Amplitude modulation reduces loudness adaptation to high-frequency tones

Abstract

Long-term loudness perception of a sound has been presumed to depend on the spatial distribution of activated auditory nerve fibers as well as their temporal firing pattern. The relative contributions of those two factors were investigated by measuring loudness adaptation to sinusoidally amplitude-modulated 12-kHz tones. The tones had a total duration of 180 s and were either unmodulated or 100%-modulated at one of three frequencies (4, 20, or 100 Hz), and additionally varied in modulation depth from 0% to 100% at the 4-Hz frequency only. Every 30 s, normal-hearing subjects estimated the loudness of one of the stimuli played at 15 dB above threshold in random order. Without any amplitude modulation, the loudness of the unmodulated tone after 180 s was only 20% of the loudness at the onset of the stimulus. Amplitude modulation systematically reduced the amount of loudness adaptation, with the 100%-modulated stimuli, regardless of modulation frequency, maintaining on average 55%-80% of the loudness at onset after 180 s. Because the present low-frequency amplitude modulation produced minimal changes in long-term spectral cues affecting the spatial distribution of excitation produced by a 12-kHz pure tone, the present result indicates that neural synchronization is critical to maintaining loudness perception over time.

FIG. 1.

Loudness adaptation to a 12-kHz tone, amplitude modulated at 100% depth, as a function of time. Each symbol represents a different modulation frequency. Error bars represent plus or minus one standard error of the mean. The solid lines represent fits to an exponential decay [see Eq. (2) in the text].

FIG. 2.

Loudness adaptation to a 12-kHz tone, amplitude modulated at 4 Hz, as a function of time. Symbols, error bars, and solid lines as in Fig. 1.

FIG. 3.

Loudness adaptation to a 12-kHz tone, amplitude modulated at 4 Hz, as a function of modulation depth. Error bars as in Fig. 1. Each symbol represents a different measurement time after tone onset. The dashed lines represent the boundaries of the range of thresholds for amplitude modulation detection. The solid lines represent fits to an exponential decay [see Eq. (3) in the text].