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Biomedical subjects

Andrew J Oxenham

Publications and source records attributed to Andrew J Oxenham.

At least 19 recordsLinked to original sources
PubMed2007

Cortical FMRI activation to sequences of tones alternating in frequency: relationship to perceived rate and streaming.

Human listeners were functionally imaged while reporting their perception of sequences of alternating-frequency tone bursts separated by 0, 1/8, 1, or 20 semitones. Our goal was to determine whether functional magnetic resonance imaging (fMRI) activation of auditory cortex changes with frequency separation in a manner predictable from the perceived rate of the stimulus. At the null and small separations, the tones were generally heard as a single stream with a perceived rate equal to the physical tone presentation rate. fMRI activation in auditory cortex was appreciably phasic, showing prominent peaks at the sequence onset and offset. At larger-frequency separations, the higher- and lower-frequency tones perceptually separated into two streams, each with a rate equal to half the overall tone presentation rate. Under those conditions, fMRI activation in auditory cortex was more sustained throughout the sequence duration and was larger in magnitude and extent. Phasic to sustained changes in fMRI activation with changes in frequency separation and perceived rate are comparable to, and consistent with, those produced by changes in the physical rate of a sequence and are far greater than the effects produced by changing other physical stimulus variables, such as sound level or bandwidth. We suggest that the neural activity underlying the changes in fMRI activation with frequency separation contribute to the coding of the co-occurring changes in perceived rate and perceptual organization of the sound sequences into auditory streams.

Acoustic Stimulation↗
PubMed2006

Masking by inaudible sounds and the linearity of temporal summation.

Many natural sounds, including speech and animal vocalizations, involve rapid sequences that vary in spectrum and amplitude. Each sound within a sequence has the potential to affect the audibility of subsequent sounds in a process known as forward masking. Little is known about the neural mechanisms underlying forward masking, particularly in more realistic situations in which multiple sounds follow each other in rapid succession. A parsimonious hypothesis is that the effects of consecutive sounds combine linearly, so that the total masking effect is a simple sum of the contributions from the individual maskers. The experiment reported here tests a counterintuitive prediction of this linear-summation hypothesis, namely that a sound that itself is inaudible should, under certain circumstances, affect the audibility of subsequent sounds. The results show that, when two forward maskers are combined, the second of the two maskers can continue to produce substantial masking, even when it is completely masked by the first masker. Thus, inaudible sounds can affect the perception of subsequent sounds. A model incorporating instantaneous compression (reflecting the nonlinear response of the basilar membrane in the cochlea), followed by linear summation of the effects of the maskers, provides a good account of the data. Despite the presence of multiple sources of nonlinearity in the auditory system, masking effects by sequential sounds combine in a manner that is well captured by a time-invariant linear system.

Auditory Perception↗
PubMed2006

Influence of musical and psychoacoustical training on pitch discrimination.

This study compared the influence of musical and psychoacoustical training on auditory pitch discrimination abilities. In a first experiment, pitch discrimination thresholds for pure and complex tones were measured in 30 classical musicians and 30 non-musicians, none of whom had prior psychoacoustical training. The non-musicians' mean thresholds were more than six times larger than those of the classical musicians initially, and still about four times larger after 2h of training using an adaptive two-interval forced-choice procedure; this difference is two to three times larger than suggested by previous studies. The musicians' thresholds were close to those measured in earlier psychoacoustical studies using highly trained listeners, and showed little improvement with training; this suggests that classical musical training can lead to optimal or nearly optimal pitch discrimination performance. A second experiment was performed to determine how much additional training was required for the non-musicians to obtain thresholds as low as those of the classical musicians from experiment 1. Eight new non-musicians with no prior training practiced the frequency discrimination task for a total of 14 h. It took between 4 and 8h of training for their thresholds to become as small as those measured in the classical musicians from experiment 1. These findings supplement and qualify earlier data in the literature regarding the respective influence of musical and psychoacoustical training on pitch discrimination performance.

Acoustic Stimulation↗
PubMed2006

Level dependence of auditory filters in nonsimultaneous masking as a function of frequency.

