Positive Schroeder Phase Research Articles

The effect of masker type on the sensitivity of the digits in noise test for detecting hearing loss

The Digits-in-Noise (DIN) test, a quick hearing-loss screening tool with high test-retest reliability and good sensitivity and specificity, can substitute for an audiogram when calibrated audiometry is unavailable. Traditionally, listeners hear three digits in speech-shaped noise, but other masker types might more effectively distinguish normal hearing thresholds (NHTs) from elevated hearing thresholds (EHTs). This study compared the ability of different energetic and informational maskers to detect low-frequency pure-tone averages >20 dB HL. First, DIN perception was adaptively measured (N = 1149 subjects) to determine the 80% correct speech-reception threshold (SRT80) for each of six masker types: female, male, and multi-gender multitalker babble; positive and negative Schroeder-phase complex tones; and speech-shaped noise. Then, for each masker type, a different group of subjects was tested at the corresponding SRT80. As expected, preliminary results suggest that a positive Schroeder-phase masker produces the greatest differences in DIN scores between NHTs and EHTs. Sensitivity and specificity rates for EHT detection for each masker, along with intersubject variability and order effects, will be explored. [The views expressed in this abstract are those of the authors and do not reflect the official policy of the Department of Army/Navy/Air Force, Department of Defense, or U.S. Government.]

Duifhuis pitch: neuromagnetic representation and auditory modeling.

When a high harmonic is removed from a cosine-phase harmonic complex, we hear a sine tone pop out of the perception; the sine tone has the pitch of the high harmonic, while the tone complex has the pitch of its fundamental frequency, f0. This phenomenon is commonly referred to as Duifhuis Pitch (DP). This paper describes, for the first time, the cortical representation of DP observed with magnetoencephalography. In experiment 1, conditions that produce the perception of a DP were observed to elicit a classic onset response in auditory cortex (P1m, N1m, P2m), and an increment in the sustained field (SF) established in response to the tone complex. Experiment 2 examined the effect of the phase spectrum of the complex tone on the DP activity: Schroeder-phase negative waves elicited a transient DP complex with a similar shape to that observed with cosine-phase waves but with much longer latencies. Following the transient DP activity, the responses of the negative and positive Schroeder-phase waves converged, and the increment in the SF slowly died away. In the absence of DP, the two Schroeder-phase conditions with low peak factors both produced larger SFs than cosine-phase waves with large peak factors. A model of the auditory periphery that includes coupling between adjacent frequency channels is used to explain the early neuromagnetic activity observed in auditory cortex.

Acoustic reflexes to Schroeder-phase harmonic complexes in normal-hearing and hearing-impaired individuals

Harmonic complexes generated with positive or negative Schroeder-phases may result in differences in cochlear excitation, even though their long-term spectra and amplitudes are equal. As a measure of possible differences in cochlear excitation resulting from these harmonic complexes, thresholds and growth of the acoustic reflex were assessed in normal-hearing and hearing-impaired subjects. Harmonic complexes with fundamental frequencies of 50, 100, and 200 Hz were constructed with positive and negative-Schroeder phases. In normal-hearing subjects, acoustic reflex thresholds for the 50- and 100-Hz fundamental waveforms were typically lower for negative Schroeder-phase complexes than for positive Schroeder phase stimuli. At the highest fundamental frequency of 200 Hz, there were no significant threshold differences due to phase. Hearing-impaired subjects showed a similar pattern for thresholds between the two phase selections, but with smaller differences than those observed in normal-hearing subjects. At levels above reflex threshold, the magnitude of the acoustic reflex was greater for the negative-phase than the positive-phase stimuli for the lowest fundamental frequency, but no significant differences were observed at fundamental frequencies of 100 and 200 Hz. These results are consistent with generally greater cochlear excitation in response to negative than to positive Schroeder-phase stimuli when the fundamental frequency is sufficiently low. Increased excitation may reflect a synchronization of response across a wide band of frequencies in the cochlea when the rate of frequency sweep within periods of these harmonic complexes is appropriately matched to timing characteristics of the traveling wave.

Frequency dependent latency and the envelope following response

Auditory evoked potentials (AEPs) reflect synchronous neural firing. Because of frequency-dependent latencies associated with basilar membrane transduction, click stimuli produce neural activity across a wide range of frequencies that is not synchronous. The envelope following response (EFR), one class of AEP, was measured in response to harmonics of 100 Hz, with components in either negative- or positive-Schroeder phase. The most robust EFRs were obtained with the negative-Schroeder stimuli, which are associated with a frequency sweep from low to high frequency, counteracting transduction latencies. Results obtained with these stimuli could be used in an objective assessment of peripheral encoding of low frequencies.

Psychophysical estimates of cochlear phase response: masking by harmonic complexes.

