Discrimination of Melodic Contour

The right STG has been implicated in contour processing (Liegeois-Chauvel et al., 1998). MR's lesion encroaches on the STG, thus the finding suggesting preservation of MR's contour processing appears anomalous. Peretz's (2003) discrimination tasks, however, only assessed MR's ability to process contour in basic sequences. Thus, four discrimination tasks designed by Trehub, Bull and Thorpe (1984) were recreated and administered to further interrogate MR's contour processing. With the exception of items from the 'Identical' condition, melodic pairs were subject to the follow manipulations: (1) Transposition, in which the absolute pitch of individual tones was changed but the pitch ratios of successive tones remained intact; (2) Contour-violation, wherein ascending tones became descending tones or vice versa; (3) and Octave-dispersed/contour-preservation, where contour and chroma (note name) were retained but without regard to the octave from which the individual notes were drawn. Randomly chosen pairs from each of the conditions were combined to form a 24-item test. Participants were told to respond 'same' when the contour of the melodies was the same irrespective of transposition or octave change, or 'different' if otherwise. A time limit of seven seconds was imposed. An example of an original melody and all transformed comparisons is depicted in Figure 2.3.

Contrary to MR's normal performance on Peretz's (2003) Contour task, MR failed to discriminate same and different contours in Trehub et al.'s (1984) more subtle Contour- violation and Transposition tasks (z = 3.05, p < 0.01 and z = 4.92, p < 0.001 respectively). This suggests a failure for contour representation. Moreover, MR performed significantly below the controls on the Identical task (z = 4.10, p < 0.001). This result suggests a low-level perceptual deficit, which may have prevented the formation of melodic representations.

Octave dispersion facilitated MR's ability to recognise preserved contour in melodic pairs, as shown by his score within the normal range (see Table 2.3). Octave dispersion exaggerates contour cues, which may explain MR's improved performance.

Table 2.3

Percentage of Correct Responses for Trehub et al.'s (1984) Melodic Contour Discrimination Tasks

a MR's five matched controls completed this task
**p < 0.01, ***p < 0.001

To test the possible contribution of exaggerated contour cues on MR's normal contour-preserved performance, a Linear-transformation/contour-preserved task created by Massaro, Kallman, and Kelly (1980) was recreated. This task reduced the total pitch range of the original sequence by half, but preserved contour and the relative size of the original intervals. Randomly chosen melodic pairs from the Linear-transformation/contour-preserved condition and the Octave-dispersed/contour-preserved condition were combined to form a test of 24 items.

As shown in Table 2.4, MR's normal performance on the Octave-dispersed/contour- preserved task was replicated. MR performed significantly below controls on the Linear- transformed/contour-preserved condition (see Table 2.4), indicating that he can reliably represent the contours of novel melodies only when the contour cues are substantial.

Table 2.4

Percentage of Correct Responses for Massaro et al.'s (1980) Preserved Contour Tasks

a MR's five matched controls completed this task
**p < 0.01, ***p < 0.001

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