Perception of suprathreshold amplitude modulation and intensity increments: Weber's law revisited

The perceived strength of intensity fluctuations evoked by suprathreshold sinusoidal amplitude modulation (AM) and the perceived size of intensity increments were compared across levels of a wideband noise and a 1-kHz tone. For the 1-kHz tone, the comparisons were made in quiet and in a high-pass noise. The data indicate that suprathreshold modulation depths and intensity increments, perceived as equivalent across levels, follow a pattern resembling Weber's law for noise and the "near miss" to Weber's law for a tone. The effect of a high-pass noise was largely consistent with that observed for AM and increment detection. The data suggest that Weber's law is not a direct consequence of the dependence of internal noise on stimulus level, as suggested by multiplicative internal noise models. Equal loudness ratios and equal loudness differences (computed using loudness for the stationary portions before and after the increment) accounted for the increment-matching data for noise and for the tone, respectively, but neither measure predicted the results for both types of stimuli. Predictions based on log-transformed excitation patterns and predictions using an equal number of intensity just-noticeable differences were in qualitative, but not quantitative, agreement with the data.