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Rhythm is fundamental to music and speech, yet little is known about how even simple rhythmic patterns are processed. Here we investigated the processing of isochronous rhythms in the short inter-onset-interval (IOI) range (IOIs < 250–400 ms) using a perceptual-learning paradigm. Trained listeners (n=8) practiced anisochrony detection with a 100-ms IOI marked by 1-kHz tones, 720 trials per day for 7 days. Between pre- and post-training tests, trained listeners improved significantly more than controls (no training; n=8) on the anisochrony-detection condition that the trained listeners practiced. However, the learning on anisochrony detection did not generalize to temporal-interval discrimination with the trained IOI (100 ms) and marker frequency (1 kHz) or to anisochrony detection with an untrained marker frequency (4 kHz or variable frequency vs. 1 kHz), and generalized negatively to anisochrony detection with an untrained IOI (200 ms vs. 100 ms). Further, pre-training thresholds were correlated among nearly all of the conditions with the same IOI (100-ms IOIs), but not between conditions with different IOIs (100-ms vs. 200-ms IOIs). Thus, it appears that some task-, IOI-, and frequency-specific processes are involved in fast-rhythm processing. These outcomes are most consistent with a holistic rhythm-processing model in which a holistic “image” of the stimulus is compared to a stimulus-specific template.

Extended high frequencies (EHF), above 8 kHz, represent a region of the human hearing spectrum that is generally ignored by clinicians and researchers alike. This article is a compilation of contributions that, together, make the case for an essential role of EHF in both normal hearing and auditory dysfunction. We start with the fundamentals of biological and acoustic determinism – humans have EHF hearing for a purpose, for example, the detection of prey, predators, and mates. EHF hearing may also provide a boost to speech perception in challenging conditions and its loss, conversely, might help explain difficulty with the same task. However, it could be that EHF are a marker for damage in the conventional frequency region that is more related to speech perception difficulties. Measurement of EHF hearing in concert with otoacoustic emissions could provide an early warning of age-related hearing loss. In early life, when EHF hearing sensitivity is optimal, we can use it for enhanced phonetic identification during language learning, but we are also susceptible to diseases that can prematurely damage it. EHF audiometry techniques and standardization are reviewed, providing evidence that they are reliable to measure and provide important information for early detection, monitoring and possible prevention of hearing loss in populations at-risk. To better understand the full contribution of EHF to human hearing, clinicians and researchers can contribute by including its measurement, along with measures of speech in noise and self-report of hearing difficulties and tinnitus in clinical evaluations and studies.

A key component of musical proficiency is the ability to discriminate between and identify musical intervals, or fixed ratios between pitches. Acquiring these skills requires training, but little is known about how to best arrange the trials within a training session. To address this issue, learning on a musical-interval comparison task was evaluated for two four-day training regimens that employed equal numbers of stimulus presentations per day. A regimen of continuous practice yielded no learning, but a regimen that combined practice and stimulus exposure alone generated clear improvement. Learning in the practice-plus-exposure regimen was due to the combination of the two experiences, because two control groups who received only either the practice or the exposure from that regimen did not learn. Posttest performance suggested that this improvement in comparison learning generalized to an untrained stimulus type and an untrained musical-interval identification task. Naive comparison performance, but not learning, was better for larger pitch-ratio differences and for individuals with more musical experience. The reported benefits of the practice-plus-exposure regimen mirror the outcomes for fine-grained discrimination and speech tasks, suggesting that a general learning principle is involved. In practical terms, it appears that combining practice and stimulus exposure alone is a particularly effective configuration for improving musical-interval perception.

Perceptual learning is an enhancement in discriminability of similar stimuli following experience with those stimuli. Here, we examined the efficacy of adding additional active training following a standard training session, compared with additional stimulus exposure in the absence of associated task performance. Mice were trained daily in an odor-discrimination task, and then, several hours later each day, received 1 of 3 different manipulations: 1) a second active-training session, 2) non-task-related odor exposure in the home cage, or 3) no second session. For home-cage exposure, odorants were presented in small tubes that mice could sniff and investigate for a similar period of time as in the active discrimination task each day. The results demonstrate that daily home-cage exposure was equivalent to active odor training in supporting improved odor discrimination. Daily home-cage exposure to odorants that did not match those used in the active task did not improve learning, yielding outcomes similar to those obtained with no second session. Piriform cortical local field potential recordings revealed that both sampling in the active learning task and investigation in the home cage evoked similar beta band oscillatory activity. Together the results suggest that odor-discrimination learning can be significantly enhanced by addition of odor exposure outside of the active training task, potentially because of the robust activity evoked in the olfactory system by both exposure paradigms. They further suggest that odorant exposure alone could enhance or maintain odor-discrimination abilities in conditions associated with olfactory impairment, such as aging or dementia.

Auditory skills improve with practice. Producing across-day learning requires a sufficient number of trials per session; additional trials in the same session produce no additional improvement. Thus, trials appear to integrate within a session to reach a learning threshold, after which there is a refractory period during which trials do not contribute to learning. The refractory period duration must be

Young adults with normal hearing (YNH) can improve their sensitivity to basic acoustic features with practice. However, it is not known to what extent the influence of the same training regimen differs between YNH listeners and older listeners with hearing impairment (OHI)--the largest population seeking treatment in audiology clinics. To examine this issue, we trained OHI listeners on a basic auditory task (spectral modulation detection) using a training regimen previously administered to YNH listeners (≈ 1 h/session for seven sessions on a single condition). For the trained conditions on which pretraining performance was not already at asymptote, the YNH listeners who received training learned more than matched controls who received none, but that learning did not generalize to any untrained spectral modulation frequency. In contrast, the OHI-trained listeners and controls learned similar amounts on the trained condition, implying no effect of the training itself. However, surprisingly the OHI-trained listeners improved over the training phase and on an untrained spectral modulation frequency. These population differences suggest that learning consolidated more slowly, and that training modified an aspect of processing that had broader tuning to spectral modulation frequency, in OHI than YNH listeners. More generally, these results demonstrate that conclusions about perceptual learning that come from examination of one population do not necessarily apply to another.