Auditory filter bandwidths were measured using nonsimultaneous masking, as a function of signal level between 10 and 35 dB SL for signal frequencies of 1, 2, 4, and 6 kHz. The brief sinusoidal signal was presented in a temporal gap within a spectrally notched noise. Two groups of normal-hearing subjects were tested, one using a fixed masker level and adaptively varying signal level, the other using a fixed signal level and adaptively varying masker level. In both cases, auditory filters were derived by assuming a constant filter shape for a given signal level. The filter parameters derived from the two paradigms were not significantly different. At 1 kHz, the equivalent rectangular bandwidth (ERB) decreased as the signal level increased from 10 to 20 dB SL, after which it remained roughly constant. In contrast, at 6 kHz, the ERB increased consistently with signal levels from 10 to 35 dB SL. The results at 2 and 4 kHz were intermediate, showing no consistent change in ERB with signal level. Overall, the results suggest changes in the level dependence of the auditory filters at frequencies above 1 kHz that are not currently incorporated in models of human auditory filter tuning.

Acoustic Stimulation↗
PubMed2006

Effects of introducing unprocessed low-frequency information on the reception of envelope-vocoder processed speech.

This study investigated the benefits of adding unprocessed low-frequency information to acoustic simulations of cochlear-implant processing in normal-hearing listeners. Implant processing was simulated using an eight-channel noise-excited envelope vocoder, and low-frequency information was added by replacing the lower frequency channels of the processor with a low-pass-filtered version of the original stimulus. Experiment 1 measured sentence-level speech reception as a function of target-to-masker ratio, with either steady-state speech-shaped noise or single-talker maskers. Experiment 2 measured listeners' ability to identify two vowels presented simultaneously, as a function of the F0 difference between the two vowels. In both experiments low-frequency information was added below either 300 or 600 Hz. The introduction of the additional low-frequency information led to substantial and significant improvements in performance in both experiments, with a greater improvement observed for the higher (600 Hz) than for the lower (300 Hz) cutoff frequency. However, performance never equaled performance in the unprocessed conditions. The results confirm other recent demonstrations that added low-frequency information can provide significant benefits in intelligibility, which may at least in part be attributed to improvements in F0 representation. The findings provide further support for efforts to make use of residual acoustic hearing in cochlear-implant users.

Adult↗
PubMed2006

Detection and F0 discrimination of harmonic complex tones in the presence of competing tones or noise.

Normal-hearing listeners' ability to "hear out" the pitch of a target harmonic complex tone (HCT) was tested with simultaneous HCT or noise maskers, all bandpass-filtered into the same spectral region (1200-3600 Hz). Target-to-masker ratios (TMRs) necessary to discriminate fixed fundamental-frequency (F0) differences were measured for target F0s between 100 and 400 Hz. At high F0s (400 Hz), asynchronous gating of masker and signal, presenting the masker in a different F0 range, and reducing the F0 rove of the masker, all resulted in improved performance. At the low F0s (100 Hz), none of these manipulations improved performance significantly. The findings are generally consistent with the idea that the ability to segregate sounds based on cues such as F0 differences and onset/offset asynchronies can be strongly limited by peripheral harmonic resolvability. However, some cases were observed where perceptual segregation appeared possible, even when no peripherally resolved harmonics were present in the mixture of target and masker. A final experiment, comparing TMRs necessary for detection and F0 discrimination, showed that F0 discrimination of the target was possible with noise maskers at only a few decibels above detection threshold, whereas similar performance with HCT maskers was only possible 15-25 dB above detection threshold.

Acoustic Stimulation↗
PubMed2006

The relationship between frequency selectivity and pitch discrimination: effects of stimulus level.

Three experiments tested the hypothesis that fundamental frequency (fo) discrimination depends on the resolvability of harmonics within a tone complex. Fundamental frequency difference limens (fo DLs) were measured for random-phase harmonic complexes with eight fo's between 75 and 400 Hz, bandpass filtered between 1.5 and 3.5 kHz, and presented at 12.5-dB/component average sensation level in threshold equalizing noise with levels of 10, 40, and 65 dB SPL per equivalent rectangular auditory filter bandwidth. With increasing level, the transition from large (poor) to small (good) fo DLs shifted to a higher fo. This shift corresponded to a decrease in harmonic resolvability, as estimated in the same listeners with excitation patterns derived from measures of auditory filter shape and with a more direct measure that involved hearing out individual harmonics. The results are consistent with the idea that resolved harmonics are necessary for good fo discrimination. Additionally, fo DLs for high fo's increased with stimulus level in the same way as pure-tone frequency DLs, suggesting that for this frequency range, the frequencies of harmonics are more poorly encoded at higher levels, even when harmonics are well resolved.

