The influence of external and internal noise on the detection of increments and decrements in the level of sinusoids

Abstract

This paper examines the influence of external and internal noise on the detection of increments and decrements in the level of sinusoidal pedestals. In experiment 1, the pedestals were presented either 18 dB above the masked threshold in broadband noise (condition 18-Masked) or 18 dB above the absolute threshold (condition 18-Abs). Pedestal frequencies were 250, 1000 or 4000 Hz, and increment/decrement durations ranged from 5 to 200 ms. For condition 18-Masked, thresholds decreased with increasing pedestal frequency, while for condition 18-Abs, thresholds did not change significantly with pedestal frequency. These results are consistent with the idea that, in condition 18-Masked, thresholds were influenced by the inherent fluctuations produced by the background noise at the output of the auditory filter centred at the pedestal frequency. These fluctuations would decrease in rate with decreasing centre frequency, and this might have a greater deleterious effect on performance. In contrast, the characteristics of the internal noise that presumably limited performance in condition 18-Abs do not appear to vary with pedestal frequency. In experiment 2, a 4000 Hz pedestal was used. It was presented either in quiet or in the presence of narrowband noise centred at 4000, or 7000 Hz, or both. The noise bandwidth ranged from 50 to 400 Hz. The increment/decrement duration ranged from 5 to 100 ms. The noise centred at 7000 Hz produced only a small deterioration in performance relative to that measured in quiet. The noise centred at 4000 Hz had a larger effect, and the effect increased with decreasing noise bandwidth. This is consistent with the idea that slow fluctuations at the output of the auditory filter impair increment and decrement detection more than rapid fluctuations. A model is proposed to account for the results, based on a simulated auditory filter, a compressive non-linearity, a sliding temporal integrator, a logarithmic transform and a template mechanism. Analysis using the model suggests that the effect of centre frequency observed in experiment 1, when background noise was present, cannot be explained entirely in terms of the fluctuations produced by the background noise at the output of the auditory filter centred at the pedestal frequency.