W ord recognition performance is poorer. Recognition of One-, Two-, and Three-Pair Dichotic Digits under Free and Directed Recall

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1 J Am Acad Audiol 10 : (1999) Recognition of One-, Two-, and Three-Pair Dichotic Digits under Free and Directed Recall Anne Strouse* Richard H. Wilson' Abstract A one-, two-, and three-pair dichotic digit test was administered to 1 subjects (20-79 years) with varying degrees of hearing sensitivity. The test was carried out under free- and directed-recall response conditions. The results indicated that recognition performance decreased as a function of increasing age. Statistical analysis using analysis of covariance indicated that differences in performance between age groups were not owing to differences in hearing sensitivity. Thus, with the effects of hearing sensitivity removed, age alone accounted for a significant portion of the variability in the data among age groups. Normative data for the free-recall condition are reported for the two lists of randomly interleaved one-, two-, and three-pair digits included on the Tonal and Speech Materials forauditory Perceptual Assessment, Disc 2.0 (1998) compact disc. A comparison of performance in free- and directed-recall conditions indicated three patterns of results. First, 58 percent of the subjects had normal recognition performance for one-, two-, and three-pair digits under both recall conditions. Second, 39 percent of the subjects had a deficit in the free-recall condition, with normal performance in the directed-recall condition, which was interpreted as representing primarily a cognitive problem. Third, the remaining 3 percent of the subjects showed a deficit in both free- and directed-recall conditions, which was attributed primarily to an auditory problem. Key Words : Aging, auditory perception, compact disc, dichotic listening tests, speech perception, uncertainty Abbreviations : ANCOVA = analysis of covariance, ANOVA = analysis of variance, CD = compact disc, rau = rationalized arcsine unit W ord recognition performance is poorer among older listeners as compared to younger listeners, particularly when listening in the presence of noise or competing sounds (Dubno et al, 1984). Unfortunately, speech perception problems, especially in the elderly population, cannot be predicted solely from the degree of peripheral hearing loss. Rather, speech perception problems reflect both peripheral and central components (Jerger et al, 1989, 19b). In addition to the well-documented age-related decline in pure-tone sensitivity, the *Veteran Affairs Medical Center, Mountain Home, Tennessee ; 'Departments of Surgery and Communicative Disorders, East Tennessee State University, Johnson City, Tennessee Reprint requests Anne Strouse, VA Medical Center, Audiology (126), Mountain Home, TN literature suggests that with advancing age, changes occur within the central auditory system. Since aging is accompanied by a decline in cognitive as well as auditory function, it is important to eamine the etent to which the auditory problems of the elderly can be eplained by concomitant decline in such cognitive functions as memory and attention (CHABA Working Group, 1988). Dichotic listening tasks, in which different auditory stimuli are presented to the two ears simultaneously, can place sufficient auditory and cognitive demands on the auditory system to assess auditory perceptual abilities. Dichotic listening tasks can be made progressively more difficult by lengthening the stimuli (e.g., a hierarchy of one to three digit pairs) (Strouse and Wilson, 1999) and/or by varying the response mode (e.g., free recall vs directed recall) (Jerger et al, 19c). In a free-recall task, stimuli are pre- 557

2 Journal of the American Academy of Audiology/Volume 10, Number 10, November/December 1999 sented dichotically, and the listener must monitor and report items heard in both ears. In a directed-recall task, stimuli are presented dichotically, but the listener is only required to report items heard in one ear. The ear to be reported is cued prior to stimulus presentation. Jerger et al (19c) proposed that the comparison of performance in free- versus directed-recall conditions can differentiate between the two principal factors underlying performance on dichotic tasks : (1) an auditory/structural factor deriving from the anatomy of the auditory pathways and (2) a task-related/cognitive factor deriving from the demands placed on cognitive processing such as memory and speed of mental processing. Specifically, Jerger et al (19) hypothesized that when recognition performance is poor in the free-recall condition, but improves substantially in the directed-recall condition, the problem is primarily in the cognitive domain since memory and attention abilities are insufficient for successful performance when both ears must be monitored simultaneously (Bayles and Kaszniak, 1987 ; Jerger et al, 19b). In contrast, when performance is below normal in both the free- and directed-recall conditions, then the problem is interpreted to be primarily in the auditory domain. Performance does not improve by reducing cognitive demands. Jerger and colleagues eamined this paradigm previously using the Dichotic Sentence Identification Test (Chmiel and Jerger, 1996 ; Fifer et al, 1983 ; Jerger et al, 19a, 1991, 1994). The current study eamined this interpretation using a dichotic digit paradigm. Dichotic digit materials are ideal for use with elderly listeners because digits (1) are relatively immune to the effects of cochlear hearing loss (Musiek, 1983 ; Speaks et al, 1985, Musiek et al, 1991) and (2) have demonstrated high intertest reliability for both young and elderly adult listeners (Strouse and Hall, 1995 ; Humes et al, 1996 ; Strouse & Wilson, 1999). Moreover, the digit stimuli are generally familiar to most listeners. The new Department of Veterans Affairs compact disc (CD), Tonal and Speech Materials for Auditory Perceptual Assessment, Disc 2.0 (1998), contains two lists of randomly interleaved one-, two-, and three-pair digits in a freerecall paradigm. Using this free-recall paradigm, data from our laboratory on young and elderly listeners with hearing impairment confirm that as the compleity of the task increases from easy (one pair) to difficult (three pair), recognition performance decreases systematically (Wil- son and Jaffe, 1996 ; Strouse and Wilson, 1999). The primary purpose of the current study was to describe normative recognition data for 20- to 79-year-old listeners under the free-recall condition on the interleaved one-, two-, and threepair dichotic digit paradigm. A secondary purpose was to eamine the hypothesis proposed by Jerger and colleagues by evaluating recognition performance of the randomly interleaved one-, two-, and three-digit paradigm in both free- and directed-recall conditions. Materials METHOD The preparation of the stimulus materials is detailed in an earlier paper (Strouse and Wilson, 1999). Briefly, the digital waveform files (1, 2, 3, 4, 5, 6, 8, 9, and 10, spoken by a male) used on the Tonal and Speech Materials for Auditory Perceptual Assessment, Disc 1.0 (Department of Veterans Affairs, 1992) were edited so that the onset of the stimulus coincided with the start of the data file. A silent interval was added to the end of the file to equalize the file lengths to the longest digit (561 msec). A 500-msec silent interval, which served as the interdigit interval for the multipair digit sets, was added to the end of each one-pair dichotic digit file. The two- and three-pair files were made by combining as required two or three of the compiled one-pair dichotic digit files, with an interstimulus interval of 4, 5, and 6 sec following the one-, two-, and three-pair stimuli, respectively. The following two rules were used in the compilation of each multipair digit list : (1) no digit was repeated in a stimulus set and (2) each of the 72 one-pair dichotic sets was used once (randomly) in each presentation position. In this manner, a list of 108 items for the free-recall condition was compiled that contained 36 stimulus sets of the one-, two-, and three-pair digits. Additionally, a 10-item practice list was compiled. Both lists were recorded on CD (Pinnacle, Model RDC- 0). The 108-item list subsequently was divided into two lists of 54 stimulus sets (18 each of the one-, two-, and three-pair digit sets) and recorded as Tracks 7 and 8 on Version 2.0 of the VA-CD Tonal and Speech Materials for Auditory Perceptual Assessment (1998). For the directed-recall condition, the words left and right (recorded by the speaker who recorded the digits) were added as required 500 msec before each digit file. Thus, the word left (or right) was presented binaurally, 500 msec 558

