Stimulus Length Uncertainty with Dichotic Digit Recognition · Abbreviations: ANOVA = analysis of variance, CD = compact disc, CV = consonant-vowel D ichotic listening is a condition

J Am Acad Audiol 10 : 219-229 (1999)

Stimulus Length Uncertainty with Dichotic Digit Recognition Anne Strouse* Richard H. Wilson*

Abstract

Dichotic digit listening was made more difficult by interleaving one-, two-, three-, and four-pair digits within a test list in which the subjects did not know a priori the number of digit pairs presented on a given trial, thereby introducing listener uncertainty . Twenty right-handed (mean age = 26.8 years) and 20 left-handed adults (mean age = 24.0) with normal hearing and 40 right-handed adults in the 60- to 75-years age range (mean age = 67.2) with mild-to-mod-

erate sensorineural hearing loss were evaluated . Compared to traditional paradigms, the uncertainty of stimulus length (one, two, three, or four pairs) reduced overall recognition per-formance on stimuli presented to both ears of all groups, but the reduction was more pronounced for the left ear than for the right ear. The recognition performance of the right-handed sub-jects was more homogeneous than the performance of the left-handed subjects . In comparison to the young subjects, the 60- to 75-year-old group had substantially reduced recognition performance .

Key Words : Aging, auditory perception, dichotic listening tests, handedness, speech per-ception, uncertainty

Abbreviations: ANOVA = analysis of variance, CD = compact disc, CV = consonant-vowel

D

ichotic listening is a condition in which different auditory stimuli are presented to the ears simultaneously. Since the

introduction of dichotic digits by Broadbent (1954) and the subsequent refinement by Kimura (1961) and Bryden (1963), tests of dichotic lis-tening have proven effective in the evaluation of central auditory processing in both children and adults (Berlin and McNeil, 1976; Jerger et al, 1990 ; Bellis, 1996) . Clinical application of such tests is based largely on the observation that recognition performance is better on mate-rials presented to the right ear than is recogni-tion performance on materials presented to the left ear (for a review, see Hugdahl, 1988) .

In the dichotic listening paradigm, there is a direct relation between the difficulty of the test

*Auditory Research Laboratory, Veteran Affairs Medical Center, Mountain Home, Tennessee, and 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 37684 . E-mail : ANNE .STROUSEQMED.VA.GOV

stimuli and the size of the right-ear advantage . As the difficulty of the test stimuli increases, the

magnitude of the ear advantage also increases . This has been shown in both children and adult populations (Musiek et al, 1984 ; Speaks et al,

1985 ; Hugdahl, 1988) . In general, the more lin-guistically similar and the more closely aligned the stimuli are, the more difficult the listening task (Bellis, 1996) . Thus, easier dichotic tasks involve the simultaneous presentation of totally dissimilar stimuli to the ears (e.g ., speech to one ear and broadband noise to the other) . More difficult dichotic tasks involve the simultaneous presentation of very similar stimuli to the two ears (e.g., consonant-vowels (CVs) like ba, da, ka, pa, ta, and ga . Dichotic digits fall somewhere between the two extremes on the difficulty con-tinuum of dichotic listening, since the stimuli are closely aligned but only lightly linguistically loaded (Bellis, 1996) . Dichotic digit materials are ideal for use with elderly listeners because dig-

its (1) are relatively immune to the effects of cochlear hearing loss (Musiek, 1983 ; Speaks et al, 1985 ; Musiek et al, 1991), and (2) have demon-strated high intertest reliability for both young and elderly adult listeners (Strouse and Hall,

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Journal of the American Academy of Audiology/Volume 10, Number 4, April 1999

1995 ; Humes et al, 1996). Moreover, the digit stimuli are generally familiar to most listeners and the test requires little time to administer and score.

Currently, one-pair and two-pair dichotic digit tests are commercially available for clini-cal use. The one-pair digits are the easiest in that there is limited information to which the lis-tener has to respond, with recognition perfor-mance near 100 percent for listeners with normal hearing (Noffsinger et al, 1994) and with periph-eral hearing loss (Speaks et al, 1985). Although two-pair digits begin to increase difficulty, most listeners are still able to respond with accuracy (Musiek, 1983). In some cases, then, effective clinical use of dichotic digit tests is limited because the test is often too easy.