Natural sounds are characterized by complex patterns of sound intensity distributed across both frequency (spectral modulation) and time (temporal modulation). Perception of these patterns has been proposed to depend on a bank of modulation filters, each tuned to a unique combination of a spectral and a temporal modulation frequency. There is considerable physiological evidence for such combined spectrotemporal tuning. However, direct behavioral evidence is lacking. Here we examined the processing of spectrotemporal modulation behaviorally using a perceptual-learning paradigm. We trained human listeners for ∼1 h/d for 7 d to discriminate the depth of spectral (0.5 cyc/oct; 0 Hz), temporal (0 cyc/oct; 32 Hz), or upward spectrotemporal (0.5 cyc/oct; 32 Hz) modulation. Each trained group learned more on their respective trained condition than did controls who received no training. Critically, this depth-discrimination learning did not generalize to the trained stimuli of the other groups or to downward spectrotemporal (0.5 cyc/oct; −32 Hz) modulation. Learning on discrimination also led to worsening on modulation detection, but only when the same spectrotemporal modulation was used for both tasks. Thus, these influences of training were specific to the trained combination of spectral and temporal modulation frequencies, even when the trained and untrained stimuli had one modulation frequency in common. This specificity indicates that training modified circuitry that had combined spectrotemporal tuning, and therefore that circuits with such tuning can influence perception. These results are consistent with the possibility that the auditory system analyzes sounds through filters tuned to combined spectrotemporal modulation.

Normal perception depends, in part, on accurate judgments of the temporal relationships between sensory events. Two such relative-timing skills are the ability to detect stimulus asynchrony and to discriminate stimulus order. Here we investigated the neural processes contributing to the performance of auditory asynchrony and order tasks in humans, using a perceptual-learning paradigm. In each of two parallel experiments, we tested listeners on a pretest and a posttest consisting of auditory relative-timing conditions. Between these two tests, we trained a subset of listeners ∼1 h/d for 6–8 d on a single relative-timing condition. The trained listeners practiced asynchrony detection in one experiment and order discrimination in the other. Both groups were trained at sound onset with tones at 0.25 and 4.0 kHz. The remaining listeners in each experiment, who served as controls, did not receive multihour training during the 8–10 d between the pretest and posttest. These controls improved even without intervening training, adding to evidence that a single session of exposure to perceptual tasks can yield learning. Most importantly, each of the two groups of trained listeners learned more on their respective trained conditions than controls, but this learning occurred only on the two trained conditions. Neither group of trained listeners generalized their learning to the other task (order or asynchrony), an untrained temporal position (sound offset), or untrained frequency pairs. Thus, it appears that multihour training on relative-timing skills affects task-specific neural circuits that are tuned to a given temporal position and combination of stimulus components.

Humans use several cues to determine the position of sound sources in space. Here we provide a review of the literature on how practice influences sound-localization performance in human adults. We divided this literature into two categories: adaptation to alterations in sound-localization cues and learning with normal, unaltered cues. For altered cues, partial adaptation has been reported with a variety of cue transformations, except in the extreme case in which the cues were reversed between the two ears. For normal cues, improvement has been observed in some but not all experiments, though the most recent data generally show learning. Thus, the reviewed data indicated that human adults can recalibrate, as well as refine the use of, sound-localization cues, and that training regimens can be developed to enhance sound-localization performance in individuals with normal and impaired localization abilities.

Noise that is amplitude modulated at rates ranging from 40 to 850 Hz can elicit a sensation of pitch. Here, the processing of this temporally based pitch was investigated using a perceptual-learning paradigm. Nine listeners were trained (1 hour per day for 6-8 days) to discriminate a standard rate of sinusoidal amplitude modulation (SAM) from a faster rate in a single condition (150 Hz SAM rate, 5 kHz low-pass carrier). All trained listeners improved significantly on that condition. These trained listeners subsequently showed no more improvement than nine untrained controls on pure-tone and rippled-noise discrimination with the same pitch, and on SAM-rate discrimination with a 30 Hz rate, although they did show some improvement with a 300 Hz rate. In addition, most trained, but not control, listeners were worse at detecting SAM at 150 Hz after, compared to before training. These results indicate that listeners can learn to improve their ability to discriminate SAM rate with multiple-hour training and that the mechanism that is modified by learning encodes (1) the pitch of SAM noise but not that of pure tones and rippled noise, (2) different SAM rates separately, and (3) differences in SAM rate more effectively than cues for SAM detection.

What is the time course of human attention in a simple auditory detection task? To investigate this question, we determined the detectability of a 20-msec, 1000-Hz tone presented at expected and unexpected times. Twelve listeners who expected the tone to occur at a specific time after a 300-msec narrowband noise rarely detected signals presented 150-375 msec before or 100-200 msec after that expected time. The shape of this temporal-attention window depended on the expected presentation time of the tone and the temporal markers available in the trials. Further, though expecting the signal to occur in silence, listeners often detected signals presented at unexpected times during the noise. Combined with previous data, these results further clarify the listening strategy humans use when trying to detect an expected sound: Humans seem to listen specifically for that sound, while ignoring the background in which it is presented, around the time when the sound is expected to occur.