Harmonic complexes with identical component frequencies and amplitudes but different phase spectra may be differentially effective as maskers. Such harmonic waveforms, constructed with positive or negative Schroeder phases, have similar envelopes and identical long-term power spectra, but the positive Schroeder-phase waveform is typically a less effective masker than the negative Schroeder-phase waveform. These masking differences have been attributed to an interaction between the masker phase spectrum and the phase characteristic of the basilar membrane. To explore this relationship, the gradient of stimulus phase change across masker bandwidth was varied by systematically altering the Schroeder-phase algorithm. Observers detected a signal tone added in-phase to a single component of a masker whose frequencies ranged from 200 to 5000 Hz, with a fundamental frequency of 100 Hz. For signal frequencies of 1000-4000 Hz, differences in masking across the harmonic complexes could be as large as 5-10 dB for phase gradients changing by only 10%. The phase gradient that resulted in a minimum amount of masking varied with signal frequency, with low frequencies masked least effectively by stimuli with rapidly changing component phases and high frequencies masked by stimuli with more shallow phase gradients. A gammachirp filter was implemented to model these results, predicting the qualitative changes in curvature of the phase-byfrequency function estimated from the empirical data: In some cases, small modifications to the gammachirp filter produced better quantitative predictions of curvature changes across frequency, but this filter, as implemented here, was unable to accurately represent all the data.

Basilar membrane responses to broadband stimuli.

Basilar membrane (BM) responses to two types of broadband stimuli-clicks and Schroeder-phase complexes--were recorded at several sites at the base of the chinchilla cochlea. Recording sites (characteristic frequency, CF, in the range of 5.5-18 kHz) span the 1-4-mm basal region of the basilar membrane. BM responses to clicks consisted of undamped oscillations with instantaneous frequency that increased over time until it reached a value around CF. The time constant of this glide is CF dependent. Throughout the entire region under study, BM vibration exceeded umbo motion by up to 60 dB. Nonlinear properties of BM responses to clicks resemble those found in the more studied 8-10-kHz region. Amplitude spectra of Schroeder-phase complex stimuli, which consist of a series of sinusoidal components summed in negative (-SCHR) and positive Schroeder phase (+SCHR), are flat. The envelope of BM responses to +SCHR stimuli contains valleys, or dips, that are wider than those found in responses to the -SCHR stimuli. Hence, BM responses to the former stimuli are "peakier" than responses to the latter. Differences in response waveforms are less obvious in linear cochleae. Suppression of a near-CF tone by -SCHR stimuli was larger than that evoked by +SCHR stimuli.

Effects of hearing impairment and presentation level on masking period patterns for Schroeder-phase harmonic complexes.

Masking period patterns (MPPs) for Schroeder-phase harmonic complexes containing equal-amplitude harmonics of a 100-Hz fundamental were determined for 5-ms tonal probes at 4,000 and 1,000 Hz. Maskers consisted of harmonics 2-50 (200-5,000 Hz bandwidth) for 4,000-Hz probes and harmonics 2-20 (200-2,000 Hz) for 1,000-Hz signals. Masked thresholds were determined for probe onsets 153, 155.5, 158, 160.5, and 163 ms following masker onset (masker duration=460 ms). Overall, results were similar for both probe frequencies. For listeners with normal hearing, MPPs for positive Schroeder-phase complexes masking 60 dB SPL probes were highly modulated and became flatter when probe level was increased to 80 dB SPL. MPPs were less modulated for listeners with sensorineural hearing loss than for normally hearing listeners at both 60 and 80 dB SPL probe levels. Thresholds in negative Schroeder-phase maskers were more similar across the two groups of listeners and across differences in probe position and probe level. The findings support an interpretation involving differences in the shape of the basilar-membrane waveform generated by each masker and possible influences of nonlinear cochlear processing on these internal responses. For normally hearing listeners, 60 dB SPL probes were most difficult to detect when temporally positioned so that probe frequency and masker instantaneous frequency were closely matched. For 80 dB SPL probes and for hearing-impaired listeners, probes presented at these same positions were often more easily detected than probes at other positions. The latter result appears to involve benefit associated with in-phase addition of the probe to a portion of the masker similar to the probe in both frequency and phase. This benefit was reduced or entirely eliminated when probe phase was altered so that this in-phase addition did not occur.

Growth of spectral contrast enhancement in Schroeder-phase harmonic complexes

Discrimination of spectral differences in harmonic complexes is better for waveforms that are highly modulated and presented at high levels relative to lower-level stimuli with flatter temporal envelopes [M. R. Leek and V. Summers, J. Acoust. Soc. Am. 94, 2074–2082 (1993)]. This may reflect an enhancement of contrast between spectral peaks and valleys related to nonlinear cochlear processing. To further explore this relationship, discrimination of harmonic complexes with slightly different peak frequencies was measured at several stimulus levels. Peak-to-background differences required for discrimination were taken as a measure of the spectral contrast preserved in the auditory system. Phases were selected to generate either flat or peaked internal waveforms (negative or positive Schroeder phase), and peak frequency regions were near 2, 3, or 4 kHz. In normal-hearing listeners, the positive Schroeder waveforms produced lower spectral contrast thresholds than the negative Schroeder stimuli at all tested intensities and peak regions, with the greatest differences at moderate stimulus levels. Generally, hearing-impaired listeners demonstrated smaller threshold differences due to phase selection. These results are consistent with normal cochlear processing that is nonlinear at moderate levels, but more linear at low and high levels, and with more linear processing in impaired ears. [Work supported by NIH.]

