APPLIED COGNITIVE PSYCHOLOGY, VOL. 10, 105-1 19 (1996)

The Short-Term Memory of Profoundly Deaf People for Words, Signs, and Abstract Spatial Stimuli

KAREN LOGAN, MURRAY MAYBERY and JANET FLETCHER

University of Western Aushafia

SUMMARY

The short-term memory (STM) of 25 deaf and 20 hearing adults fluent in Australian Sign Language (Auslan) was tested using both free- and serial-recall versions of three tasks. On two tasks, where stimuli were presented as either written words or Auslan signs, hearing subjects performed significantly better than deaf subjects. This difference was attributed to the facility of the hearing subjects in translating these two classes of languagebased st imul i into phonological codes, which have a preferred status in STM. On the third, language-free task, which was an adaptation of the Corsi Blocks test, the deaf and hearing subjects performed at comparable levels, indicating that differences in their STM became evident only with the introduction of languagebased factors. Analyses restricted to the deaf subjects showed that performances on the language-based STM tasks correlated positively with scores on a reading comprehension test. Also, deaf subjects who reported an oral education outperformed their counterparts, who reported a total communication (oral plus signed English) education on the languagebased STM tasks. Thus, for this diverse adult deaf sample, proficiency in STM for language-based material, skill in reading, and report of an oral rather than total communication education appear to covary.

Research with hearing subjects examining short-term memory (STM) of language- based items has indicated that phonological coding is the primary mode of representation used (Conrad, 1972, 1979). This conclusion was reached upon examination of the types of errors made in short-term memory tasks. Intrusion errors, for instance, when a letter or word was misrecalled, were often items which had a phonological similarity to an item on the original list (Baddeley, 1986; Conrad, 1964). This suggested that the primary method of coding used by hearing subjects had a phonological base (Conrad, 1964). In addition, research with hearing subjects had shown that the recall of serial information is reduced when the items to be remembered are phonologically similar (Conrad and Hull, 1964), reinforcing claims that the representation of such items is of a phonological nature.

Such results posed the question of how the profoundly deaf population, who have restricted auditory coding mechanisms, encode information in STM (Wallace and

Correspondence to Karen Logan/Murray Maytery, Psychology Department, University of Western Australia, Perth, Western Australia 6907. The authors would like to thank the staff and members of the WA Deaf Society for their enthusiasm and support. Particularly, we thank Karen Htndry and Danielle Ashworth for their invaluable assistance.

CCC 08884080/96/020105-15 Q 1996 by John Wiley & Sons, Ltd.

Received 10 August 1994 Accepted 30 May 1995

106 K. Logan et al.

Corballis, 1973). In a pioneering study, Blair (1957) proposed that the memory of deaf and hearing children differs significantly, but the direction of difference depends upon the nature of the memory task. Blair (1957) also noted that for the deaf children tested, performance on all of the memory tests correlated to some extent with measures of reading achievement. Blair inferred from this that a deaf child’s deficiency in memory span and lower reading achievement levels may have a common psychological basis.

Since this initial research, studies using deaf subjects have found that printed stimuli such as letters or words can be encoded in a number of different ways. Studies have demonstrated coding on the basis of shape (Conrad, 1971; Wallace and Corballis, 1973), or via the use of fmgerspelling codes (Hanson, Liberman and Shankweiler, 1984; Wallace and Corballis, 1973).

The nature of signs in any sign language is that meaning is conveyed by the production of formational parameters including hand configuration, location, movement and place of articulation (Hanson, 1982; Johnston, 1989; Moores, 1978; Poimer, Bellugi and Tweeney, 1981). Cherological encoding, based on the formational parameters of signs, has been demonstrated using analyses of intrusion errors in tasks relating to the recall of signs and even print (Bellugi, Klima and Siple, 1975; Bellugi and Siple, 1974; Hamilton and Holzman, 1989; Hirsh- Pasek and Treiman, 1982; Krakow and Hanson, 1985; Shand, 1982).

