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.