Cochlear Dispersion Shapes Processing of Frequency Sweeps

Abstract

The acoustic environment is often a superposition of competing sounds that, through interactions within the auditory system, can mask or reduce the audibility of target signals. While masking by stationary sounds, such as pure tones, is well studied, the mechanisms underlying masking by dynamic sounds remain less understood. In normally hearing listeners, upward frequency sweeps, where instantaneous frequency increases over time, are typically more effective at masking tonal and speech signals than downward sweeps. This directional sensitivity is thought to reflect cochlear mechanical processes, including traveling-wave dispersion—which may be enhanced or compensated by the sweep—and active nonlinear interactions, as it diminishes with hearing impairment and increasing stimulus intensity. Here, we examined cochlear vibrations in mice of either sex to investigate how a characteristic frequency tone response is suppressed by competing sounds with varied temporal properties, including sweep direction and rate of frequency change. For sweep rates at least ten times slower than the natural dispersion rate, up- and downsweep suppressors produced similar suppression. However, as sweep rate approached the dispersion rate, upsweep stimuli became more effective suppressors than downsweep ones. Sensitivity to sweep direction was strongest at lower sweep intensities and decreased with increasing sweep level, but not with probe level. These differences reflect changes in suppression tuning: the low-frequency side depended primarily on sweep intensity, whereas the high-frequency side was shaped by the temporal properties of the sweep. Together, these findings identify a cochlear mechanical basis for perceptual asymmetries in masking by sweeps observed in human listeners.