Evidence has also been found that lists of formationally similar signs can produce performance decrements in the recall of such signs analogous to those reported for hearing subjects when rhyming words are used (Hanson, 1982; Hanson and Lichtenstein, 1990; Shand, 1982). Thus, signs that differ in only one formational parameter may be easily confused in recall tasks (e.g. Bellugi, Klima and Siple, 1975; Hamilton and H o b a n , 1989; Hanson, 1982; Krakow and Hanson, 1985). For example, within Auslan (Australian sign language), the word ‘which’ may be

what today Figure 1. Auslan signs that can easily be confused due to the similarity of their formational

parameters.

Memory of Profoundly Deaf People 107

mistaken for the word ‘maybe’, or the word ‘what’ for ‘today’ (see Figure 1). Hamilton and H o h a n (1989) argued that the confusion of cherologically similar signs evident with their STM tasks reflected post-encoding stages of processing, that is, it did not simply reflect the misperception of their signed stimuli. An important factor in STM research with deaf people has been the connection to

reading ability. Many researchers have assumed that deafness prevents access to English phonology. Hanson (1989) argued that deaf people may indeed have access to phonological information that could be used to support skilled reading. Hanson (1989) claimed that to assume that deaf readers lack access to phonology as a result of their deafness, is to confuse a sensory deficit with a cognitive one. She extended her argument by stating that deaf people could learn about the phonology of English from the motor events involved in speech production, from experience in lipreading or from their experience with orthography through reading.

This notion has been supported in recent studies showing evidence of phonological coding amongst deaf subjects during reading (Hamilton and Holzman, 1989; Hanson et al., 1984; Lichtenstein, 1985). Such studies have shown that deaf subjects who had higher reading skills and were more academically successful, were more likely to use phonological encoding when presented with printed material for written recall (Hamilton and H o h a n , 1989; Hanson et al. 1984; Hirsh-Pasek and Treiman, 1982; Krakow and Hanson, 1985; Lichtenstein, 1985). Krakow and Hanson (1985) suggested that well-educated deaf signers may not translate into their primary sign language when the information to be recalled is in English. Furthermore, there is evidence from deaf samples, tested for written recall of words or even signs, that the greater the reliance on phonological coding, the larger the memory span (Hanson, 1982; Hanson and Lichtenstein, 1990; Lichtenstein, 1985). Thus, although causal links have not been established, it does seem that skill in reading, reliance on phonological coding, and proficiency in STM for language-based material, covary for deaf samples.

However, it should be noted that profoundly deaf readers of the English language are placed at an immediate disadvantage as they are required to read an orthography that more closely represents the phonological structure of spoken English (Hanson, 1989; Mattingly, 1972). No such relationship exists between sign language and English orthography (Krakow and Hanson, 1985).

Baddeley (1979) and Crowder (1978) have suggested that the use of a phonological code may be especially well suited to the retention of order of verbal information and that it contains unique properties that gives it a preferred status in STM. If this is correct, whether or not a deaf subject is able to make use of a phonological code should influence histher performance on tasks of serial order recall (Hanson and Lichtenstein, 1990), but not necessarily free recall (Kyle, 1989). Hanson (1982) did find evidence of phonological coding among deaf subjects when they were required to provide ordered, rather than free, recall.

This has resulted in the speculation that the STM coding of deaf bilinguals (who are competent in both sign and English) is flexible and can change depending on stimulus input or on task requirement (Krakow and Hanson, 1985). It appears that deaf subjects use cherological codes for signs, and a variety of codes-phonological, cherological or visual-for print (Hamilton and H o b a n , 1989). The type of code used, however, is likely to be influenced by the required method of recall.