Currently, the Department of Veterans Affairs (VA) compact disc (CD) Tonal and Speech Materials for Auditory Perceptual Assessment, Disc 2.0 is being prepared for national distrib-ution . In addition to the one-pair digits con-tained on the first version of the CD, version 2.0 also contains two lists each of two-pair and three-pair digits . Kimura (1961) originally sug-gested three-pair dichotic digits . The three-pair digits are more difficult than the two-pair dig-its, and many studies using three-pair digits report right-ear performance superior to left-ear performance for various populations of lis-teners (Kimura, 1961; Bryden, 1963 ; Wilson et al, 1968) . Bryden (1963) demonstrated a right-ear advantage for three- and four-pair digits and found no difference in performance between ears for five-pair digits, suggesting that mem-ory requirements were too great given such a large amount of material to recall . An earlier paper from this laboratory (Wilson and Jaffe, 1996) reported on the recognition performance of young and elderly adult listeners using one-to four-pair dichotic digits . Results showed that as the complexity of the listening task increased from easy (one pair) to difficult (four pair), there was a corresponding significant decrease in recognition performance . When the dichotic task involved little difficulty (one- and two-pair stim-uli), there was no significant right-ear effect . When the listening task became more difficult, as with the three- and four-pair stimuli, the right-ear advantage increased and large between-group differences emerged .

In addition to the material variable (num-ber of pairs) that determines the difficulty of dichotic digit tests, a second variable that can be manipulated to alter performance is listener uncertainty about the characteristics of the tar-

get stimulus . For example, early detection exper-iments (Creelman, 1960 ; Green, 1961) demon-strated better performance when the signal frequency was known to the listener than when the signal frequency was unknown to the lis-tener. Similar effects have been shown in the temporal domain in which the time of signal onset was the uncertainty variable (Egan et al, 1961 ; Green and Weber, 1980). Version 2.0 of the VA CD contains two lists of randomly inter-leaved one-, two-, and three-pair digits, thus introducing uncertainty to the dichotic digit test. This paradigm allows the clinician to eval-uate performance for one-, two-, and three-pair digit stimuli within one test session. The use of a hierarchical set of dichotic digits enables a wider range of recognition performances to be investigated, which will be useful in differenti-ating the perceptual abilities of various sub-jects and subject groups . At the same time, the uncertainty variable increases the difficulty of the dichotic digits test .

The purpose of the current experiment was to evaluate the effect of manipulating stimulus length uncertainty on dichotic digit recognition . The study introduced uncertainty in the listen-ing task by interleaving in a quasi-random man-ner the number of digit pairs (one, two, three, or four pair) presented during each stimulus trial . The following demonstrates the quasi-ran-dom order: three pair, four pair, two pair, one pair, two pair, four pair, three pair, one pair, etc. In this uncertainty paradigm, the listener did not know a priori the number of digit pairs involved with each stimulus trial. This is in contrast to the Wilson and Jaffe (1996) study, where the stimulus conditions were ordered sys-tematically with the one-pair digit condition given first, followed by the two-pair condition, etc. Thus, the subjects study knew a priori the number of digit pairs involved with each stim-ulus presentation or trial.

METHOD

T he 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 Per-ceptual Assessment, Disc 1.0 CD (Department of Veterans Affairs, 1992) were edited so that the onset of the stimulus (the first voltage departure from baseline) coincided with the start of the data file . A silent interval was concatenated to the end of the file to equalize the file lengths to 561 msec, which was the duration of the longest digit (9). The files were compiled to form the 72

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Stimulus Length Uncertainty and Dichotic Digit Recognition/Strouse and Wilson

possible one-pair dichotic stimulus sets of the nine-digit pair combinations . No digits were repeated within a pair. A 500-msec silent inter-

val, which served as the interdigit interval for the multipair digit sets, was concatenated to

the end of each one-pair dichotic digit file . The

two-pair, three-pair, and four-pair files were made by concatenating as required two, three,

and four of the compiled one-pair dichotic digit

files . 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 (e.g ., with the two-pair list, the "3-6" was used once

as the first pair of a stimulus set and once as the

second pair of a different stimulus set) . In this

manner, one-, two-, three-, and four-pair lists of

dichotic digits were constructed, each consisting of 72 stimulus sets . For the uncertainty para-

digm, four lists of 72 stimulus sets were compiled, each consisting of 18 stimulus sets from the

one-, two-, three-, and four-pair digits, preceded by 10 practice stimulus sets . Within each uncer-tainty list, the stimulus sets from each of the

four-paired conditions were randomized by blocks of four with the 24 possible combinations of four

used three times in each list . Each block of four consisted of a one-, two-, three-, and four-pair

digit set . For example, the first 10 stimulus

items with the first randomization were three

pair, four pair, two pair, one pair, two pair, four

pair, three pair, one pair, one pair, and two pair.

