Biomedical subjects

R D Patterson

Publications and source records attributed to R D Patterson.

At least 37 records · Page 2Linked to original sources

PubMed1999

Quantifying the distortion products generated by amplitude-modulated noise.

When sinusoidal amplitude modulation (SAM) is applied to noise or tone carriers, the stimuli can generate audible distortion products in the region of the modulation frequency. As a result, when bandpass-filtered SAM noise is used to investigate temporal processing, a band of unmodulated noise is typically positioned at the modulation frequency to mask any distortion products. This study was designed to investigate the distortion products for bandpass noise carriers, and so reduce ambiguity about the form of this distortion and its role in perception. The distortion consists of two distortion-noise bands and a distortion tone at the modulation frequency. In the first two experiments, the level and phase of the distortion tone are measured using two different experimental paradigms. In the third experiment, modulation-frequency difference limens are measured for filtered SAM noise and it is shown that performance deteriorates markedly when the distortion tone is canceled. In a fourth experiment, masked threshold is measured at low frequencies for bands of high-frequency, unmodulated noise with the same levels and spectra as the SAM noises in the earlier experiments. The results confirm that unmodulated noise also produces quadratic distortion which may explain some aspects of earlier reports on remote masking.

Adolescent↗

PubMed1998

Analysis of temporal structure in sound by the human brain.

For over a century, models of pitch perception have been based on the frequency composition of the sound. Pitch phenomena can also be explained, however, in terms of the time structure, or temporal regularity, of sounds. To locate the mechanism for the detection of temporal regularity in humans, we used functional imaging and a 'delay-and-add' noise, which activates all frequency regions uniformly, like noise, but which nevertheless produces strong pitch perceptions and tuneful melodies. This stimulus has temporal regularity that can be systematically altered. We found that the activity of primary auditory cortex increased with the regularity of the sound. Moreover, a melody composed of delay-and-add 'notes' produced a distinct pattern of activation in two areas of the temporal lobe distinct from primary auditory cortex. These results suggest a hierarchical analysis of time structure in the human brain.

Auditory Cortex↗

PubMed1998

Temporal dynamics of pitch strength in regular interval noises.

The pitch strength of rippled noise and iterated rippled noise has recently been fitted by an exponential function of the height of the first peak in the normalized autocorrelation function [Yost, J. Acoust. Soc. Am. 100, 3329-3335 (1996)]. The current study compares the pitch strengths and autocorrelation functions of rippled noise (RN) and another regular-interval noise, "AABB." RN is generated by delaying a copy of a noise sample and adding it to the undelayed version. AABB with the same pitch is generated by taking a sample of noise (A) with the same duration as the RN delay and repeating it to produce AA, and then concatenating many of these once-repeated sequences to produce AABBCCDD.... The height of the first peak (h1) in the normalized autocorrelation function of AABB is 0.5, identical to that of RN. The current experiments show the following: (1) AABB and RN can be discriminated when the pitch is less than about 250 Hz. (2) For these low pitches, the pitch strength of AABB is greater than that for RN whereas it is about the same for pitches above 250 Hz. (3) When RN is replaced by iterated rippled noise (IRN) adjusted to match the pitch strength of AABB, the two are no longer discriminable. The pitch-strength difference between AABB and RN below 250 Hz is explained in terms of a three-stage, running-autocorrelation model. It is suggested that temporal integration of pitch information is achieved in two stages separated by a nonlinearity. The first integration stage is implemented as running autocorrelation with a time constant of 1.5 ms. The second model stage is a nonlinear transformation. In the third model stage, the output of the nonlinear transformation is long-term averaged (second integration stage) to provide a measure of pitch strength. The model provides an excellent fit to the pitch-strength matching data over a wide range of pitches.

Acoustics↗

PubMed1998

Modeling temporal asymmetry in the auditory system.