3 Dichotic Digit Recognition under Free and Directed Recall/Strouse and Wilson before the simultaneous presentation of different digit stimulus sets to the right and left ears. Again, a list of 108 stimulus sets was compiled that consisted of 36 of each of the one-, two-, and three-pair digit sets. Half of the stimulus sets (18 sets of each of the one-, two-, and three-pair digits) were recorded on CD as cued to the left ear and the other half of the digit sets cued to the right ear. Different 108-item stimulus sets were used for the free- and directed-recall conditions. Subjects A total of 1 right-handed subjects (129 male, 51 female) were evaluated, ranging in age from 20 to 79 years. There were 30 subjects in each of the following si age groups : 20 to 29 years (mean age = 23.5 years), 30 to 39 years (mean age = 34.6 years), 40 to 49 years (mean age = 45.6 years), 50 to 59 years (mean age = 55.2 years), to 69 years (mean age = 64.6 years), and to 79 years (mean age = 72.9 years). The subjects were recruited from within the audiology clinic and from the community and were paid for their participation. The mean audiometric thresholds for each group are shown in Table 1. For each subject, the severity of hearing loss did not eceed the mild-to-moderate range and asymmetry of the pure-tone thresholds never eceeded 15 db. An analysis of variance (ANOVA) indicated a significant difference in pure-tone thresholds for the right (F [5, 1741 = 29.4, p <.0001) and the left (F [5, 1741 = 31.5, p <.0001) ears. Post hoc analysis using the Tukey test revealed that for both ears, threshold data were not significantly different (p <.05) for adjacent age groups (i.e., 20- vs 30-year group, 30- vs 40-year group, 40- vs 50-year group, etc.) ; however, statistically significant differences (p >.05) were found for comparisons between the remaining age groups. Word recognition performance on a recorded version of the Northwestern University Auditory Test No. 6 (NU-6) (Department of Veterans Affairs, 1991) was >_ percent in each ear and within 10 percent between ears for each subject. There was no significant difference in word recognition performance between the si age groups (F [5, 1741 = 1.8, p >.05). Procedures The dichotic digits were reproduced by a CD player (Sony, Model CDP-497) and fed through an audiometer (Grason-Stadler, Model 10) to TDH-50 earphones encased in P/N 510CO17-1 cushions. All stimuli were presented at db HL (ANSI, 1996). For the free-recall conditions, the subjects were instructed to recall in any order the digit pairs presented to both ears. For the directed-recall conditions, the subjects were instructed to recall only the digits presented to the ear precued relative to the stimulus presentation (Bryden et al, 1983). For eample, the subject first heard right and then Table l Mean Thresholds (db HL)* and Standard Deviations for the 30 Subjects in Each of the Age Groupings Frequency (Hz) Age Group Left Ear (5.5) 8.0 (4.1) 4.8 (4.0) 4.5 (4.6) 5.1 (5.0) 8.2 (6.8) (7.0) 8.7 (5.4) 7.2 (6.1) 7.5 (5.8) 13.5 (12.0) 12.6 (11.8) (6.2) 8.0 (5.1) 8.5 (5.6) 10.1 (7.4) 29.0 (23.4) 29.5 (21.9) (8.3) 11.8 (6.5) 12.8 (9.3) 19.7 (13.9) 35.8 (22.5) 43.5 (24.5) (9.6) 15.2 (9.0) 17.7 (10.7) 24.3 (14.5) 50.5 (23.6).6 (23.1) (6.6) 15.8 (7.4) 16.0 (9.2) 23.7 (11.7) 45.0 (18.2) 55.2 (17.8) Right Ear (5.8) 5.7 (5.5) 5.0 (6.0) 4.2 (4.4) 4.2 (4.5) 9.3 (6.5) (8.1) 9.2 (7.0) 5.5 (6.1) 4.5 (6.9) 10.8 (15.4) 14.3 (12.4) (8.0) 8.5 (5.6) 7.2 (6.5) 8.2 (8.0) 26.3 (24.1) 31.7 (20.1) (6.9) 12.0 (6.9) 12.2 (8.6) 17.7 (15.2) 35.2 (21.8) 43.3 (24.2) (7.0) 16.0 (10.6) 18.0 (11.8) 23.5 (15.4) 45.3 (22.3) 59.5 (23.8) (7.0) 16.0 (8.1) 18.0 (9.0) 25.8(14.2) 47.2 (17.6) 57.2 (19.5) -ANSI (1996). 559

