The effect of temporal stimulus characteristics in maintenance of the acoustic reflex.

In normal listeners, acoustic reflex decay (ARD) typically occurs for high- but not for low-frequency tones. In patients with acoustic neuromas, decay can be obtained at all frequencies, presumably due to poor neural synchrony. These observations have led us to hypothesize that resistance to decay is due to robust encoding of temporal fine structure of the eliciting stimulus. For a 4-kHz stimulus, ARD is reduced by sinusoidal amplitude modulation (SAM), a result attributed to the low-frequency pattern of SAM providing the temporal characteristics necessary to maintain the reflex. If this interpretation is correct, then further reductions in ARD should be seen for stimuli having temporal characteristics that even more closely resemble the neural response to low-frequency stimulus fine structure. On the other hand, if other perceptual qualities of a SAM tone are responsible for the effect (e.g., rate pitch), then manipulations of perceived sound quality, rather than temporal characteristics per se, should produce similar effects. The experiment reported here included a reference condition, (1) 5-kHz pure tone, and three "temporal" manipulations, composed of a 5-kHz tone multiplied by (2) a raised 100-Hz sinusoid, (3) a noise sample, lowpass filtered at 100 Hz, and (4) a half-wave rectified 100-Hz sinusoid. Additional conditions manipulated perceived pitch. These stimuli spanned 4.5-8 kHz, including a reference condition, (5) Gaussian noise, and a stimulus associated with a 100-Hz pitch, (6) iterated rippled noise. Results show the greatest reductions in ARD with the half-wave rectified stimulus, thought to most closely mimic the temporal characteristics of a low-frequency tone. Little or no reduction in ARD was associated with the iterated rippled noise, suggesting that perceived pitch does not play an important role in maintaining the acoustic reflex.