Effects of envelope fluctuations on gap detection

The inherent fluctuations present in narrowbands of noise may limit the ability to detect gaps in the noise; 'dips' in the noise may be confused with the gap to be detected. For subjects with cochlear hearing loss, loudness recruitment may effectively magnify the fluctuations and this could partly account for the reduced ability to detect gaps in noise bands that is usually found in subjects with cochlear hearing loss. In the present experiments we tested these ideas by processing noise bands to alter the amount of envelope fluctuation. The envelopes of the noise bands were raised to a power, N. Powers greater than 1 result in expansion of the envelope (magnified fluctuations, simulating loudness recruitment), while powers less than 1 result in compression of the envelope (decreased fluctuations). Thresholds for detecting gaps in processed noise bands centered at 1 kHz were measured as a function of noise bandwidth and of N. To prevent the detection of spectral changes introduced by the gap or by the processing, stimuli were either presented in background noise, or at a low sensation level (20 dB). Three normally hearing subjects, two subjects with unilateral cochlear hearing loss and two subjects with bilateral cochlear hearing loss were tested. Gap thresholds generally increased with increasing N. This effect was large for small noise bandwidths (50 Hz or less) and smaller for larger noise bandwidths (200 Hz or more). For both the normal and impaired ears, gap thresholds at narrow bandwidths were improved relative to those for unprocessed noise bands (N = 1) by compressing the envelope fluctuations (N < 1). The results support the idea that fluctuations in narrowband noises affect gap detection, and that loudness recruitment may adversely affect the ability to detect gaps in noise bands. They also show that compression of the fluctuations in the noise can improve gap detection.