Oticon Alta

Temporal Processing Deficits in Children with Dyslexia

The primary goal of pediatric habilitation among audiologists and speech language pathologists is the development of age-appropriate language and reading skills so children can be educated. Driven by solid evidence that children with hearing impairments struggle to develop normal language skills, few would debate the importance of audition for normal language acquisition.

Therefore, all children with speech, language or learning disabilities should be assessed by an audiologist for hearing problems. Children diagnosed with hearing impairment should be provided remediation through appropriate hearing aid(s), cochlear implants(s), amplification system(s) or assistive listening device(s). In this way, the speech language pathologist would be assured that impediments due to hearing loss would be adequately overcome, prior to the initiation of speech language therapy.

While the role of audition per se is well understood, the role of auditory processing and auditory processing disorders (APDs) in the development of language and reading is more controversial.

Auditory processing includes a variety of skills, which can be divided into several major categories (ASHA, 1996). Deficits in these categories can be identified through standard behavioral tests, but how those deficits interfere with normal language and reading development is not clearly understood. To better understand the impact of auditory processing and APDs on language and reading development, researchers have begun to evaluate auditory processing skills in children with known language and reading deficits.

Reading problems during the early years of education interfere dramatically with successful achievement. As many as 10% (or more) of the children in school today have a variety of learning disabilities that can result in poor reading skills. Of those, half may be diagnosed as dyslexic (Snowling, 1998).

Dyslexia is defined as a language-based reading disorder that is primarily attributed to weaknesses in phonological processing. Audiologically, most dyslexic children have normal hearing thresholds, but many may have an undiagnosed auditory processing disorder (Moncrieff & Musiek, 2002). It has been suggested that unless APDs are diagnosed, addressed and remediated, a pre-exisitng APD may impede traditional therapies designed to facilitate typical phonological processing in dyslexic children (Cacace & MacFarland, 1998). This implies that prior to initiation of therapy, children with dyslexia should be referred to the audiologist for a comprehensive audiologic evaluation and APD testing and evaluation.

Auditory processing disorders may be defined as weaknesses in perception and/or cognition following the input of an auditory stimulus. From initial neural firing at the receptor cells within the cochlea to the complex interactions that occur within the cortex, auditory processing occurs at many levels throughout the brain. Throughout early childhood, auditory structures in the brain depend upon innervation in the cochlea and ascending synaptic activity for normal development of neurons (Rubel & Fritzsch, 2002). Children need exposure to a variety of simple and complex auditory stimuli from their environment to develop normal auditory pathways and abilities. The acoustic stimuli must adequately and appropriately excite ascending neural pathways in order for the brain to develop normally. Any disruption in acoustic input from the environment or from the neural pathways, can lead to behavioral deficits that are commonly characterized as auditory processing disorders.

Audiologists routinely assess five auditory processing categories: auditory closure, auditory figure-ground, binaural interaction, binaural integration and temporal processing. Because of a reported link between temporal processing deficits and deficits in phonics skills in reading impaired children, there has been a surge in research related to this topic (Tallal, Miller & Fitch, 1999). Results from recent research efforts related to temporal processing in dyslexic children will be the primary focus of this review.

Temporal processing can be divided into two primary categories, temporal integration and temporal resolution (Eddins & Green, 1995). Temporal integration includes tasks in which the intensity and duration of the signal interact such as in threshold determination and signal thresholds during different types of masking. Temporal resolution includes tasks of temporal order judgment like the tone-order task used by Tallal, but also includes gap detection, masking level difference, detection of amplitude modulation, and detection of temporal asynchrony.

Temporal Integration

When asked to detect a probe tone in the presence of a masking noise, Rosen and Manganari (2001) found that dyslexic teenagers performed similarly to control children with forward and simultaneous masking of the tones. During backward masking dyslexic teenagers demonstrated thresholds for the tones that were significantly elevated. These results were consistent with a genuine auditory deficit and similar to results reported previously when other groups of children were tested under the same three masking conditions (Wright, Lombardino, King, Puranik, Leonard, & Merzenich, 1997). To relate this backward masking deficit to a speech analogue, Rosen and Manganari also presented two different contrasting CV pairs to children. They hypothesized that discrimination of the contrast /ba/ vs. /da/ would be difficult for the dyslexic children because the steady state vowel would backward mask the two consonants. Alternatively, they hypothesized that discrimination would be normal for /ab/ vs. /ad/ because the vowel preceded the consonant and masking would only occur in the forward direction. When tested, the dyslexic children were indeed poorer than control children at discriminating consonant vowel pairs when the syllable cluster occurred in either direction. When the researchers then tested the dyslexic children's ability to perceive non-speech analogues of the CV contrasts by presenting them with second formants in isolation, 75% of the dyslexic children performed similarly to the control children.

