Detection and intensity discrimination of Gaussian-shaped tone pulses as a function of duration.

Abstract

Van Schijndel et al. [J. Acoust. Soc. Am. 105, 3425-3435 (1999)] proposed that the auditory system partitions the spectro-temporal domain into frequency-time (f-t) windows and that the characteristics of these windows could be explored by measuring intensity discrimination for Gaussian-shaped tone pulses presented just above their detection threshold in noise. They reasoned that for a long-duration tone pulse, the auditory representation would be maximally compact in the frequency domain, but would spread across several f-t windows in the time domain. For a very-short-duration tone pulse, the auditory representation would be maximally compact in the time domain, but would spread across several f-t windows in the frequency domain. There should be some intermediate duration at which the auditory representation is compact in both the time and frequency domains and for which intensity-discrimination performance should worsen, due to the limited opportunity for multiple looks. Their data for signal frequencies of 1 and 4 kHz were consistent with this expectation; intensity discrimination was poorest at a duration of about 3-5 ms at 1 kHz and 1 ms at 4 kHz (durations are specified between 6.8-dB-down points on the envelope). This experiment attempted to replicate those results and to extend them to a wider range of frequencies and levels. Intensity discrimination of Gaussian-shaped tone pulses was measured at three levels: 10 dB above absolute threshold or above masked threshold in a pink noise with a spectrum level of either 15 or 40 dB at 1 kHz. The signal frequency was 0.25 kHz (durations from 2 to 320 ms), 1 kHz (durations from 0.5 to 80 ms), or 4 kHz (durations from 0.1 to 20 ms). Three normally hearing subjects were tested. At 1 and 4 kHz, performance was poorest overall for the 15-dB pink noise level, and thresholds showed a peak at intermediate durations (about 3-5 ms at 1 kHz and 1 ms at 4 kHz). Such peaks were still apparent, but smaller in the no-noise condition and were almost absent at the higher noise level. For the 0.25-kHz signal frequency, peaks were not observed consistently at any level, although two subjects showed small peaks for durations around 10 ms. An explanation is offered for the results in terms of the level and frequency dependence of basilar-membrane input-output functions.