Sound sources in the environment produce waves that are almost invariably asymmetric in time, and human listeners are highly sensitive to temporal asymmetry. The spectral analysis and neural transduction processes in the cochlea enhance temporal asymmetry, as do time-domain models of cochlear processes, but it appears that the resulting asymmetry is not sufficient to explain the observed perceptual asymmetry. In the auditory image model (AIM) of hearing, the temporal asymmetry in the neural activity produced by the cochlea is further enhanced by the "strobed" temporal integration that converts the neural activity pattern into an auditory image, and the temporal asymmetry in the auditory image is sufficient to explain the perceptual asymmetry. Modern versions of the "duplex model" of pitch have time-domain cochlea simulations that produce neural activity with temporal asymmetry similar to that produced by AIM. In the final stage, however, they apply autocorrelation to the neural pattern and autocorrelation is a symmetric process in time. In this paper the effect of autocorrelation on temporal asymmetry is examined in a range of auditory models with varying forms of auditory filterbank, compression, and neural transduction. It is concluded that autocorrelation does not enhance temporal asymmetry and often reduces it, and that autocorrelogram models cannot explain the magnitude of the perceptual asymmetry in their current form. Then, the original version of strobed-temporal-integration is reviewed with regard to temporal asymmetry, and the delta-gamma theory of temporal asymmetry [Irino and Patterson, J. Acoust. Soc. Am. 99, 2316-2331 (1996)] is used to develop a new version of strobed-temporal-integration that is more robust and physiologically more plausible.

Auditory Perception↗

PubMed1998

Amplitude compression in cochlear implants artificially restricts the perception of temporal asymmetry.

This paper presents a study in which five cochlear implantees were asked to discriminate the timbre of stimuli with temporally asymmetric envelopes. Stimuli were damped and ramped sinusoids presented acoustically. They were transformed by the speech processor of the implant and were presented through one electrode. All cochlear implantees could discriminate the damped and ramped sinusoids when the half-life was 4 ms, the carrier frequency was 400 Hz, and the period of the envelope was 50 ms. In a second experiment, timbre discrimination performance was measured as a function of half-life for two cochlear implantees. Both showed that timbre discrimination was possible over the range 1-24 ms. In normal-hearing listeners, the range is 1-64 ms and in cochlear implantees, stimulated directly without the speech processor, the range is 1-300 ms. At long half-lives, the decrease in discrimination performance observed with the speech processor appears to be due to the amplitude compression applied by the device. The present results suggest that it may be important to ensure that cochlear implants do not restrict temporal asymmetry unduly when applying compression to control level.

Adult↗

PubMed1997

A comparison of detection and discrimination of temporal asymmetry in amplitude modulation.

Two compound experiments were performed to compare the detection of amplitude modulation with the discrimination of modulator shape when the modulators have strong temporal asymmetry. In experiment 1, an adaptive procedure was used to measure detection and discrimination as a function of modulation frequency from 4 to 400 Hz. In experiment 2, the method of constant stimuli was used to measure psychometric functions for detection and discrimination at one modulation frequency, 8 Hz. The asymmetric modulators were time-reversed pairs. Thus their envelope spectra are identical and models based on the envelope spectrum would predict no effect of asymmetry on detection or discrimination at any modulation depth. The detection results show, as predicted, that the direction of asymmetry does not affect the detectability of modulation in either experiment. In contrast, the discrimination results show that direction of asymmetry is readily discriminable for modulation frequencies less than about 50 Hz, indicating that envelope-spectrum models will require modification if they are to be extended to include discrimination of temporal asymmetry.

Auditory Perception↗

PubMed1997

Discrimination of temporal asymmetry in cochlear implantees.

Several studies have recently demonstrated that normal-hearing listeners are sensitive to short-term temporal asymmetry in the envelopes of sinusoidal or noise carriers. This paper presents a study in which cochlear implantees were presented trains of current pulses with temporally asymmetric envelopes through one channel of an implant that stimulates the auditory nerve directly, thereby bypassing cochlear processes. When the level of the stimuli was adjusted to fit their audibility range, the implantees were able to discriminate temporal asymmetry over a much wider range than normal-hearing listeners. The results suggest that the perception of temporal asymmetry is limited by compression in the normal cochlea.