Rosen and Manganari concluded that the backward masking deficit in children with dyslexia appeared to have no clear-cut relationship with the children's perception of consonants in syllable initial positions. Even though dyslexic children were better at discriminating non-speech analogues, the authors disagreed with earlier conclusions by Mody and colleagues (1997) that this supports a speech-specific deficit. They recognized that non-speech analogues used in this and in Mody's experiment were acoustically far simpler than the speech contrasts used. Therefore, they hypothesized there may be an important effect of acoustic complexity on the deficit observed in dyslexic children.

Results in the Rosen and Manganari study were also variable. Approximately 40% of the dyslexic children performed normally on backward masking and discrimination of the non-speech contrast. In another study of backward masking, children with reading disabilities were divided into two subgroups on the basis of oral language skills (McArthur & Hogben, 2001). When tested, only a subset (41%) of the children with concomitant reading and oral language deficits demonstrated significantly elevated thresholds during the backward masking recognition task. The remaining 59% of the children with reading plus oral language deficits had backward masking thresholds comparable to control children, as did reading disabled children with normal oral language included in the study. These results are consistent with Rosen and Manganari and others that suggest that some, but not all, children with reading disorders will demonstrate auditory processing deficits (Adlard & Hazen, 1998; Manis, McBride-Chang, Seidenberg, Keating, Doi, Munson, & Petersen, 1997).

Temporal Resolution

Gap Detection and Masking Level Difference

A study investigating gap detection found that younger dyslexic children between ages of 6 and 9 performed significantly poorer than age-matched control children, but when the same test was administered to dyslexic children ages 10 to 13, they were able to perform gap detection normally (Hautus, Setchell, Waldie, & Kirk, 2003). These results suggest that temporal resolution deficits found in younger children through gap detection tests may improve as children mature and may not be directly involved in language and reading deficits after age 10. Hautus and colleagues noted that the presence of these deficits in younger children may serve as an early indicator of a more persistent deficit in perception and language. As such, gap detection tests may serve as reasonable screening tools to assess younger children for presence of this type of temporal processing deficit.

When tested on tasks of gap detection and binaural masking level difference, reading disabled children ages 7 to 14 did not perform differently from non-impaired children (Breier, Fletcher, Foorman, Klaas, & Gray, 2003). Because a majority of children in this study were older than 9 years, these results for gap detection may be similar to those found by Hautus and colleagues. The authors did notes that across all temporal processing tests, performance by children with a co-morbid attention deficit disorder, whether or not they also had a reading disability, was reduced. These results highlight the importance of assuring that children who are assessed for language, reading, and auditory processing disorders also be assessed for attention deficit disorder.

Temporal Order Judgment

Since first described by Tallal (1980), tests of the link between deficits in temporal order judgment and reading disorders have produced inconsistent results. Several researchers who failed to identify systematic differences in temporal order judgment (TOJ) between dyslexic and control children have suggested that large individual differences in performance on this task may be linked to verbal labeling skill rather than temporal processing (Nittrouer, 1999; Marshall, Snowling & Bailey, 2001; Heath, Hogben, & Clark, 1999). Others have supported Tallal's theory that dyslexic children falter in the processing of rapid auditory information when they found that TOJ performance was poorer in dyslexic children than in controls unless the two stimuli used in the task were artificially slowed, after which dyslexic children's performance reached that of the control group (Rey, DeMartino, Espesser & Habib, 2002). Rey and colleagues went on to add that TOJ performance in the dyslexic group also correlated with tests of phonological processing, lending substantial support to the rapid auditory deficit hypothesis.

Another study that utilized Tallal's TOJ task to explore the relationship between temporal auditory processing and measures of phonological awareness and reading reported no significant correlation was found (Bretherton & Holmes, 2003). Approximately half (48%) of the reading disabled children performed at greater than one standard deviation below control children on the tone-order judgment task. When the dyslexic children were divided into two subgroups on the basis of their tonal TOJ performance and assessed on several measures of phonological awareness, order processing of consonant-vowel speech sounds, or severe reading difficulties, there was no consistent relationship between tone-order deficits and these other skills. One possible explanation for the difference between these two studies could be that Rey and colleagues tested dyslexic children with a TOJ task that involved two consonants (/p/ and /s/) within a syllable cluster while Bretherton and Holmes tested children with contrasting tones. It is possible that TOJ performance with speech analogues may correlate better with phonological awareness and reading skills than performance using non-linguistic tonal stimuli.