Objective: Individuals with autism spectrum disorders (ASD) show enhanced perceptual and memory abilities in the domain of pitch, but also perceptual deficits in other auditory domains. The present study investigated their skills with respect to “echoic memory,” a form of short-term sensory memory intimately tied to auditory perception, using a developmental perspective. Method: We tested 23 high-functioning participants with ASD and 26 typically developing (TD) participants, distributed in two age groups (children vs. young adults; mean ages: 11 and 21 years). By means of an adaptive psychophysical procedure, we measured the longest period for which periodic (i.e., repeated) noise could be reliably discriminated from nonperiodic (i.e., plain random) noise. On each experimental trial, a single noise sample was presented to the participant, who had to classify this sound as periodic or nonperiodic. Results: The TD adults performed, on average, much better than the other three groups, who performed similarly overall. As a function of practice, the measured thresholds improved for the TD participants, but did not change for the ASD participants. Thresholds were not correlated to performance in a test assessing verbal memory. The variance of the participants’ response biases was larger among the ASD participants than among the TD participants. Conclusion: The results mainly suggest that echoic memory takes a long time to fully develop in TD humans, and that this development stops prematurely in persons with ASD.

This study investigated the development of auditory frequency and temporal resolution using simultaneous and backward masking of a tone by a noise. The participants were 6- to 10-year-old children and adults. On the measure of frequency resolution (the difference in the detection threshold for a tone presented either in a bandpass noise or in a spectrally notched noise), 6-year-old children performed as well as adults. However, for the backward masking task, 6-yearolds had, on average, 34 dB higher thresholds than adults. A negative exponential decay function fitted to the backward masking data for subjects of all ages indicated that adult-like temporal resolution may not be reached until about 11 years of age. These results show that, measured by masking, frequency resolution has reached adult-like performance by 6 years of age, whereas temporal resolution develops beyond 10 years of age. Six-year-old children were also assessed with tests of cognitive ability. Improvements in both frequency and temporal resolution were found with increasing IQ score.

Detection performance for a masked auditory signal of fixed frequency can be substantially degraded if there is uncertainty about the frequency content of the masker. A quasimolecular psychophysical approach was used to examine response strategies in masker-uncertainty conditions, and to investigate the influence of uncertainty when the number of different masker samples was limited to ten or fewer. The task of the four listeners was to detect a 1000-Hz signal that was presented simultaneously with one of ten ten-tone masker samples. The masker sample was either fixed throughout a block of two-interval forced-choice trials or was randomized across or within trials. The primary results showed that: (1) When the signal level was low and the masker sample differed between the two intervals of a trial, most listeners based their responses more on the presence of specific masker samples than on the signal. (2) The detrimental effect of masker uncertainty was clearly evident when only four maskers were randomly presented, and grew as the size of the masker set was increased from two to ten. (3) The slopes of psychometric functions measured with the same masker samples differed among the fixed and two random-masker conditions. (4) There were large differences in the influence of masker uncertainty across masker samples and listeners. These data demonstrate the great susceptibility of human listeners to the influence of masker uncertainty and the ability of quasimolecular investigations to reveal important aspects of behavior in uncertainty condition.

While it is commonly held that the capacity to learn is greatest in the young, there have been few direct comparisons of the response to training across age groups. Here, adolescents (11–17 years, n = 20) and adults (≥18 years, n = 11) practiced detecting a backward-masked tone for ∼1 h/day for 10 days. Nearly every adult, but only half of the adolescents improved across sessions, and the adolescents who learned did so more slowly than adults. Nevertheless, the adolescent and adult learners showed the same generalization pattern, improving on untrained backward- but not forward- or simultaneous-masking conditions. Another subset of adolescents (n = 6) actually got worse on the trained condition. This worsening, unlike learning, generalized to an untrained forward-masking, but not backward-masking condition. Within sessions, both age groups got worse, but the worsening was greater for adolescents. These maturational changes in the response to training largely followed those previously reported for temporal-interval discrimination. Overall, the results suggest that late-maturing processes affect the response to perceptual training and that some of these processes may be shared between tasks. Further, the different developmental rates for learning and generalization, and different generalization patterns for learning and worsening imply that learning, generalization, and worsening may have different origins.

A simultaneously masked auditory signal becomes easier to detect as its onset is delayed from masker onset. This ‘‘enhancement effect’’ depends upon masking components presented outside of the signal’s frequency channel, indicating the participation of an across‐channel process. The nature of this process has not been clearly established due to the difficulty of separating the influence of excitation and inhibition in simultaneous masking. Here, the forward‐masked threshold of a 20‐ms, 1000‐Hz tone was determined in the presence of one or two masking tones [M1 (1000 Hz, 50 dB) and M2 (variable from 500 to 3000 Hz, 70 dB)]. When the masker duration was 500 ms, adding M2 to M1 reduced signal threshold when M2 was presented at 1150 Hz (suppression), but had no influence on performance when M2 was above about 1300 Hz. In contrast, when the masker duration was 40 ms, adding M2 to M1 did not lower threshold at 1150 Hz, and always increased threshold when M2 was presented above 1300 Hz. This latter threshold inc...

