Forward masking of amplitude modulation across ears and its tuning in the modulation domain

Disentangling the effects of hearing loss and age on amplitude modulation frequency selectivity

The processing and perception of amplitude modulation (AM) in the auditory system reflect a frequency-selective process, often described as a modulation filterbank. Previous studies on perceptual AM masking reported similar results for older listeners with hearing impairment (HI listeners) and young listeners with normal hearing (NH listeners), suggesting no effects of age or hearing loss on AM frequency selectivity. However, recent evidence has shown that age, independently of hearing loss, adversely affects AM frequency selectivity. Hence, this study aimed to disentangle the effects of hearing loss and age. A simultaneous AM masking paradigm was employed, using a sinusoidal carrier at 2.8 kHz, narrowband noise modulation maskers, and target modulation frequencies of 4, 16, 64, and 128 Hz. The results obtained from young (n = 3, 24-30 years of age) and older (n = 10, 63-77 years of age) HI listeners were compared to previously obtained data from young and older NH listeners. Notably, the HI listeners generally exhibited lower (unmasked) AM detection thresholds and greater AM frequency selectivity than their NH counterparts in both age groups. Overall, the results suggest that age negatively affects AM frequency selectivity for both NH and HI listeners, whereas hearing loss improves AM detection and AM selectivity, likely due to the loss of peripheral compression.

Age-related reduction of amplitude modulation frequency selectivity

The perception of amplitude modulations (AMs) has been characterized by a frequency-selective process in the temporal envelope domain and simulated in computational auditory processing and perception models using a modulation filterbank. Such AM frequency-selective processing has been argued to be critical for the perception of complex sounds, including speech. This study aimed at investigating the effects of age on behavioral AM frequency selectivity in young (n = 11, 22-29 years) versus older (n = 10, 57-77 years) listeners with normal hearing, using a simultaneous AM masking paradigm with a sinusoidal carrier (2.8 kHz), target modulation frequencies of 4, 16, 64, and 128 Hz, and narrowband-noise modulation maskers. A reduction of AM frequency selectivity by a factor of up to 2 was found in the older listeners. While the observed AM selectivity co-varied with the unmasked AM detection sensitivity, the age-related broadening of the masked threshold patterns remained stable even when AM sensitivity was similar across groups for an extended stimulus duration. The results from the present study might provide a valuable basis for further investigations exploring the effects of age and reduced AM frequency selectivity on complex sound perception as well as the interaction of age and hearing impairment on AM processing and perception.

Amplitude-modulation forward masking for listeners with and without hearing loss

Amplitude-modulation (AM) forward masking was measured for listeners with normal hearing and sensorineural hearing loss at 4000 and 1000 Hz, using continuous and noncontinuous masker and signal carriers, respectively. A low-fluctuation noise (LFN) carrier was used for the "continuous carrier" conditions. An unmodulated low-fluctuation noise (U-LFN), an unmodulated Gaussian noise (U-GN), and an amplitude-modulation low-fluctuation noise (AM-LFN) were maskers for the "noncontinuous carrier" conditions. As predicted, U-GN yielded more masking than U-LFN and similar masking to AM-LFN, suggesting that U-GN resulted in AM forward masking. Contrary to predictions, no differences in masked thresholds were observed between listener groups.

Informational masking in the modulation domain

Uncertainty regarding the frequency spectrum of a masker can have an adverse effect on the ability to focus selective attention on a target frequency channel, yielding informational masking (IM). This study sought to determine if uncertainty regarding the modulation spectrum of a masker can have an analogous adverse effect on the ability to focus selective attention on a target modulation channel, yielding IM in the modulation domain, or "modulation IM." A single-interval, two-alternative forced-choice (yes-no) procedure was used. The task was to detect 32-Hz target sinusoidal amplitude modulation (SAM) imposed on a broadband-noise carrier in the presence of masker SAM imposed on the same carrier. Six maskers, spanning the range from 8 to 128 Hz in half-octave steps, were tested, excluding those that fell within a two-octave protected zone surrounding the target. Psychometric functions (d'-vs-target modulation depth) were measured for each masker under two conditions: a fixed (low-uncertainty/low-IM) condition, in which the masker was the same on all trials within a block, and a random (high-uncertainty/high-IM) condition, in which it varied randomly from presentation-to-presentation. Thresholds and slopes extracted from the psychometric functions differed markedly between the conditions. These results are consistent with the idea that IM occurs in the modulation domain.

Frequency selectivity in the modulation domain estimated using forward masking: Effects of masker modulation depth and masker-signal delay

The threshold for detecting amplitude modulation (AM) of a sinusoidal or noise carrier is elevated when the signal AM is preceded by masker AM applied to the same carrier. This effect, called AM forward masking, shows selectivity in the AM domain, consistent with the existence of a modulation filter bank (MFB). In this paper we explore the effect of two factors that can influence AM forward masking, using an 8-kHz sinusoidal carrier and a range of masker AM frequencies, f_m, both below and above the signal AM frequency, f_s, of 40 Hz. The first factor was the time delay, t_d, between the end of the masker AM and the start of the signal AM. The second was the AM depth, m, of the masker, which was either 1 or 0.25. The AM forward masking patterns in all conditions showed tuning in the AM domain; signal thresholds were highest when f_m was close to f_s. The amount of AM forward masking decreased with increasing t_d in a similar way for all f_m, so the shapes of the masking patterns did not change markedly with t_d. Remarkably, the amount of AM forward masking decreased by only about 3 dB (a non-significant effect) when the masker m was decreased from 1 to 0.25. This result appears to be inconsistent with an explanation of AM forward masking in terms of adaptation in a MFB or in terms of a sliding temporal integrator.