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Researchers further suggest that certain characteristics that vary between deaf subjects, such as their degree of hearing loss, language background and educational history, as well as the specific context and demands of a particular task, may all influence the type of coding used (Bonvillian, Rea, Orlansky and Slade, 1987; Conrad, 1979; Hanson, 1982). An additional factor governing the performance of deaf subjects on STM tasks

when compared to hearing is the nature of the material to be recalled. The tasks previously cited have all involved language-based material, whether it be the recall of letters, printed Engllsh words or ASL or BSL signs, and, compared to hearing subjects, the deaf have consistently performed at a lower level. However, Blair (1957) reported non-significant differences between deaf and hearing subjects on the recall of non-verbal material. Particular tasks, such as the Knox Cube test, even showed that the deaf had superior recall of visual materials (Blair, 1957). This led to the suggestion by Cumming and Rodda (1985) that when language factors are held to a minimum, the deaf will perform as well as the hearing. It is when language factors are involved that differences between deaf and hearing emerge (Conrad, 1979).

In the present study, free and serial recall for printed words, signs, and visuo- spatial stimuli were assessed for a congenitally profoundly deaf group and a hearing group fluent in sign. A reading test was also administered. The classes of stimuli for the STM tasks were selected, firstly, to represent the primary language differences of the deaf and hearing subjects (signs and words), and secondly, to provide a contrast between language-based and language-free stimuli (signs and words versus visuo- spatial stimuli). The visuo-spatial tasks were computerised versions (see, e.g., Smyth and Scholey, 1992) of the Cord Blocks task (Milner, 1971).

There are two sources of evidence that the stimuli for the Corsi tasks-spatial locations marked by uniform stimuli (in our case, positions on a screen marked by illuminating squares )-are not encoded linguistically. First, several studies have shown that concurrent performance of a language-based task (e.g. counting ‘1 2 3 4 5’ repeatedly) has a negligible effect on memory for sequences of these visuo-spatial stimuli, but a substantial detrimental effect on memory for sequence of language- based items (Smyth, Pearson and Pendleton, 1988; Smyth and Scholey, 1992, 1994). Second, several neuropsychological studies report patterns of dissociation inconsistent with a language-based representation of the Corsi stimuli. For example, De Renzi and Nichelli (1975) described two patients with severely impaired Corsi-task performance, but normal retention of sequences of digits, and two additional patients with spared Corsi-task performance, but impaired retention of the digit sequences. A similar double dissociation was reported by Hanley, Young and Pearson (1991). Thus there is substantial evidence to support characterising the Corsi tasks as language-free.

It was predicted that deaf subjects would perform better on the STM tasks involving signs compared to those involving words. Hearing subjects on the other hand, were predicted to perform better on the STM tasks involving words compared to those involving signs. These predictions reflect the primary language differences of the two groups. Nevertheless, when comparing the two subject groups, the hearing subjects were expected to outperform the deaf subjects for both the sign and word stimuli conditions. This is because the signs chosen for th is study can be readily tradslated into phonological codes, which may be optimal for the short-term retention of language-based material, and with which the hearing are more adept.

Memory of Profoundly Deaf People 109

However, it was predicted that the deaf and hearing subjects would show comparable performance levels for the language-free visuo-spatial STM tasks, for which phonological coding should be available to neither group.

It was also expected that hearing subjects would show a reduced serial-versus-free recall difference on the language-based tasks (words and signs) compared to deaf subjects. This was because the hearing subjects were expected to be more competent in their use of phonological coding, which would assist them in the recall of order information, and thereby close the gap between serial and free recall. This interaction between subject group and recall condition (serial versus free) was not expected for the visuo-spatial stimuli, where phonological coding was not anticipated, and therefore would not be available to provide an advantage to the hearing subjects in serial recall.

If any deaf subjects indicated the use of phonological coding, it was expected that they would be the more proficient readers. These subjects would be expected to show improved serial recall across the language-based tasks when compared to other deaf subjects. Accordingly, among the deaf subjects, access to and use of phonological coding should correlate with reading proficiency. Therefore, better readers were expected to show improved serial recall across the language-based tasks when compared to other deaf subjects.

METHOD

Subjects

Two groups of subjects participated in this study. A group of 25 congenitally profoundly deaf subjects were obtained via their association with the Western Australian (WA) Deaf Society. Their ages ranged from 23 to 67years, with a mean age of 37 years. Subjects were asked to indicate their degree of hearing loss if it were known, and of those who replied, the range was from 75 dB to lOOdB hearing loss, with an average hearing loss of 94dB. Of those who did not know, all indicated that they were profoundly deaf, and all subjects used Auslan as their primary forin of communication. In addition, staff at the WA Deaf Society indicated that all subjects had a hearing loss in the profound range.