Some individuals complain of hearing difficulties in the presence of background noise even in the absence of clinically significant hearing loss (obscure auditory dysfunction). Previous evidence suggests that these listeners have impaired frequency resolution, but there has been no thorough characterization of auditory filter shapes in this population. Here, the filter shapes of adults (n = 14) who self-reported speech recognition problems in noise and performed poorly on a sentence-in-noise perception test despite having clinically normal audiograms were compared to those of controls (n = 10). The filter shapes were evaluated using a 2-kHz probe with a fixed level of 30, 40, or 50 dB sound pressure level (SPL) and notched-noise simultaneous maskers that were varied in level to determine the masker level necessary to just mask the probe. The filters of the impaired group were significantly wider than those of controls at all probe levels owing to an unusual broadening of the upper slope of the filter. In addition, absolute thresholds were statistically indistinguishable between the groups at the standard audiometric frequencies, but were elevated in the impaired listeners at higher frequencies. These results strengthen the idea that this population has a variety of hearing deficits that go undetected by standard audiometry.

Fluctuations in sound amplitude provide important cues to the identity of many sounds including speech. Of interest here was whether the ability to detect these fluctuations can be improved with practice, and if so whether this learning generalizes to untrained cases. To address these issues, normal-hearing adults (n = 9) were trained to detect sinusoidal amplitude modulation (SAM; 80-Hz rate, 3-4 kHz bandpass carrier) 720 trials/day for 6-7 days and were tested before and after training on related SAM-detection and SAM-rate-discrimination conditions. Controls (n = 9) only participated in the pre- and post-tests. The trained listeners improved more than the controls on the trained condition between the pre- and post-tests, but different subgroups of trained listeners required different amounts of practice to reach asymptotic performance, ranging from 1 (n = 6) to 4-6 (n = 3) sessions. This training-induced learning did not generalize to detection with two untrained carrier spectra (5 kHz low-pass and 0.5-1.5 kHz bandpass) or to rate discrimination with the trained rate and carrier spectrum, but there was some indication that it generalized to detection with two untrained rates (30 and 150 Hz). Thus, practice improved the ability to detect amplitude modulation, but the generalization of this learning to untrained cases was somewhat limited.

Delaying the onset of a signal relative to the onset of a sim ultaneous notched masker often improves the ability of listeners to ‘hear o u t’ the signal at both threshold and suprathreshold levels. Viemeister & Bacon ( J. acoust. Soc. Am. , 71, 1502-1507 (1982)) suggested th at such auditory enhancement effects could be accounted for if the suppression produced by the masker on the signal frequency adapted, thereby releasing the signal from suppression. In support of their hypothesis, Viemeister & Bacon reported that a masker preceded by an enhancer having no com ponent at the signal frequency produced more forward masking than did the masker by itself. Here evidence is provided from five new experiments showing that adaptation of psychophysical two-tone suppression is inadequate to account either for auditory enhancement effects or for the enhanced forward masking demonstrated by Viemeister & Bacon.

Humans rely on interaural level differences (ILDs) to determine the location of sound sources, particularly for high-frequency sounds. Previously, ILD-discrimination performance with a 4-kHz pure tone was reported to improve with multi-hour training. Here the effect of the same training regimen on ILD discrimination with a 4-kHz tone sinusoidally amplitude modulated (SAM) at 0.3 kHz was examined. Ten of the 16 trained listeners improved more than untrained controls, demonstrating training-induced learning. However, compared to the learning previously obtained with the 4-kHz pure tone, learning with the SAM tone was less predictable based on starting performance, took longer to complete, and was characterized by specificity to stimulus type (SAM vs pure tones) rather than stimulus frequency. These differences demonstrate an influence of amplitude modulation on learning of ILD discrimination. This influence suggests that the auditory system makes use of amplitude envelope information in determining ILD-discrimination performance, a form of interaction between time and level processing in the binaural system.

The threshold of a 1250‐Hz tonal signal was measured in the presence of five noise bands (each 50 Hz wide, centered at 850, 1050, 1250, 1450, and 1650 Hz) under five conditions of uncertainty about the waveform type (‘‘correlated’’ or ‘‘uncorrelated’’), and/or the specific waveform sample to be presented. The waveform type was correlated when the temporal envelopes of all of the noise bands were the same, and was uncorrelated when the temporal envelope of the band centered on the signal differed from the common envelope of the other bands. At the low‐uncertainty end of the continuum of conditions, the same waveform type was presented throughout an entire block of trials, and, in addition, the same waveform sample was presented on the two observation intervals of a single trial (but changed across trials). At the high‐uncertainty end of the continuum, both the waveform type and the waveform sample were chosen at random for every observation interval. Threshold estimates obtained from trials in which both observation intervals contained the same waveform type were not affected by uncertainty about the waveform sample within a trial, nor by uncertainty about the waveform type introduced across trials. Thus the comodulation masking release, or CMR (the difference in the thresholds obtained with the uncorrelated and correlated waveforms), calculated from these types of trials was robust across all of the uncertainty conditions. However, on those trials in which one correlated interval and one uncorrelated interval were paired, threshold estimates were influenced by a bias for listeners to choose the uncorrelated interval as the signal interval, whether or not it actually contained the signal. This bias reveals the importance of recognizing the contribution of the nonsignal interval in experiments involving masker uncertainty. Parallel results were obtained using the comodulation detection difference (CDD) task. In some conditions, marked individual differences were observed.