Results from another recent study support this possibility. When tested on TOJ tasks with two and three elements, reading disabled children performed more poorly on tasks involving shorter inter-stimulus intervals between consonant-vowels, but not between tones (Breier, Gray, Fletcher, Foorman & Klaas, 2002). They added that the relationship between TOJ measures and phonological processing measures were stronger for tasks involving speech analogues than for non-speech stimuli. Breier and colleagues argued children performed more poorly on the task with syllables because acoustic features in the syllables were more complex than tones. This is in agreement with the conclusions drawn by Rosen and Manganari (2001) in their backward masking experiment that acoustic complexity may negatively interfere with temporal processing and suggests that complexity may be an important factor for temporal integration and temporal resolution.

Interestingly, when Rey and colleagues manipulated complexity in their TOJ task by simplifying the syllable structure from CCV to CVC, performance by the dyslexic children did not improve. None of the clusters used in their experiment formed real words, thereby limiting activation to the phonological channel and avoiding lexical access throughout the experiment. This result suggests that in tasks that activate phonological processes, changes in complexity across different phonological stimuli may have less impact on performance by dyslexic listeners than changes in complexity that contrast a phonological stimulus (CV) with an acoustic stimulus (tone). Since all of the stimuli involved did not create words, it suggests that complex components within phonological processes can be used to demonstrate performance deficits in dyslexic children, and further, the deficits are not necessarily specific to speech.

Temporal Asynchrony

Children with reading disability, whether or not they had a co-morbid attention deficit disorder, did perform more poorly on one temporal acuity task involving the detection of a tone onset asynchrony (Breier, et al., 2003). The authors argued that the presence of a deficit on this temporal acuity task without evidence of any deficit on gap detection, also a temporal acuity task, suggests sensitivity to backward masking in children with reading disorders. They pointed out that because the task required that the children detect the presence of two successive signals, the first tone was susceptible to backward masking effects of the second tone presented. This would suggest that a tone onset asynchrony task is perhaps both a temporal integration task and a task of temporal resolution and that some mechanisms involved in tone onset asynchrony may overlap between the two types of temporal processing skills. It also lends further support to the observation that on backward masking tasks, dyslexic children are more likely to perform poorly regardless of whether the stimulus is tonal or speech-like.

Summary

Several researchers have reported deficits in auditory temporal processing in approximately half the dyslexic children studied. While this demonstrates that auditory processing deficits may be present in a significant number of dyslexic children, it also substantiates that temporal processing deficits are not necessary to cause language and reading disorders witnessed in these children. Whether they are sufficient to cause the language and reading disorders found in a subset of dyslexic children is still unresolved. The presence of temporal processing deficits in a significant number of dyslexic children tested across a number of studies does justify continued research into this important question. These studies also highlight the important distinction that not all children with reading disorders will demonstrate similar deficits on any one auditory processing task. Because there is little homogeneity among children with dyslexia, therapists involved in diagnosing and treating dyslexia should perform comprehensive evaluations to determine what underlying deficits are present in each dyslexic child they intend to treat.

In addition to evaluations for speech and language disorder, clinicians who are assessing and treating children with dyslexia should recommend testing for auditory processing disorders prior to initiating remediation. While it has been hypothesized that children with a co-morbid auditory processing disorder may not benefit from reading therapy to the same extent as children without APDs, there is little direct evidence from clinicans to support this hypothesis. If children are evaluated and grouped on the basis of auditory processing skills before remediation begins, pre-therapy information could be used to learn whether or not co-morbid APDs negatively interfere with therapeutic efforts.

It should be noted that temporal processing deficits have received a substantial amount of attention in recent years, but other types of APDs may also be diagnosed in dyslexic children. It would be a mistake to focus on one type of APD to the exclusion of others. Children with language and reading disorders should routinely be evaluated by a complete battery of tests performed by an audiologist trained in the diagnosis of auditory processing disorders. In addition to tests of temporal processing, children should be assessed on auditory closure, auditory figure ground, binaural interaction and binaural integration. It is also recommended that parents and teachers be given an opportunity to assess the child's auditory skills by completing a comprehensive questionnaire.

Several therapies that purport to remediate temporal processing deficits are available and research studies have begun to evaluate their effectiveness. In addition, research is underway to develop therapies to remediate other types of APD. To the extent that effective therapies become available, it is hoped that children with any type of APD might be able to enroll in an appropriate program designed to address their deficit.

Audiologists and speech language pathologists are invaluable members of the comprehensive therapy team providing full diagnostic evaluations and appropriate remedial efforts for children diagnosed with dyslexia.


REFERENCES

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