The materials were output from a 16-bit digital-

to-analog converter (Antex, Model SX-10) and

recorded on digital audiotape (DAT) (Sony, Model

PCM 2500A,B) with four, six, six, and eight sec-

onds following the one-, two-, three-, and four-

pair stimuli, respectively. Three subject groups were studied . The

dichotic stimuli were presented at 70 dB HL to

20 right-handed (mean age = 26.8 years) and 20

left-handed young adults (mean age = 24.0) with

normal hearing (<_20 dB at octave intervals from

250-8000 Hz) (ANSI, 1989) and 40 right-handed

adults in the 60- to 75-year age range (mean age

= 67.2 years) with bilaterally symmetric mild-to-moderate sensorineural hearing loss (see Table 1 for the mean audiogram) . There was no more than a 10-dB difference between ears for pure-tone air-conduction thresholds for any sub-ject for the frequencies 500, 1000, 2000, and 4000 Hz. Recorded Northwestern University No. 6 word recognition scores were >-80 percent in each ear and were within 10 percent between ears for each subject. Handedness was estab-lished based on the hand used by the subject to write and eat. The dichotic digits were presented from a DAT (Sony, Model DTC-59ES) through an audiometer (Grason-Stadler, Model 10) to TDH-50 earphones encased in P/N 510CO17-1 cush-ions . The subjects were instructed to respond using a free-recall paradigm following each set of stimulus pairs (e.g ., with the three-pair con-dition, the responses were following presentation of the six stimuli). All subjects participated in two trials during one test session. The verbal responses of the subjects were audio recorded and subsequently scored by two audiologists .

The <30 right-handed group also was admin-istered a dichotic CV test . The test stimuli con-

sisted of the six CV nonsense syllables (pa, ta, ka, ba, da, and ga) (Studdert-Kennedy and Shankweiler, 1970) recorded dichotically on the

Tonal and Speech Materials for Auditory Per-

ceptual Assessment, Disc 1.0 CD (1992) . The

dichotic CVs were presented simultaneously at

70 dB HL. Two trials were administered, each

consisting of 30 test items . Two responses were

required for each stimulus pair . The channels were alternated so that channel 1 was presented

to the left ear of the even-numbered subjects and

to the right ear of the odd-numbered subjects . All subjects were practiced on the dichotic

listening task before data collection was initiated. Practice consisted of three to five presentations each of the one-, two-, three-, and four-pair digit stimuli or CV stimuli. During the practice inter-val, subjects responded verbally and were queried periodically concerning their comfort in performing the listening/response task . When the subject reported feeling comfortable with

Table 1 Mean Thresholds (dB HL)* and SDs for the 40 Subjects in the 60- to 75-Years Group

Frequency (Hz)

Ear 250 500 1000

Left 20.8 (10 .0) 19.8 (11 .9) Right 20 .1 (10 .7) 18 .3 (11 .5)

*ANSI, 1989

22 .6 (15 .1) 22 .3 (13 .2)

2000 4000 6000

36.0 (20 .2) 58 .7 (20 .0) 69 .4 (19 .6) 32.6 (17 .9) 56 .8 (19 .6) 67 .0 (19 .5)

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the task, which typically was after 10 or so total practice items, data collection began.

RESULTS AND DISCUSSION

Table 2 Mean Percent Correct Recognition and SDs for the One-, Two-, Three-, and

Four-Pair Dichotic Digits Presented to the Right and Left Ears of the Three Subject Groups

T he mean percent correct dichotic digit recog-nition data (and standard deviations) for

Trial 1, Trial 2, and both trials combined are listed in Table 2 . Mean data for both trials com-bined are plotted in Figure 1 . In the figure, data from the four stimulus conditions (one, two, three, and four pair) are depicted for the two <30-years groups and 60- to 75-years group . Data were examined using a mixed model analy-sis of variance (ANOVA) with stimulus pair, ear, and trial as within-subjects factors and group as the between-subjects factor. For the analysis, the data across presentation position were aver-aged for each subject for the two-, three-, and four-pair conditions . The data in Figure 1 and Table 2 and the results of the ANOVA revealed that the main effects for the three independent variables were significant (stimulus pair : F [3,2311= 386.2, p < .0001 ; ear : F [1,771= 41.8, p < .0001 ; trial : F [1,771 = 27.4, p < .0001) . The recognition performances of the three subject groups also were significantly different (F [2,771 = 42.6 ; p < .0001) . A post hoc analysis using Tukey's HSD Test revealed a significant (p < .01) difference in performance between the 60- to 75-years group and both the <30-years right- and left-handed groups . There was no significant difference between the performances of the <30 right- and left-handed groups . These main effects and significant interaction effects are described in detail in the subsequent sections .

