s books and games. This rhyme matching game had four alternatives arranged around a central probe, similar to the visual matching stimuli used in previous experiments. However, the matching objective was different in this experiment. Participants were to select the surrounding picture whose name rhymed with that of the center picture. Each of the four choices belonged to a different set of rhyming stimuli. Within each block of trials, the four surrounding pictures remained constant. Only the central picture changed after each response, always rhyming with one of the four alternatives. For example, during one particular test sequence, pictures of the choices nail, book, cat, and ring were shown, with a picture of a hook in the center. The correct choice would be the picture of

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the book. After feedback on that response, if a picture of a snail appeared, the picture of the nail was to be selected; and so on. A variety of matching trials were constructed, using balanced permutations of stimuli, to make the task as consistently challenging as possible. The same four choice pictures were maintained for blocks of trials that lasted until a memory trial occurred. A different central probe picture was presented on each trial until all 16 available probes rhyming with one of the picture choices was used once, and then these same probes were repeated in a different random order. Participants received immediate feedback in the rhyme matching game similar to that provided in the visual matching game. The memory probe pictures representing bee, bow, tea and toe were the same as those used in the first experiment. Procedure Auditory-alone task. The initial familiarization and auditory task practice were the same as in the first experiment. Visual-alone task. Unlike the earlier task with geometric shapes, the rhyme matching task required familiarization trials to teach the appropriate name for each picture and ensure that participants could recall that name. (Although these simple pictures directly illustrated their names, other names could apply as well.) Therefore, during the familiarization trials, each picture was presented for 2 s, accompanied by a digital recording of its name heard on the headphones. The pictures were shown in rhymed sets. After each set of five pictures was shown, participants were asked to name each picture as they pointed to it. The experimenter corrected any errors and did not continue until the participant could provide the appropriate name for each picture. Next, the experimenter explained the rhyme matching task and showed the participant a sample trial. Additional assistance was provided until the participant could make four consecutive correct choices without the experimenter’s intervention. The participant was given feedback on every matching trial by the appearance of a star for a correct answer. After the rhyme matching practice, baseline control data were collected. The participant was told to find the correct match as quickly as possible to see how many correct answers (and how many stars) he or she could win in a minute. At the end of this one-minute trial, the participant saw the same animated feedback as provided in the previous experiment. Children were also given colored stickers to paste in their albums. Dual task. After the auditory and visual practice and the single-task trial, the participant was asked to continue trying to match the pictures as quickly as possible while hearing words bee, bow, tea, and toe. As in the dual task condition in the first experiment, participants were told to try to ignore the words so that they would not interfere with the matching game, except when the pictures of the spoken items appeared, at which time the picture represent-

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ing the word heard last was to be selected. All other aspects of this experiment were as in Experiment 1. Visual-alone follow-up task. The final task was another 1 min trial of the visual matching task performed without auditory stimuli. The entire session lasted approximately 50–55 min. Results

Auditory Memory Performance Auditory-alone task. In the auditory-alone practice session, performance was perfect for adults and nearly perfect for children at all test delays. Across test delays, children in both age groups correctly identified over .99 of the consonants and .99 of the vowels. Auditory memory within the dual task. In the dual-task experimental conditions, performance was worse than the single-task auditory practice at the 1s delay, and it declined across the longer delays as shown in Table 2. Memory declined more severely over time for the younger groups. Memory performance was examined statistically in a 2 1 3 1 3 mixed analysis of variance (ANOVA) of the proportion correct with phoneme type (consonants versus vowels) and test delay (1, 5, and 10 s) as within-participant factors and age (first grade, third grade, and adult) as a between-participant factor. There was a significant main effects of phoneme type, F(1,69) Å 4.44, MSe Å .02, p õ .05. Vowel identification (M Å .89) was more accurate than consonant identification (M Å .87). Also, there were significant main effects of test delay, F(2,138) Å 20.92, MSe Å .03, p õ .0001, and age, F(2,69) Å 6.29, MSe Å .07 p õ .01. Moreover, the Age 1 Test Delay interaction reached significance, F(2,138) Å 3.18, MSe Å .02, p õ .05. The Phoneme Type 1 Test Delay interaction that was statistically significant in Experiment 1 was not significant here, F(2,138) õ 1, nor did any other interactions with phoneme type approach significance. The three-way interaction was not significant, F(4,138) Å 1.43. Because phoneme type did not interact with age or test delay, that distinction is perhaps less relevant in this experiment. Thus, it seems important to repeat the analysis ignoring the phonemes and scoring by word, when the response is counted as correct only if both phonemes are correctly identified. Word recognition scores for the adults and children are plotted in the right panel of Fig. 1. An ANOVA of word recognition scores yielded results very similar to the phoneme recognition data, with significant effects for test delay, F(2,138) Å 28.86, MSe Å .03, p õ .0001, age, F(2,69) Å 9.68, MSe Å .07, p õ .001, and the Age 1 Test Delay interaction, F(2,138) Å 4.15, MSe Å .03, p õ .005. One of the most important types of result was the Age 1 Test Delay interactions not found in Experiment 1. To determine whether this finding

