Michael J. Shailer

Gap detection as a function of frequency, bandwidth, and level

The threshold for detection of a temporal gap in a noiseband was measured. A notched noise masker was used to restrict listening to a limited spectral region. Threshold was measured as a function of center frequency, bandwidth, and level. For a signal bandwidth of one-half the center frequency, the gap threshold decreased from 22.5 ms for a center frequency of 0.2 kHz to 3.2 ms at 8.0 kHz: a wideband condition provided an estimate of 2.3 ms, a value in agreement with previously published estimates. Bandwidth manipulation showed that the variation with frequency was not due to changes in absolute bandwidth alone. The effect of changes in level was determined at three frequencies, 0.4, 1.0, and 6.5 kHz, using a signal bandwidth of half the center frequency. At all frequencies gap threshold decreased as the signal spectrum level was raised from 10 to 25 dB, but a further increase to 40 dB showed no additional improvement. At frequencies up to about 1.0 kHz, the variation of gap threshold with frequency matches well the reciprocal of the bandwidth of the auditory filter, as determined from masking experiments using a notched-noise masker. This suggests that the temporal response of the auditory filter may limit gap detection at low frequencies.

Frequency and intensity difference limens for harmonics within complex tones

A two-interval, two-alternative forced choice task was used to estimate frequency difference limens (DLs) for individual harmonics within complex tones, and DLs for the periodicity (i.e., number of periods per s) of the whole complexes. For complex tones with equal-amplitude harmonics, the DLs for the lowest harmonics were small (less than one percent). The DLs increased rather abruptly around the fifth to seventh harmonic. The highest harmonic in each complex was also well discriminated, and the discriminability of a single high harmonic was markedly improved by increasing its level relative to the other components. The DL for a complex tone was generally smaller than the frequency DL of its most discriminable component. The DL for a complex was found to be predictable from the DLs of the harmonics comprising the complex, using a formula derived by Goldstein [J. Acoust. Soc. Am. 54, 1496-1516 (1973)] from his optimum processor theory for the formation of the pitch of complex tones. The DL for a complex is sometimes primarily determined by high harmonics, such as the highest harmonic, or a harmonic whose level exceeds that of adjacent harmonics. We also measured intensity DLs for individual harmonics within complex tones. The intensity DLs were smallest for low harmonic numbers, and for the highest harmonic in a complex. An excitation-pattern model was used to determine whether the frequency DLs of harmonics within complex tones could be explained in terms of place mechanisms, i.e., in terms of changes in the amount of excitation at appropriate frequency places. We conclude that place mechanisms are not adequate, and that information about the frequencies of individual harmonics is probably carried in the time patterning of neural impulses.

Gap detection and the auditory filter: Phase effects using sinusoidal stimuli

Psychometric functions were determined for the detection of temporal gaps in sinusoidal signals at center frequencies between 0.2 and 2.0 kHz. A continuous notched-noise masker was used to restrict listening to the signal frequency region. The gap always started when the signal was at a positive-going zero crossing. There were three different conditions for the starting phase of the signal at the termination of the gap. In the standard-phase condition the signal restarted at a positive-going zero crossing, in the reversed-phase condition at a negative-going zero crossing, and in the preserved-phase condition at the phase the signal would have had if the gap had not been present. In the standard-phase and reversed-phase conditions the psychometric functions were nonmonotonic, showing oscillations with a period equal to that of the signal; maxima in the functions for the standard-phase condition coincided with minima in the functions for the reversed-phase condition, and vice versa. In the preserved-phase condition the psychometric functions were monotonic and the 75% points were roughly independent of center frequency, having a value of about 5 ms. The general form of the results can be modeled by a filter bank followed by a square-law device and a temporal integrator, but good agreement between the data and the model could not be attained across the whole range of gap durations. The deviations between data and model suggest that subjects are sensitive to the brief transitions in phase (or, equivalently, in frequency) in some conditions.