Sound source localization on the horizontal plane is primarily determined by interaural time differences (ITDs) for low-frequency stimuli and by interaural level differences (ILDs) for high-frequency stimuli, but ITDs in high-frequency complex stimuli can also be used for localization. Of interest here is the relationship between the processing of high-frequency ITDs and that of low-frequency ITDs and high-frequency ILDs. A few similarities in human performance with high- and low-frequency ITDs have been taken as evidence for similar ITD processing across frequency regions. However, such similarities, unless accompanied by differences between ITD and ILD performance on the same measure, could potentially reflect processing attributes common to both ITDs and ILDs rather than to ITDs only. In the present experiment, both learning and variability patterns in human discrimination of ITDs in high-frequency amplitude-modulated tones were examined and compared to previously obtained data with low-frequency ITDs and high-frequency ILDs. Both patterns for high-frequency ITDs were more similar to those for low-frequency ITDs than for high-frequency ILDs. These results thus add to the evidence supporting similar ITD processing across frequency regions, and further suggest that both high- and low-frequency ITD processing is less modifiable and more noisy than ILD processing.

Listeners learn non-native phonetic contrasts given practice and feedback, but what is the dose-response curve for that training? Does testing without feedback affect that response? We trained native English speakers on a non-native phonetic contrast between pre-voicing and voicing on two consecutive days, varying the number of daily training trials across groups. Practice with feedback for 240, 120, or 60 trials/day all led to a similar steepening of the category-boundary slope, but practice with feedback for 30 trials/day yielded no learning. To determine whether the daily pre- and post-training tests without feedback (120 trials/test) affected this outcome, we trained another group 60 trials/day with feedback, but removed all but the final post-test. This group did not learn, indicating the tests without feedback facilitated improvement. Finally, to establish whether the benefit from these tests came from task performance, we trained another group 60 trials/day with feedback, but replaced the original ...

The acquisition of many perceptual skills proceeds over a course of days. However, little is known about how much daily training is needed for such learning to occur. Here we investigated this question by examining how varying the number of training trials per day affected learning over multiple days on two auditory discrimination tasks: frequency discrimination and temporal-interval discrimination. For each task, we compared improvements in discrimination thresholds between different groups of listeners who were trained for either 360 or 900 trials per day for 6 days. Improvement on frequency discrimination required >360 trials of training per day while learning on temporal-interval discrimination occurred with 360 training trials per day, and additional daily practice did not increase the amount of improvement. It therefore appears that the accumulation of improvement over days on auditory discrimination tasks may require some critical amount of training per day, that training beyond that critical amount yields no additional learning on the trained condition, and that the critical amount of training needed varies across tasks. These results imply that perceptual skills are transferred from short- to long-term memory (consolidated) daily, but only if a task-specific initiation requirement has been met.

Between 3 and 6 per cent of children who are otherwise unimpaired have extreme difficulties producing and understanding spoken language1. This disorder is typically labelled specific language impairment. Children diagnosed with specific language impairment often have accompanying reading difficulties (dyslexia)2, but not all children with reading difficulties have specific language impairment3. Some researchers claim that language impairment arises from failures specific to language or cognitive processing4–6. Others hold that language impairment results from a more elemental problem that makes affected children unable to hear the acoustic distinctions among successive brief sounds in speech7–11. Here we report the results of psycho-physical tests employing simple tones and noises showing that children with specific language impairment have severe auditory perceptual deficits for brief but not long tones in particular sound contexts. Our data support the view that language difficulties result from problems in auditory perception, and provide further information about the nature of these perceptual problems that should contribute to improving the diagnosis and treatment of language impairment and related disorders.

Language-based learning disorders such as dyslexia affect millions of people, but there is little agreement as to their cause. New evidence from behavioral measures of the ability to hear tones in the presence of background noise indicates that the brains of affected individuals develop more slowly than those of their unaffected counterparts. In addition, it seems that brain changes occurring at ≈10 years of age, presumably associated with puberty, may prematurely halt this slower-than-normal development when improvements would normally continue into adolescence. The combination of these ideas can account for a wide range of previous results, suggesting that delayed brain development, and its interaction with puberty, may be key factors contributing to learning problems.

A crucial aspect of the central nervous system is that it can be modified through experience. Such changes are thought to occur in two learning phases: acquisition—the actual period of training—and consolidation—a post-training period during which the acquired information is transferred to long-term memory. My coworkers and I have been addressing these principles in auditory perceptual learning by characterizing the factors that induce and those that prevent learning during the acquisition and consolidation phases. We also have been examining how these factors change during development and aging and are affected by hearing loss and other conditions that alter auditory perception. Application of these principles could improve clinical training strategies. Further, though learning is the foundation for behavioral rehabilitation, the capacity to learn can itself be impaired. Therefore, an individual’s response to perceptual training could be used as an objective, clinical measure to guide diagnosis and treat...

Exposure to an acoustic stimulus can facilitate learning when encountered near in time to practice on a perceptual task. Here, we explored the possibility that this learning enhancement arises in part because the stimulus exposures encountered in the absence of practice amplify the internal representation of the stimulus and this amplification then remains during the subsequent practice period. If this is the case, learning should be disrupted if the stimulus representation is instead suppressed during stimulus exposure without practice, because that suppression should also spread to the practice period. To test this idea, we trained listeners on a non-native phonetic-contrast categorization task using regimens in which a period of practice followed a period of stimulus exposure without practice in each daily session. We manipulated the extent to which listeners presumably suppressed the auditory stimuli that were presented without accompanying practice by varying the attentional demands of a visual task performed during their presentation. Learning decreased markedly as the attentional demand during these periods increased. Thus, it appears that the magnitude of the internal stimulus representation affects learning and that changes in this magnitude can spread beyond the time in which they are induced to promote or interfere with learning. [Work supported by NIH.]

