Neurobiology of (central) Auditory Processing Disorder and Language-based Learning Disability

(Central) auditory processing disorder (abbreviated here APD) is defined as a deficit in the processing of auditory information, despite normal hearing thresholds, that primarily involves the auditory modality (ASHA, 2005; Jerger & Musiek, 2000). This umbrella definition encompasses a wide variety of perceptual and cognitive manifestations. APD thus cannot be reduced to a single anatomic site or impaired process in the auditory system. The question of whether such a nonspecific definition can really benefit research, diagnosis, and treatment notwithstanding, in this chapter we will focus on the physiologic processes thought to underlie the perception of auditory aspects that fall within the realm of APD. We will review evidence for specific physiologic processes and anatomic sites contributing to normal and abnormal auditory processing. In particular we review studies of the physiology and anatomy of: (1) auditory temporal processing , (2) auditory perception in noise, (3) representation and discrimination of acoustic features and (4) binaural processing. We also present evidence for (5) training-related neural plasticity of these processes where such evidence exists, focusing on training studies aimed at populations with symptoms of APD. The physiologic processes reviewed here with their accompanying perceptual correlates are summarized in Table 4–1. To our knowledge, physiologic and anatomic studies of APD diagnosed populations have been rare. On the other hand, many studies have focused on auditory processing in populations diagnosed with language-based learning disabilities (e.g., specific language impairment [SLI] and