Temporal modulation transfer functions for band-limited noise in subjects with cochlear hearing loss

The modulation depth required for the detection of sinusoidal amplitude modulation was measured as a function of modulation rate, giving temporal modulation transfer functions (TMTFs). The carrier was a one-octave wide noise centred at 2 kHz, and it was presented in an unmodulated background noise lowpass filtered at 5 kHz. Three subjects with unilateral cochlear hearing loss were tested. For each subject, the normal ear was tested both at the same sound pressure level (SPL) and at the same sensation level (SL) as the impaired ear. The TMTFs were essentially the same for the normal and impaired ears, both at equal SPL and at equal SL. The better ears of three subjects with bilateral cochlear losses were also tested. Again, TMTFs were essentially the same as obtained for normal ears. These results suggest that temporal resolution is not necessarily adversely affected by cochlear hearing loss, at least as measured by this task.

Detection of temporal gaps in bandlimited noise: Effects of variations in bandwidth and signal‐to‐masker ratio

Thresholds were measured for the detection of a temporal gap in a bandlimited noise signal presented in a continuous wideband masker, using an adaptive forced-choice procedure. In experiment I the ratio of signal spectrum level to masker spectrum level (the SMR) was fixed at 10 dB and gap thresholds were measured as a function of signal bandwidth at three center frequencies: 0.4, 1.0, and 6.5 kHz. Performance improved with increasing bandwidth and increasing center frequency. For a subset of conditions, gap threshold was also measured as bandwidth was varied keeping the upper cutoff frequency of the signal constant. In this case the variation of gap threshold with bandwidth was more gradual, suggesting that subjects detect the gap using primarily the highest frequency region available in the signal. At low center frequencies, however, subjects may have a limited ability to combine information in different frequency regions. In experiment II gap thresholds were measured as a function of SMR for several signal bandwidths at each of three center frequencies: 0.5, 1.0, and 6.5 kHz. Gap thresholds improved with increasing SMR, but the improvement was minimal for SMRs greater than 12-15 dB. The results are used to evaluate the relative importance of factors influencing gap threshold.

Auditory filter shapes at 8 and 10 kHz

Auditory filter shapes were derived from notched-noise masking data at center frequencies of 8 kHz (for three spectrum levels, N0 = 20, 35, and 50 dB) and 10 kHz (N0 = 50 dB). In order to minimize variability due to earphone placement, insert earphones (Etymotic Research ER2) were used and individual earmolds were made for each subject. These earphones were designed to give a flat frequency response at the eardrum for frequencies up to 14 kHz. The filter shapes were derived under the assumption that a frequency-dependent attenuation was applied to all stimuli before reaching the filter; this attenuation function was estimated from the variation of absolute threshold with frequency for the three youngest normally hearing subjects in our experiments. At 8 kHz, the mean equivalent rectangular bandwidths (ERBs) of the filters derived from the individual data for three subjects were 677, 637, and 1011 Hz for N0 = 20, 35, and 50 dB, respectively. The filters at N0 = 50 dB were roughly symmetrical, while, at the lower spectrum levels, the low-frequency skirt was steeper than the high-frequency skirt. The mean ERB at 10 kHz was 957 Hz. At this frequency, the filters for two subjects were steeper on the high-frequency side than the low-frequency side, while the third subject showed a slight asymmetry in the opposite direction.

Comodulation masking release as a function of level

These experiments examine the effects of masker level on the magnitude of comodulation masking release (CMR). In experiment 1, threshold was measured for detecting a 2000-Hz signal in noise bands 100 or 3200 Hz wide, centered at the signal frequency. The noise was either amplitude modulated by a low-pass-filtered noise, or was unmodulated. At noise spectrum levels of 30 and 50 dB, thresholds were lower in the 3200-Hz-wide modulated noise than in the 100-Hz-wide modulated noise or the 3200-Hz-wide unmodulated noise, indicating a CMR. The magnitude of this CMR decreased at a noise spectrum level of 10 dB, and was very small at a spectrum level of -10 dB. In experiment 2, threshold was measured for a 700-Hz signal centered in a 20-Hz wide band of noise (the on-frequency band, OFB), both in the presence and absence of eight flanking bands (FBs) whose envelopes were either identical with that of the OFB (correlated condition) or were uncorrelated. Thresholds were lower in the correlated than in the uncorrelated condition, indicating a CMR. When the OFB and the FBs were presented to the same ear, the CMR decreased when the spectrum level of all bands was below 30 dB, or when the spectrum level of the FBs was decreased below 40 dB keeping the level of the OFB constant at 40 dB. When the OFB and the FBs were presented to opposite ears, the CMR decreased when the spectrum level of all bands was decreased below 30 dB or when the spectrum level of the FBs was decreased below 40 dB, keeping the level of the OFB fixed at 40 or 60 dB. However, the CMR was almost independent of the spectrum level of the OFB (over the range 10-70 dB) when the spectrum level of the FBs was held constant at 60 dB. The results are interpreted in terms of perceptual grouping mechanisms. Implications for the measurement of CMR in hearing-impaired subjects are also discussed.