Adult↗

PubMed1996

Temporal asymmetry in the auditory system.

When a damped exponential with a half-life of 4-8 ms is repeated every 25-50 ms and used to modulate a sinusoid or a wideband noise, it suppresses the sound quality typically associated with the carrier. When the envelopes of these "damped" sounds are reversed in time, producing "ramped" sounds, a continuous component with the sound quality of the carrier is restored to the perception. This paper presents an experiment that measures the temporal asymmetry revealed by this perceptual contrast. A ramped sinusoid or noise with a given half-life was presented with a damped sinusoid or noise having the same or greater half-life, to determine the damped half-life required to produce a continuous component with the equivalent relative strength in the two sounds. The results with sinusoidal carriers show that the half-life of the damped sound has to be, on average, about five times the half-life of the ramped sound if the tonal component of the two perceptions is to have the same relative strength. The asymmetry for the noise carrier is about half that of the sinusoidal carrier and, again, the damped sound has the greater matching half-life. Several multichannel auditory models based on a gammatone filterbank are used to try to explain the data in terms of traditional leaky integration, but they produce neither sufficient asymmetry nor the correct pattern of asymmetry. A "delta-gamma" theory is then developed to provide a framework for understanding temporal asymmetry in the auditory system. The theory is used to compare the temporal asymmetry produced by several auditory models and to explain when and how they can accommodate the perceptual asymmetry observed in the experiments.

Auditory Perception↗

PubMed1995

The time course of auditory segregation: concurrent vowels that vary in duration.

Human listeners perform well when identifying both members of simultaneous steady-state vowel pairs, even when the vowels start and stop at the same time, are presented monaurally, have approximately equal intensities, and have the same fundamental frequency (f0). The sensation described by listeners is of one dominant, vowel "colored" by the second, less easily identified, or nondominant vowel. Introducing a small separation in f0 between the vowels improves performance and listeners now report that there is a sensation of two voice sources rather than one. It has been suggested that listeners use an f0-guided segregation strategy in identifying two vowels that differ in f0. An experiment is reported in which four listeners attempted to identify both members of a pair of concurrent vowels which varied in duration from a single cycle of the stimulus waveform (one pitch period) up to eight cycles. A dominant vowel was identified with near 100% accuracy even in the single-cycle condition, whereas identification of the nondominant vowel showed a slow improvement up to eight cycles. A difference in f0 between the vowels improved identification of the nondominant vowel, but between three and four cycles of the vowels were necessary for this advantage. It is first concluded that the improvement in performance with stimulus duration is due to an improvement in identification of the nondominant vowel; and, second, a difference in f0 is not required for segregation of the dominant vowel which is available from stimuli which are too brief to provide a useful estimate of f0.

Adult↗

PubMed1995

Time-domain modeling of peripheral auditory processing: a modular architecture and a software platform.

A software package with a modular architecture has been developed to support perceptual modeling of the fine-grain spectro-temporal information observed in the auditory nerve. The package contains both functional and physiological modules to simulate auditory spectral analysis, neural encoding, and temporal integration, including new forms of periodicity-sensitive temporal integration that generate stabilized auditory images. Combinations of the modules enable the user to approximate a wide variety of existing, time-domain, auditory models. Sequences of auditory images can be replayed to produce cartoons of auditory perceptions that illustrate the dynamic response of the auditory system to everyday sounds.

Auditory Perception↗

PubMed1990

Auditory warning sounds in the work environment.