Auditory Threshold↗
PubMed2006

The relationship between frequency selectivity and pitch discrimination: sensorineural hearing loss.

This study tested the relationship between frequency selectivity and the minimum spacing between harmonics necessary for accurate fo discrimination. Fundamental frequency difference limens (fo DLs) were measured for ten listeners with moderate sensorineural hearing loss (SNHL) and three normal-hearing listeners for sine- and random-phase harmonic complexes, bandpass filtered between 1500 and 3500 Hz, with fo's ranging from 75 to 500 Hz (or higher). All listeners showed a transition between small (good) fo DLs at high fo's and large (poor) fo DLs at low fo's, although the fo at which this transition occurred (fo,tr) varied across listeners. Three measures thought to reflect frequency selectivity were significantly correlated to both the fo,tr and the minimum fo DL achieved at high fo's: (1) the maximum fo for which fo DLs were phase dependent, (2) the maximum modulation frequency for which amplitude modulation and quasi-frequency modulation were discriminable, and (3) the equivalent rectangular bandwidth of the auditory filter, estimated using the notched-noise method. These results provide evidence of a relationship between fo discrimination performance and frequency selectivity in listeners with SNHL, supporting "spectral" and "spectro-temporal" theories of pitch perception that rely on sharp tuning in the auditory periphery to accurately extract fo information.

Adult↗
PubMed2005

Neuromagnetic correlates of streaming in human auditory cortex.

The brain is constantly faced with the challenge of organizing acoustic input from multiple sound sources into meaningful auditory objects or perceptual streams. The present study examines the neural bases of auditory stream formation using neuromagnetic and behavioral measures. The stimuli were sequences of alternating pure tones, which can be perceived as either one or two streams. In the first experiment, physical stimulus parameters were varied between values that promoted the perceptual grouping of the tone sequence into one coherent stream and values that promoted its segregation into two streams. In the second experiment, an ambiguous tone sequence produced a bistable percept that switched spontaneously between one- and two-stream percepts. The first experiment demonstrated a strong correlation between listeners' perception and long-latency (>60 ms) activity that likely arises in nonprimary auditory cortex. The second demonstrated a covariation between this activity and listeners' perception in the absence of physical stimulus changes. Overall, the results indicate a tight coupling between auditory cortical activity and streaming perception, suggesting that an explicit representation of auditory streams may be maintained within nonprimary auditory areas.

Acoustic Stimulation↗
PubMed2005

Effects of envelope-vocoder processing on F0 discrimination and concurrent-vowel identification.

OBJECTIVE: The aim of this study was to examine the effects of envelope-vocoder sound processing on listeners' ability to discriminate changes in fundamental frequency (F0) in anechoic and reverberant conditions and on their ability to identify concurrent vowels based on differences in F0. DESIGN: In the first experiment, F0 difference limens (F0DLs) were measured as a function of number of envelope-vocoder frequency channels (1, 4, 8, 24, and 40 channels, and unprocessed) in four normal-hearing listeners, with degree of simulated reverberation (no, mild, and severe reverberation) as a parameter. In the second experiment, vowel identification was measured as a function of the F0 difference between two simultaneous vowels in six normal-hearing listeners, with the number of vocoder channels (8 and 24 channels, and unprocessed) as a parameter. RESULTS: Reverberation was detrimental to F0 discrimination in conditions with fewer numbers of vocoder channels. Despite the reasonable F0DLs (<1 semitone) with 24- and 8-channel vocoder processing, listeners were unable to benefit from F0 differences between the competing vowels in the concurrent-vowel paradigm. CONCLUSIONS: The overall detrimental effects of vocoder processing are probably are due to the poor spectral representation of the lower-order harmonics. The F0 information carried in the temporal envelope is weak, susceptible to reverberation, and may not suffice for source segregation. To the extent that vocoder processing simulates cochlear implant processing, users of current implant processing schemes are unlikely to benefit from F0 differences between competing talkers when listening to speech in complex environments. The results provide further incentive for finding a way to make the information from low-order, resolved harmonics available to cochlear implant users.

