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Contact author: Cynthia M. Zettler, who is now at the Waisman Center, University of Wisconsin–Madison, 1500 Highland Avenue, Madison, WI 53705. E-mail: zettler{at}waisman.wisc.edu.
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Abstract |
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Method: Eighty-one 7- to 10-year-old children (30 with and 51 without RD) and 20 adults without RD listened to CMR stimuli through headphones. The difference between reference and modulated masker thresholds provided a measure of CMR.
Results: Hierarchical regression analyses indicated that reading status did not predict thresholds or CMR. An analysis of variance revealed significantly less CMR for children than for adults.
Conclusions: This research does not support the hypothesis that children with RD have difficulty processing temporal and spectral auditory information as measured by the CMR paradigm. In contrast with some previous research (K. Veloso, J. Hall, & J. Grose, 1990), this study suggests that CMR is continuing to develop beyond 10 years of age. Future research using a CMR paradigm with older children (10–16 years of age) would further illuminate the developmental picture of CMR.
KEY WORDS: reading disabilities, temporal processing, auditory development
Up to 20% of children in the United States exhibit difficulty learning to read (National Institutes of Health [NIH], 1994). A number of hypotheses have arisen to explain this difficulty, including hypotheses about phonological processing and/or auditory perceptual deficits. Proponents of the phonological processing deficit hypothesis (Liberman, 1997; Mody, Studdert-Kennedy, & Brady, 1997; Nittrouer, 1999) assert that children who have difficulty learning to read share a fundamental deficit in their ability to process letter–sound relationships that causes impaired word decoding ability and results in a reading disability (RD). Thus, the impairment is hypothesized to be a language-based deficit. Alternatively, proponents of an auditory perceptual deficit hypothesis (Tallal, 1980) state that because receptive language must first be processed via the auditory system, a perceptual deficit at some level in the auditory system may impair a child's understanding of phonology, resulting in difficulty learning to read.
The auditory perceptual deficit hypothesis in reading disabilities (RD) has had two areas of focus: (a) temporal processing impairments and (b) spectral processing impairments. Temporal processing concerns the ability of the auditory system to detect changes in sound over time (Moore, 1997). Because speech has a natural pattern of modulation over time that influences its intelligibility (cf. Menell, McAnally, & Stein, 1999), studies of amplitude modulation (AM), a sinusoidal fluctuation in the amplitude of a sound wave, may provide insight into any auditory temporal deficit in individuals with reading impairments. Menell and colleagues (1999) measured the processing of AM signals by estimating temporal modulation transfer functions (TMTF) and recording amplitude modulation following responses (AMFR), an electrophysiological correlate of TMTF, for a group of adults diagnosed with dyslexia and a group of adults who had no history of reading difficulties. TMTF results revealed significantly higher thresholds for detecting AM across modulation rates from 10 to 320 Hz for participants with dyslexia relative to control listeners. In addition, the AMFRs of the participants with dyslexia indicated a reduced sensitivity to AM than that of the control listeners at all rates tested (10, 20, 40, 80, and 160 Hz), further supporting the suggestion that individuals with dyslexia show decreased responsiveness to AM stimuli.
TMTFs also have been assessed in children with dyslexia. Lorenzi, Dumont, and Fullgrabe (2000) measured TMTFs and the identification of unprocessed or processed (removal of spectral information, leaving only temporal envelope cues) vowel–consonant–vowel speech stimuli in 6 children diagnosed with dyslexia as well as in 6 children and 6 adult control participants to determine whether children with dyslexia have impaired temporal-envelope perception. Although the unimpaired child and adult groups did not significantly differ, results indicated that children with dyslexia showed higher thresholds for sinusoidal amplitude modulation (SAM) than either the child or adult control groups at the lowest (4 Hz) and highest (1024 Hz) modulation frequencies tested and also showed higher between-listener variability than their unimpaired counterparts. In addition, the children with dyslexia evidenced poorer performance than either control group in their perception of both unprocessed and processed speech envelope noise. The authors suggested that the children with dyslexia had difficulty gleaning information from the temporal envelope of the sounds that could result in impaired speech perception.