Stimulus Pair Effects

Figure 2 shows percent correct recognition by ear and presentation position for the one-, two-, three-, and four-pair dichotic digits pre-sented to the three subject groups using the interleaved uncertainty paradigm . In compari-son, data from a similar experiment with the <30-year-old, right-handed subjects, using a sys-tematic ordering of digit pairs (filled circles), are shown (Wilson and Jaffe, 1996). For the uncertainty paradigm, the best performance was obtained by the <30 right-handed group (circles), followed closely by the <30 left-handed group (triangles), with the worst performance by the 60- to 75-years right-handed group (squares).

The results from the ANOVA (stimulus pair) and the data from the three subject groups indi-cate that, as the complexity of the listening task

Trial 1 Trial 2 Both Trials

<30 years, right-handed (n = 20) One pair

Left ear 99.4 (1 .7) 99 .4 (1 .7) 99 .4 (1 .5) Right ear 100.0 (0 .0) 99 .7 (1 .2) 99 .9 (0 .6) (RE -LE) 0.6 (1 .7) 0.3 (2 .2) 0.5 (1 .6)

Two pair Left ear 94 .9 (5 .6) 95.8 (3 .7) 95.3 (4 .1) Right ear 98 .8 (1 .4) 97 .2 (2 .5) 98.0 (1 .3) (RE - LE) 3 .9 (6 .2) 1 .4 (3 .8) 2 .6 (4 .2)

Three pair Left ear 80.7 (9 .2) 86 .9 (6 .1) 83.8 (6 .9) Right ear 90.0 (6 .3) 93 .2 (5 .8) 91 .6 (5 .5) (RE - LE) 9.3 (10 .1) 6 .4 (7 .4) 7 .8 (7 .8)

Four pair Left ear 67.4 (11 .6) 73 .1 (9 .5) 70.2 (8 .6) Right ear 82.2 (8 .6) 85 .6 (9 .4) 83.9 (8 .0) (RE- LE) 14 .8 (15 .5) 12.6 (11 .9) 13 .7 (11 .0)

<30 years, left-handed (n = 20) One pair

Left ear 99 .4 (2 .5) 100.0 (0 .0) 99 .7 (1 .2) Right ear 99.2 (2 .0) 100.0 (0 .0) 99.6 (1 .0) (RE - LE) -0.2 (2 .2) 0 .0 (0 .0) -0 .1 (1 .1)

Two pair Left ear 89.3 (9 .5) 93 .1 (6 .8) 91 .2 (7 .3) Right ear 97.6 (2 .6) 98 .5 (2 .9) 98 .1 (2 .4) (RE - LE) 8.3 (8 .6) 5 .4 (5 .9) 6 .9 (5 .7)

Three pair Left ear 75 .1 (15 .9) 82 .8 (14 .5) 78.9 (14 .8) Right ear 91 .7 (4 .7) 91 .9 (6 .8) 91 .8 (4 .9) (RE -LE) 16.6(15.7) 9 .1 (13 .8) 12.8(14.1)

Four pair Left ear 65 .1 (17 .4) 69 .4 (17 .8) 67 .3 (17 .2) Right ear 78 .2 (11 .9) 79.0 (11 .4) 78 .6 (11 .1) (RE -LE) 9.4 (9 .7) 9.6(19 .9) 11 .3(19 .1)

60-75 years, right-handed (n = 40) One pair

Left ear 84.6 (17 .6) 85.8 (17 .3) 85.2 (16 .6) Right ear 91 .8 (9 .8) 91 .8 (10 .7) 91 .8 (8 .9) (RE- LE) 7.2 (14 .0) 6 .0 (13 .3) 6 .6 (12 .0)

Two pair Left ear 68 .1 (19 .7) 67.6 (22 .0) 67.9 (20.2) Right ear 86.4 (10 .0) 87 .4 (11 .5) 86.9 (9.7) (RE- LE) 18.3 (18 .9) 19 .8 (20 .9) 19.0 (18 .6)

Three pair Left ear 54.3 (15 .6) 63 .6 (15 .8) 58 .9 (14 .9) Right ear 77.3 (12 .9) 75 .3 (13 .3) 76 .3 (12 .1) (RE - LE) 23.0 (22 .0) 11 .8 (21 .0) 17 .4 (20 .1)

Four pair Left ear 50 .7 (15 .9) 53.6 (15 .8) 52 .1 (15 .1) Right ear 67 .9 (12 .7) 67.8 (15 .8) 67 .8 (13 .1) (RE - LE) 17 .2 (21 .4) 14 .2 (24 .9) 15 .7 (21 .5)

increased from easy (one pair) to difficult (four pair), there was a corresponding significant decrease in recognition performance. As can be

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<30 Years Right Handed

<30 Years Left Handed

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Figure 1 The percent correct recognition for the one-, two-, three-, and four-pair digit materials presented to the right ear and to the left ear for the three subject groups .

