Abstract
Difference limens (DLs) for modulation depth of sinusoidally amplitude modulated (SAM) wideband noise were measured as a function of the modulation depth of the standard, the frequency of modulation (25, 100, 400 Hz), and the spectrum level of the noise (0, 20, 40 dB SPL). Preliminary results for one subject show that psychometric functions are linear and parallel in Log d′ and 10 Log(m12 − m02), where m1 and m0 are the modulation depths of the variable and standard stimuli, respectively. In the main study, three subjects were tested using a 2IFC adaptive procedure. The DL [10 Log(m12 − m02)] is at, or slightly above, the modulation detection threshold when m0 is small. For m0 above modulation threshold, the DL is a positively accelerating function of 20 Log(m0). Weber's Law does not describe the discrimination of modulation depth over any appreciable range. Across all conditions, DLs tend to converge to values around −9 dB when the level of the standard is −5 dB (m0 = 56%) and generally cannot be measured beyond this level. The effects of noise level on the DL are, in general, small. [Supported by NIH grant NS12125 and by USPHS HD07151 traineeship from the Center for Research in Human Learning.]
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