The second group of subjects consisted of 20 hearing people who were familiar with Auslan. Subjects were either accredited Auslan interpreters, students completing their interpreting course at a metropolitan tertiary institution, or staff at the WA Deaf Society. Their ages ranged from 18 to 53 years with a mean age of 33 years. All subjects were fluent in Auslan, with a minimum of two years' experienced

Apparatus and procedure

General procedure All subjects were tested individually on each of the seven tasks in a single 75-minute session. The six tasks involving STM were the free and serial recall versions of: (i) a

'A measure of Auslan proficiency was not available. However, the hearing subjects as a group, were undoubtedly less proficient than the deaf sample, for whom Auslan was the primary form of communication. This lack of comparability of the samples is not cruciaJ since our rcsults show that the hearing sample outperformed the deaf sample on STM for signed stimuli.

110 K. Logan et al.

memory for signs task, (ii) a memory for printed English words task and (iii) the Corsi Blocks task. The seventh task was the passage comprehension task of the Woodcock Reading Mastery Test-Revised (WRMT-R), Form G, assessing reading comprehension. Order of presentation of the six memory tasks was counterbalanced as accurately as possible, given the numbers in each subject group.

There were ten trials for each of the six memory tasks, six trials at length six, followed by four trials at length seven. The first two trials were treated as practice trials. For the serial-recall tasks, subjects were instructed to report the items of each sequence in the order in which they had been presented. For the free-recall tasks, they were instructed to report the items in any order.

Instructions for each of the tasks were presented to all subjects in printed English on their response sheet. In addition, hearing subjects had their instructions read out to them by the experimenter, whilst deaf subjects had the instructions signed by an experienced Auslan interpreter. To ensure that exactly the same information was conveyed to each deaf subject, the signed instructions were recorded on videotape.

Corsi task The language-free STM task consisted of a computerised version of the Corsi Blocks task. The task was presented to subjects on a Hyundai 386 IBM compatible computer and an NEC Microtouch colour monitor with a 36cm touch-sensitive screen. Ten 1.5 x 1.5 cm yellow square outlines appeared on the computer screen at randomly distributed locations and remained visible throughout the task. The experimenter initiated each trial when the subject was ready. Squares lit up one at a time for 1.5 s per square to form sequences of six or seven squares. At the end of each sequence the ten squares turned red for a period of 1.5 s, then back to yellow and this was the subject's cue to respond. The responses were recorded using the touch- sensitive computer screen and squares lit up when touched, to indicate that the response had been recorded.

Stimuli for the sign and word memory tasks A set of 64 signs was selected from Johnston's (1989) Auslan dictionary. All signs chosen were nouns, had a one-word English equivalent, and had only one meaning in Auslan (some Auslan signs may have a related meaning that is normally distinguished by the accompanying lip patterns). Signs were chosen with the assistance of the Auslan coordinator of Western Australia, and a qualified community worker who is an interpreter for the deaf. A pilot study ensured that the signs chosen were easily understood by both groups of subjects. There were four lists of signs, each containing the same 64 items, but in a different random order. There were four corresponding lists of English words. These four lists were assigned at random to the four sign and word tasks for each subject, but so that each list was used approximately equally often for each task in each subject group.

Signs task For consistency of presentation, the memory for signs task was presented to subjects on videotape. The signer for the recording was the Auslan coordinator of WA, who is a native signer of Auslan and is profoundly and congenitally deaf. Thus the signing reflected the common dialect of Auslan in WA.

Memory of Profoundly Deaf People 11 1

The signer was framed from forehead to waist for maximum visibility. Signs were videotaped at the rate of one sign per 1.5 s, with lapses of approximately 10 s at the end of each sequence to allow for subjects’ responses. This interval could be extended if subjects required more time to respond. The subjects responded by writing the English word equivalents for the signs on a prepared response sheet. Consistent with past research, the signs were filmed with lip patterns suppressed.