Spectral processing is the ability of the auditory system to perceive and use the frequency information present in sounds to aid in signal detection. Both band-pass and notched-noise masking paradigms have been used to assess both temporal and spectral abilities in children with RD. Masking studies using a spectral notch in a noise masker enable a measure of spectral processing by allowing the manipulation of the width of the notch required to determine the optimal detection of a signal. Whereas backward masking is hypothesized to tap into temporal processing ability, the addition of a spectral notch to the noise adds the measure of spectral processing ability. Rosen and Manganari (2001) reported significantly elevated thresholds (nearly 30 dB) in eight 11- to 14-year-old children with dyslexia compared with 8 age-matched controls in a backward masking condition when a 1000-Hz, pure-tone signal was presented 20 ms prior to the onset of a 300-ms bandpass masker. Although both groups' performance improved in a backward masking notched condition relative to the bandpass condition, performance improved significantly only for the participants with dyslexia (i.e., the presence of the notch in the masker reduced the difficulty of the task for the participants with dyslexia by about 16 dB).
Using stimuli similar to those presented by Rosen and Manganari (2001), Montgomery, Morris, Sevcik, and Clarkson (2005) assessed backward masking with a bandpass masker and with a notched-noise masker in a group of 26 children (7 to 10 years of age) with RD and 26 similar-aged, typically developing control children, finding that RD status predicted performance in both conditions. Given the significantly poorer performance in both the bandpass and the notched-noise condition by the children with RD, the authors suggested that a complex relationship between auditory processing and RD was likely, involving both the temporal and spectral components of sound. Interestingly, Montgomery et al. (2005) found that participants with RD did not show the dramatic improvement in performance found by Rosen and Manganari (2001) in the notched-noise condition. Although performance improved for both groups overall in their study, the improvement found by the control group in the notched-noise condition was actually greater than that of the participants with RD (Montgomery et al., 2005). A possible explanation for this disparity is that the participants in the Montgomery et al. (2005) study were younger than those in the Rosen and Manganari (2001) report. This age difference may be one reason for the variation in findings here, as the control participants in the Rosen and Manganari (2001) study did not show much backward masking to begin with, leaving little room for improvement in a notched-noise condition and suggesting that the ability is mature in typical 11- to 14-year-old children. Consequently, the evaluation of central auditory processing abilities, in both typical and impaired readers, is one that requires careful consideration of the typical course of development for the tasks being used.
Possible auditory spectral and temporal processing deficits reported in some children with RD suggest that the assessment of children with RD using auditory measures that allow the manipulation of both temporal and spectral components would further help to explicate the nature of any auditory deficit. Further, research indicating that persons with reading disabilities show a reduced sensitivity to AM suggests that AM could be a useful tool in exploring the limits of any auditory deficit in other nonspeech paradigms. Although AM is a component of speech, AM itself is a nonspeech element of sound. If the auditory deficit found in some individuals with reading impairments is limited to speech stimuli, one would not expect to find the reduced sensitivity for AM found by Lorenzi et al. (2000) and Menell et al. (1999) or the elevated thresholds in backward masking tasks found by Montgomery et al. (2005) and Rosen and Manganari (2001). Alternatively, if the auditory deficit is not limited to speech stimuli, then one would expect to find the deficit in a variety of auditory tasks. Given that Menell et al. (1999) showed that adults with dyslexia displayed a reduced sensitivity to AM, it is pertinent to explore whether this finding would apply in the context of a more complex nonspeech auditory task—one in which the use of AM is an essential component of signal extraction. In the present study, we explored the auditory perceptual deficit hypothesis in children with reading disabilities using a complex masking task as the vehicle for exploring perception of AM.
The comodulation masking release (CMR) paradigm provides a way to explore temporal and spectral processing ability in children with RD using a single paradigm. The comparison of AM information across multiple frequencies yields a measure of spectral processing, whereas the perception of AM involves temporal processing. CMR is an auditory phenomenon that allows listeners to separate signals from noise in the natural environment through the perceptual grouping of different stimuli. Most environmental stimuli are not independent; they are grouped on the basis of their common spectral and temporal properties (Nelken, Rotman, & Yosef, 1999). CMR is a measure of a listener's ability to use coherent bands of amplitude-modulated noise in multiple frequency regions to aid signal detection relative to conditions where only the on-signal band of AM noise is present. Listeners detect similarities in the AM of noise across auditory filters as being perceptually distinct from a signal, and signal extraction becomes easier. Thus, the addition of noise results in lower thresholds.