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TABLE 2 Recognition Accuracy of Auditory Stimuli by Age Group and Test Delay in the Dual-Task Condition for Children and Adults in Experiment 2 Test delay (s) Scoring unit

1

5

10

0.93 0.17

0.81 0.24

0.72 0.25

0.94 0.15

0.76 0.23

0.75 0.18

0.91 0.14

0.88 0.15

0.83 0.20

0.95 0.13

0.94 0.11

0.82 0.19

0.95 0.13

0.92 0.14

0.87 0.21

0.98 0.07

0.97 0.08

0.93 0.14

First-grade children Consonants M SD Vowels M SD

Third-grade children Consonants M SD Vowels M SD

Adults Consonants M SD Vowels M SD

resulted from the change in the attended task or the increase in the participant age range, the child data were analyzed again, without the adult group, but this made little difference. This analysis yielded significant effects of test delay, F(2,92) Å 18.49, MSe Å .03, p õ .0001, age, F(1,46) Å 4.23, MSe Å .08 p õ .05, and the Age 1 Test Delay interaction, F(2,92) Å 3.63, MSe Å .03, p õ .05. Likewise, an ANOVA of memory performance by word, excluding adults, yielded significant effects of test delay, F(2,92) Å 26.10, MSe Å .04, p õ .0001, age, F(1,46) Å 4.79, MSe Å .09, p .05, and the Age 1 Test Delay interaction, F(2,92) Å 3.48, MSe Å .04, p õ .05. Visual task performance. Performance on the rhyme matching task was measured in terms of accuracy and RT. Accuracy and RT means by trial blocks are shown in Fig. 3. Accuracy scores were consistently high (over .80), though not quite as high as in Experiment 1.

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FIG. 3. Mean accuracy (top) and reaction time (bottom) in the visual matching task for each trial block in single-task (unfilled shapes) and dual-task (filled, connected shapes) conditions for first grade children (triangles), third grade children (circles), and adults (squares) in Experiment 2.

Overall means for accuracy and RT were calculated by averaging the two control blocks and averaging the 12 experimental blocks (see Table 3). These speed and accuracy measures were analyzed in separate 3 1 2 ANOVAs with age as a between-participant factor and task type (single or dual task) as a within-participant factor. Analysis of accuracy produced a significant effect for age, F(2,69) Å 4.23, MSe Å .01, p õ .05, but no reliable Task Type 1 Age interaction F(2,69) Å 2.33, p ú .05. The accuracy analysis did show a reliable effect of task type, F(1,69) Å 28.94, MSe Å 0.001, p õ .0001, but the difference was in the opposite direction to that expected from dual-task trade-offs. The accuracy was actually lower in the single-task condition, .91, than in the dual-task condition, .96. An examination of the accuracy over trials, shown in Fig. 3, reveals that this reversal is probably due to fatigue, so that the last control trial has a disproportionate number of errors. A similar analysis of RTs yielded a significant main effect for age, F(2,69) Å 46.79,

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TABLE 3 Mean Accuracy and Reaction Time (RT) on the Rhyme Matching Task in Single- and DualTask Conditions for Children and Adults in Experiment 2 Condition Measurement

Accuracy RT (s)

Single-task

Dual-task

First-grade children 0.87 5.71

0.93 4.81

Third-grade children Accuracy RT (s)

0.92 3.75

0.97 3.61

Adults Accuracy RT (s)

0.95 2.04

0.97 1.90

MSe Å 3.3, p õ .0001. The older groups were faster. There was no statistically significant effect for Task Type, F(1,69) Å 1.50, nor for the interaction of Task Type 1 Age, F(2,69) Å 1.01, p ú .05. This pattern of results for the rhyme matching task is very similar to that of the visual matching task in Experiment 1. In both experiments, the data provide no evidence of tradeoffs between the visual and auditory tasks. Discussion