4 Journal of the American Academy of Audiology/Volume 10, Number 10, November/December 1999 digits were presented simultaneously to each ear. The task of the subject was to recall only the digits presented to the right ear. All of the subjects were practiced on the dichotic listening task before data collection. Practice consisted of three to five presentations each of the one-, two-, and three-pair digit stimuli in free- or directed-recall conditions. During the practice sequence, the subjects responded verbally and were queried periodically concerning their comfort in performing the listening/response task. When the subject reported feeling comfortable with the task, which typically was after 10 or so total practice items, data collection began. Both free- and directed-recall conditions were evaluated. The order of presentation of the two test conditions was counterbalanced among the subjects who participated in two trials during separate test sessions. Thus, during each test session, the subject was administered 108 free-recall items preceded by 10 practice items (test time of -14 minutes) and 108 directed-recall items preceded by 10 practice items (test time of -20 minutes). All testing was conducted in a double-walled sound booth. RESULTS he dichotic digit data were scored by presentation position (i.e., each digit in the pair T was scored separately). Thus, for the three-pair digits, there were si possible numbers to be scored as correct or incorrect. For the analysis of the two- and three-pair conditions, the data from the presentation positions were averaged for each subject. Prior to statistical analyses, all percentage scores were transformed into rationalized arcsine units (raus) (Studebaker, 1985). This had the effect of minimizing the relationship between mean score and variance that is characteristic of percentage scores, while providing a scoring unit that is similar to percentages. The data epressed in raus were used for statistical analysis ; however, figures and tables in the following discussion of results are epressed in percent correct. Trial Effects The mean data from the si age groups by trial are shown in Table 2. Although an ANOVA revealed that the main effect for trials was significant (F [1, 1741 = 23.8, p <.0001), inspection of the mean data in Table 2 indicates that the largest percent difference between trials for Table 2 Means and Standard Deviations for the Right and Left Ears of Trials 1 and 2 in Free- and Directed-Recall Conditions Left Ear Trial 1 Trial 2 Right Ear Left Ear Right Ear Free Recall years One pair 99.6 (1.2) 99.7 (1.1) 99.5 (1.3) 99.9 (0.5) Two pair 96.9 (4.7) 97.6 (3.7) 98.1 (3.2) 99.0 (1.3) Three pair 89.8 (8.5) 92.7 (7.1) 93.1 (7.2) 95.4 (3.6) years One pair 96.5 (9.1) 98.3 (6.1) 98.7 (3.3) 99.5 (1.3) Two pair 86.5 (16.4) 95.1 (5.4) 89.2 (15.3) 94.8 (5.9) Three pair 76.0 (16.2) 85.2 (8.6).1 (15.3) 88.2 (8.8) years One pair 96.6 (7.4) 98.9 (4.1) 97.9 (3.7) 98.3 (5.6) Two pair 88.3 (13.0) 94.3 (8.1) 89.8 (14.6) 95.4 (4.8) Three pair 77.3 (16.3) 83.3 (13.2) 78.6 (16.1) 86.8 (8.6) years One pair 93.5 (8.6) 96.9 (5.1) 95.6 (9.9) 98.1 (4.1) Two pair 78.4 (16.2).7 (8.3) 82.8 (17.4) 91.2 (9.2) Three pair 68.2 (15.8) 77.0 (13.2) 73.8 (15.5) 79.9 (13.2) -69 years One pair 93.6 (14.3) 96.9 (6.3) 96.5 (6.0) 99.0 (2.1) Two pair 83.8 (16.3) 91.9 (9.4) 88.0 (10.7) 94.0 (7.0) Three pair 72.0 (16.2).2 (13.8) 76.6 (12.9) 84.1 (13.6) -79 years One pair.3 (13.0) 95.2 (9.9) 93.7 (9.3) 97.8 (3.8) Two pair 74.3 (16.7).6 (10.9) 78.8 (17.0).6 (11.5) Three pair 64.5(18.4) 78.3(14.0).2 (18.4).6 (11.7) Directed Recall years One pair 98.5 (2.9) 98.9 (3.4) 96.5 (5.2) 98.9 (2.7) Two pair 96.1 (4.9) 97.8 (2.7) 96.9 (4.0) 98.1 (2.4) Three pair 95.3 (5.7) 98.4 (2.8) 96.3 (3.8) 98.6(l.9) years One pair 96.3 (7.7) 97.8 (4.5) 94.4 (11.5) 98.3 (3.0) Two pair 94.5 (8.3) 95.5 (8.0) 91.7 (10.5) 95.7 (7.2) Three pair 94.2 (9.8) 96.2 (8.4) 91.8 (11.7) 95.9 (7.9) years One pair 93.9 (11.0) 97.8 (4.3) 95.7 (7.8) 97.2 (8.9) Two pair 94.0 (9.2) 96.0 (6.2) 94.6 (8.6) 97.2 (6.0) Three pair 92.6 (11.1) 95.4 (7.2) 94.4 (7.0) 97.5 (4.1) years One pair 95.0 (8.6) 96.5 (7.1) 94.8 (9.0) 97.2 (6.2) Two pair 94.5 (5.8) 96.5 (6.4) 95.5 (6.3) 95.5 (7.6) Three pair 92.5 (8.2) 96.1 (6.2) 93.2 (9.4) 96.1 (6.1) -69 years One pair 95.6 (9.5) 98.5 (4.6) 98.4 (3.5) 99.8 (3.9) Two pair 94.5 (8.4) 97.8 (4.9) 95.7 (4.6) 94.2 (6.7) Three pair 94.3 (9.8) 97.9 (4.2) 95.3 (4.9) 98.6 (2.3) -79 years One pair.4 (16.6) 97.0 (8.4) 94.2 (11.5) 98.0 (3.9) Two pair.9 (9.5) 95.7 (9.4) 92.0 (11.8) 96.6 (6.8) Three pair 89.8 (13.3) 96.2 (9.6) 92.6 (12.5) 97.6 (5.1) the free-recall condition was for the - to 79- year group. For the three-pair digit condition, there was a difference of 5.7 percent between 5