To assess the performance of children with RD using the CMR paradigm against that of typically developing children, the developmental course of CMR must first be clarified. Studies indicate that the development of some auditory tasks can be protracted in children. Veloso, Hall, and Grose (1990) assessed frequency resolution in adults and 6-year-old listeners and reported that the 6-year-old children showed adultlike thresholds at all notch widths tested (0, 300, and 600 Hz). In a later study, Hall and Grose (1991) determined that children as young as 4 years of age showed frequency resolution similar to that of adults but that processing efficiency, defined as the ease with which the central auditory system processes information relative to the peripheral auditory system (Hill, Hartley, Glasberg, Moore, & Moore, 2004), was markedly reduced as indicated by higher thresholds relative to adults in all conditions.
Tasks that require temporal processing may undergo a more protracted course of development than peripherally mediated tasks requiring spectral processing. Hartley, Wright, Hogan, and Moore (2000) assessed backward masking in 6- to 10-year-old typically developing children, finding immature temporal resolution in the 6-year-old children and continuing age-related improvements in temporal resolution in the 10-year-old children, leading the authors to suggest that adultlike temporal resolution might not be in place until age 11 years.
Although few studies of CMR have been conducted with children, research suggests that at least some of the mechanisms involved in CMR are developed by middle childhood. Veloso et al. (1990) tested CMR in six 6-year-old children and in six adults. They reported similar CMR for adults and children (8.4 and 8.2 dB, respectively) when thresholds for detection of a 1000-Hz signal were measured in three comodulated bands of noise relative to a single (reference) band of modulated noise. This finding led the authors to conclude that the across-channel processes necessary for signal detection in CMR were in place by 6 years of age. Nonetheless, the masked thresholds of the children were significantly higher than those of adults in all masking conditions tested, suggesting that central processing efficiency still might be immature in these children.
To determine the ability of children diagnosed with RD to use temporal and spectral information in a complex masking task and to further clarify the developmental course of CMR, the present study assessed CMR in a large sample of children with and without a reading disability and in normal adults. It was anticipated that if children with a reading disability possess relatively poor temporal processing and frequency resolution, they would show less CMR than similar-aged children without a reading disability. It also was anticipated that children with RD would show poorer processing efficiency, as evidenced by higher thresholds, than that shown by children without RD. In addition, the children's performance was compared with that of a group of naive adult listeners to further the developmental understanding of CMR.
Method
Participants
To assess CMR in children, a group of children was recruited from a larger study designed to examine the impact of a child's language on reading. All children were fluent English speakers. Nine children were classified as having learned also to read in Spanish, according to caregiver report. However, the performance of these 9 children on the CMR task was not significantly different (p > .05) from the other children, so their results were included in the final analyses. Children were recruited from public schools in the Fulton County Public School System in the Atlanta, Georgia, metropolitan area and came from a wide range of socioeconomic backgrounds. Informed consent was obtained from the parents of the child participants, and assents were obtained from all child participants. Children received their choice of a colorful adhesive sticker for their participation.
Eighty-one 7- to 10-year-old children (41 females, 40 males; M = 8.95 years of age) participated. Thirty-one children were classified as having a reading disability, and 50 children had typically developing reading skills. Children were classified as having a reading disability according to either an IQ discrepancy definition or a low-achievement (LA) definition using the Basic Reading Skills (Word Identification and Word Attack subtests) component of the Woodcock–Johnson Tests of Achievement (3rd ed.; WJ-III; Woodcock, McGrew, & Mather, 2001) and the Matrices subtest of the Kaufman Brief Intelligence Test (K-BIT; Kaufman & Kaufman, 1990), a measure of nonverbal intelligence (NVIQ). Children classified according to the IQ discrepancy criteria had both a standard score above 70 on the K-BIT and a reading regression–corrected discrepancy that was 1 SE or greater than the estimate. Children classified according to the LA criteria had both a standard score above 70 on the K-BIT and a reading-achievement scaled score at or below 85.
