Detection of decrements and increments in sinusoids at high overall levels.

Thresholds for the detection of decrements in level of sinusoidal signals were measured as a function of duration (2, 4, 6, 10, and 14 ms), level (70, 80, and 90 dB SPL) and frequency (250, 500, 1000, 2000, and 4000 Hz). Seven normally hearing listeners were tested at each frequency (with different subjects for each frequency). Thresholds for detecting a 10-ms increment in level were also measured. The sinusoids were presented in a background noise low-pass filtered at 5 kHz, which was intended to mask spectral splatter associated with the decrement or increment. Performance improved with increasing frequency for all decrement and increment durations. Performance also tended to improve with increasing level at 2000 and 4000 Hz. The results were analyzed using a four-stage model consisting of an auditory filter centered on the signal frequency, a compressive nonlinearity, a sliding temporal integrator and a decision mechanism. The analysis indicated that the improved performance with increasing frequency and increasing level could be attributed partly to off-frequency listening; for the two highest center frequencies, subjects probably made use of the output of an auditory filter centered above the signal frequency, where changes in excitation level associated with an increment or decrement were magnified. The measurements at 4000 Hz were repeated using a broadband background noise (15-kHz bandwidth), which would prevent the use of information from auditory filters centered far above the signal frequency. Performance was poorer than when low-pass noise was used, but still improved somewhat with increasing level. The slight improvement in performance with increasing level can be accounted for by a reduced compressive linearity at high levels. A good fit to the data could be obtained by assuming that the equivalent rectangular duration (ERD) of the temporal integrator was invariant with level, but that the compressive nonlinearity varied with level in a similar way to basilar-membrane input-output functions. The nonlinearity appears to be somewhat less compressive at 250 Hz than at higher center frequencies. The ERD is about 7 ms regardless of center frequency.