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MR's Environmental Agnosia

Dawn Senathi-Raja

Peretz's Taxonomic Model

MR exhibited impaired recognition of environmental sounds. His environmental agnosia is thereby proportionate relative to his music agnosia. Cases exhibiting a similar pattern of association exist in the literature, but are scarce (Metz-Lutz & Dahl, 1984; Takahashi et al., 1992; Yaqub, Gascon, Al-Nosha, & Whitaker, 1988). However, disproportionate impairment of music recognition, relative to environmental sound recognition, is more common (Eustache et al., 1990; Mendez & Geehan, 1988; Tanaka et al., 1987), and has encouraged Peretz's continued focus at the level of cognition. A higher-level focus has given rise to her notion that the recognition of environmental sounds, compared with musical sounds, relies on a differentiated system (Peretz et al., 1994). MR's co-morbid environmental and music agnosia do not support Peretz's suggestion that the environmental and musical auditory domains represent separate and dissociable processing systems. However, MR's dual deficits are aligned with Peretz's interpretation of associated impairments generally, whereby a lesion may cut across adjacent but separate modules (Peretz et al., 1994). Alternatively, a lower-level perceptual deficit may underlie both of these 'independent' processes.

Pitch Perception Account

MR's environmental agnosia can be explained in terms of his specific spectral processing deficit for tones presented over short durations. His difficulty in identifying individual environmental sounds was most pronounced for items composed of rapidly changing broad acoustic spectrums, compared with items comprising slower changing narrowband spectrum. Hence, the disturbance of MR's spectral ability for rapidly changing pitches can be considered to underlie his impaired ability to recognise environmental sounds with broad spectral content.

Auditory Scene Analysis Account

MR was impaired in segregating sounds from an auditory scene. This may have resulted from either, or both, incorrect pitch processing or dysfunctional Gestalt grouping mechanisms. To reconstruct a useful representation of reality, grouping mechanisms allow appropriate linkages between elements of sound to be formed and inhibit the formation of inappropriate groupings (Deutsch, 1999). Given MR's declining performance as additional elements were added to groups of environmental sounds, it appears that his grouping mechanisms are highly sensitive to interference. MR's impaired sequential and simultaneous streaming explains why he was neither able to track pitches from a single environmental source, nor prevent independent environmental sound sources from fusing. MR's lesion is consistent with current theories of anatomical-functional correlation for these grouping mechanisms (Patterson, Uppenkamp, Johnsrude, & Griffiths, 2002).

MR exhibited an auditory spatial deficit. The spatial movement of sound through air, which has a pervasive role in auditory scene analysis, is used to discover the time and frequency pattern of a sound source (Bregman, 2002). As a result, MR's impaired ability to analyse an environmental auditory scene is presumably related to his low-level spatial sound deficit in accordance with Bregman's theory. MR's reported difficulty for locating various environmental sounds, such as the sound of a dog barking, also accords with his sound localisation deficit. MR's right hemisphere lesion outside the primary auditory cortex supports the proposed correlation between this anatomical location and spatial processing of sound patterns (Pinek & Brouchon, 1992).

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