Two of the primary cues used to localize the sources of sounds are interaural level differences (ILDs) and interaural time differences (ITDs). We conducted two experiments to explore how practice affects the human discrimination of values of ILDs and ongoing ITDs presented over headphones. We measured discrimination thresholds of 13 to 32 naive listeners in a variety of conditions during a pretest and again, 2 weeks later, during a posttest. Between those two tests, we trained a subset of listeners 1 h per day for 9 days on a single ILD or ITD condition. Listeners improved on both ILD and ITD discrimination. Improvement was initially rapid for both cue types and appeared to generalize broadly across conditions, indicating conceptual or procedural learning. A subsequent slower-improvement stage, which occurred solely for the ILD cue, only affected conditions with the trained stimulus frequency, suggesting that stimulus processing had fundamentally changed. These different learning patterns indicate that practice affects the attention to, or low-level encoding of, ILDs and ITDs at sites at which the two cue types are processed separately. Thus, these data reveal differences in the effect of practice on ILD and ITD discrimination, and provide insight into the encoding of these two cues to sound-source location in humans.

Observers with dyslexia show a significant increase in backward pattern masking, but the slopes of their psychometric functions do not differ from controls, suggesting a perceptual rather than a cognitive basis for this deficit.

Recent behavioral evidence supports the idea that some individuals with reading and language disorders are impaired in their perception of nonlinguistic auditory and visual information. More sophisticated measurement paradigms and analysis techniques are leading to a clearer understanding of these deficits and to possibilities for their remediation.

The neural representation of sensory events depends upon neural synchrony. Auditory neuropathy, a disorder of stimulus-timing-related neural synchrony, provides a model for studying the role of synchrony in auditory perception. This article presents electrophysiological and behavioral data from a rare case of auditory neuropathy in a woman with normal hearing thresholds, making it possible to separate audibility from neuropathy. The experimental results, which encompass a wide range of auditory perceptual abilities and neurophysiologic responses to sound, provide new information linking neural synchrony with auditory perception. Findings illustrate that optimal eighth nerve and auditory brainstem synchrony do not appear to be essential for understanding speech in quiet listening situations. However, synchrony is critical for understanding speech in the presence of noise.

This paper concerns the characterization of performance and perceptual learning of somatosensory interval discrimination. The purposes of this study were to define (1) the performance characteristics for interval discrimination in the somatosensory system by naive adult humans, (2) the normal capacities for improvement in somatosensory interval discrimination, and (3) the extent of generalization of interval discrimination learning. In a two-alternative forced choice procedure, subjects were presented with two pairs of vibratory pulses. One pair was separated in time by a fixed base interval; a second pair was separated by a target interval that was always longer than the base interval. Subjects indicated which pair was separated by the target interval. The length of the target interval was varied adaptively to determine discrimination thresholds. After initial determination of naive abilities, subjects were trained for 900 trials per day at base intervals of either 75 or 125 msec for 10–15 d. Significant improvements in thresholds resulted from training. Learning at the trained base interval generalized completely across untrained skin locations on the trained hand and to the corresponding untrained skin location in the contralateral hand. The learning partially generalized to untrained base intervals similar to the trained one, but not to more distant base intervals. Learning with somatosensory stimuli generalized to auditory stimuli presented at comparable base intervals. These results demonstrate temporal specificity in somatosensory interval discrimination learning that generalizes across skin location, hemisphere, and modality.

Individuals with reading disorders have difficulty reading despite having normal intelligence. Many also exhibit a number of auditory perceptual deficits. According to one proposal, these deficits arise because perceptual development in these individuals is delayed in childhood and then halted, for skills on which improvements would normally continue into adolescence, around age 10 (presumably due to brain changes associated with puberty). If so, perceptual skills with long developmental courses should be impaired in adults with reading disorders. While perceptual development is typically examined through naive performance, this prediction was tested here using a perceptual‐learning paradigm. Ten adults with reading disorders were trained on a temporal‐interval discrimination task using a multiple‐session training regimen known to yield learning reliably in adults, but rarely in adolescents [Huyck and Wright, Developmental Science (in press). Seven of the ten did not improve with training. Thus, the response to training in the adults with reading disorders more closely resembled that of adolescent than of adult controls. This result is consistent with the idea that perceptual development may be delayed and then halted in individuals with reading disorders and highlights prolonged maturational changes in a perceptual skill other than naive performance.

The effects of absolute and relative masker duration were investigated using one- and two-tone forward maskers. In the absolute-duration conditions, the duration (D) of a one-component on-frequency (M1; Dm1-alone) or off-frequency (M2; Dm2-alone) masker or the common duration of a two-component masker (Dm1 = Dm2) was varied. Masked threshold increased as duration increased in all three conditions. In the relative-duration conditions, the duration of one masker component was fixed at a relatively long value and the duration of the other component was varied (Dm1Dm2 or Dm2Dm1). In the Dm1Dm2 condition, masked threshold decreased as the duration of Dm2 was increased. In the Dm2Dm1 conditions, masked threshold increased and then decreased slightly as the duration of Dm1 was increased. Masked threshold was frequently lower in the M1+M2 than in the M1-alone conditions, indicating psychophysical suppression. Further, masked threshold in the Dm2Dm1 condition was always higher than in the corresponding absolute-duration condition (Dm1 = Dm2) having the same M1 duration, indicating masker enhancement. A two-stage neural model was proposed to account for these forward-masking data.

