The effects of temporal asymmetry on the detection and perception of short chirps

There is an intriguing contrast between the physiological response to short frequency sweeps in the brainstem and the perception produced by these sounds. Dau et al. (2000) demonstrated that optimised chirps with increasing instantaneous frequency (up-chirps), designed to compensate for spatial dispersion along the cochlea, enhance wave V of the auditory brainstem response (ABR), by synchronising excitation of all frequency channels across the basilar membrane. Down-chirps, that is up-chirps reversed in time, increase cochlear phase delays and therefore result in a poor ABR wave V. In this study, a set of psychoacoustical experiments with up-chirps and down-chirps has been performed to investigate how these phase changes affect what we hear. The perceptual contrast is different from what was reported at the brainstem level. It is the down-chirp that sounds more compact, despite the poor synchronisation across channels and phase delays up to 20 ms. The perceived 'compactness' of a sound is apparently more determined by the fine structure of excitation within each peripheral channel than by between-channel phase differences. This suggests an additional temporal integration mechanism at a higher stage of auditory processing, which effectively removes phase differences between channels.