This experiment used a verbal primary task (rhyme matching) rather than the nonverbal primary task (visual matching) used in Experiment 1. As expected, the verbal task generated more memory errors and thus lowered performance levels. This experiment also revealed age differences more directly attributable to memory storage than the main effect reported in Experiment 1. The interaction between age and test delay, found here, suggests faster forgetting in younger participants. These results cannot be explain simply in terms of poorer encoding or less reliable recognition judgements in the younger subjects. A remaining concern is that part of the interaction with age may be an artifact of ceiling effects. Adults’ average word recognition accuracy was quite high at the shortest delay (see the right panel of Fig. 1), and some made no mistakes in this condition. However, this concern is alleviated by the analysis that excluded adults. The children scored lower than the adults and had word recognition scores at the shortest test delay very similar to one

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another. Yet, the Age 1 Test Delay interaction emerged even when only those two groups were compared. Once again, there was no evidence that adults or children were dividing attention between the auditory stimuli and the visual matching task. The only significant task effect was a counterintuitive dual-task advantage in accuracy. Participants seemed to perform the rhyme matching task at least as well when they received the auditory stimuli concurrently as when they did not. GENERAL DISCUSSION

The present study examined several aspects of the development of memory for ignored speech. One aspect is the overall level of performance. Experiment 1 showed that fourth grade children could recall ignored speech at a higher level than first grade children. A comparable age difference was not obtained when the children attended to the speech sounds, for which performance was near ceiling in both groups. In this first experiment, though, the rate of forgetting of ignored speech across silent postlist delays of 1, 5, and 10 s was not noticeably different in the two age groups. Therefore, the overall age difference possibly might be attributed to a factor other than memory decay, such as automatic stimulus encoding or rehearsal. The superiority of vowel recall over consonant recall in the ignored speech condition (as in the adult study by Cowan et al., 1990) suggests that the participants relied to a large extent on an acoustic code, which is more useful for the acoustically simpler vowel phonemes (Cole, 1973; Crowder, 1971, 1973; Darwin & Baddeley, 1974; Pisoni, 1973). For a number of reasons it has been assumed (e.g., Broadbent, 1958; Cowan, 1988) that this sensory code is formed automatically, in the absence of attention, more completely than other codes. That assumption would explain, for example, why it is much easier to focus attention on the basis of physical features of a stimulus channel than on the basis of categorical or semantic features (Cherry, 1953; Johnston & Heinz, 1978). Experiment 2 examined memory for ignored speech in children and adults when the primary task, rhyme matching, required verbal processing. The results suggest a developmental difference in memory decay for ignored words when the attended task is verbal. Younger children forgot the ignored speech faster than older children or adults. Although it is not entirely clear whether the relevant memory store here is acoustic, phonetic, or some combination, the results are consistent with Keller and Cowan (1994), who reported a developmental difference in the decay of memory for tone pitch in children and adults. Keller and Cowan suggested their results were evidence for developmental change in an automatic sensory store. This developmental change could also explain the developmental differences in the decay of memory for ignored speech. The absence of performance tradeoffs between the visual matching task

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and auditory memory, in both of these experiments, helps to verify that the supposed ‘‘ignored speech’’ actually was, for the most part, ignored. It suggests that there was little if any sharing of a central attentional resource between the two tasks. In particular, accuracy and speed on the visual matching task was not better in a single-task situation than in the dual-task situation. Of course, one can still argue that these procedures were not sensitive enough to detect subtle tradeoffs. We agree that much more study needs to be done to distinguish between strategic and structural developmental changes in memory. Nevertheless, we believe that this study, along with the tone memory study of Keller and Cowan (1994) discussed earlier, provides some of the first evidence of developmental changes in the automatic, structural aspects of memory. Hasher and Zacks’ (1979) generalization about the lack of development in automatic encoding processes may not apply to auditory sensory memory. There are remaining issues about memory for ignored speech that can be resolved at present only tentatively. One of these is the nature of the memory code for ignored speech. Cowan et al. (1990) suggested that both auditory (sensory) and phonetic codes are used. The pattern of memory across testing delays was said to depend on how those codes were combined. Auditory sensory memory is more useful for vowels than for stop consonants, whereas phonetic memory is equally useful for all types of phonemes. When the task allowed the formation of both codes, but the phonetic code was subsequently eroded by silent reading (Cowan et al., Experiment 1), the result was a Phoneme Type 1 Test Delay interaction. It can be attributed to the increasing superiority of vowels over consonants across test delays, as both codes decay but only the phonetic code also is eroded by silent reading interference. However, when the attended task was reading in a whisper, which may have impaired the initial phonetic encoding of the ignored speech, the outcome included no such interaction. In that situation, it was suggested that the result provided a fairly pure index of auditory sensory memory. The pattern in the present study is similar. The visual matching task of Experiment 1 did not tie up phonetic encoding, and the result included a Phoneme Type 1 Test Delay interaction. In contrast, the rhyme-matching task of Experiment 2 did tie up phonetic encoding, and the result included main effects of Phoneme Type and Test Delay but no interaction of these variables. Despite the absence of a three-way interaction between age, test delay, and phoneme type in Experiment 2, it can be seen in Table 2 that the first-grade children did not display the vowel phoneme advantage in the 5-s condition. We have no definitive account of this anomaly, but one possible explanation is that under some circumstances the use of vowel sound information sometimes was blocked in young children when they concentrated on other vowels within the rhyming task.