Steady state response as an indicator of basilar membrane travel time

It has been suggested that neural firing in response to a broadband stimulus can be maximized by a frequency sweep that counteracts the frequency-specific delays of basilar membrane propagation. To test this, synchrony of neural discharge across frequency was estimated by measuring steady state responses (SSRs) elicited by harmonic complexes whose components were in either negative or positive Schroeder phase. These stimuli reduce to periodic frequency glides, ramping upward (negative Schroeder) or downward (positive Schroeder) in frequency, but having unfiltered envelopes that are identical. The data from three normal-hearing listeners support the basic notion that an upward frequency glide can lead to improved neural synchrony. Higher levels of neural synchrony occurred at higher rates for higher center frequencies, consistent with shorter travel times as a function of frequency for higher center frequencies. The derived travel times were consistently faster than those typically predicted via passive models of basilar membrane wave propagation. Interpretation of these data require caution in light of possible effects of unbalanced evoked responses from the various frequency regions stimulated, nonlinear summation of responses across channels, or nonlinearity of frequency-dependent delays in basilar membrane propagation. [Work supported by NIH NIDCD RO1-DC01507.]

Masking of tones and speech by Schroeder-phase harmonic complexes in normally hearing and hearing-impaired listeners

Tone detection and sentence recognition were measured for normally hearing and hearing-impaired listeners using maskers consisting of harmonic series with components summed in positive or negative Schroeder phase. Each task was carried out with the signal set at 60, 70, or 80 dB SPL. For listeners with normal hearing, positive Schroeder-phase complexes masked tones and sentences less than negative Schroeder-phase maskers. In the two experimental tasks, to achieve the same amount of masking, positive Schroeder-phase complexes had to be set as much as 12–15 dB higher than negative Schroeder-phase complexes. Large phase effects were observed on both tasks at all three test levels. The two maskers were more nearly equal in effectiveness in the presence of cochlear damage. The findings support an interpretation that involves differences in the shape of the basilar-membrane waveform generated by each masker and active cochlear processing which enhances the internal signal-to-masker ratio for signals presented in the positive Schroeder masker. This spectral enhancement appears to require nonlinear active gain that is characteristic of normal auditory processing at moderate presentation levels. The results of the sentence recognition task suggest that group differences observed in tone detection transfer fairly directly to speech perception under masking.

Masking period patterns of Schroeder-phase complexes: effects of level, number of components, and phase of flanking components.

Masking period patterns (MPPs) were obtained for maskers consisting of harmonics 2-20 of a 100-Hz fundamental. The signal was always a 5-ms 1100-Hz sinusoid presented 152, 154, 156, 158, or 160 ms after the start of a 400-ms masker. Experiment 1 replicated the finding that, for a masker level of 69 dB component, the shape of the MPP depended strongly on the phases of the components: Summing them in positive Schroeder phase led to a threshold variation of about 18 dB across the MPP, but summing them in negative Schroeder phase produced a flat MPP [A. Kohlrausch and A. Sander, J. Acoust. Soc. Am. 97, 1817-1829 (1995)]. Reducing the level of the positive-phase masker resulted in a systematic flattening of the MPP, whereas the negative-phase MPPs were flat both at high and at low levels. Experiment 2 showed that removing all components of a positive-phase masker except those close to the signal raised thresholds at the minimum of the MPP. In contrast, a similar manipulation applied to the negative-phase masker produced a uniform elevation of the MPP. Experiment 3 showed that an analogous effect could be obtained by manipulating the phases of masker components remote from the signal. It is shown that several features of the data can be simulated using a nonlinear model of the auditory periphery [C. Giguère and P.C. Woodland, J. Acoust. Soc. Am. 95, 331-342 (1994)].

Excitation produced by Schroeder-phase complexes: evidence for fast-acting compression in the auditory system.

A series of experiments compared the excitation produced in an auditory filter centered on 1100 Hz by two complexes, both of which consisted of harmonics 2-20 of a 100-Hz fundamental. When the components had a level of 69 dB SPL each, summing them in positive Schroeder phase produced substantially less forward masking of an 1100-Hz signal than when the components were summed in negative Schroeder phase. This difference decreased with decreases in overall masker level. Listeners also reported that the components of the positive-phase masker close to 1100 Hz were quieter than the corresponding components in the negative-phase masker. The data are explained using Kohlrausch and Sander's [J. Acoust. Soc. Am. 97, 1817-1829 (1995)] finding that the response of an 1100-Hz auditory filter to the positive-phase complex shows marked peaks and dips, whereas that to the negative-phase complex does not. It is argued that the peaks in the response to the positive-phase masker are attenuated by fast-acting compression in the auditory system, thereby reducing the excitation produced by that sound. It is also argued that, compared to the power functions commonly used to model "excess masking" and the growth of loudness, the present data reflect greater compression at high levels but less compression at low levels.