Words task The words for this task were displayed on the VGA computer screen in 5cm type and words within a sequence were presented at intervals of 1.5 s. At the end of each sequence, the word ‘RECALL‘ appeared on the screen, indicating it was time for subjects to recall the sequence. Responses were written as for the signs task.

Method of scoring on the free and serial recall versions of the STM tasks Free-recall responses were scored as to the proportion of items correct, irrespective of order. Serial-recall responses were scored for the longest sequence in correct order, with the number of items in this sequence expressed as a proportion of the total number of items presented. Using this method, the reported sequence bicycle, shower, holiday, family, night, play, for the stimulus sequence play, family, bicycle, shower, holiday, night, would be scored as having three out of six items correct (proportion of items correct = 0.50). Even though the words bicycle, shower and holiday are not in their correct absolute positions, they are in correct order with reference to each other. This method of scoring allowed unambiguous scoring of incomplete sequences such as holiday, night (proportion ofitemscorrect = 0.33). Although illustrated with words, the same procedures were used to score the sign and Corsi tasks. The reported analyses are based on proportions computed across the four test trials at each of the two sequence lengths, six and seven, because preliminary analyses showed that sequence length did not interact with any other variable.

Woodcock reading test At the end of six STM tasks, subjects completed subsection six from the WRMT-R, Form G. This test is concerned with reading comprehension. Subjects were shown a sentence (sometimes accompanied by a picture) and were asked to write down the word that was missing as indicated by a blank space in the sentence. The test was scored according to the manual, whereby testing was discontinued after six consecutive errors and each correct answer was scored as one point.

RESULTS

Comparison of deaf and hearing subjects on visuo-spatial memory

For the visuo-spatial task, a 2 x 2 mixeddesign ANOVA was conducted on the proportion of items scored correct, with Group (deaf or hearing) as a between-subjects

*Separate analyses were conducted for the language-based tasks (words and signs) on the one hand, and the visuo-spatial Corsi task on the other. This was for two reasons. First, the central predictions map directly onto effects in thesc analyses. Therefore, had a more inclusive analysis been conducted, follow-up tests analogous to these effects would have been required to evaluate these predictions precisely. Second. a different level of chance succcss was expected for the visuo-spatial task compared to the two language- based tasks. This w a s because there were ten fmed response options for the visuo-spatial task. whereas the response options were virtually unlimited for the language-based tasks.

112 K. Logan et al.

factor, and Recall Type (free or serial) as a repeated-measures factor. The deaf and hearing subjects performed comparably on both the serial- and free-recall versions of the visuo-spatial task, with neither the Group main effect nor the Group x Recall Type interaction approaching significance. The only significant effect was the main effect of Recall Type, F(1,43) = 285.00, P< 0.001, with mean = 0.66 [standard deviation (SD) = 0.141 for serial recall, and mean = 0.93 (SD =0.07) for free recall.3

Comparison of deaf and hearing subjects on memory for words and signs

For the language-based tasks, a 2 x 2 x 2 mixed-design ANOVA was conducted on the proportion of items correct, with Group (deaf vs hearing) as a between-subjects factor, and Stimulus Type (words vs signs) and Recall Type (free vs serial) as repeated-measures factors.

Two effects involving the Group factor were significant. First, the Group main effect yielded F(1,43) = 62.49, Pe 0.001. As expected for these language-based tasks, the hearing subjects (mean = 0.67, SD = 0.15) outperformed the deaf subjects (mean = 0.45, SD = 0.14). Second, the Group x Stimulus Type interaction, which is depicted in Figure 2, yielded F(1,43) = 40.18, P< 0.001. There are several noteworthy features to this interaction, which are confirmed by post-boc t-tests conducted with a Bonferroni adjustment (Pedhazur, 1973). In particular, the deaf subjects scored higher for sign stimuli compared to word stimuli [t(24) = 2.61, P= 0.01 1, whilst the hearing subjects scored higher for word stimuli compared to sign stimuli [t(19)=6.21, PcO.OOl]. These differences are consistent with the primary languages of the two subject groups. Nevertheless, as predicted, the hearing group outperformed the deaf group on both of the language-based tasks, with t(43) = 4.62, P<O.OOl, for sign stimuli, and t(43)=9.50, P<O.OOl, for word stimuli.

