Phase effects in masking by harmonic complexes in birds

Abstract

Masking by harmonic complexes depends on the frequency content of the masker and its phase spectrum. Harmonic complexes created with negative Schroeder phases (component phases decreasing with increasing frequency) produce more masking than those with positive Schroeder phases (increasing phase) in humans, but not in birds. The masking differences in humans have been attributed to interactions between the masker phase spectrum and the phase characteristic of the basilar membrane. In birds, the similarity in masking by positive and negative Schroeder maskers, and reduced masking by cosine-phase maskers (constant phase), suggests a phase characteristic that does not change much along the basilar papilla. To evaluate this possibility, the rate of phase change across masker bandwidth was varied by systematically altering the Schroeder algorithm. Humans and three species of birds detected tones added in phase to a single component of a harmonic complex. As observed in earlier studies, the minimum amount of masking in humans occurred for positive phase gradients. However, minimum masking in birds occurred for a shallow negative phase gradient. These results suggest a cochlear delay in birds that is reduced compared to that found in humans, probably related to the shorter avian basilar epithelia.

Psychophysical evidence of damaged active processing mechanisms in Belgian Waterslager Canaries

“…Thirdly, previous studies in normal-hearing birds have shown that maskers with C values of +1.0 and −1.0 produce similar amounts of masking (Leek et al 2000; Dooling et al 2001; Lauer et al 2006). Interestingly, BWC show a larger difference in the amount of masking by these maskers (4.94 dB) than non-BWC (1.07 dB) (Dooling et al 2001; Lauer et al 2006).…”

“…Average thresholds for detecting 2800 Hz tones in harmonic complex maskers for BWC are shown in Figure 4 along with mean thresholds replotted from Lauer et al (2006) for three normal-hearing non-BWC tested using identical methods. Several of the non-BWC also participated in Experiments 1 and 2 of the present study.…”

“…Recent studies in birds and humans have examined the effects of temporal waveform shape on masking when long-term frequency cues are held constant but component starting phases are selected according to the Schroeder algorithm (Schroeder 1970; Leek et al 2000; Dooling et al 2001; Lauer et al 2006). The complex that produces the least amount of masking within a set of Schroeder-phase maskers is thought to mirror the phase response of the cochlea, at least in mammals.…”

“…Changing the scalar alters the rate of the frequency change within each period, so that scalars closer to zero produce more rapid frequency changes than those close to ±1.0. Time waveforms for several of the harmonic complexes are shown below the x-axis in Figure 4 (see also Lauer et al 2006). …”

“…The procedures were identical to those described by Lauer et al (2006). Birds were trained to detect the presence of a 2800 Hz tone embedded in a harmonic complex masker.…”