Delaying the onset of a signal relative to the onset of a simultaneous notched masker often improves the ability of subjects to ‘‘hear out’’ the signal at both threshold and suprathreshold levels. Viemeister and Bacon [J. Acoust. Soc. Am. 71, 1502–1507 (1982)] suggested that such signal‐enhancement effects might be attributable to adaptation of the suppression directed from the masker to the signal, thereby releasing the signal from suppression. In support of their hypothesis, Viemeister and Bacon reported that a masker, preceded by an enhancer having no component at the signal frequency, produced more forward masking than did the masker by itself. Here the masker enhancement described by Viemeister and Bacon, signal enhancement, and two‐tone suppression were measured in the same six subjects. Parametric manipulations of the masker‐enhancement stimulus produced results similar to those previously reported for parallel investigations of signal enhancement, indicating that the two types of enhancement are closely related effects. Although the present data reveal an inverse relationship between the amounts of suppression and enhancement, suggesting that the two processes may be interrelated, no support was obtained for the hypothesis that adaptation of suppression can account for enhancement.

The pattern of sound energy spread across frequency and time (spectro‐temporal modulation) is a crucial stimulus cue for sound identification. Improvements in the sensitivity to this modulation could potentially aid performance on numerous real‐world tasks, yet it is unknown how practice influences this sensitivity. To investigate this issue, normal‐hearing adults (n=8) were trained ∼1 h/day for 7 days to discriminate between noises with the same spectro‐temporal modulation drifting upward in audio frequency but with different modulation depths. Performance on the trained condition and on four untrained conditions was examined both before and after the training phase. Depth‐discrimination thresholds improved significantly on the trained condition and on an untrained condition with a downward‐drifting modulation. Discrimination thresholds also improved in subsets of listeners on untrained isolated component modulations (spectral or temporal) of the trained spectro‐temporal modulation. Finally, the ability ...

Recent evidence suggests that the development of naive performance on some auditory perceptual tasks can continue well into adolescence. Of interest here was whether there are sex differences in the maturation rate on three such tasks, temporal‐interval discrimination, tone detection in forward masking, and tone detection in backward masking. To investigate this issue, performance was compared between males and females in multiple age groups on these three tasks (n per sex in each age group=9–20). There were no sex differences for backward masking. However, males reached adult‐like performance earlier than females on the other two tasks: forward masking (males at ∼12 years, females at ∼15 years) and temporal‐interval discrimination (males at 14 years, females at >14 years, on each of three separate conditions). Surprisingly, the male advantage occurred only during adolescence. Thus, on two of the three late‐developing auditory abilities examined, males matured more quickly than females, but did not differ...

Masking sounds can be continuously present, gated simultaneously with the signal, or gated somewhat prior to the signal. This continuum of relative onset times was explored using waveforms of the sort commonly employed in studies of comodulation masking release (CMR). There was a 50‐Hz masker band centered on the 1250‐Hz tonal signal, and four 50‐Hz flanker bands centered at 850, 1050, 1450, and 1650 Hz. In some conditions, all four flanker bands had the same temporal envelope, and the masker band either had that same envelope (correlated presentations) or a different envelope (uncorrelated presentations). In other conditions, all five bands had different temporal envelopes (all‐uncorrelated presentations). The masker band and/or the four flanker bands were either gated nearly simultaneously with the signal (burst conditions) or were gated prior to the signal by a duration that was systematically varied (fringed conditions). The eight listeners could be partitioned into three groups on the basis of their response to these fringing manipulations. Two listeners (the large fringers) showed a gradual improvement in detectability with increasing fringe duration (called a temporal decline of masking), while three others (the small fringers) showed little improvement in detectability. For the remaining three subjects, there was evidence of a ‘‘learning’’ effect that changed them from large fringers to small fringers over a 10‐week period of listening. When present, the temporal decline of masking was greater for the correlated than for the uncorrelated comodulation condition; as a consequence, the difference in detectability between them (the comodulation masking release or CMR) increased with fringe duration. By fringing the masker and flanker bands separately and in combination, it was revealed that the temporal declines of masking were primarily attributable to the fringing of the flanker bands. In contrast, large CMRs required long fringes on both the masker and flanker bands. The above results were obtained with 50‐ms signals, but generally similar data were obtained with a signal duration of 240 ms. The difficulties raised for experimentalists and theorists by such long‐term practice effects are discussed.

The pattern of peaks and valleys of sound level spread across audio frequency (spectral modulation) is a key stimulus cue used for sound identification and localization. However, little is known about how practice can influence the ability to detect this modulation. Here we trained three separate groups of normal‐hearing adults to detect one of three different spectral modulations. Listeners practiced distinguishing a noise with a flat spectrum from one with a sinusoidal spectral modulation 1 hr/day for 7 days. We varied the modulation depth adaptively to determine the detection threshold. On average, the groups trained to detect 0.5 cyc/oct or 1 cyc/oct spectral modulation across a 200–1600 Hz carrier improved significantly, while the group trained to detect 2 cyc/oct spectral modulation across a 400–3200 Hz carrier did not. Further, it took multiple sessions for the 0.5‐cyc/oct group to reach optimal performance, but only one session for the 1‐cyc/oct group. Thus it appears that the current training had a smaller and more rapid effect as spectral modulation frequency increased from 0.5 to 2 cyc/oct (if we assume no influence of carrier). Pre‐training thresholds also decreased over this range, suggesting that there is less room to improve at higher spectral modulation frequencies.

Adults can learn to hear the phonetic distinctions of another language with practice. It is commonly assumed that such improvements require active performance of the target task throughout training. However, we have seen that, for other perceptual tasks, similar performance gains can be achieved by replacing some active‐performance trials with passive stimulus exposures. Here we examined the learning of a non‐native phonetic contrast using several training protocols that varied in the proportion of active and passive trials during the practice period. Native, monolingual English speakers were trained to identify a Thai phonetic contrast along a voice‐onset‐time continuum. A group given half active training and half passive exposures showed as much improvement in the category boundary as a group given active training for the entire practice period. The active‐passive group out‐performed a group given half active training and half training on an unrelated task with no passive exposures. Finally, a group giv...