The other main effects in the analysis, those of Stimulus Type and Recall Type, were also significant, with F(1,43)=4.55, Pq0.05, and F(1,43)=92.29, PcO.001, respectively. However, these effects were qualified by a Stimulus Type x Recall Type interaction, F(1,43) = 9.68, P< 0.01. As is evident from Figure 3, the difference in serial and free recall is more pronounced for the sign stimuli compared to the word stimuli. That is, the requirement to report items in order under serial recall had a more deleterious effect when the stimuli were signs rather than words. One interpretation of this pattern is that phonological codes can be generated more

3The problem of merent levels of chance success (see Footnote 1) is also relevant to the comparison of free and serial recall, at least for the visuo-spatial task, where responses are constrained to the ten squares. For example, with free recall of a sequence of length 7, any seven responses must result in at least four items correct. A lower level of chance success applies to serial recall, where responses are scored with respect to order. Although we are interested in interactions rather than simply the serial- versus free-rccall difference, we nevertheless decided to rcanalyse the visuespatial responses using a stringent scoring criterion that drastically reduoed the levels of chance accuracy. Accordingly, we reanalysed the sequences of length 6, scoring each sequence as correct only if all items composing it were correctly reported (for serial recall this meant in the correct order as well). The number of sequences correct (maximum 4) was then submitted to a Group (deaf vs hearing) x Recall Type (serial vs free) ANOVA. Consistent with the analysis reported in the body of the text, the ody significant effect was that of Recall Type, L7(1,43)=52.54, P<O.OOl, with neither of the effects involving Group approaching significance. Mean number of q u e n w comct was 1.53 for serial recall and 2.91 for free recall. Since this method of scoring r e d m the level of chance success to 0.02 sequences correct for free recall. it is unlikely that the pattern of effects we report is due to differential levels of chance success for free versus serial recall.

Memory of Profoundly Deaf People 1 13

TYPE OF STIMULI Figure 2. Proportion of items recalled correctly as a function of Group and Stimulus Type.

readily for words than for signs, and this form of coding especially benefits retaining the order of items under serial recall.

Finally, Group did not enter into any interactions with Recall Type. Thus the hearing group did not show a reduced serial- versus free-recall difference on either of the language-based tasks, compared to the deaf group. We had expected this form of interaction, given the assumption that the hearing subjects would be the more adept users of phonological codes.

Reading level of deaf and hearing subjects

In addition to the central analysis on recall accuracy, a t-test was carried out to examine the differences in reading scores between deaf and hearing subjects. Deaf subjects (mean= 29.68, SD= 7.12) scored significantly lower than hearing subjects

114 K. Logan et al.

TYPE OF RECALL

Figure 3. Proportion of items recalled correctly under free and serial recall for signs and words.

(mean = 60.75, SD =4.41) on the passage comprehension section of the WRMT-R, t(43) = 4.13, P < 0.001.

Characteristics of deaf subjects

A number of analyses were carried out using the data from deaf subjects only. This was done in order to examine two of the variables cited in past literature that were thought to influence the performance of deaf subjects on memory tasks. The variables assessed were reading level and type of education.

A correlational analysis was undertaken to determine the relationship between a deaf person’s reading level and his or her performance under the free- and serial- recall versions of the three memory tasks. Reading level was found to be significantly correlated with the serial recall of the signs task (r=0.49, P<O.05), serial recall of

Memory of Profoundly Deaf People 115

the words task (r=0.46, P<0.05) and the free recall of the Corsi task (r=0.47, P < 0.05).