Data are averaged across presentation position for each

condition.

seen in Figure 2 for all subject groups, the per-cent correct recognition decreased more between

the one-pair and four-pair conditions for the materials presented to the left ear than for the materials presented to the right ear. Larger dif-ferences were observed between the easy and dif-

ficult listening tasks for the 60- to 75-years group than for either of the two <30-years groups .

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PRESENTATION EAR and POSITION

Figure 2 The percent correct recognition for the mate-rials presented to the right ear and to the left ear by pre-sentation position and by stimulus length for the <30-years right-handed group (circles), <30-years left-handed group (triangles), and the 60- to 75-years right-handed group (squares). The filled circles depict data (n = 20) from the Wilson and Jaffe (1996) study in which

uncertainty of stimulus length was not a variable .

Similar differences between performance have been noted in past investigations that have used a dichotic digit paradigm in which stimulus length uncertainty was not a variable (Bryden, 1963 ; Johnson et al, 1979 ; Wilson and Jaffe, 1996).

Figure 3 shows mean percent correct recog-nition by ear and presentation position of the randomly ordered dichotic digits (open circles)

and systematically ordered dichotic digits (filled circles, Wilson and Jaffe, 1996) presented to

<30-years right-handed adults . Compared to the Wilson and Jaffe data, the present findings indicate a further decrease in recognition per-formance for the <30-years right-handed subjects when uncertainty is added to the listening task .

Mean correct identification for dichotic CV mate-rials (squares) is added for comparison . The data in Figure 3 indicate that by the four-pair dichotic digit condition, especially at the third and fourth presentation positions, performance

on the digits approached that obtained with the

dichotic CVs . Mean identification scores for dichotic CVs were 70.4 percent and 59.4 percent for the right and left ears, respectively, result-ing in a statistically significant difference (t [19] = 17.1, p < .0001) . The dichotic CV scores are almost identical to previous reports (Berlin et al,

1978 ; Noffsinger, 1985, Wilson and Leigh, 1996) using CVs that were spoken by the speaker used in the current study.

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PRESENTATION EAR and POSITION

Figure 3 The mean percent correct recognition by ear and presentation position for Trials 1 and 2 of the ran-domly ordered dichotic digits (triangles) and dichotic CVs (squares) presented to the <30-years right-handed adults . Mean percent correct recognition for systemati-cally ordered dichotic digits (closed circles) is added for

comparison (Wilson and Jaffe, 1996).

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Also of interest from the data in Figures 2 and 3 is the morphology of the data points for the mean four-pair dichotic digit recognition data . The U-shaped function likely reflects the "serial-position curve of free recall" described in the memory literature (Deese, 1957; Murdock, 1962 ; Bartz, 1968 ; Jahnke, 1968). Typically, the serial position curve is characterized by a pri-macy effect (early occurring items) and a recency effect (later occurring items). The primacy effect is usually more precipitous, although smaller in magnitude than the recency effect . For the cur-rent data, the decreasing performance over the initial presentation positions coincides with the primacy portion of the serial-position curve and the increased performance observed with the fourth presentation position coincides with the recency effect .

In addition to memory-related factors, lis-teners in all groups found the four-pair stimu-lus sets very difficult, making the dichotic task unpleasant . For these reasons, the use of four-pair digits has limitations as a clinical tool for evaluating dichotic listening performance.

100

40 YRS. RIGHT HANDED <50 YRS. LEFT HANDED 80.75 YRS. RIGHT HANDED

Ear Effects

The data in Table 2 represent the overall mean percent correct recognition scores for dichotic digit materials in percent as well as the differences in performances on the materials presented to the ears (i .e ., right-ear score minus left-ear score) for the three groups of subjects . Individual data are plotted in Figure 4 as bivari-ate plots of the subject responses for trials 1 and 2 combined. In each panel, subject performance on materials presented to the right ear is plot-ted on the abscissa, whereas subject perfor-mance on the material presented to the left ear is plotted on the ordinate . The filled symbols rep-resent the mean data for each of the conditions, and the diagonal line in each panel represents equal performance. Thus, datum points below the diagonal line indicate that right-ear per-formance was better than left-ear performance.