5 Dichotic Digit Recognition under Free and Directed Recall/Strouse and Wilson trial 1 and trial 2, with recognition performance on trial 2 (.2%) better than recognition performance on trial 1 (64.5%). As each token in the free-recall condition equaled 2.8 percent (%/36 items possible for each of the one-, two-, and three-pair digit sets), the 5.7 percent difference was equivalent to two tokens. In the directed-recall condition, the largest percent difference between trials was for the - to 79-year group, where there was a 3.8 percent improvement on trial 2 (94.2%) compared to trial 1 (.4%) for the left ear in the one-pair digits condition. With each token in the directed-recall condition equivalent to 5.6 percent (%/18 items possible for the right and left ears in each of the one-, two-, and three-pair digit sets), this trial difference was less than one token. Because of the small between-trial differences, in addition to the fact that one trial typically would be used for clinical purposes due to the test length, the dichotic digit data from trial 1 only were used for the remainder of the analyses. Accordingly, statistical analysis using the trial 1 data was performed. For the dependent variable (percent correct recognition), the influence of (1) age group, (2) stimulus condition, (3) ear, (4) stimulus pair, and (5) interactions of these factors was assessed using a mied model ANOVA with condition, ear, and stimulus pair as within-subject factors and age group as the between-subject factor. The results indicated that the main effects for the three independent variables were significant (condition : F 11, 1741= 210.9, p <.0001 ; ear : F [1, 1741 = 73.3, p <.0001 ; stimulus pair : F [2, 3481= 465.6, p <.0001). The main effect of age group also was significant (F [5, 1741 = 7.8, p <.0001). To eamine the effects of hearing loss, the three-frequency pure-tone average (500, 0, and 2000 Hz) and the four-frequency pure-tone average (500, 0, 2000, and 4000 Hz) for the right and left ears were used as covariates in separate analyses of covariance (ANCOVA) that eamined the same variables. The results of the ANCOVA using the three- (F [5, 1721 = 3.6, p <.01) and four-frequency (F [5, 1721 = 3.2, p <.01) pure-tone average were not different than the ANOVA results, indicating that the differences in performance between groups were not owing to differences in hearing sensitivity. Thus, with the effects of hearing sensitivity removed, age alone accounted for a significant portion of the variability in the data between age groups. The main effects and significant interaction effects for these data are described in detail in subsequent sections. Condition Effects Figure 1 shows percent correct recognition by ear and presentation position for the one-, two-, and three-pair dichotic digits presented to the si subject groups in the free- (upper panels) and directed-recall (lower panels) conditions. The means and standard deviations for the left and right ears for each condition are listed in Table 3. The data in the table are shown by presentation position for the one-, two-, and three-pair digit conditions and are averaged across presentation position for two- and three-pair digit conditions in the right two columns. The data for all subject groups indicated that the recognition performance in the free-recall condition was poorer than recognition in the directed-recall condition. For eample, in the free-recall condition, the mean recognition performance for the three-pair dichotic digits presented to the left ear ranged from 89.8 percent (20- to 29-year group) to 64.5 percent (- to 79-year group). In contrast, performance on the three-digit pairs presented to the left ear in the directed-recall condition was better, with a smaller intergroup range (95.3% and 89.8% for the 20- to 29-year and - to 79-year groups, respectively). For the most part, the data in Figure 1 are orderly and systematic. Generally, as the age of the group increased, there was a decrease in recognition performance. There was, however, one eception. The performances on the two- and three-pair digits in the free-recall condition by the - to 69- year group were better than the performances by the 50- to 59-year group. We are unable to account for this discrepancy. Ear Effects The ANOVA indicated a significant difference between scores on the materials presented to the right and left ears (F [1, 1741 = 73.3 ; p <.0001). As previously reported by Kimura (1961), when the dichotic task involved little difficulty as with the one-pair stimuli, there was only a small, if any, right-ear effect. When the listening task became more difficult, as with the three-pair stimuli, the difference between ear preference increased. Consider first the percent correct recognition data from the free-recall conditions displayed in the upper panels of Figure 2. The data are displayed separately for one-, two-, and three-pair digits within each condition. The right-ear scores were higher than the left-ear scores for all subject groups. The ear differences for the youngest 561

6 Journal of the American Academy of Audiology/Volume 10, Number 10, November/December 1999 ONE-PAIR TWO-PAIR THREE-PAIR a g 0 FREE RECALL ~fl o DIRECTED RECALL Left Ear Right Ear Left Ear Right Ear Left Ear Right Ear PRESENTATION POSITION Figure 1 The mean percent correct recognition for the one-, two-, and three-pair digit materials presented to the right and left ears by presentation position for the si subject age groups in free- (upper panels) and directed-recall conditions (lower panels). age group (20-29 years) were small for one-, two-, and three-pair digits (0.1%, 0.7%, and 2.9%, respectively) and were not statistically different by paired t-test (p >.05). Because of the near ceiling performance, there was little difference between right and left ears. With the eception of the 20- to 29-year group, however, performance on materials presented to the right ear was significantly (p <.05) better than performance on materials presented to the left ear. Analysis of the data for the free-recall condition revealed a significant ear by group interaction (F [5, 174] = 2.5 ; p <.05). As can be seen in Figure 2, scores obtained for the right ear were better than scores obtained for the left ear and the right-ear advantage increased as age increased. The linear equations used to fit the mean data for the left ear (Xs) and right ear (Os) are shown at the bottom of each panel in the figure. In the equations, the coefficient of defines the slope of the function. The slope of the function describes the change in performance as a function of age and is epressed as percent/decade. All slopes of the linear functions for the free-recall conditions are negative, which indicates a decrease in dichotic digit recognition as a function of age. For one-pair digits in the free-recall condition (upper left panel), the slopes are percent/decade and percent/decade for the right and left ears, respectively, indicating that the performance on the materials presented to the left ear decreased faster across age than the performance on the materials presented to the right ear. This result was statistically significant by paired t-test 562

