Amplitude modulated (AM) or frequency modulated (FM) sounds are perceived as varying in loudness or pitch respectively. The higher temporal level of hundreds of milliseconds or seconds for sound patterns parallels the level of low modulation rates at tens of Hz or less (Kay, 1982; Moore & Sek, 1995). Thereby, a low modulation task, developed by Griffiths et al. (2001), assessed MR's temporal pattern processing for continuous sound by requiring him to detect a sinusoidally modulated tone.
As shown in Table 2.11, MR's 75 percent threshold was within the normal range for both FM and AM tasks (see Appendix L for MR's psychometric function), suggesting preserved higher-order temporal structure for pitch and loudness cues of continuous patterned sound occurring over a longer temporal window.
Table 2.11
Seventy-five Percent Thresholds for Griffiths et al.'s (2001) Higher-order Temporal Processing Tasks
| Psychophysical task | MR's threshold | Controls threshold (SD)a | |
|---|---|---|---|
| Gap detectionb | 2.30 | 1.70 (0.35) | |
| Low frequency modulationce | |||
| 2 Hz FM | 0.16 | 0.01 (0.18) | |
| 40 Hz FM | -1.13 | -1.22 (0.16) | |
| Low amplitude modulationde | |||
| 2 Hz AM | -1.20 | -1.02 (0.26) | |
| 40 Hz AM | -1.39 | -1.30 (0.14) | |
a Data based on 15 controls matched for age to MR from Griffith et al.'s (2001) norms.
b Data are expressed as log transformations of milliseconds
c Data are expressed as log transformations of the ratio of the frequency excursion to the modulation rate.
d Data are expressed as log transformations of the proportional depth of modulation.
e The carrier frequency and duration of each sound was 500 Hz and 1 second respectively.
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