Previous investigations have reported that both right- and left-handed subjects tend to exhibit a right-ear advantage more often than a left-ear advantage on dichotic tasks (Lake and Bryden, 1976 ; McKeever et al, 1984 ; Bryden, 1988; Wilson and Leigh, 1996). This finding has been widely interpreted as reflecting the spe-cialization of the left cerebral hemisphere for speech in almost all right handers and in a majority of left handers, as well as the superi-ority of contralateral over ipsilateral ear-cortex

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PERCENT CORRECT RECOGNITION-RIGHT EAR

Figure 4 Bivariate plot of percent correct recognition by right ear (abscissa) and left ear (ordinate) for the three subject groups . The larger symbols in each panel represent the mean data .

pathways (Kimura, 1967 ; Springer, 1986). The present data support this finding. The ANOVA showed a significant difference between scores on the materials presented to the right and left ears . As the mean data indicate, right-ear scores were significantly greater than left-ear scores for each of the three subject groups (<30 right-handed : t [191= 5.37, p < .0001; <30 left-handed: t [19] = 3.91, p < .001 ; 60-75 years: t [39] = 5.80, p < .0001) .

Results of the present study show a signif-icant interaction between stimulus pair and ear (F [3,771 = 21.8 ; p < .0001) for the dichotic digit materials, indicating that as stimulus difficulty increased, both right- and left-ear scores decreased, but that the left-ear scores decreased more than right-ear scores . As previously reported, 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- and four-pair stimuli, the difference between ear preference increased (Kimura, 1961 ; Bartz, 1968). Kimura also noted a tendency for right-handed subjects to yield a right-ear advan-tage and left-handed subjects to show a left-ear advantage, which was not observed in the pre-

224


sent data . As McKeever et al (1984) have noted, many of the studies of handedness have not found a significant difference between handed-ness groups . It has been suggested that this finding may be due, in part, to inaccurate clas-sification of right- and left-handed subjects . Sev-eral authors have reported that use of self-report methods to identify handedness (when com-pared to actual tested analysis of handedness) is poor for identification of pure left-handed-ness (Benton et al, 1962 ; Satz et al, 1967 ; Knox and Boone, 1970), indicating that self-reports of handedness should be verified by motor laterality testing. Using strict criteria for determining handedness, Knox and Boone (1970) reported that when dichotic tasks become difficult, lis-tening lateralization occurred in the ear ipsi-lateral to the tested side of handedness .

Virtually all dichotic studies, however, have found at least a trend for left-handers to show a reduced laterality effect, as was evidenced in the present data . Bryden (1988) summarized 18 studies in which both left- and right-handed subjects were tested and in which individual subject data were reported . Although only 10 of the 18 studies showed a significant handedness effect, it was clear that the general pattern was that a higher proportion of right-handers showed a right-ear advantage compared to left-handers . Overall, from the Bryden analysis, about 82 percent of right-handers and 64 percent of left-handers yielded a right-ear advantage.

The analysis of the current data also revealed a significant ear by group interaction (F [2,771 = 3.9 ; p < .05), with mean data indi-cating performance on materials presented to the right ears was more similar among subject groups than was performance on the materials presented to the left ears of the three subject groups . As was depicted in Figure 1, the right-ear advantage exhibited by the 60- to 75-years group was larger than the right-ear advantage exhibited by the two <30-years groups . This fording is in agreement with previous studies reporting an increasing left-ear deficit in dichotic listening with age (Johnson et al, 1979 ; Jerger et al, 1990 ; Wilson and Jaffe, 1996). One may

speculate that this ear differential, which increases with the complexity of the listening

task, is the result of decreased neural processes and neural efficiency associated with aging, which has more of an effect on inputs to the left auditory periphery/right cortical mechanism than on inputs to the right auditory periph-ery/left cortical mechanism. Indeed, it has been proposed that there is a greater decline in right-

hemisphere than in left-hemisphere function

with increasing age (Goldstein and Shelly, 1981) . It has also been suggested that this pattern of

findings may be explained by a decline in the effi-

ciency of interhemispheric transfer associated with aging (Goldstein and Braun, 1974; Jerger

et al, 1995) . The right-ear advantage in young

subjects derives from the fact that input to the

left ear must cross the corpus callosum to reach

the left hemisphere for processing . An age-

related decline in interhemispheric transfer via

the corpus callosum would put the left ear at a

greater disadvantage relative to the right ear since the right ear has direct access to the left

hemisphere . Cognitive factors such as a decline

in memory skills or information-processing strategies may also contribute to decreased

scores in elderly listeners (Craik, 1965) . A final relation is apparent from the data in