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A related issue is whether the age difference that we observed resulted from a difference in the persistence of memory, a difference in the quality of memory encoding, or both. Hitch, Halliday, Dodd, and Littler (1989) found that the phonetic encoding of spoken words appeared stable from a young age, in contrast to the phonetic recording of pictures. Specifically, they found that the length of words in a spoken list affected short-term memory for those words even in young children, whereas the length of the names of pictures to be recalled affected short-term memory only relatively late in childhood. The difference presumably was that young children form a phonetic code for speech automatically but do not do so for pictures. Indeed, when the pictures were labeled for the young children, they did show word length effects (Hitch, Halliday, Schaafstal, & Hefferman, 1991). If a full phonetic encoding of speech turns out to take place at an early age even for ignored lists, then it will follow that the age difference in memory for ignored speech must be a difference in memory persistence, not encoding. Our finding of a difference in the delay effect between first- and third-grade children in Experiment 2, in the absence of an initial level difference at the 1-s delay, also seems consistent with this interpretation. Nevertheless, there may well be encoding differences between children and adults that also affect performance. There are bound to be lingering questions that cannot be resolved within this first study of developmental changes in the persistence of memory for ignored speech. It can be said, however, that there is a developmental increase in the persistence of at least one automatic (attention-free) component of memory, in addition to the well-known developmental increase in the use of mnemonic strategies. In practical terms, the pattern of results suggests that one might expect some after-the-fact comprehension of (and perhaps compliance with) spoken instructions given to children who are not paying attention at the time; but the comprehension in such nonideal circumstances may well be poorer in younger children, and poorer in children who are busy with a verbal as opposed to a nonverbal task. It should be a priority for future research to distinguish between maturational and experiential influences on memory for ignored speech. REFERENCES Baddeley, A. D. (1986). Working memory. Oxford, England: Clarendon Press. Bjorklund, D. F. (1995). Children’s thinking: Developmental function and individual differences. Pacific Grove, CA: Brooks/Cole. Broadbent, D. E. (1958). Perception and communication. New York: Pergamon. Cherry, E. C. (1953). Some experiments on the recognition of speech, with one and with two ears. The Journal of the Acoustical Society of America, 25, 975–979. Cole, R. A. (1973). Different memory functions for consonants and vowels. Cognitive Psychology, 4, 39–54. Colombo, M., D’Amato, M., Rodman, H., & Gross, C. (1990). Auditory association cortex lesions impair auditory short-term memory in monkeys. Science, 247, 336–338.