Comodulation masking release in subjects with unilateral and bilateral hearing impairment

Three subjects with unilateral cochlear hearing loss and three subjects with bilateral cochlear hearing loss were tested in three experiments. In the first, their auditory filter shapes were measured for center frequencies of 700 and 2000 Hz, using the notched-noise method. The auditory filters were generally broader for the impaired than for the normal ears. In experiment 2, the threshold for detecting a 2000-Hz signal centered in a band of noise was measured as a function of the noise bandwidth for a Gaussian noise, and for that same noise multiplied (modulated) by a second noise low-pass filtered at 12.5 Hz. For the Gaussian noise, thresholds increased up to a certain noise bandwidth and then flattened off. This bandwidth was usually greater for the impaired than for the normal ears, consistent with the broader auditory filters of the impaired ears. For the modulated noise, thresholds tended to decrease when the noise bandwidth was increased beyond a certain value, indicating comodulation masking release (CMR). The decrease occurred at wider bandwidths for the impaired than for the normal ears. For the unilaterally impaired subjects, the amount of decrease was smaller for the impaired than for the normal ears when tested at equal SPL, but not when tested at equal SL. In experiment 3, the threshold for detecting a 700-Hz signal centered in a 20-Hz-wide band of noise (the on-frequency band, ONB) was measured in the presence of eight flanking bands (FBs) whose envelopes were either identical with that of the ONB (correlated condition) or were uncorrelated. CMR was defined as the difference in threshold between the correlated and uncorrelated conditions. The ONB and the FBs were presented either to the same ear (monaural condition) or to opposite ears (dichotic condition). CMRs tended to be greatest at high levels of the ONB and the FBs. CMRs in the monaural condition were smaller for hearing-impaired than for normal ears. However, at high levels, CMRs in the dichotic condition were similar for normal, bilaterally impaired, and unilaterally impaired subjects. In the latter case, CMRs were similar when the ONB was presented to the normal ear and to the impaired ear of each subject.(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation discrimination interference for narrow‐band noise modulators

The discrimination of the depth of amplitude modulation of a signal carrier frequency can be disrupted by the presence of other modulated carriers (maskers), an effect called modulation discrimination interference (MDI). This paper examines whether MDI is influenced by the similarity in the envelope pattern of the signal and masker. A narrow-band noise (centered at 10 Hz) was used as the signal modulator. The first experiment used masker modulators that were narrow-band noises identical in spectral characteristics to the signal modulator. The masker modulators were either identical to the signal modulator, negatively correlated with it, or uncorrelated with it. The amount of MDI was similar for all three cases. In experiment 2, the masker was sinusoidally modulated at rates varying from 2 to 64 Hz. The results showed a broad tuning for modulation rate, comparable to that found for sinusoidal modulation of the signal. The maximum amount of MDI produced by the sinusoidally modulated masker was similar to that produced by the noise-modulated maskers when modulation depths were expressed as their root-mean-square values. It is concluded that similarity of the moment-by-moment envelope pattern of the signal and masker modulators plays only a minor role in MDI, although similarity in modulation rate has some influence.

Dichotic interference effects in gap detection

Thresholds for detecting a temporal gap in a 20-Hz-wide band of noise (the target) were measured for the target alone, and in the presence of multiple 20-Hz-wide flanking bands presented to the opposite ear. The flanking bands caused gap thresholds to increase, and this effect was greater at higher levels of the flanking bands. The impairment to gap detection was greater when the flanking bands were comodulated with the target (i.e., had the same envelope) than when they were not comodulated, except at very low and high levels of the flanking bands. A series of supplementary experiments was conducted to investigate why the difference between comodulated and noncomodulated bands was reduced at high levels. The results suggest that this was not due to inter-aural crosstalk. It may have been partly caused by: (1) a central masking effect that reduced the effective sensation level of the target band at high levels of the contralateral flanking bands; (2) reduced independence of the flanking bands owing to broadening of the auditory filters at high levels. The results are discussed in terms of perceptual grouping processes.