During the reading screening, children were administered the Elision subtest of the Comprehensive Test of Phonological Processing (CTOPP; Wagner, Torgesen, & Rashotte, 1999) as a measure of the child's phonological awareness. Children also were administered the Fluency and Passage Comprehension subtests of the WJ-III to provide further descriptive information regarding the sample. Eleven children (9 with RD, 2 without) had raw scores on the fluency measure too low to be classified as standard scores on the WJ-III. To include them in the descriptive analyses, the minimum standard score of 54 was used, subtracting 1 point to reflect the lowest score in the sample.
Because children with RD commonly evidence comorbid deficits in attention (Brier, Gray, Fletcher, Foorman, & Klaas, 2002), an auditory attention task also was administered to each child. The task, based on one developed by Keith (1994), was implemented to assess the child's auditory attentional state prior to CMR testing and should not be construed as a measure related to a diagnosis of an attention deficit (i.e., attention-deficit/hyperactivity disorder). Children were classified according to norms determined by Keith (1994), with sliding age-based criteria for classification, whereby children who scored above the specified cutoff (i.e., had more errors than the average-age peer) were classified as inattentive, and those who scored below the specified cutoff were classified as attentive. Fifteen of the 31 children with RD and 13 of the 50 children without RD in the study were classified as inattentive according to the auditory continuous performance test. Table 1 summarizes the demographic and psychometric characteristics of the children grouped by RD status and of the entire sample of children.
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Stimuli
Auditory attention. The auditory continuous performance test consisted of two orally presented test lists in which a target letter "X" occurred 21 times in a list of 100 randomly sequenced letters. Letter strings were repeated 6 times in a list. The letter "S" was excluded because of its phonological similarity to the target; otherwise, all remaining letters in the English alphabet were used. One 23-letter practice list contained five instances of the target letter. Sound pressure levels (SPLs) for each of the two test lists were between 54 and 62 dB(A) SPL for List 1 and between 65 and 78 dB(A) SPL for List 2. Auditory continuous performance test stimuli were recorded by a female speaker at a rate of 1 stimuli per second onto a Sony TCD-D100 digital audiotape player and were played to the children through Sennheiser HD 25-1 headphones.
CMR. CMR stimulus parameters were selected by using the parameter level that brought about the maximal amount of masking release according to results in the existing CMR literature. Specifically, a parameter value (e.g., signal frequency, modulation rate/depth) was chosen if it led to the greatest amount of CMR achieved across each of the previous studies. This procedure led to the greatest chance that the CMR stimuli would bring about the maximum masking release possible by the participants.
A pure-tone signal and two noise maskers were generated. The signal stimulus was a 1000-Hz pure-tone signal having a 400-ms duration, including a 50-ms cosine2 rise/fall time. Signal levels ranged from 14 dB to 100 dB SPL. The on-signal masker was a 75 dB(A) SPL 20-Hz wide bandpass noise centered on the signal frequency (990–1010 Hz) that was 100% amplitude modulated at 10 Hz. The duration of the masker was 600 ms. Thus, the signal and masker were gated on simultaneously, and the signal was gated off before the masker. The comodulated masker included the on-signal masker in combination with eight flanking bands comodulated at a rate of 10 Hz. Each of the flanking bands was 20-Hz wide and was separated from the others by 100 Hz, resulting in flanking bands of 590–610 Hz, 690–710 Hz, 790–810 Hz, 890–910 Hz, 1090–1110 Hz, 1190–1210 Hz, 1290–1310 Hz, and 1390–1410 Hz. The comodulated masker's level was set at a maximum of 75 dB SPL, including all eight flanking bands.
A custom software program digitized the signals and maskers for CMR with a 20-kHz sampling rate using a Tucker–Davis Technologies (TDT) array processor board. The signal and maskers were sent through a TDT 16-bit D/A converter, were low-pass filtered at 10 kHz, were routed through a TDT programmable attenuator, and then were summed before being sent through a headphone buffer. Participants listened to the sounds diotically via Sennheiser HD 25-1 headphones.
Procedure
Children. Children were first administered the K-BIT, then the auditory continuous performance test, and then CMR. In a separate testing session, children were administered the Basic Reading Skills component and the Fluency and Passage Comprehension subtests of the WJ-III as well as the Elision subtest of the CTOPP. Each testing session lasted approximately 1–1.5 hr.