Figure 4 and Table 2. Although the right-ear effect is obvious for all groups, individual datum points for the <30 left-handed subjects are more dispersed than the datum points for <30 right-handed subjects, indicating increased variabil-ity in performance among the left-handed group. This relation also is apparent in the standard deviations listed in Table 2 . The average stan-dard deviations for the four conditions were 3.9

percent and 5.3 percent for the right and left ears of the <30-years right-handed subjects versus 4.9 percent and 10.1 percent for the right and left ears of the <30-years left-handed subjects . Thus, as other investigators have reported (Piazza, 1980 ; McKeever et al, 1984 ; Bryden, 1988; Emmerich et al, 1988 ; Wilson and Leigh, 1996), left-handed subjects as a group were less homogenous in their performance on dichotic lis-tening tasks than were right-handed subjects . In addition, performance by the 60- to 75-years group showed increased variability as compared to the two <30-years groups . The average stan-dard deviations for the four conditions were 11 .0 percent and 16.7 percent for the right and left ears of the 60- to 75-years subjects .

Trial Effects

The individual data from the three groups by trial are shown in Figure 5. The figure is a bivariate plot of the percent correct recognition for Trial 1 (abscissa) and percent correct recog-nition for Trial 2 (ordinate). The filled and larger symbols represent the mean data for each of the conditions, and the diagonal line in each panel represents equal performance. Thus, datum points above the diagonal line indicate

225


40 YRS. RIGHT HANDED e30 YRS. LEFT HANDED

1 O0

8o

60

40 20

1 PAIR x xo

PAIRS . _ _ _ _ _ . .

0x 0

/

xxxp

x7

x * x x

3 PAIRS

w y, o ~'

XXX

I 4 PAIRS 0 X

Xx X.

X 0 XX

0

X

XX

.

. . . . . . . . . .

0

. 0

Wo 00

X

. .. . .. . .. .. .. . . . 0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100

PERCENT CORRECT RECOGNITION-TRIAL #1

Figure 5 Bivariate plot of percent correct recognition by Trial 1 (abscissa) and Tria12 (ordinate) across the three subject groups (X = LE, O = RE). The larger symbols in each panel represent the mean data .

that recognition performance on Trial 2 was better than recognition performance on Trial 1 .

Although, in the ANOVA, the main effect for trials was significant, inspection of the mean data in Table 2 and Figure 5 indicates that the largest percent differences between trials for each group occurred for the left ear in the three-pair condition. Differences of 6.2 percent, 7.7 per-cent, and 9.3 percent were observed for the <30-years right-handed, <30-years left-handed, and 60- to 75-years right-handed groups, respec-tively. As each token was equal to 3.3 percent, these differences amounted to two to three tokens. Thus, all differences between the two tri-als were less than three tokens . The interaction of trial and ear also was significant (F [1,771 = 9.41 ; p < .01) . The mean data in Table 2 indicate that differences between the mean scores across trials were greater by a factor of two or more for the left ear than for the right ear for all subject groups . This relationship was most pronounced for the three- and four-pair conditions . The between-trial differences for the <30 right-handed group ranged from 1.6 percent to 3.4 per-cent for the right ear and from 0.9 percent to 6.2 percent for the left ear. For the <30-years left-

60-75 YRS. RIGHT HANDED handed group, differences ranged from 0.2 per-cent to 0.9 percent for the right ear and from 3.8 percent to 7.7 percent for the left ear. Finally, for the 60- to 75-years right-handed group, between-trial difference scores ranged from 0.1 percent to 1.0 percent for the right ear and from 0.5 percent to 9 .3 percent for the left ear. The over-all consistency of recognition performance across trials is in agreement with previous studies of dichotic listening using digits and CVs as stim-uli (Ryan and McNeil, 1974; Speaks et al, 1982; Musiek et a1,1991; Strouse and Hall, 1995 ; Wil-son and Leigh, 1996).