7 Dichotic Digit Recognition under Free and Directed Recall/Strouse and Wilson Table 3 Means and Standard Deviations for Digit Materials Presented to the Right and Left Ears for Si Subject Groups Left-Ear Presentation Position Right-Ear Presentation Position Left-Ear Average Right-Ear Average, Free Recall years One pair 996 (1.2) 99.7 (1.1) 99.6 (1.2) 99.7 (1.1) Two pair 96.9 (48) 96.8 (5.1) 97.8 (3.7) 97.5 (4.5) 96.9 (4.7) 97.6 (3.7) Three pair 94.9 (55) 89.4 (9.6) 85.2 (13.7) 95.8 (4.5).2 (11.2) 91.9 (9.6) 89.8 (8.5) 92.7 (7.1) years One pair 96.5 (1.9) 98.3 (6.1) 96.5 (9.1) 98.3 (6.1) Two pair 86.6 (17.4) 86.5 (16.4) 95.8 (4.8) 94.4 (6.7) 86.5 (16.4) 95.1 (5.4) Three pair 82.3 (167) 74.0 (17.4) 71.7 (196) 92.8 (5.0) 81 1 (11.8) 81.7 (13.2) 76.0 (16.2) 85.2 (8.6) years One pair 96.6 (7.4) 98.9 (4.1) 96.6 (7.4) 98.9 (4.1) Two pair 87.7 (13A) 88.9 (14.8) 92.9 (11.3) 95.7 (6.0) 88.3 (13.0) 94.3 (8.1) Three pair 83.9 (14.0) 75.6 (18.6) 72,4 (22.6) 86.2 (14.2) 78.3 (19.7) 85.5 (11.6) 77.3 (16.3) 83.3 (13.2) years One pair 93.5 (8.6) 96.9 (5.1) 93.5 (8.6) 96.9 (5.1) Two pair 79.0 (16.1) 77.9 (20.5).0 (8.2) 91 3 (10.6) 78.4 (16.2).7 (8.3) Three pair 74.1 (163) 64.0 (19.2) 66.5 (18.6) 82.0 (13.0) 71.2 (18.1) 77.9 (14.7) 68.2 (15.8) 77.0 (13.2) -69 years One pair 93.6 (14.3) 96.9 (6.3) 93.6 (14.3) 96.9 (6.3) Two pair 833 (16.6) 84.2 (17 3) 92.1 (7.4) 91.6 (13.3) 83.8 (16.3) 91.9 (9.4) Three pair 77.8 (19.8) 69.8 (18.6) 68.3 (17.9) 86.9 (11.7) 74.6 (17.1) 791 (18.0) 72.0 (16.2).2 (13.8) -79 years One pair.3 (13.0) 95.2 (9.9).3 (13.0) 95.2 (9.9) Two pair 74.5 (20.5) 74.2 (19.0) 89.2 (13.0) 92.0 (10.5) 74.3 (16.7).6 (10.9) Three pair 72.0 (21.6).7 (20.1).8 (22.8) 84.3 (13.4).9 (18.2) 79.9 (14.5) 64.5 (18.4) 78.4 (14.0) Directed Recall years One pair 98.5 (29) 98.9 (3.4) 98.5 (2.9) 98.9 (3.4) Two pair 94.6 (76) 97.6 (4.0) 97.2 (4.1) 98.3 (3.0) 96.1 (4.9) 97.8 (2.7) Three pair 93.5 (8.8) 95.6 (6.8) 96.7 (5.0) 97.2 (4.1) 98.7 (3.2) 99.3 (2.4) 95.3 (5.7) 98.4 (2.8) years One pair 963 (7.7) 97.8 (4.5) 96.3 (7.7) 97.8 (4.5) Two pair 93.1 (8.0) 95.9 (10.4) 95.2 (7.3) 95.7 (9.4) 94.5 (8.3) 95.5 (8,0) Three pair 93.7 (10.5) 95.4 (7.9) 93.5 (13.3) 96.3 (5.5) 96.1 (12.2) 96.1 (11.2) 94.2 (9.8) 96.2 (8.4) years One pair 93.9 (10.9) 97.8 (4.3) 93.9 (11.0) 97.8 (4.3) Two pair 93.3 (9.3) 94.6 (9.8) 94.8 (7.7) 97.2 (5.0) 94.0 (9.2) 96.0 (6.2) Three pair 91 1 (12.3) 92.2 (13.1) 94.4 (11.3) 93.7 (10.1) 95.6 (7.5) 97.0 (4.8) 92.6 (11.1) 95.4 (7.2) years One pair 95.0 (86) 96.5 (7.1) 95.0 (8.6) 96.5 (7.1) Two pair 91.2 (8.9) 94.6 (9.8) 95.6 (8.9) 97.4 (5.0) 94.5 (5.8) 96.5 (6.4) Three pair.9 (10.5) 93.7 (8.7) 93.0 (10.3) 94.8 (9.2) 96.5 (6.4) 97.0 (5.2) 92.5 (8.2) 96.1 (6.2) -69 years One pair 95.6 (9.5) 98.5 (4.6) 95.6 (9.5) 98.5 (4.6) Two pair 94.8 (7.7) 94.3 (10.3) 96.5 (6.4) 99.1 (4.2) 94.5 (8.4) 97.8 (4.9) Three pair 94.6 (102) 93.5 (13.5) 94.8 (8.0) 97.4 (5.4) 98.2 (3.7) 98.2 (4.5) 94.3 (9.8) 97.9 (4.2) -79 years One pair.4 (16.6) 97.0 (8.4).4 (16.6) 97.0 (8.4) Two pair 89.3 (15.0) 92.6 (12.0) 94.1 (11.0) 97.4 (8.4).9 (9.5) 95.7 (9.4) Three pair 887 (12.8) 89.6 (18.4) 91.1 (12,2) 94.8 (12.5) 97.0 (8.7) 96.9 (8.7) 89.8 (13.3) 96.2 (9.6) The data are shown by presentation position for the one-, two-, and three-pair digits in the first two columns and are averaged across presentation position for two- and three-pair digits in the last two columns of the table. (p <.05). For two- and three-pair digits in the free-recall condition, again the slope of the function for the left ear (-3.94 and percent/decade, respectively) is about twice as steep as the slope of the function for the right ear (-1.35 and percent/decade, respec- 563

8 Journal of the American Academy of Audiology/Volume 10, Number 10, November/December 1999 ONE-PAIR TWO-PAIR THREE-PAIR 40 FREE RECALL 1 1 T 1 lx DIRECTED RECALL t 1 } YHE = X ype = X y1,e yle = yle = yle = y y.e= yqe= yqe = yle = yle = yle = V ' 1b Oq 0 ' 9o ~ 1 ~ ;L ' S ' V h ' ;cp 'l ' do 40 44o,~o IV 40 yo ~oti ~o5 '~oop~oh~4o6 1101~ AGE GROUPS Figure 2 The mean percent correct recognition and standard deviations for the right (Os) and left (Xs) ears as a function of age group in free- (upper panels) and directed-recall (lower panels) conditions. Data are displayed separately for one-, two-, and three-pair digits within each condition. The linear equations used to fit the data for the left and right ears are shown at the bottom of each panel. tively). The results from the right and left ears for the two- and three-pair digits also were significantly different (p <.05). The mean data for the directed-recall condition are shown in the lower three panels of Figure 2. As with the free-recall conditions, recognition performance in the directed-recall conditions was better on the materials presented to the right ear than on the materials presented to the left ear for one-, two-, and three-pair digits. Only the scores in the three-pair condition, however, reached statistical significance by paired t-test (p <.05). The largest difference between right- and left-ear scores in the threepair, directed-recall condition was 6.4 percent for the - to 79-year group (see Table 3). The slopes of the linear functions for the right ear are essentially flat (-0.08, 0.14, and 0.08%/decade for one-, two-, and three-pair digits, respectively), indicating consistent performance across age. In comparison to the slope data for the right ear, the slope data for the left ear show a slightly greater decrease in recognition performance with age (-0.91, -0.43, and -0.50%/decade for the one-, two-, and three-pair digits, respectively). Stimulus Pair Effects The results from the ANOVA on the data from the si subject groups indicate that in the directed-recall condition, recognition performance among the one-, two-, and three-pair digits was not significantly different (F [2, 348] = 1.9 ; p >.05). For the free-recall condition, however, as the compleity of the listening task increased from easy (one pair) to difficult (three pair), there was a corresponding significant decrease in recognition performance (F [2, 348] = 5.0 ; p <.0001). In addition, there was a direct relation between difficulty of the task (one, two, or three pair) and age. As age increased, there was an increase in the difference in performance on the easy and difficult tasks. There was also a significant interaction effect between stimulus pair and ear (F [2, 348] = 16.0 ; p <.0001). As 564

