Over the years, it has become clear to me that hearing plays an important role in the acquisition of early literacy skills, particularly when one considers the relationships among phonological awareness, temporal integration, the impact of training, and the effects of deprivation, such as early otitis media, on the auditory system. This article will focus on the interrelationships among hearing, phonological processing, reading, and dyslexia, as linked by definitions, neuroanatomy, and neuroimaging.
One of the linkages to study is definitions. For one, learning disabilities, according to the Learning Disabilities Association of America, is defined as a neurological disorder that interferes with a person's ability to store, process, or produce information. This can affect a person's ability in the areas of listening, speaking, reading, writing, and mathematics (National Center for Learning Disabilities, NCLD). The federal law (PL 94-121a.5) defines specific learning disability as a disorder in one or more of the basic psychological processes involved in understanding or in using language, spoken or written (Nicolosi et al., 2004). In a broad perspective, learning disabilities implicate listening and the inability to take in the spoken message.
According to the NCLD (2002) dyslexia is the most common learning disability and it is described as a difficulty with language processing as it affects reading, writing, and spelling. According to the International Dyslexia Association, it is neurobiological, characterized by difficulties with accurate and/or fluent word recognition and poor spelling and decoding resulting from a deficit in the phonological component of language (Lyon, Shaywitz, & Shaywitz, 2003).
Dyslexia is a developmental language disorder whose defining characteristic is difficulty in phonological processing and is often genetically transmitted. Phonological processing disorders include problems storing, retrieving, and using phonological codes in memory as well as deficits in phonological awareness and speech production. We see these difficulties in school-age children when they have difficulty learning to decode and spell printed words. Such difficulties lead to problems in comprehension and writing.
According to Catts and Kamhi (1999), one half of children with reading problems also have other language problems. Along with phonological processing, we see limitations in vocabulary, morphology, syntax, and narrative comprehension. In the ASHA Guidelines on Roles and Responsibilities of Speech-Language Pathologists with Respect to Reading and Writing in Children and Adolescents (2000), spoken language provides the foundation for the development of reading and writing. Children with reading and writing problems frequently have difficulties with spoken language. Conceivably, literacy problems may have their foundations in spoken-language difficulties.
The single best predictor of reading success has been known to be phonological awareness. That is, children need to be aware of sounds of speech in order to acquire sound-letter correspondence knowledge and use this knowledge to decode the printed word. If a child is not aware of the sounds contained in a word, the child will have difficulty associating sounds with letters.
According to Sally Shaywitz (2003), for children with dyslexia, phonemes are less well developed. As a consequence, children when speaking may have a hard time selecting the appropriate phoneme. The phoneme is the fundamental element of the language system, the "essential building block of all spoken and written words" (p. 41). It is at the lowest level of the language hierarchy, relegated to processing the distinctive sound elements of language. The upper language hierarchy involves semantics-words, syntax-grammar, and discourse-connected speech. Dyslexia involves a weakness within the language system at the phonologic level.
Shaywitz believes that before words can be identified, understood, stored in memory, or retrieved from it, they must be broken down into phonemes. It is the phoneme that gets processed by the brain's language system. Shawitz contends that the reader needs to convert the letters of words on a page into their sounds and appreciate that words are composed of smaller segments or phonemes. Dyslexics perceive words as an "amorphous blur," without appreciating the underlying segmental nature, and fail to recognize the internal sound structure of words.
In several instances, Shaywitz talks about the "sound structure." Is she not referring to hearing and the involvement of the auditory system? It appears that the auditory system plays a role here. Doesn't the child who begins to learn to read and learns to "sound out the word," identifying the first, middle, last position of sounds in words, rely on hearing those sounds?
When children begin to link the letters to sounds, they link what they see on paper to what they hear in spoken language. They learn that words can be broken apart based on the sound structure, and that print letters represent these sounds. When children have learned this linkage, they have learned the "alphabetic principle" and hence, they are ready to read.
Shaywitz, a pediatrician, not an audiologist, does not discuss the role of audition in sound identification, segmentation, or phonological awareness. I contend that to do those tasks, one needs to have an intact auditory mechanism and be able to hear the sounds of words. The phonological awareness skills of sound deletion, sound categorization, sound blending, and syllable segmentation are identified as the key ingredients in the identification of children in need of early intervention. The children who are unable to make such associations are at risk for reading disabilities.
Unfortunately, the linkage is not all that clear, at least in specific areas that have been studied as components to reading, such as temporal or temporal order skills. For instance, there are individuals who have shown that problems in perceiving rapidly occurring sounds lead to poor phonological representation, which lead to difficulties in phonological awareness and reading (Tallal, 1980). Poor readers have deficits in judging rapidly presented non-speech stimuli. Such individuals are similar to those with phonological impairments.
Other studies indicated that poor readers have non-speech perceptual deficits (Helenuis, Uutela, & Hari, 1999), and others have not found the relationship to be significant (Nitrouer, 1999). Researchers such as Breier et al. (2002) found that good and poor readers differed only in speech perception, not tone perception, and with no relation to temporal factors, such as interstimulus intervals (a temporal skill).
Some researchers question whether measures of phonological awareness are related to auditory processing and reading. In a study of 500 children, Share et al. (2002) found no significant relationship between auditory temporal processing in kindergarten children and their later phonological awareness and decoding skills in second grade. Poor readers with temporal processing deficits were not less skilled on phonological processing in later grades than poor readers with no history of temporal processing deficits. However, the researchers only looked at temporal skills. Are there other hidden variables in the processing of acoustic cues that contribute to sound segmentation and awareness? Is it durational cues, temporal ordering, frequency patterning?