A factor related to test-retest reliability is test consistency, meaning that a listener should show the same direction of ear advantage on test and retest, except in cases in which the size of the ear advantage is small (Speaks et al, 1982). To examine consistency of the ear advantage across trials, the percentage of ear advantage was calculated as the difference between percent correct right ear and percent correct left ear (RE - LE). Within the <30 right-handed group, four subjects (20%) reversed the ear advantage direction from Trial 1 to Trial 2. In the same group on Trial 1, 18 of the 20 subjects (90%) showed a right-ear advantage, whereas two sub-jects showed a left-ear advantage. Such left-ear preferences in normal right-handed subjects, although not common, have been reported pre-viously (Studdert-Kennedy and Shankweiler, 1970 ; Berlin et al, 1973 ; Speaks and Niccum, 1977). For the <30 left-handed group, one of the 20 subjects (5%) showed a reversal in direction of ear advantage between trials . On Trial 1, 17 of the left-handed subjects had a right-ear advan-tage, whereas three subjects had a left-ear advantage for digit recognition across the four stimulus pair conditions . Finally, for the 60- to 75-years group, two of the 40 subjects (5%) showed a reversal in direction of ear advantage from Trial 1 to Trial 2, with four subjects main-taining a left-ear advantage across trials . Over-all, only 7 of the 80 subjects (8.8%) in the present study showed a reversal in direction of ear advantage across trials . This finding is consid-erably lower than the frequency of reversals reported by Pizzamiglio et al (1974), who found that 30 percent of their 91 listeners reversed the direction of ear advantage between two trials of a dichotic digits test . Similar inconsistencies have been reported for dichotic CV tests (Blum-stein et al, 1975 ; Speaks and Niccum, 1977). Other CV studies, however, demonstrate that ear preference and the relative size of the ear advan-tage do not change significantly over successive

226


trials (Troendle et al, 1973 ; Ryan and McNeil, 1974) .

formance approached that obtained with the dichotic CVs.

SUMMARY

T he data from this study, based on a hierar-chy of 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, indicate the following :

1. Recognition performance on materials pre-sented to the right ear is better than recog-nition performance on materials presented to the left ear.

2. The uncertainty of stimulus length (one, two, three, or four pairs) reduces overall recognition performance on dichotic digit stimuli presented to both ears, but the effect is more pronounced on the left ear than on the right ear. Thus, the uncertainty variable enhances the right-ear advantage.

3 . Although older subjects maintain the right-ear advantage for dichotic digit stimuli, the recognition performance on materials pre-sented to both ears is poorer than the recog-nition performance by either of the <30-years groups of listeners . Performance on the materials presented to the right ears was more similar between groups than was per-formance on materials presented to the left ears . Thus, the right-ear advantage was larger for the 60- to 75-years group versus the two <30-years groups .

4. Recognition performance between two trials is relatively stable for the right ear, and left ear scores improve by 6 percent to 9 percent, which is at least twice the improve-ment demonstrated by the right ear. Direc-tion of ear advantage is consistent between trials .

5. Dichotic digit recognition by young adult right-handed subjects is more homogenous than dichotic digit recognition by young adult left-handed subjects . Likewise, per-formance of the 60- to 75-year group is more variable than performance of the two <30-years groups .

6. Dichotic digit recognition for the right and left ears by the <30-years right-handed group was better than recognition perfor-mance for dichotic CV stimuli. By the four-pair dichotic digit condition, especially at the third and fourth presentation positions, per-

From these data, we propose that the uncer-tainty paradigm is a useful tool for evaluating dichotic digit performance . Evaluation of four-pair digit performance is not recommended, however, due to the reported difficulty of the task and the increased contribution of cognitive fac-tors required for correct recognition . Accord-ingly, the uncertainty paradigm recorded on the VA CD Tonal and Speech Materials forAuditory Perceptual Assessment, Disc 2.0 does not include four-pair dichotic digits . The suggested para-digm allows the clinician to evaluate perfor-mance for one-, two-, and three-pair digit stimuli within a single test session . At the same time, the uncertainty variable increases the difficulty of the dichotic digits test . The use of this hier-archical set of interleaved dichotic digits enables a wider range of recognition performances to be investigated, which will be useful in differ-entiating the perceptual abilities of various sub-jects and subject groups .

Acknowledgment . This work was supported by Rehabilitation, Research and Development Service and by Medical Research Service, Department of Veterans Affairs. Appreciation is expressed to Anne Oyler, M.S . and Renee Sumrall, M.S . for assistance with data col-lection and to John Kalbfleisch, Ph.D . for assistance with statistical analysis .

This paper was presented at the Annual Convention of the American Academy of Audiology, April 1997, Fort Lauderdale, FL .

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Documents

Stimulus Length Uncertainty with Dichotic Digit Recognition · Abbreviations: ANOVA = analysis of variance, CD = compact disc, CV = consonant-vowel D ichotic listening is a condition