9 Dichotic Digit Recognition under Free and Directed RecalUStrouse and Wilson the stimulus difficulty increased, recognition performance on the materials presented to both the right and left ears decreased with the larger decrease in performance associated with the materials presented to the left ear. Figure 3 shows mean percent correct recognition performance for one-, two-, and threepair digits for each age group in the free-recall condition. The linear equations used to fit the data for the left (Xs) and right ears (0s) are shown at the bottom of each panel. The differences between the functions for the right and left ears represent the right-ear advantage or, conversely, the left-ear disadvantage. All slopes of the linear functions (%/digit pair) are negative with the slopes for the left-ear data steeper than the slopes for the right-ear data. For the right ear, the slope of the function increased from -3.5 percent/pair for the 20- to 29-year group to -8.4 percent/pair for the - to 79-year group. Likewise, for the left ear, the slope of the function increased from -4.9 percent/pair for the 20- to 29-year group to percent/pair for the - to 79-year group. Thus, the slope data indicated that (1) the performance on the materials presented to the left ear decreased faster as a function of task difficulty than performance on materials presented to the right ear and (2) that subject groups above the 20- to 29-year group ehibited an increase in the difference in performance on the easy and difficult tasks. The percent correct performance for the three-pair condition in the 50- to 59-, - to 69-, and - to 79-year groups illustrated in Figure 3 was near chance, especially in the left ear. If the responses for the three-pair digits were based strictly on chance performance, then the likelihood of correctly guessing si of the nine possible digits would be 67 percent. In Figure 3, the percent correct recognition for the threepair condition was 68.2 percent, 72 percent, and 64.5 percent in the left ear for the 50- to 59-, - to 69-, and - to 79-year groups, respectively. Based on chance, the results suggest that the subjects in the 50- to 59-, and - to 69-year groups performed just above chance and subjects in the - to 79-year group performed slightly below chance. Because the response task of the subjects did not require a response to all stimuli, however, often the number of responses was YEARS YEARS YEARS 0 50 YAE = yae = ype = yle = yle = yle = YEARS -69 YEARS -79 YEARS o o 1Z 50 yqe = yqe = yqe = yle = yle = NUMBER OF DIGIT PAIRS = yle Figure 3 The mean percent correct recognition for the right (Os) and left (Xs) ears for one-, two-, and three-pair digits for each age group in the free-recall condition. The linear equations used to fit the data for the left and right ears are shown at the bottom of each panel. 565

10 Journal of the American Academy of Audiology/Volume 10, Number 10, November/December 1999 Free Recall vs Directed Recall ~O1P 9g A ' 4~9 6 ' ~O QO y0 AGE GROUPS Figure 4 The percent of subject responses per presentation as a function of age for one- (squares), two- (triangles), and three-pair (circles) dichotic digits. fewer than the number of stimulus presentations. It became instructive then to eamine the relation between the number of responses and the number of stimulus presentations, which, in effect, adjusted the level of chance performance. Figure 4 shows the percent of responses per presentation as a function of age. The data represent how many digits the subjects within each age group reported in each condition, regardless of correctness. Recall that if the subject always reported si digits in the three-pair condition, chance would be 67 percent. The data in Figure 4, however, indicate that in the threepair condition for the - to 79-year group, the subjects actually responded to only 69.3 percent of the digits presented, which would reduce chance performance to 46 percent ([ ]/9). A similar pattern was observed for the one- and two-pair digits with 94 percent and percent responses, respectively. When viewed from this perspective, all correct responses were above chance performance. The data in Figure 4 also show that as the task increased in difficulty from one- to three-pair digits, the percentage of responses per presentation decreased for all age groups. In the 20- to 29-year group, subjects responded to percent of the one-pair digits and 96 percent of three-pair digits. For comparison, in the - to 79-year group, subjects responded to 94 percent of one-pair digits and only 69.3 percent of three-pair digits. These results suggest that as the response task increased in difficulty, the subjects became more conservative in their responses and that this effect became more pronounced as a function of age. To eamine the hypothesis proposed by Jerger et al (19a and 1996), the recognition performance of each subject for the free-recall dichotic digit condition was scored according to the criteria in Figure 5 and Table 4. The data in the figure show mean percent correct recognition for one-, two-, and three-pair data for the left and right ears. The solid lines above and below the mean data represent the 99.7 percent confidence interval that was calculated as the mean percent correct recognition score ±3 standard errors. The 99.7 percent confidence intervals for the free-recall condition are listed in Table 4 for each age group. The 95 percent confidence intervals (representing ±2 standard errors) also are provided. For the current study, the lower cutoff of the 99.7 percent confidence interval was used to categorize "normal" and "belownormal" performance for the one-, two-, and three-pair digits within each age group. On the basis of these normative findings, overall performance for each subject was categorized as normal or below normal according to criteria delineated by Jerger et al (19c). According to this criteria, if subject performance was normal for both free-recall and directed-recall conditions, then overall performance was categorized as normal. A total of 104 of the 1 subjects (58%) were in this normal category. If recognition performance was below normal in the free-recall condition and normal in the directed-recall condition, then overall performance was again categorized as normal since the dichotic deficit in the free-recall condition was eliminated by lessening the cognitive demands of the task in the directed-recall condition. According to the hypothesis, the reduced performance for the free-recall condition could be attributed to cognitive rather than auditory-specific factors. There were subjects (39%) in this second normal group. Finally, if recognition performance was below normal in both free- and directed-recall conditions, then overall performance was categorized as below normal, interpreted as an auditory-specific deficit. The reduced memory and attention requirements in the directedrecall task did not result in increased recognition performance. Before categorizing a subject as having an auditory-specific deficit, the subject scored below the normal cutoff for a least one ear in both the free- and directed-recall conditions. There were si subjects (3%) in this category. Of the si subjects, one was from the 566