Acoustic Stimulation↗
PubMed2005

Comparing different estimates of cochlear compression in listeners with normal and impaired hearing.

A loss of cochlear compression may underlie many of the difficulties experienced by hearing-impaired listeners. Two behavioral forward-masking paradigms that have been used to estimate the magnitude of cochlear compression are growth of masking (GOM) and temporal masking (TM). The aim of this study was to determine whether these two measures produce within-subjects results that are consistent across a range of signal frequencies and, if so, to compare them in terms of reliability or efficiency. GOM and TM functions were measured in a group of five normal-hearing and five hearing-impaired listeners at signal frequencies of 1000, 2000, and 4000 Hz. Compression values were derived from the masking data and confidence intervals were constructed around these estimates. Both measures produced comparable estimates of compression, but both measures have distinct advantages and disadvantages, so that the more appropriate measure depends on factors such as the frequency region of interest and the degree of hearing loss. Because of the long testing times needed, neither measure is suitable for clinical use in its current form.

Adolescent↗
PubMed2005

An autocorrelation model with place dependence to account for the effect of harmonic number on fundamental frequency discrimination.

Fundamental frequency (f0) difference limens (DLs) were measured as a function of f0 for sine- and random-phase harmonic complexes, bandpass filtered with 3-dB cutoff frequencies of 2.5 and 3.5 kHz (low region) or 5 and 7 kHz (high region), and presented at an average 15 dB sensation level (approximately 48 dB SPL) per component in a wideband background noise. Fundamental frequencies ranged from 50 to 300 Hz and 100 to 600 Hz in the low and high spectral regions, respectively. In each spectral region, f0 DLs improved dramatically with increasing f0 as approximately the tenth harmonic appeared in the passband. Generally, f0 DLs for complexes with similar harmonic numbers were similar in the two spectral regions. The dependence of f0 discrimination on harmonic number presents a significant challenge to autocorrelation (AC) models of pitch, in which predictions generally depend more on spectral region than harmonic number. A modification involving a "lag window"is proposed and tested, restricting the AC representation to a limited range of lags relative to each channel's characteristic frequency. This modified unitary pitch model was able to account for the dependence of f0 DLs on harmonic number, although this correct behavior was not based on peripheral harmonic resolvability.

Acoustic Stimulation↗
PubMed2005

Comparing F0 discrimination in sequential and simultaneous conditions.

In an influential study, Carlyon and Shackleton [J. Acoust. Soc. Am. 95, 3541-3554 (1994)] measured listeners' performance (d') in fundamental-frequency (F0) discrimination between harmonic complex tones (HCTs) presented simultaneously in different spectral regions and compared their performance with that found in a sequential-comparison task. In this Letter, it is suggested that Carlyon and Shackleton's analysis of the simultaneous-comparison data did not adequately reflect their assumption that listeners were effectively comparing F0's across regions. A reanalysis consistent with this assumption is described. The new results suggest that under the assumption that listeners were effectively comparing F0 across regions, their performance in this task was substantially higher than originally estimated by Carlyon and Shackleton, and in some conditions much higher than expected from the performances measured in a traditional F0-discrimination task with sequential HCTs. Possible explanations for this outcome, as well as alternative interpretations, are proposed.

Discrimination, Psychological↗
PubMed2004

A neural representation of pitch salience in nonprimary human auditory cortex revealed with functional magnetic resonance imaging.

Pitch, one of the primary auditory percepts, is related to the temporal regularity or periodicity of a sound. Previous functional brain imaging work in humans has shown that the level of population neural activity in centers throughout the auditory system is related to the temporal regularity of a sound, suggesting a possible relationship to pitch. In the current study, functional magnetic resonance imaging was used to measure activation in response to harmonic tone complexes whose temporal regularity was identical, but whose pitch salience (or perceptual pitch strength) differed, across conditions. Cochlear nucleus, inferior colliculus, and primary auditory cortex did not show significant differences in activation level between conditions. Instead, a correlate of pitch salience was found in the neural activity levels of a small, spatially localized region of nonprimary auditory cortex, overlapping the anterolateral end of Heschl's gyrus. The present data contribute to converging evidence that anterior areas of nonprimary auditory cortex play an important role in processing pitch.

