Have you ever wondered what makes a particular audiologic test especially sensitive to auditory processing disorder (APD)? Have you wondered why particular treatment strategies offer great potential to improve APD? Surely, you've wondered about the underlying cause of APD. The key to answering all three questions is, in a word, neurobiology.
Neurobiology focuses on the anatomy, physiology, and chemistry of the nervous system. Neurobiology guides our understanding of brain functions ranging from basic synaptic mechanisms to complex human behavior, and it presents extraordinary implications for the practical matters of diagnosis and therapy. Indeed, APD can be defined as difficulties in the perceptual processing of auditory information in the central nervous system, and in the neurobiologic activity that underlies those processes.
All aspects of audition, from pure-tone hearing to complex spoken language processing, rely on the transmission of neural information across synapses. Information about sound encoded at the cochlea must be transmitted to the brain through a complex network of neural synapses. Synaptic transmission is dependent on chemical processes.
These neurochemical processes serve an important role in the structure and function of the brain, including structural and functional hemispheric asymmetry and plasticity.1,2 Suspected neurobiologic sources of APD include deficient interhemispheric transfer via the corpus callosum and lack of appropriate hemispheric lateralization.3
APD TESTS ROOTED IN NEUROBIOLOGY
All behavioral tests used today to diagnose APD have neurobiologic roots in the early work of an Italian team of physicians who began developing more sensitive tests to quantify the auditory difficulties reported by their patients with compromised central auditory nervous systems.4,5 A few years later, Kimura introduced dichotic testing and formulated a neurobiologic model to explain the physiology of the central auditory nervous system (CANS) underlying dichotic perception.6
Today's auditory training approaches that exercise central auditory processes (e.g., interaural intensity difference training, interhemispheric transfer training) derive from our understanding of neurobiology and can be traced to the work of these pioneers.
We now know that the basic principles of neurobiology—maturation, myelination, plasticity, stimulation, and deprivation—carry significant implications for both diagnostic and therapeutic approaches. In formulating APD test batteries, audiologists must consider a plethora of variables with neurobiologic footprints, including age appropriateness of tasks and response mode, availability of age- (and gender-) appropriate norms, demands on other modalities (e.g., vision), attention, memory, and intellect, and the patient's physical state and medications.
Likewise, therapy for APD is derived from three pivotal, neuroscience-based principles: (1) intensive training exploits plasticity and cortical reorganization, (2) extensive training maximizes generalization and reduces functional deficits, and (3) salient reinforcement induces learning.7 These principles translate into an aggressive management approach that relies on the plasticity of the central auditory nervous system to improve auditory processes through intensive (bottom-up) auditory training coupled with more extensive (top-down) training of cognitive, metacognitive, and language resource allocation strategies to buttress residual auditory processing deficits.8
Our increased understanding of neurobiology drives efforts to develop more sensitive behavioral tests of central auditory function, as well as electrophysiologic, electroacoustic, and neuroimaging procedures that may soon transform clinical auditory processing test batteries. These more powerful tools will elucidate brain structure-function relationships and the neural processes underlying higher cortical functions.3,9 Likewise, cumulative developments in auditory and cognitive neuroscience are being translated into auditory training approaches and strategies training that may improve auditory function and listening.10,11
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