MR's Music Agnosia

Peretz's Taxonomic Model

MR exhibited impaired recognition for familiar musical instruments and impaired perceptual matching for the melodic components of novel tunes. Combined with his established deficits in recognising song lyrics, familiar tunes, and rhythmic processing (Beckwith, 2003), MR exhibits each of the agnosias in the musical branch of Peretz's taxonomy, with the exception of metricality. The selective sparing of MR's metricality lends support to Peretz's notion of distinct subsystems for the temporal and melodic organisation of tunes, as well as separate modules for rhythmic and metric processing within the temporal dimension (Peretz & Coltheart, 2003).

Within melodic organisation, MR's contour representation was impaired, supporting Peretz's claim that the right STG is necessary for this process (Liegeois-Chauvel et al., 1998). MR showed impaired performance for tasks that provided a "clean" assessment of contour representation function. MR's normal performance on Peretz's Contour task may reveal a task difficulty effect or a cognitive approach effect. Her task was based on multi-componential stimuli involving temporal cues and contained tones that were presented relatively slowly. Thus, MR may have mentally labelled the outcome of the first melody (up/down/same) and then compared that verbal label with the second melody. The unsubtle cues found in the Octave-dispersed/contour-preserved task may have allowed MR to use a similar strategy. This strategy appears to have been thwarted when more demands were placed on the cognitive system, as shown by MR's inability to discriminate linearly-transformed/contour-preserved melodies.

MR's combination of impaired contour and interval processing supports Peretz's theory that contour representations provide the anchorage points for encoding interval information (Peretz, 1990). Based on the frequent co-occurrence of impaired tonality and intervallic processing, Peretz has further claimed that intervallic processing is required for the perception of tonality since the musical scale is dependent on precise intervals (Peretz, 1993b, 2001a). MR's case provides additional evidence to her claim.

Although the lowest levels of Peretz's taxonomy, melodic and temporal organisation, explain MR's music agnosia, they do not account for his musical illusions or his incorrect perception of identical melodies. Due to the confounding influence of an apparent perceptual deficit, MR's findings challenge Peretz's assumption that melodic impairments arise solely as a result of an impaired and isolable contour/interval/tonality subsystem (Peretz & Babai, 1992; Peretz & Coltheart, 2003; Peretz & Kolinsky, 1993). Instead, MR's case suggests that perceptual deficits may underlie his melodic disorders.

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