Children were tested in a small, quiet room in the child's school, where the child was seated at a table in front of a portable computer and listened to the sounds through headphones. To ensure consistency in the auditory environment across rooms and schools, sound baffles were placed around the perimeter of the child and computer. Ambient noise levels in the elementary schools were between 43.6 and 52.8 dB(A) SPL.
For the auditory continuous performance test, each child was randomly assigned to receive one of the two lists of letters. Children were instructed to lay a hand flat on the table where the child was seated, raise the thumb every time he or she heard only the letter "X" spoken, and then place the hand flat again. The practice list was administered before testing to ensure that the child understood the task. The experimenter coded the child's responses during presentation of both the practice and test lists. The auditory continuous performance test was administered immediately prior to the CMR task to determine the most accurate representation of the child's attentional state at the time of auditory testing.
Adults. Adult participants received only the CMR task and did not receive the reading, NVIQ, or attention testing. Adults' test sessions lasted approximately 45 min to 1 hr. Adults were tested in a university laboratory in a room that was not sound attenuated, to make the testing environments between the adults and the children as similar as possible. The ambient noise level in the laboratory was between 43.6 and 52.8 dB(A) SPL.
All participants. Two conditions used to determine CMR were presented to each participant: One condition consisted of the signal plus the on-signal masker (reference condition), and the other condition consisted of the signal and the masker containing flanking bands (modulated-masker condition). The order of presentation for the two conditions was counterbalanced across participants, with participants randomly assigned to receive either the reference condition or the modulated masker condition first. In both conditions, three threshold estimates were obtained for all participants unless the first two thresholds fell within 5 dB of one another. An average threshold was calculated for each participant in each condition, and the difference between the averages for the reference and modulated-masker conditions was taken as the estimate of CMR.
To ensure that the participant understood the task, practice trials were instituted prior to testing in each condition. Through headphones, the participants heard samples of the tone, the tone with the noise, and the noise alone. Using the computer keyboard, they indicated whether the tone was presented or not by pressing the right arrow key (labeled with a "Y" to indicate the presence of the tone) or the left arrow key (labeled with an "N" to indicate the absence of the tone). After the completion of practice trials, testing in the relevant condition began. All participants were tested with a single-interval yes–no procedure because previous research suggests that children with RD may have problems making judgments of temporal order (Montgomery, 2002; Tallal, 1980). Signal levels and threshold estimates were determined according to a maximum-likelihood algorithm. This technique estimates reliable thresholds with fewer trials compared to other psychophysical methods (Gu & Green, 1994). The first trial presented a stimulus level between 95 and 100 dB for which it was assumed that all listeners with normal hearing would be able to hear the tone within the noise masker, and the second trial presented the 24-dB stimulus level for which it was assumed that all listeners with normal hearing would not be able to hear the tone within the masker. Signal levels on subsequent trials were based on the participant's responses to the previous trials. The listener's threshold was extrapolated at the 71% point on the psychometric function having the maximum likelihood at the end of 24 trials. To assess any false-alarm bias in the responses, six "catch" trials at the 14-dB stimulus level were randomly interspersed among the "true" trials (Gu & Green, 1994). The intertrial interval was a minimum of 300 ms. If the participant failed to respond to a trial after 5 s, the program prompted him or her to do so. Therefore, the intertrial interval varied from trial to trial. After each threshold estimate, the participant was rewarded with a 30- to 55-s audiovisual display that consisted of music from Walt Disney films and the associated visual images.
Results
A repeated-measures analysis of variance (ANOVA) with condition as the within-subjects factor and order as the between-subjects factor revealed that order of presentation did not significantly influence threshold (p > .05). Thus, thresholds were combined across order for descriptive purposes and for data analysis. Pearson product–moment correlations revealed that neither CMR nor reference or modulated masker thresholds were correlated significantly with any of the reading measures given. Table 2 displays CMR and mean thresholds for each condition.
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R2 = .008, p > .05), and the overall model became insignificant (p > .05). Neither attention status (entered third) nor RD status (entered fourth) was significant (p > .05), and the overall model remained nonsignificant (p > .05). Group means for the reference condition for children classified by reading status are displayed in Figure 2.
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R2 = .007, p > .05), and the overall model became insignificant (p < .05). Neither attention status (entered third) nor RD status (entered fourth) was significant (p > .05), and the overall model was not significant (p > .05). Group means for the modulated masker condition for children classified by reading status are displayed in Figure 3.