Adult↗
PubMed2004

Correct tonotopic representation is necessary for complex pitch perception.

The ability to extract a pitch from complex harmonic sounds, such as human speech, animal vocalizations, and musical instruments, is a fundamental attribute of hearing. Some theories of pitch rely on the frequency-to-place mapping, or tonotopy, in the inner ear (cochlea), but most current models are based solely on the relative timing of spikes in the auditory nerve. So far, it has proved to be difficult to distinguish between these two possible representations, primarily because temporal and place information usually covary in the cochlea. In this study, "transposed stimuli" were used to dissociate temporal from place information. By presenting the temporal information of low-frequency sinusoids to locations in the cochlea tuned to high frequencies, we found that human subjects displayed poor pitch perception for single tones. More importantly, none of the subjects was able to extract the fundamental frequency from multiple low-frequency harmonics presented to high-frequency regions of the cochlea. The experiments demonstrate that tonotopic representation is crucial to complex pitch perception and provide a new tool in the search for the neural basis of pitch.

Cochlear Implants↗
PubMed2004

Masker phase effects in normal-hearing and hearing-impaired listeners: evidence for peripheral compression at low signal frequencies.

The presence of cochlear-based compression at low frequencies was investigated by measuring phase effects in harmonic maskers. In normal-hearing listeners, the amount of masking produced depends strongly on the phase relationships between the individual masker components. This effect is thought to be determined primarily by properties of the cochlea, including the phase dispersion and compressive input-output function of the basilar membrane. Thresholds for signals of 250 and 1000 Hz were measured in harmonic maskers with fundamental frequencies of 12.5 and 100 Hz as a function of the masker phase curvature. Results from 12 listeners with sensorineural hearing loss showed reduced masker phase effects, when compared with data from normal-hearing listeners, at both 250- and 1000-Hz signal frequencies. The effects of hearing impairment on phase-related masking differences were not well simulated in normal-hearing listeners by an additive white noise, suggesting that the effects of hearing impairment are not simply due to reduced sensation level. Maximum differences in masked threshold were correlated with auditory filter bandwidths at the respective frequencies, suggesting that both measures are affected by a common underlying mechanism, presumably related to cochlear outer hair cell function. The results also suggest that normal peripheral compression remains strong even at 250 Hz.

Acoustic Stimulation↗
PubMed2004

Sequential F0 comparisons between resolved and unresolved harmonics: no evidence for translation noise between two pitch mechanisms.

Carlyon and Shackleton [J. Acoust. Soc. Am. 95, 3541-3554 (1994)] suggested that fundamental-frequency (F0) discrimination performance between resolved and unresolved harmonics is limited by an internal "translation" noise between the outputs of two distinct F0 encoding mechanisms, in addition to the encoding noise associated with each mechanism. To test this hypothesis further, F0 difference limens (DLF0s) were measured in six normal-hearing listeners using sequentially presented groups of harmonics. The two groups of harmonics presented on each trial were bandpass filtered into the same or different spectral regions, in such a way that both groups contained mainly resolved harmonics, both groups contained only unresolved harmonics, or one group contained mainly resolved and the other only unresolved harmonics. Three spectral regions (low: 600-1150 Hz, mid: 1400-2500 Hz, or high: 3000-5250 Hz) and two nominal F0s (100 and 200 Hz) were used. The DLF0s measured in across-region conditions were well accounted for by a model assuming only two sources of internal noise: the encoding noise estimated on the basis of the within-region results plus a constant noise associated with F0 comparisons across different spectral regions, independent of resolvability. No evidence for an across-pitch-mechanism translation noise was found. A reexamination of previous evidence for the existence of such noise suggests that the present negative outcome is unlikely to be explained by insufficient measurement sensitivity or an unusually large across-region comparison noise in the present study. While the results do not rule out the possibility of two separate pitch mechanisms, they indicate that the F0s of sequentially presented resolved and unresolved harmonics can be compared internally at no or negligible extra cost.

Acoustic Stimulation↗