Other Auditory Variables
Other studies linking auditory skills to reading include the work of Moncrieff and Musiek (2002) in which a Dichotic Digits Task (a test used in the assessment of auditory processing developed by Frank Musiek), a measure of temporal integration, was used to differentiate readers with dyslexia from those that were not dyslexic. Results of the Dichotic Digits Test indicated control subjects to be within normal limits for both ears whereas the group with dyslexia demonstrated more interaural asymmetry, particularly due to the right ear performing better than the left.
On a Competing Words Test, all 11-year-olds revealed a right ear advantage, but such an advantage was greater in the children with dyslexia. The left ear appears to improve more than the right in time, reducing the right ear advantage. As children's hearing maturates by age 11, a persistent presence of interaural asymmetry may be correlated with auditory processing problems. All the children with dyslexia scored below average in at least one of the dichotic listening tasks and revealed higher right ear advantage percentages compared to their match peers.
On a measure of durational patterning and frequency patterning, Walker et al. (2002) found that on measures of temporal processing, individuals with reading disorders displayed deficits in discriminating between rapidly presented acoustic stimuli. Studies have suggested that adults with reading disorders also demonstrate difficulty with phonemic discrimination tasks, decoding skills, and phonological processing.
Through the use of frequency and pattern duration tests, 18 college students (nine reading disordered and nine controls) showed no significant difference for the Frequency Pattern test, but a significant difference was found between groups for the Duration Pattern test. The group with reading impairment demonstrated more errors in discrimination of frequency and tone patterns. Correlations between reading ability and temporal processing were found, particularly in word recognition and pattern processing. Such findings suggest that there may be a causal relationship between temporal processing skills and efficiency of decoding. It appears then that temporal processing deficits may be linked with language and reading skills.
Variables Involving Cortical Activation
On electrophysiologic measures, such as Auditory Brainstem Response (ABR), including middle latency response (MLRs) and late cortical responses (P1, P2, N1, P3), and behavioral tests used to evaluate auditory processing in 10 gender-matched children with learning disability, results indicate poorer behavioral scores in the group with learning disability. Minor ABR latency differences between the two groups were found in cortical responses (P1 was earlier and P3 was later and smaller in the learning disability group; Purdy, Kelly, & Davies, 2002).
Some of the controversies are being resolved with the current use of electromagnetic resonance studies (fMRI). From review of the neuroimaging studies, dyslexics show a disruption in white matter connectivity between posterior and frontal regions, which supports a neurobiological etiology. It has been shown by Elise Temple of Cornell University's Department of Human Development (May, 2002) that developmental dyslexics have brain disruption responses to phonological and rapid auditory processing demands, as well as white matter abnormalities.
Neuroimaging data indicate that brain processes related to sound structure of language are disrupted in dyslexia. This research links disruptions in both phonological and auditory processing in dyslexia to abnormalities in neural processing. Although dyslexic individuals exhibit some abnormalities of visual processing, a strong consensus emerges proposing that a central difficulty in dyslexia is the processing of speech sounds, known as phonological processing (which includes rhyming, syllable counting, and sounding out pseudowords). It has been shown that dyslexics have a behavioral deficit in processing rapidly changing auditory information. The rapid processing hypothesis suggests that deficits in rapid auditory processing impair the ability to discriminate auditory cues necessary to distinguish phonemes.
Functional Magnetic Resonance Imaging (fMRI) neuroimaging and position emission tomography (PET) have been used to study dyslexic adults and children to see functional organization of the brain. Neuroimaging studies of phonological processing in adult dyslexics who were performing a rhyme detection test under fMRI report a reduction or absence of activity in the left hemisphere temporoparietal cortex and neural disruption in phonological processing was noted.
Among a variety of age groups, in explicit or implicit tasks, ability levels, analysis techniques, and language, all studies show decreased activity in left hemisphere posterior language regions-temporoparietal cortex in dyslexic subjects, as compared to normal reading subjects.
Shaywitz (2003), using fMRI, found decreased activity in temporoparietal regions-superior temporal gyrus and angular gyrus-during phonological processing of both letter and pseudo rhyme.
Other studies show reduced activity during phonological processing (Temple et al., 2001) in the left temporoparietal area of the left lobe in 8-to 10-year-olds. Left temporal dysfunction occurred in children with dyslexia from different cultural and linguistic backgrounds (UK, France, Italy). All showed decreased activity in the left temporoparietal regions-superior, middle, and inferior gyri, the region that is typically activated during auditory tasks.
Event-related potentials suggest that neural processing of rapid auditory stimuli is disrupted in dyslexics. Normal readers show activation in the left prefrontal cortex, middle and superior frontal gyri during rapid stimuli, whereas readers with dyslexia show no left prefrontal response to the rapid stimuli.
Likewise, neuroanatomical dysfluencies appear to exist for individuals with (C)APD, when one studies synchronicity of the ABR. Ectopic areas and abnormal symmetry are seen in both individuals with APD and ADHD.
We are fortunate to have technology to shed light on the functionality of neuroanatomical structures involved in reading-or not involved, as the case may be-for individuals with dyslexia, to provide a growing understanding of cortical involvement. Through the use of such technology, we will be able to objectively observe what was previously measured behaviorally.
I have discussed only a small segment of the role of the auditory system in reading and phonological processing using definitions contributed by other professions, and the results of cortical and neuroanatomical studies and imaging. There are other areas of interrelationships to be explored, such as the deprivation effects of otitis media and the impact of remediation and specific training programs. With time and technology, the controversy regarding the role of audition in reading may yet be resolved as longitudinal and neuroanatomical studies using fMRI provide information only heretofore conjectured.