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Significant relationships were found between NVIQ and masked thresholds. On average, children with higher NVIQ had lower thresholds in both the reference and modulated masker conditions, suggesting a relationship between nonverbal intellectual functioning and signal detection in these masking tasks. Processing efficiency is reduced when thresholds in both the reference and the modulated masker conditions are elevated, regardless of the amount of CMR measured. As processing moves higher into the central auditory system, more basic cognitive skills might well have an increasing impact on auditory performance. In this case, NVIQ predicting processing efficiency (i.e., reference and modulated masker condition thresholds) would be a logical finding.
The finding that CMR was not reduced in children diagnosed with RD in the present study is consistent with previous research with adult listeners with RD. Amitay, Ahissar, and Nelken (2002) assessed auditory processing, including CMR, in 23 adults with reading disabilities and 27 adult controls and found that CMR was not reduced in the sample of listeners with RD. It is of interest that Amitay et al. (2002) measured CMR using the modulated–unmodulated difference, as this method differed from the one used in the present study and suggests that multiple methods used to measure CMR yield similar results between listeners with and without RD.
Because both adults (Menell et al., 1999) and children (Lorenzi et al., 2000) with RD are found to have a reduced sensitivity to AM stimuli, the ability of children with RD to perform the CMR task without measurable difficulty was somewhat unexpected. However, there are various reasons that might explain why a significant effect was not found. Menell et al. (1999) reported a significant difference between adults with RD and those without RD in their detection of AM. However, comparisons between those results and the present study must be made cautiously, as those participants qualified for RD status by being at least 2 SDs below the mean on a measure of reading ability, whereas in the present study, participants met criteria for RD status by scoring at least 1 SD below the mean. Selection criteria used by Amitay et al. (2002) also differed from that of the present study, as those participants were recruited based on self-report of reading difficulties. No standardized reading measures were available for the native language of the participants (Hebrew), but participants classified as RD by Amitay et al. (2002) did have nonword reading scores at least 1 SD below the control group. Although children in the present study were relatively well matched on all variables excluding the reading measures, it is possible that a stricter diagnostic criterion would result in a stronger relationship between RD status and CMR.
Secondly, because the participants in the Amitay et al. (2002) and Menell et al. (1999) studies were adults, the possibility remains that the relationship between RD status and auditory processing (including CMR) may change developmentally. Given the recent findings that children with language impairments show auditory thresholds on a variety of tasks that bear similarity to those of children approximately 4 years younger (e.g., Wright & Reid, 2002), it is possible that the course of auditory development for children with RD differs from that of typically developing children, which might become evident when using participants outside the range of ages used here.
The stimuli used in the present study also may explain the lack of a significant finding. The low rate of AM (10 Hz) may have been too slow to extract any temporal differences in abilities across participants, as those rates below about 16 Hz do not tax the temporal auditory system but, rather, depend on the ability of the ear itself to resolve the AM information in the sound (Moore, 1997). Because the stimuli were designed to elicit maximal CMR, it is possible that more challenging stimuli (e.g., a higher rate of AM) would bring about the expected pattern of results. However, given that the rate of AM that is important for the perception of speech falls between 4 and 16 Hz, using a higher rate of AM in CMR may not be justified if one intends on explaining a possible relationship between the perception of AM as required for CMR and that required for speech perception. If CMR appears poor for persons with RD at higher rates of AM outside of that required for speech, then conclusions regarding any similarities between the use of AM in CMR and speech perception would be confounded.
CMR, however, is not a pure AM detection task, and other cues (e.g., spectral cues) are available to the listener that may be used to aid in overcoming a possible AM-detection deficit. The possibility remains that across-channel cues may serve to counter the impact of any AM processing deficit of children with RD in the CMR paradigm, resulting in no discernible impact on CMR. However, given the relatively close proximity of the eight flanking bands in the modulated masker condition, it is possible that within-channel cues were used in signal detection by listeners in the present study. Additional within-channel spectral and temporal information provided by the relatively closely spaced flanking bands may have precluded the need to depend on the across-channel information typical of CMR. These additional peripherally based cues may have provided extra information on signal detection to any listeners with temporal processing difficulties, making the task easier and resulting in the similar thresholds found between the groups in the present study. Thus, further research with flanking bands placed further apart is needed to evaluate the possibility of any cross-channel processing deficit.
