Virginia M. Richards
Mark Ian Sanderson
Dept. of Psychol., 3815 Walnut St., Univ. of Pennsylvania, Philadelphia, PA 19104
In an effort to examine the shift from modulation-based discrimination for narrow-band complex stimuli to across-frequency spectral comparisons for wideband complex stimuli, the mutual discriminability of 11 stimuli was determined. The five ``base'' stimuli were the sum of three tones. By altering the relative phases of the tones, modulation depths ranging from maximum (SAM) to minimum (QFM) were generated using five equal log steps. The central frequency was 1000 Hz, and modulation rates ranged from 20 to 240 Hz. Three additional stimuli had envelopes identical to three of the base stimuli, but had periodic fine structures. Three other stimuli had fine structures identical to three of the base stimuli, but no envelope modulation. For observers tested thus far, where the modulation rate was 20 Hz, discriminability is well described as being based on changes in modulation depth. Where the modulation rate was 240 Hz, the discrimination appears to depend on changes in long term power spectra. For intermediate modulation rates, the data do not appear to reflect a gradual change from modulation-based detection to power spectrum-based detection, but the intervening cues appear not to be well defined by our stimuli.