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Coltheart, M. (1984). Sensory memory: A tutorial review. In H. Bouma & D. G. Bouwhuis (Eds.), Attention and performance X: Control of language processes. London: Erlbaum. Cowan, N. (1984). On short and long auditory stores. Psychological Bulletin, 96, 341–370. Cowan, N. (1988). Evolving conceptions of memory storage, selective attention, and their mutual constraints within the human information processing system. Psychological Bulletin, 104, 163–191. Cowan, N., & Kielbasa, L. (1986). Temporal properties of memory for speech in preschool children. Memory and Cognition, 14, 382–390. Cowan, N., Lichty, W., & Grove, T. (1990). Properties of memory for unattended spoken syllables. Journal of Experimental Psychology: Learning, Memory, and & Cognition, 16, 258–269. Cowan, N., & Saults, J. S. (1995). Memory for speech. In H. Winitz (ed.), Human communication and its disorders (Vol. 4). Timonium, MD: York Press. Crowder, R. G. (1971). The sound of vowels and consonants in immediate memory, Journal of Verbal Learning and Verbal Behavior, 10, 587–596. Crowder, R. G. (1973). The sounds of speech in precategorical acoustic storage. Journal of Experimental Psychology, 93, 14–24. Crowder, R. G. (1983). The purity of auditory memory. Philosophical Transactions of the Royal Society of London, B 302, 251–265. Darwin, C. J., & Baddeley, A. D. (1974). Acoustic memory and the perception of speech. Cognitive Psychology, 6, 41–60. Dempster, F. N., & Rohwer, W. D. (1983). Age differences and modality effects in immediate and final free recall. Child Development, 54, 30–41. Dickstein, P., & Tallal, P. (1987). Attentional capabilities of reading-impaired children during dichotic presentation of phonetic and complex nonphonetic sounds. Cortex, 23, 237–249. Engle, R. M., Fidler, D. S., & Reynolds, L. H. (1981). Does echoic memory develop? Journal of Experimental Child Psychology, 32, 459–473. Eriksen, C., & Johnson, H. J. (1964). Storage and decay characteristics of nonattended auditory stimuli. Journal of Experimental Psychology, 68, 28–36. Flavell, J. H., Miller, P. H., & Miller, S. A. (1993). Cognitive development (3rd ed.), Englewood Cliffs, NJ: Prentice Hall. Frank, H. S., & Rabinovitch, M. S. (1974). Auditory short-term memory: Developmental changes in precategorical storage. Child Development, 45, 522–526. Hasher, L., & Zacks, R. T. (1979). Automatic and effortful processes in memory. Journal of Experimental Psychology: General, 108, 356–388. Hitch, G. J., Halliday, M. S., Dodd, A., & Littler, J. E. (1989). Development of rehearsal in short-term memory: Differences between pictorial and spoken stimuli. British Journal of Developmental Psychology, 7, 347–362. Hitch, G. J., Halliday, M. S., Schaafstal, A. M., & Heffernan, T. M. (1991). Speech, ‘‘inner speech,’’ and the development of short-term memory: Effects of picture-labeling on recall. Journal of Experimental Child Psychology, 51, 220–234. Johnston, W. A., Heinz, S. P. (1978). Flexibility and capacity demands of attention. Journal of Experimental Psychology: General, 107(4), 420–435. Kail, R. (1990). The development of memory in children (3rd ed.), New York: W. H. Freeman & Co. Kausler, D. (1990). Automaticity of encoding and episodic memory processes. In E. A. Lovelace (Ed.), Aging and cognition: Mental processes, self-awareness, and interventions. Amsterdam: North-Holland. Keller, T., & Cowan, N. (1994). Developmental increase in the duration of memory for tone pitch. Developmental Psychology, 30(6), 855–863.

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Lane, D. M., & Pearson, D. A. (1982). The development of selective attention. Merrill-Palmer Quarterly, 28, 317–337. Lu, Z., Williamson, S., & Kaufman, L. (1992). Behavioral lifetime of human auditory sensory memory predicted by physiological measures. Science, 258, 1668–1670. Obrzut, J. E., Obrzut, A., Bryden, M. P., & Bartels, S. G. (1985). Information processing and speech lateralization in learning-disabled children. Brain and Language, 25, 87–101. Pelham, W. (1979). Selective attention deficits in poor readers? Dichotic listening, speeded classification, and auditory and visual central and incidental learning tasks. Child Development, 50, 1050–1061. Penney, C. G. (1989). Modality effects and the structure of short-term verbal memory. Memory & Cognition, 17, 398–422. Pisoni, D. B. (1973). Auditory and phonetic memory codes in the discrimination of consonants and vowels. Perception and Psychophysics, 13, 253–260. Posner, M. I., & Snyder, C. R. R. (1975). Facilitation and inhibition in the processing of signals. In P. M. A. Rabbit & S. Dornic (Eds.), Attention and performance V (pp. 669–682). New York: Academic Press. Rabinowicz, T. (1980). The differentiate maturation of the human cerebral cortex. In F. Falkner & J. M. Tanner (Eds.), Human growth 3: Neurobiology & Nutrition. New York: Plenum Press. Sams, M., Hari, R., Rif, J., & Knuutila, J. (1993). The human sensory memory trace persists about 10 sec: Neuromagnet evidence. Journal of Cognitive Neuroscience, 5, 363–370. Siegler, R. S. (1991). Children’s thinking. Englewood Cliffs, NJ: Prentice Hall. Sipe, S., & Engle, R. W. (1986). Echoic memory processes in good and poor readers. Journal of Experimental Psychology: Learning, Memory, and Cognition, 12, 402–412. Snodgrass, J. G., & Corwin, J. (1988). Perceptual identification thresholds for 150 fragmented pictures from the Snodgrass and Vanderwart picture set. Perceptual and Motor Skills, 67, 3–36. Snodgrass, J. G., & Vanderwart, M. (1980). A standardized set of 260 pictures: norms for naming agreement, familiarity, and visual complexity. Journal of Experimental Psychology: Human Learning and Memory, 6, 174–215. RECEIVED: June 8, 1995;

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