The finding that reading status did not predict CMR in the current sample of children may reflect a complex relationship between auditory processing and reading skills, with deficits being auditory task specific. Previous research indicates that children with RD have difficulties with some measures of temporal processing and not others. For example, Montgomery (2002) found similar performance between children with or without RD on the precedence effect but found impaired performance by the children with RD on judgments of temporal order up to a 400-ms delay. However, Montgomery et al. (2005) assessed backward masking in a sample of 52 children (26 with RD), finding that RD status predicted performance for this temporal measure in accordance with other previous research (Rosen & Manganari, 2001). The backward masking task, similar to CMR, requires central processing. Unlike backward masking, the individual conditions presented in CMR bear more similarity to the peripherally based simultaneous masking condition, wherein the signal is temporally centered within the masker rather than being presented immediately prior to the onset of the masker, as in the backward masking condition. It is noteworthy to mention that simultaneous masking also was explored by both Montgomery et al. (2005) and Rosen and Manganari (2001) in the same children and adolescents with and without RD mentioned previously, and no significant differences were found between groups with this masking task. Thus, it remains possible that the simultaneous nature of the masking task in the CMR paradigm also may contribute to the lack of a significant finding in the present study and others that measured performance with CMR stimuli.
Banai and Ahissar (2006) recently reported finding differences in the pattern of impairments across auditory tasks in a group of seventh- and eighth-grade female children diagnosed with dyslexia as well as having additional learning disabilities. Children with dyslexia performed similarly to typically developing children on measures of identification and discrimination of both tones and speech sounds. However, the children with dyslexia had difficulties making judgments of temporal order and in making comparisons about one sound being longer or shorter, or higher or lower, than another. The authors suggested that the type of task, along with the demands placed on working memory by the task, influenced whether or not children with dyslexia had difficulty performing the task. This suggestion, along with the results of the present study, indicates that CMR appears to be a task that is not sensitive to differences in reading ability.
Development of CMR
The large numbers of participants at different ages permitted a more complete evaluation of the development of the processing of CMR stimuli than was possible with the small samples in previous research. When collapsed across RD status, the mean CMR in children rose by 6.03 dB from age 7 years to age 10 years, yet this difference was not statistically significant. Likewise, both reference and modulated masker thresholds tended to decrease with increasing age. Finally, group comparisons between children and adults revealed that CMR increased significantly and both reference and modulated masker thresholds decreased significantly with age between childhood and adulthood. In the present study, the amount of CMR increased from 1.54 dB (SD = 8.79) in the 7-year-olds to 7.57 dB (SD = 13.48) in the 10-year-olds. This nonsignificant improvement with age has been observed in other studies assessing CMR in children (Hall, Grose, & Dev, 1997) and in the present study may be due to a combination of reduced statistical power and high threshold variability in the CMR task conditions in each age group. The large group of children in the present study showed less CMR than did the six 6-year-old children tested by Veloso et al. (1990; 4.02 dB vs. 8.2 dB, respectively). Furthermore, the 7-year-old children in the present sample evidenced nearly 7 dB less CMR than Veloso et al. 's (1990) 6-year-olds, suggesting potential sampling, stimuli, or methodological differences between the studies.
The children in the present sample were highly comparable across all psychometric measures tested except for the reading and phonological measures. Because Veloso et al. (1990) did not report the psychometric characteristics of the children, the comparability of their sample with the present one cannot be determined. Some other attribute of the children in the Veloso et al. (1990) study may have increased the amount of CMR they reported relative to that seen here. For example, NVIQ significantly predicted CMR thresholds in the present study. If Veloso et al. 's (1990) children had a higher average IQ than the children tested here, then that might be related to the increased amount of CMR that they reported. The large sample of children of average intelligence in the present sample suggests that the present data may provide a more accurate picture of the normative development of CMR than has been gleaned from smaller samples of children.