One of the most common auditory warnings is the ambulance 'siren'. It cuts through traffic noise and commands one's attention, but it does so by sheer brute force. This 'better safe than sorry' approach to auditory warnings occurs in most environments where sounds are used to signal danger or potential danger. Flooding the environment with sound is certain to attract attention; however it also causes startled reactions and prevents communications at a crucial point in time. In collaboration with several companies and government departments, the MRC Applied Psychology Unit performed a series of auditory warning studies. The main conclusions of the research were that the number of immediate-action warning sounds should not exceed about six, and that each sound should have a distinct melody and temporal pattern. The experiments also showed that it is possible to predict the optimum sound level for a warning sound in most noise environments. Subsequently, a set of guidelines for the production of ergonomic auditory warnings was developed. The guidelines have been used to analyse the environments in both fixed-wing and rotary-wing aircraft, and to design prototype warning systems for environments as diverse as helicopters, operating theatres and the railways.

Auditory Threshold↗

PubMed1987

A pulse ribbon model of monaural phase perception.

This article presents two sets of experiments concerning the ability to discriminate changes in the phase spectra of wideband periodic sounds. In the first set, a series of local phase changes is used to modify the envelopes of the waves appearing at the outputs of a range of auditory filters. The size of the local phase change required for discrimination is shown to be strongly dependent on the repetition rate, intensity, and spectral location of the signal. In the second set of experiments, a global phase change is used to produce a progressive phase shift between the outputs of successive auditory filters, without changing the envelopes of the filtered waves. Contrary to what is often assumed, listeners can discriminate between-channel phase shifts once the total time delay across the channels containing the signal reaches 4-5 ms. In this case, however, the discrimination is largely independent of signal parameters other than bandwidth. A highly simplified model of the cochlea, consisting of an auditory filter bank and units that record the times of the larger peaks in the filter outputs, is developed to explain the two contrasting sets of results.

Acoustic Stimulation↗

PubMed1984

Sinusoidal and noise maskers in simultaneous and forward masking.

We compare psychophysical tuning curves obtained with sinusoidal and narrow-band (50-Hz wide) noise maskers in both simultaneous and forward masking. In one experiment, we examine the effects of different combinations of duration and intensity of the 1-kHz sinusoidal signal. In a second experiment, we compare tuning curves obtained with a sinusoidal signal to those obtained with a noise signal. In both experiments, a narrow-band noise is a more effective simultaneous masker than a sinusoid for masker frequencies near the signal frequency. We argue that this is probably due to the use of different detection cues in the presence of sinusoidal and noise maskers, and that the greater masking produced by the noise is not simply due to its greater variability. As observed in other studies, tuning curves are narrower in forward masking than in simultaneous masking.

Auditory Perception↗

PubMed1984

On the growth of masking asymmetry with stimulus intensity.

Masking asymmetry was investigated over a wide range of stimulus intensities for two signal frequencies, fo = 1.0 and 4.0 kHz, using both fixed-masker and fixed-signal paradigms. The masker was a notched noise with the upper and lower edges of the notch, fu and fl, respectively, placed asymmetrically about fo. For various notch widths, the asymmetry of masking was measured as the difference between the masked threshold obtained when fl was nearer fo and that obtained when fu was nearer fo. For maskers with wide notches, (fu - fl)/fo greater than 0.15, masking asymmetry changed with stimulus level; at the highest level, masked threshold was greatest when fl was nearer fo, and, at the lowest level the asymmetry reversed slightly for fo = 1.0 kHz so that masked threshold was actually greater when fu was nearer fo. Nonparallel growth of masking functions reveal changes in masking asymmetry with signal level as well as with masker level. It is concluded that the nonlinear growth of masking with level is due primarily to changes in the auditory filter, rather than changes in the detector following the filter.

Auditory Threshold↗

PubMed1984

Auditory filter asymmetry in the hearing impaired.

