Monday, May 12, 2014

Central auditory disorders: toward a neuropsychology of auditory objects

Hrvatski: Primarna slušna moždana kora.
Hrvatski: Primarna slušna moždana kora. (Photo credit: Wikipedia)
en: Acoustic radiation(red arrow). from Medial...
en: Acoustic radiation(red arrow). from Medial geniculate nucleus to Primary auditory cortex ja:聴放線(赤色の矢印)。内側膝状体から一次聴覚皮質へ。 (Photo credit: Wikipedia)
Deficits of auditory cognition are less familiar and less well understood than their visual equivalents. The objects of auditory cognition are natural sounds, but ‘auditory object’ is a problematic concept[]. An auditory object might be defined neuropsychologically as a collection of acoustic data bound in a common perceptual representation and disambiguated from the auditory scene. This definition suggests the importance of perceptual regularities whilst allowing that ‘top-down’ processes may forge associations between acoustic properties and current behavioural goals may give prominence to particular objects within the same acoustic data (e.g., in the spoken word “dog”, relevant sound objects could include the speech token ‘dog’, the speaker’s voice, emotional state, accent, etc). Even this general definition raises certain difficulties. Most everyday sounds have a complex, time-varying frequency structure (see ‘A brief acoustic primer’, Supplementary Material; available using website ), and temporal object boundaries are often difficult to determine. Furthermore, sounds, unlike visual objects, are ‘transparent’ when superimposed; and auditory objects are associated with diverse physical entities, including both discrete sources (e.g., a barking dog) and acoustic events (e.g., a gust of wind, a spoken phoneme). These various auditory object properties and categories have potentially separable neural representations and associated clinical deficits.
The literature on central auditory disorders illustrates these difficulties. Terms such as ‘cortical (or ‘cerebral’) deafness’ and ‘auditory agnosia’ (see Table 1) are widely used, but remain rather loosely defined and demarcated from one another, and progress in defining a useful taxonomy has so far been limited. This is partly attributable to difficulties extrapolating between symptom-led single-case studies and lesion-led group studies in patient populations that may or may not be representative (such as temporal lobectomy series), lack of uniformity of test materials across studies, and the rarity of strategically located brain lesions.
Table 1
Terminology of central auditory disorders
Here we review recent progress in characterising central auditory disorders, focusing on disorders of auditory object processing: the auditory agnosias. From an auditory neuroscience perspective, we are here concerned chiefly with the effects of damage affecting object processing in the putative auditory ventral (‘what’) pathway[]; however, auditory object processing entails important interactions with dorsal ‘where’ and ‘how’ pathways, particularly in the parsing of natural auditory scenes. Our approach is based on a simple operational classification of four fundamental stages likely to be involved in processing auditory objects: parsing of objects in the auditory scene; encoding of auditory properties (at the sub-object level); representation of the perceptual structures of whole objects; and recognition of objects. Key terms are summarised in Table 1. Recent studies of central auditory disorders are summarised in Table 2.!po=38.8889