In addition to sampling differences, differences in stimulus parameters may explain some of the variability in thresholds across studies. For example, some studies (Fantini, Moore, & Schooneveldt, 1993; Hall et al., 1997) have reported that simultaneous gating of signal and masker lowers CMR. The simultaneous gating of signal and masker in the present study may have pushed the amount of CMR here toward the lower end of the range as reported in previous studies. Likewise, previous studies using a 1000-Hz signal with a 20-Hz bandwidth for the masker (as used in the present study) reported CMR in adults (8–15 dB) that were similar to those of the 10-year-olds in the current study (Hall & Grose, 1990; Veloso et al., 1990).
Finally, Veloso et al. (1990) used a three-alternative, forced-choice procedure, whereas the present study used a single-interval, maximum-likelihood estimation procedure to collect thresholds. Although Gu and Green (1994) reported that thresholds with both estimation procedures result in similar thresholds, the data collected for that methodological study consisted of unimpaired adult listeners. Therefore, it cannot be assumed that children with reading impairments would show the same pattern of performance between the two methods. The task may have been made easier for the children in Veloso et al. (1990), given that they had the opportunity to compare three stimuli within each trial, which may explain their lower thresholds relative to the children in the present study. Furthermore, short-term and working memory deficits have been reported in children with RD (cf. Morris et al., 1998; Swanson, 1999) that might have played a role in the relatively higher thresholds found by the children in the present study, as a majority of children heard and practiced the stimuli only once at the beginning of each set of stimuli (unless they requested to hear them again) and had to keep the target stimulus in mind throughout a threshold run. However, this possibility cannot be determined, as working memory was not evaluated in the present sample of children. As the maximum-likelihood estimation procedure used herein was designed to alleviate potential temporal order judgment confounds, which could result from the forced-choice estimation procedure, it is apparent that methodological considerations such as these must be considered when working with individuals with impairments.
The 10-year-old children's CMR of 7.57 dB (SD = 13.48) is comparable to that reported in some previous research with adults. Estimates of adults' CMR have ranged from zero (Richards, Buss, & Tian, 1997) to approximately 15–16 dB (Grose, Hall, & Mendoza, 1995; Hall & Grose, 1990). However, as a group, the children showed significantly less CMR than adults. The trend for CMR to increase with increasing age and the significant difference between children's and adults' thresholds suggests that the abilities necessary to achieve an adultlike CMR are not fully mature until beyond 10 years of age.
The children's mean reference threshold in the present study was 84.79 dB(A) (SD = 10.48), and the children's mean modulated masker threshold was 80.95 dB(A) (SD = 8.43), both of which were significantly higher (p < .01) than those of the adults tested in the present study: 78.41 dB(A), SD = 9.83, and 67.04 dB(A), SD = 7.90, respectively. These results are in agreement with previous studies that reported differences in thresholds between children and adults by 5 dB or greater (Hall et al., 1997; Veloso et al., 1990) and support the hypothesis that processing efficiency is immature in the present sample of children. The greatest difference in thresholds between children and adults was found for the modulated masker condition, with adults' thresholds being, on average, 13.91 dB lower than those of children. The fact that processing efficiency seems particularly poor in the modulated masker condition suggests that across-channel processing may be undergoing an especially protracted course of development. Future research with flanking bands spaced at wider frequencies is needed to lend support to this possibility.
In summary, reading status did not predict CMR and did not predict thresholds in the reference or modulated masker condition. Thus, this sample of children with RD showed no deficit in achieving a benefit from AM coherence. CMR appears to be a relatively slowly developing auditory skill, with the amount of masking release increasing significantly between middle childhood and adulthood. However, the basic across-channel mechanisms needed to achieve a release from masking appear to be at least functional by 7 years of age, as measured in the present sample of children. Likewise, processing efficiency, as reflected in children's high thresholds in the masking condition, appears to be undergoing development beyond 10 years of age.
Further research that manipulates stimulus parameters is needed (i.e., a faster rate of AM than that used in the current study might tax the temporal abilities that are hypothesized to be sluggish in children with RD and increasing the spacing between flanking bands to ensure across-channel processing) to determine whether altering stimulus parameters in CMR would have a significant impact on thresholds in children with RD. Additionally, testing large groups of children of varying ages with multiple stimulus parameters will continue to clarify the developmental course of CMR.
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Acknowledgments |
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Received September 16, 2006
Revision received April 4, 2007
Accepted October 10, 2007
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