11 Dichotic Digit Recognition under Free and Directed Recall/Strouse and Wilson LEFT EAR RIGHT EAR o 0 o ONE-PAIR 0 0 o TWO-PAIR THREE-PAIR AGE IN YEARS Figure 5 The mean percent correct recognition for one-, two-, and three-pair data in the free-recall condition for the left and right ears as a function of age. The solid lines above and below the mean data represent the 99.7 percent confidence interval calculated as the mean percent correct recognition score +3 standard errors. 30- to 39-year group, one was from the 40- to 49-year group, one was from the - to 69-year group, and three were from the - to 79-year group. The deficit, in both free- and directedrecall conditions, was on the right ear for two subjects and on the left ear for four subjects. The below-normal score was present in all three digit pair conditions for one subject and was below normal in the two- and three-pair conditions for the remaining five subjects. For these five subjects, recognition performance for one-pair digits was normal. Normative Data DISCUSSION The normative data presented in this study are based on a hierarchy of randomly interleaved one-, two-, three-, and four-pair dichotic digits in which the subjects did not know a priori the number of digit pairs to be presented on a given trial. The results indicated good intertest reliability for the test materials. Although statistically different, the two trials differed by only two tokens (5.7%), a difference that was deemed insignificant for clinical purposes. The overall consistency of recognition performance across trials is in agreement with previous studies of dichotic listening using digits and consonant-vowels as stimuli (Ryan and McNeil, 1974 ; Speaks et al, 1982 ; Musiek et al, 1991 ; Strouse and Hall, 1995 ; Wilson and Leigh, 1996). Normative data are reported for the freerecall condition of the dichotic digits test as a function of age group. Statistical analysis using ANCOVA indicated that with the effects of hearing sensitivity removed, age alone accounted for a significant portion of the variability in the data between age groups. Given that the dichotic digit stimuli were presented at db HL, it is possible that the decreased recognition performance in older listeners was a result of the presentation of digit materials at a lower sensation 567

12 Journal of the American Academy of Audiology/Volume 10, Number 10, November/December 1999 Table % Confidence Intervals (± 3 Standard Errors) and 95% Confidence Intervals (± 2 Standard Errors) Calculated from the Mean Percent Correct Recognition Data for One-, Two-, and Three-Pair digits in the Free-Recall Condition for Each Age Group 99.7% Confidence Interval Left Ear Right Ear 95% Confidence Interval Left Ear Right Ear years One pair Two pair Three pair years One pair Two pair Three pair years One pair Two pair Three pair years One pair Two pair Three pair years One pair Two pair Three pair years One pair Two pair Three pair level since pure-tone audiometric thresholds differed between age groups. Several findings argue against the likelihood that the findings are a result of an increase in pure-tone thresholds as a function of increasing age. First, if reduced pure-tone sensitivity was a factor affecting performance, then we would epect reduced recognition performance on the easy one-pair items as well as the more difficult two- and three-pair items. The results indicated, however, that recognition performance was within the range of normal for one-pair dichotic digits in all age groups. Similarly, if hearing loss had an effect on dichotic digit performance, then we would epect also that subjects would demonstrate reduced recognition performance on directed-recall items. This finding was not supported by the present data. Second, the observed differences in ear performance across tasks do not suggest a general inability to hear the dichotic stimuli. Third, monosyllabic word recognition performance was evaluated in each subject at db HL, which was the same presentation level used for dichotic digit test stimuli. As indicated previously, there was no significant difference in monaural word recognition performance between groups. Finally, previous investigations using dichotic digit materials support the notion that digits are relatively immune to the effects of cochlear hearing loss (Musiek, 1983 ; Speaks et al, 1985 ; Musiek et al, 1991). These results support the conclusion that there was an overall decline in dichotic digit performance with age that cannot be easily attributed to the effects of peripheral hearing loss. The results of the ANCOVA support this conclusion. As the normative data illustrate, there was a difference between left and right ear recognition performance for one-, two-, and three-pair digits. Previous investigations on dichotic tasks report that subjects, especially right-handed subjects, tend to ehibit a right-ear advantage more often than a left-ear advantage (Lake and Bryden, 1976 ; McKeever et al, 1984 ; Bryden, 1988 ; Wilson and Leigh, 1996). The present data support this finding. The recognition performance on materials presented to the right ear was better than the recognition performance on materials presented to the left ear for free- and directed-recall conditions. Additionally, as the compleity of the listening task increased from easy to difficult, there was a corresponding 568