Deaf subjects were also asked to indicate where they went to school, and what type of teaching method predominated: oral, total communication, cued speech or a regular class. Subjects were then classified into one of two groups, those who received a strictly oral education (oral, cued speech or a regular class), or those who received oral training as an adjunct to signed English (total communication). There were 10 subjects in the oral education group and 15 in the total communication group. Two ANOVAs were conducted to compare these groups on recall performance.

First, for the visuo-spatial task, a 2 x 2 mixed-design ANOVA was conducted on the proportion of items scored correct, with Education (oral vs total) and Recall Type (free vs serial) as the factors. Neither the main effect of Education nor its interaction with Recall Type approached ~ignificance.~

Second, for the language-based tasks, a 2 x 2 ~ 2 mixed-design ANOVA was conducted on the proportion of items scored correct, with Education (oral vs total), Stimulus Type (words vs signs) and Recall Type (serial vs free) as factors. In this case, a significant main effect was found for Education, where the deaf subjects educated orally (mean =0.50, SD = 0.13) outperformed the deaf subjects educated via total communication (mean = 0.42, SD = 0.15). No other effects involving the Education variable approached significance.

DISCUSSION

As anticipated, deaf subjects performed better on the signs tasks than the words tasks, whereas hearing subjects performed better on the words tasks than the signs tasks, reflecting the different language experiences of both groups. It is important to note, however, that deaf subjects scored significantly lower than hearing subjects on both sets of tasks, indicating that language experience did not account for all aspects of performance. This pattern of results is consistent with past research showing that hearing people have larger STM spans than deaf people for language-based material generally (Bellugi er al., 1975; Conrad, 1972, 1979).

These performance differences may reflect differences in the structure of the two languages used by the subjects. The English language has a relatively fmed word order and a grammatical structure that is essentially sequential. A mechanism based on phonological codes may be the most efficient way of retaining the sequential information represented in English (Hanson, 1989), and hearing adults may use this mechanism to retain ordered information from other modalities when that information can be recoded simply into phonological forms,

English is not the most natural form of communication for deaf subjects and many years of specific instruction are required for them to become proficient in the use of the English language. The information that deaf people can gather from lipreading, reading and speech production appears to allow them access to a phonological

‘In this analysis and the next, effects not involving the Education factor are omitted. In every case, the outcomes for these effects were the same as in the earlier analyses involving hearing as well as deaf subjects.

116 K. Logan et al.

coding system, but cannot compare to the ease with which auditory information can be transformed into a phonological code by hearing subjects. Thus, a deaf person’s use of phonological coding is not likely to reach the same level of proficiency as that of a hearing person.

Auslan, the primary language for our deaf group, as with other sign languages, is less dependent on sequential order than English. Furthermore, the cherological codes derived from signs may be intrinsically less suited to the retention of ordered language-based information compared to phonological codes. Thus, the poorer STM performance of the deaf subjects for signs, as well as for words, may reflect either a reliance on less-than-optimal cherological codes, or the inefficient use of phonological codes.

The latter alternative has some support in the analyses of individual differences among deaf subjects. In particular, reading level correlated with the serial recall of both sign and word stimuli. Also, those who reported an oral education outperformed those who reported a total communication education on the language-based STM tasks. Thus a facility in using phonological codes may be augmented by an oral education, and may contribute to proficiency in both reading and STM for language-based information. This interpretation is, of course, speculative, since the data on education were gathered through retrospective self reports, and other potentially relevant variables were not controlled in this diverse sample of deaf adults.

However, the other alternative-that the poorer performance of the deaf subjects on the language-based STM tasks reflects a reliance on less-than-optimal cherological codes-has some support in our informal observations of the deaf subjects as they completed the word tasks. Thirteen of the deaf subjects showed evidence of overt signing, consistent with coding the word stimuli in a cherological form. These subjects tended to be the poorer readers, as indicated by the correlation between reading level and the existence of overt signing [point biserial correlation= -0.56, r(23)=3.24, P<O.Ol]. Also, 12 of the 13 had experienced a total communication rather than an oral education. Thus, the individual differences among the deaf subjects in STM for word and sign stimuli may at least partly reflect different language preferences, not surprisingly, a reliance on sign for those with a total communication education and a reliance on English for those with an oral education.

