Intensity discrimination and detection of amplitude modulation

Thresholds for detection of low-rate sinusoidal amplitude modulation and for detection of intensity increments were measured over a wide range of levels in an examination of the relationship between these fundamental aspects of intensity processing. As expected, thresholds measured with a continuous 1-kHz tone decrease with increasing carrier/pedestal level. For levels between 6 and 85 dB SPL the data are well described by 10 log delta I/I = 0.44.(20 log m) + D(fm), where delta I/I is the Weber fraction for increment detection, m is the modulation index at threshold, and D(fm) depends on modulation rate (fm). The relationship between the psychometric functions for modulation and increment detection is also consistent with this equation. The data indicate a clear relationship between modulation and increment detection and thus provide an important additional consideration for models of modulation processing. No existing models provide an adequate account of this relationship.