Thresholds for 2-kHz sinusoidal signals were determined in the presence of a notched-noise masker, for six normal-hearing listeners and 12 listeners with cochlear hearing losses. Following Patterson and Nimmo Smith [J. Acoust. Soc. Am. 67, 229-245 (1980)], conditions were used where the notch was placed both symmetrically and asymmetrically about the signal frequency. The auditory filter shape for both the low- and high-frequency side of the filter was calculated using the rounded-exponential form of the filter. In six hearing-impaired listeners, the auditory filter shape showed a shallow low-frequency skirt indicating pronounced susceptibility to the upward spread of masking. In two hearing-impaired listeners, the filter shape showed a shallow high-frequency skirt, indicating pronounced susceptibility to the downward spread of masking. Two other listeners with mild threshold losses had steeper and more symmetric filters than normal, suggesting either a small conductive loss or an attenuation factor of sensorineural origin not associated with a degradation of frequency resolution. In the remaining two listeners, the auditory filter had too little selectivity for its shape to be reliably determined.

Adult↗

PubMed1984

Dynamic range and asymmetry of the auditory filter.

This experiment was designed to measure the shape and asymmetry of the auditory filter over a wider dynamic range than has been measured previously. Thresholds were measured for 2-kHz sinusoidal signals in the presence of two 800-Hz-wide noise bands, one above and one below the signal frequency. The spectrum level of the noise was 45 dB (re: 20 muPa), and the noise bands were placed both symmetrically and asymmetrically about the signal frequency. The deviation of the signal frequency from the nearer edge of each noise band varied from 0 to 0.8 times the signal frequency. Each ear of six subjects was tested, and the subjects' ages ranged from 22 to 74 years. The auditory filters derived from the data were somewhat asymmetric, with steeper slopes on the high-frequency side; the degree of asymmetry varied across subjects. The asymmetry could be characterized as a uniform stretching of the (linear) frequency scale on one side of the filter. The dynamic range of the auditory filter exceeded 60 dB in the younger listeners, but the dynamic range and sharpness of the filter tended to decrease with increasing age.

Adult↗

PubMed1983

Detecting a repeated tone burst in repeated noise.

Samples of wideband noise 0.05, 0.1, 0.2, or 0.4 s in duration were digitized and then replayed cyclically to produce repeated-noise maskers. The signal was a repeating tone burst (0.4 or 1.6 kHz). It was half the duration of the noise sample, centered in the noise temporally, and it was repeated at the same point in each repetition of the noise. In the antiphasic conditions of the experiment, either the noise sample or the tone burst was inverted in alternate repetitions of the masker; in the homophasic conditions both the tone burst and noise, or neither, were inverted in alternative repetitions. If the auditory system were capable of storing detailed waveforms of sufficient length, alternate repetitions could be added or subtracted and we might expect a release from masking in the antiphasic conditions. The results show a small but significant advantage for the antiphasic conditions when the signal frequency was 0.4 kHz, but no difference with the 1.6-kHz signal.

Adult↗

PubMed1982

The deterioration of hearing with age: frequency selectivity, the critical ratio, the audiogram, and speech threshold.

The frequency selectivity of the auditory system was measured by masking a sinusoidal signal (0.5, 2.0, or 4.0 kHz) or a filtered-speech signal with a wideband noise having a notch, or stopband, centered on the signal. As the notch was widened performance improved for both types of signal but the rate of improvement decreased as the age of the 16 listeners increased from 23 to 75 years, indicating a loss in frequency selectivity with age. Auditory filter shapes derived from the tone-in-noise data show (a) that the passband of the filter broadens progressively with age, and (b) that the dynamic range of the filter ages like the audiogram. That is, the range changes little with age before 55, but beyond this point there is an accelerating rate of loss. The speech experiment shows comparable but smaller effects. The filter-width measurements show that the critical ratio is a poor estimator of frequency selectivity because it confounds the tuning of the system with the efficiency of the signal-detection and speech-processing mechanisms that follow the filter. An alternative, one-point measure of frequency selectivity, which is both sensitive and reliable, is developed via the filter-shape model of masking.

Adult↗