Simultaneous and non‐simultaneous masking tasks are often introduced to listeners in terms of the veridical relative positions of the signal and masker. Here we report that the actual perceived temporal positions of the signal and masker do not always match those descriptions. We asked normal‐hearing adults about the perceived presentation time of a brief “signal” that was presented either before, during, or after a longer “masker.” The task was either to mark the perceived presentation time of the signal on a schematic diagram of the masker, or simply to judge whether the signal occurred inside or outside of the masker. Listeners tended to perceive the signals as having occurred later than they actually had. This temporal distortion was most marked for signals that were presented near the end of the masker. Signals that were presented 25–100 ms before the end of the masker were perceived as occurring outside of the masker 50% of the time. Similar results were obtained with different masker durations (150...

Individuals with hearing loss are poorer than normal‐hearing listeners at detecting patterns of peaks and valleys of sound level spread across audio frequency (spectral modulation detection)—an important skill for speech and music perception. In this preliminary report we examined whether listeners with hearing loss could improve on spectral modulation detection with practice. We trained older adults with sensorineural hearing loss (n=7) ∼1 hr/day for 7 days to distinguish a 400–3200 Hz noiseband with a flat‐spectrum from one with a 2 cyc/oct sinusoidal spectral modulation. Modulation depth was varied adaptively to determine the detection threshold. Their mean thresholds improved significantly from 18.9 to 12.8 dB. These listeners also improved at both a lower and a higher untrained spectral modulation frequency (1 and 4 cyc/oct), but showed no change on a measure of frequency selectivity. Interestingly, a separate group of younger normal‐hearing adults who received the same training (n=7) did not improve...

The separation of auditory events in time (temporal resolution) and the summation of these events over time (temporal integration) are both essential to normal hearing. Here, it is reported that both of these abilities are affected by stimulus bandwidth in naive listeners. In a single group of 30 naive listeners, amplitude‐modulation detection thresholds were determined over a range of modulation rates, and signal‐detection thresholds over a range of signal durations, using noises of two different bandwidths (0.25 kHz and 3.80 kHz), both with a 4.40‐kHz upper cutoff frequency. Amplitude‐modulation thresholds were adjusted mathematically to compensate for reductions in modulation depth introduced by filtering after modulation. The average time constants estimated from these data were significantly larger for the narrowband than the broadband noise for both temporal resolution (5.1 versus 1.3 ms; p

The ability to discriminate changes in interaural time differences (ITDs) and interaural level differences (ILDs) varies markedly both within and across individuals. Such variability has been frequently noted, but rarely systematically investigated. Here, both forms of variability, expressed in terms of coefficients of variation (mean divided by standard deviation), were examined across a variety of ITD and ILD conditions in a total of 84 naive listeners. Three consistent patterns emerged. First, across‐listener variability was relatively constant across ITD and ILD discrimination, suggesting that this variability might be determined by factors common to the processing of both cues. Second, compared to across‐listener variability, within‐listener variability was larger for the ITD conditions but smaller for the ILD conditions, suggesting that, within individuals, ITD processing may have a higher level of internal noise than ILD processing. Third, stimulus frequency (ranging from 0.3 to 6 kHz) and stimulus type (pure tones and sinusoidally amplitude modulated tones) had little influence on either type of variability, suggesting that variability was determined by cue‐dependent, rather than stimulus‐dependent mechanisms. The systematic nature of these variability patterns illustrates that examination of variability can provide important insights into the processing of interaural cues. [Work supported by NIH.]

The two primary cues to the location of sound sources on the horizontal plane are interaural level differences (ILDs) and interaural time differences (ITDs). The malleability of the processing of these two cues in humans was investigated by examining how multiday practice affects the discrimination of different values of ILDs and ongoing ITDs presented over headphones in adults with normal hearing. On average, the listeners improved on both ILD and ITD discrimination, but the learning patterns differed between the two cue types. Improvement was initially rapid for both cue types and appeared to generalize broadly across conditions, suggesting procedural learning. However, a subsequent slower‐improvement stage occurred solely for the ILD cue and showed some specificity to the stimulus used during training. Interestingly, for ILD discrimination, both the best and worst daily threshold estimates decreased with multiday training, indicating an improvement in fundamental processing capacity. In contrast, for ITD discrimination, the best threshold estimates remained unchanged but the worst decreased, suggesting an increased ability to access already existing capacities. One interpretation of these data is that training mediates the processing of ILDs and ITDs at a stage in the auditory pathway, where these two cues are processed separately. [Work supported by NIH/NIDCD.]

Improvement on most perceptual tasks requires active performance of the target task during training. However, there has been little consideration of whether that active performance is necessary throughout the entire practice period. Suggesting that it is not, there is recent evidence that active performance on a simple frequency‐discrimination task, interleaved with passive stimulus exposures, can yield more learning than the active performance alone. The current study examines whether these findings extend to a speech‐learning task, specifically foreign‐accent adaptation. Native‐English speakers transcribed sentences spoken in English by non‐native speakers. Participants who performed the target task for half the training period and received passive stimulus exposures while performing a distracter task for the other half out‐performed participants who received the same amount of active training, but no passive exposures during the distracter task, suggesting that passive exposures enhanced learning. The active‐passive group also out‐performed participants who received only passive exposures—and performed as well as a group who performed only the target task—for the entire training period. Taken together, these results suggest that the combination of active training and passive stimulus exposure may play a key role in real‐world perceptual learning for speech. [Work supported by NIH/NIDCD.]