It is possible that the deaf subjects were adversely affected by the method of response for the sign tasks. Writing may have been more difficult for them, although there is some evidence that the written response mode does not adversely affect the performance of deaf subjects (e.g. Bellugi and Siple, 1974; Hamilton and Holzman, 1989; Krakow and Hanson, 1985). Alternatively, the translation from Auslan to Enghsh may have been more difficult for them than the hearing subjects, being a translation from a primary to a secondary language rather than the reverse.

Support was found for the hypothesis that deaf and hearing subjects would perform comparably on the Corsi task. This visuo-spatial task is not conducive to phonological coding (Hanley et al., 1991; Smyth et al., 1988). Therefore, our results favour the conclusion that hearing subjects have an advantage on STM tasks only when the items to be recalled have a language-based content that enables phonological coding. This pattern of results was anticipated by Bellugi et ul. (1975), Kyle (1989), Moores (1982) and Zwiebel and Mertens (1985).

Memory of Profoundly Deaf People 117

As one would expect, performance on free recall was superior to that on serial recall. However, in addition, this free-serial recall difference was smaller for the word stimuli compared to the sign stimuli. This probably reflects the easier access to phonological coding for words relative to signs. Phonological coding aids the retention of order information, thereby improving serial recall relative to free recall.

Contrary to expectations, the free-serial recall difference on the language-based tasks was not any less pronounced for the hearing subjects compared to the deaf subjects. Also, the Stimulus Type x Recall Type interaction discussed in the previous paragraph was not modified by Group. These outcomes are consistent with at least some deaf subjects making effective use of phonological encoding in support of serial recall. This notion is supported by the correlations calculated for the deaf sample: those who scored higher on the reading test tended to score higher on the serial recall of the language-based tasks. Thus, when phonological coding is available to a deaf subject, it appears to facilitate the serial recall of both printed words and signs, just as it does for a hearing subject.

What is surprising is that reading competence of the deaf participants correlated with performance on the visuo-spatial Corsi task. However, this correlation is consistent with an explanation advanced by Smyth and Scholey (1992) for their finding that Corsi span correlated with articulation rate, albeit at a lower level than the correlation between verbal span and articulation rate. They argued for the involvement of general purpose executive processes in the retention of order information on the Corsi span, verbal span and articulation rate tasks. Because reading requires the retention of order information, our correlation between reading competence and Corsi task performance can be explained in similar terms. If the general purpose processes postulated by Smyth and Scholey (1992) are viewed as falling under the ambit of the central executive of Baddeley and Hitch’s model of working memory (see Baddeley, 1986), then this proposal is not inconsistent with the view that the Corsi stimuli are represented in a different form to language-based stimuli. In particular, retention of the Corsi stimuli may involve the visuo-spatial sketch pad system (Hanley et al., 1991), whereas language-based information may involve the articulatory loop system. Based on phonological codes that preserve the intrinsic order of language-based information, the articulatory loop may supplement the retention of order information in tasks conducive to this form of coding.

conclusion

The results of the present study, examining the STM of deaf and hearing adults who are literate in Auslan, reinforce the notion that hearing subjects perform better than deaf subjects on measures of STM involving language-based materials. The proposition that the STM of deaf and hearing subjects would be comparable when the language demands of tasks were reduced, was supported by the finding that there were no differences between the groups on the free and serial recall of the visuo-spatial Corsi task. The finding of a reduced free-serial recall difference on the words tasks compared to the signs tasks is consistent with the notion that phonological coding supports serial recall, but there was no interaction of this result with hearing status. Some indications that phonological coding supported serial recall were found from the examination of individual differences within the deaf

118 K. Logan et al.

group, showing that reading skill was associated with better performance on the serial recall of language-based tasks.

Thus, the results of the present study are supportive of the literature showing differences between deaf and hearing groups on language-based measures of STM. The present study augments existing knowledge by demonstrating that the STM of deaf and hearing people is comparable on language-free tasks and that the differences in memory performance between the groups only become evident with the introduction of language-based factors.

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