13 Dichotic Digit Recognition under Free and Directed Recall/Strouse and Wilson decrease in recognition performance for all age groups. In the free-recall condition, recognition of one-, two-, and three-pair digits presented to the left and right ears decreased as a function of age, with the decrease faster for the left ear than for the right ear. Consequently, the right-ear advantage ehibited by the older adult groups was larger than the right-ear advantage of the younger adult groups. When results were viewed in terms of the slopes of the functions, the data for the free-recall condition indicated that (1) recognition of one-, two-, and three-pair digits presented to the left and right ears decreased as a function of age ; (2) scores obtained for the left ear decreased as a function of age faster than scores obtained for the right ear ; and (3) the slope of the functions increased as the difficulty of the task increased. These findings are in agreement with previous studies reporting a progressively larger right-ear advantage (or left-ear disadvantage) in dichotic listening with advancing age (Johnson et al, 1979 ; Jerger et al, 19c ; Wilson and Jaffe, 1996). The analysis of ear effects also indicated that when the dichotic task involved little difficulty as with the one-pair stimuli, there was only a small, if any, right-ear effect. When the listening task became more difficult, as with the three-pair stimuli, the difference between ear preference increased. For the protocol used in the current study, 108 stimulus items from the free-recall condition were presented. These stimulus items were subsequently recorded as Tracks 7 and 8 on the VA CD Tonal and Speech Materials forauditory Perceptual Assessment, Disc 2.0. Of the 108 stimulus items, 54 items were recorded as Track 7 and 54 items were recorded as Track 8. We compared recognition scores from the first half of the test (Track 7) to the second half of the test (Track 8) in an effort to determine whether significant practice effects were present. Results of an ANOVA indicated a significant main effect of half (F [1, 1741 = 33.8, p <.0001), indicating that recognition performance on the second 54 test items was better than recognition performance on the first 54 items of the test. Percent differences between first and second half scores across age groups ranged from 0.1 percent to 6.3 percent. The largest improvement (6.3%) was in the left ear for one-pair digits in the - to 79-year group. As each token in the free-recall condition equaled 2.8 percent, this difference amounted to only two tokens. Thus, it is feasible that the interleaved dichotic digits test could be administered using one list of 54 items as opposed to 108 items, so that Tracks 7 and 8 on the VA CD could be used as independent measures. Free Recall vs Directed Recall A secondary objective of this investigation was to compare recognition performance between free- and directed-recall response conditions in an effort to differentiate between auditory/structural and cognitive/task-related deficits in dichotic listening. According to previous investigators (Bayles and Kaszniak, 1987 ; Jerger et al, 19, 1994 ; Chmiel and Jerger, 1996), the free-recall condition places greater stress on attentional resources than the directed-recall condition. In the free-recall condition, the subject has to attend to and report digits heard in both ears. In the directed-recall condition, the subject hears digits presented to each ear but can focus on the digits heard in the cued ear while disregarding the digits presented to the noncued ear, resulting in a reduced demand on cognitive processing. As Jerger et al (19c) described, when recognition performance is below normal in the free-recall condition but improves substantially in the directed-recall condition, then the problem is primarily in the cognitive domain since memory and attention abilities are insufficient for successful performance when both ears must be monitored simultaneously. When performance is below normal in both the free- and directed-recall conditions, then the problem is interpreted to be primarily in the auditory domain since performance does not improve by reducing cognitive demands. Results of the present study indicate that of the 1 subjects, 58 percent showed no deficit in either ear under either response condition, 39 percent showed below-normal performance in one ear in the free-recall condition and a normal response in the directed-recall condition, and 3 percent scored below normal in both response conditions. Thus, the overall recognition performance of 97 percent of the subjects (174 of 1) was categorized as normal and the recognition performance of just 3 percent of the subjects (6 of 1) was categorized as below normal (suggesting an auditory-specific deficit). Interestingly, all subjects in this group complained of substantial listening difficulties, especially in the presence of competing noise ; however, this complaint was not eclusive to this group of listeners. The number of subjects identified in this study as possibly having an auditory-specific 569

14 Journal of the American Academy of Audiology/Volume 10, Number 10, November/December 1999 deficit is considerably less than that reported in previous studies using dichotic sentence materials in a similar paradigm (Chmiel and Jerger, 1996 ; Jerger et al, 19b, 1994) For eample, Chmiel and Jerger evaluated 115 elderly subjects over the age of years with high-frequency sensorineural hearing loss. Using the free- and directed-recall conditions for the Dichotic Sentence Identification Test, 33 of the 115 subjects (29%) were categorized as having an auditoryspecific deficit. No specific information was offered characterizing the remaining 61 percent of subjects who were categorized as normal. Using the same paradigm, Jerger et al (19) administered dichotic sentences to 172 elderly subjects (- years) with various degrees of sensorineural hearing loss. Of the total group of 172 subjects, 19 percent showed no deficit in either ear under either condition, 58 percent showed below-normal performance in one ear in the free-recall condition and a significant improvement in the directed-recall condition, and 23 percent showed a deficit in both response conditions. The smaller percentage of subjects identified in the current study as having an auditoryspecific deficit as compared to previous work may be attributed to the age range of the subjects evaluated. All subjects in the dichotic sentence investigations were over the age of years, whereas the current study evaluated subjects ranging in age from 20 to 79 years. Because the incidence of central auditory deficit increases with age (Stach et al, 19 ; Jerger, 1992), it is not surprising that a smaller percentage of subjects was identified from the population under study in this investigation. For a better comparison to previous work, a smaller sample was formed by eliminating from the original group of 1 subjects all individuals under the age of years. This resulted in a group of subjects in the age range of to 79 years. Within this group, 52 percent showed no deficit in either ear under either the free- or directed-recall condition, 42 percent showed below-normal performance in one ear in the free-recall condition and a significant improvement in the directed-recall condition, and 7 percent showed a deficit in both response conditions. The greater percentage of results consistent with cognitive deficit as a function of age is not unepected given the welldocumented age-related decline in various dimensions of cognitive function (CHABA Working Group, 1988). Because subjects in the current study were not screened to determine cognitive functioning, this finding must be con- sidered speculative. In support of the eplanation based on cognitive status, however, it has been suggested that the presence of a cognitive deficit should affect results in the free-recall condition more than in the directed-recall condition since the demands on attentional resources are presumably greater in the freerecall condition (Jerger et al, 19b, 1994). This finding is supported by the current data. The data for all subject groups indicated that the recognition performance in the free-recall condition was poorer than recognition in the directed-recall condition. The smaller number of subjects in the -to- 79 age range identified with an auditoryspecific deficit in the current study (7%) as compared to previous work may be attributed to differences in test materials and response tasks. One might speculate that for elderly subjects, the response task associated with the Dichotic Sentence Identification Test is more complicated than the response task associated with dichotic digits. Specifically, the response task of (1) identifying the sentence(s) heard from a list of 10 sentences, (2) identifying the number corresponding to the sentence, and (3) reporting the number of the sentence(s) heard involves more cognitive steps than the simple recall of digits, which are generally familiar to most listeners. CONCLUSION F rom these data, we propose that the randomly interleaved one-, two-, and three-pair digits included on the Tonal and Speech Materials forauditory Perceptual Assessment, Disc 2.0 (1998) are a useful tool for evaluating dichotic digit performance. The use of this hierarchical set of interleaved dichotic digits enables a wide range of recognition performances to be investigated. Moreover, modifying the materials to include both free- and directed-recall conditions may better delineate between auditory-specific deficits and those involving cognitive aspects of cognition. Acknowledgment. This work was supported by Rehabilitation, Research and Development Service and by Medical Research Service, Department of Veterans Affairs. The first author is on a Career Development Award sponsored by the Rehabilitation, Research and Development Service. Appreciation is epressed to Nicole Brush, M.S., and Sarah Seehorn, M.S., for assistance with data collection and to John Kalbfleisch, Ph.D., for assistance with statistical analysis. This paper was presented at the Annual Convention of the American Speech-Language-Hearing Association, San Antonio, TX. 5

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