Late maturation of auditory perceptual learning

Sequential maturation of stimulus-specific adaptation in the mouse lemniscal auditory system

Stimulus-specific adaptation (SSA), the reduction of neural activity to a common stimulus that does not generalize to other, rare stimuli, is an essential property of our brain. Although well characterized in adults, it is still unknown how it develops during adolescence and what neuronal circuits are involved. Using in vivo electrophysiology and optogenetics in the lemniscal pathway of the mouse auditory system, we observed SSA to be stable from postnatal day 20 (P20) in the inferior colliculus, to develop until P30 in the auditory thalamus and even later in the primary auditory cortex (A1). We found this maturation process to be experience-dependent in A1 but not in thalamus and to be related to alterations in deep but not input layers of A1. We also identified corticothalamic projections to be implicated in thalamic SSA development. Together, our results reveal different circuits underlying the sequential SSA maturation and provide a unique perspective to understand predictive coding and surprise across sensory systems.

Perceptual Learning of Dysarthria in Adolescence

PURPOSE

As evidenced by perceptual learning studies involving adult listeners and speakers with dysarthria, adaptation to dysarthric speech is driven by signal predictability (speaker property) and a flexible speech perception system (listener property). Here, we extend adaptation investigations to adolescent populations and examine whether adult and adolescent listeners can learn to better understand an adolescent speaker with dysarthria.

METHOD

Classified by developmental stage, adult (n = 42) and adolescent (n = 40) listeners completed a three-phase perceptual learning protocol (pretest, familiarization, and posttest). During pretest and posttest, all listeners transcribed speech produced by a 13-year-old adolescent with spastic dysarthria associated with cerebral palsy. During familiarization, half of the adult and adolescent listeners engaged in structured familiarization (audio and lexical feedback) with the speech of the adolescent speaker with dysarthria; and the other half, with the speech of a neurotypical adolescent speaker (control).

RESULTS

Intelligibility scores increased from pretest to posttest for all listeners. However, listeners who received dysarthria familiarization achieved greater intelligibility improvements than those who received control familiarization. Furthermore, there was a significant effect of developmental stage, where the adults achieved greater intelligibility improvements relative to the adolescents.

CONCLUSIONS

This study provides the first tranche of evidence that adolescent dysarthric speech is learnable-a finding that holds even for adolescent listeners whose speech perception systems are not yet fully developed. Given the formative role that social interactions play during adolescence, these findings of improved intelligibility afford important clinical implications.

Evidence that anterograde learning interference depends on the stage of learning of the interferer: blocked versus interleaved training

Training on one task (task A) can disrupt learning on a subsequently trained task (task B), illustrating anterograde learning interference. We asked whether the induction of anterograde learning interference depends on the learning stage that task A has reached when the training on task B begins. To do so, we drew on previous observations in perceptual learning in which completing all training on one task before beginning training on another task (blocked training) yielded markedly different learning outcomes than alternating training between the same two tasks for the same total number of trials (interleaved training). Those blocked versus interleaved contrasts suggest that there is a transition between two differentially vulnerable learning stages that is related to the number of consecutive training trials on each task, with interleaved training presumably tapping acquisition, and blocked training tapping consolidation. Here, we used the blocked versus interleaved paradigm in auditory perceptual learning in a case in which blocked training generated anterograde-but not its converse, retrograde-learning interference (A→B, not B←A). We report that anterograde learning interference of training on task A (interaural time difference discrimination) on learning on task B (interaural level difference discrimination) occurred with blocked training and diminished with interleaved training, with faster rates of interleaving leading to less interference. This pattern held for across-day, within-session, and offline learning. Thus, anterograde learning interference only occurred when the number of consecutive training trials on task A surpassed some critical value, consistent with other recent evidence that anterograde learning interference only arises when learning on task A has entered the consolidation stage.

Auditory processing remains sensitive to environmental experience during adolescence in a rodent model

Elevated neural plasticity during development contributes to dramatic improvements in perceptual, motor, and cognitive skills. However, malleable neural circuits are vulnerable to environmental influences that may disrupt behavioral maturation. While these risks are well-established prior to sexual maturity (i.e., critical periods), the degree of neural vulnerability during adolescence remains uncertain. Here, we induce transient hearing loss (HL) spanning adolescence in gerbils, and ask whether behavioral and neural maturation are disrupted. We find that adolescent HL causes a significant perceptual deficit that can be attributed to degraded auditory cortex processing, as assessed with wireless single neuron recordings and within-session population-level analyses. Finally, auditory cortex brain slices from adolescent HL animals reveal synaptic deficits that are distinct from those typically observed after critical period deprivation. Taken together, these results show that diminished adolescent sensory experience can cause long-lasting behavioral deficits that originate, in part, from a dysfunctional cortical circuit.

Effects of Gap Position on Perceptual Gap Detection Across Late Childhood and Adolescence

The ability to detect a silent gap within a sound is critical for accurate speech perception, and gap detection has been shown to have an extended developmental trajectory. In certain conditions, the detectability of the gap decreases as the gap is placed closer to the beginning of the signal. Early in development, the detection of gaps shortly after signal onset may be especially difficult due to immaturities in the encoding and perception of rapidly changing sounds. The present study explored the development of gap detection from age 8 to 19 years, specifically when the temporal placement of the gap varied. Performance improved with age for all temporal placements of the gap, demonstrating a gradual maturation of gap detection abilities throughout adolescence. Younger adolescents did not benefit from increasing gap onset times, while older adolescents' thresholds gradually improved as gap onset time lengthened. Regardless of age, listeners learned between the two testing days but did not improve within days. Younger adolescents had poorer thresholds for the last block of testing on the second day, returning to baseline performance despite learning between days. These data support earlier studies showing that gaps are harder to detect near stimulus onset and confirm that gap detection abilities continue to mature into adolescence. The data also suggest that younger adolescents do not receive the same benefit of increasing gap onset time and respond differently to repeated testing than older adolescents and young adults.

Long-Term Training-Induced Gains of an Auditory Skill in School-Age Children As Compared With Adults

The few studies that compared auditory skill learning between children and adults found variable results, with only some children reaching adult-like thresholds following training. The present study aimed to assess auditory skill learning in children as compared with adults during single- and multisession training. It was of interest to ascertain whether children who do not reach adult-like performance following a single training session simply require additional training, or whether different mechanisms underlying skill learning need to reach maturity in order to become adult-like performers. Forty children (7-9 years) and 45 young adults (18-35 years) trained in a single session. Of them, 20 children and 24 adults continued training for eight additional sessions. Each session included six frequency discrimination thresholds at 1000 Hz using adaptive forced-choice procedure. Retention of the learning-gains was tested 6 to 8 months posttraining. Results showed that (a) over half of the children presented similar performance and time course of learning as the adults. These children had better nonverbal reasoning and working memory abilities than their non-adult-like peers. (b) The best predicting factor for the outcomes of multisession training was a child's performance following one training session. (c) Performance gains were retained for all children with the non-adult-like children further improving, 6 to 8 months posttraining. Results suggest that mature auditory skill learning can emerge before puberty, provided that task-related cognitive mechanisms and task-specific sensory processing are already mature. Short-term training is sufficient, however, to reflect the maturity of these mechanisms, allowing the prediction of the efficiency of a prolonged training for a given child.

Neural Variability Limits Adolescent Skill Learning

Skill learning is fundamental to the acquisition of many complex behaviors that emerge during development. For example, years of practice give rise to perceptual improvements that contribute to mature speech and language skills. While fully honed learning skills might be thought to offer an advantage during the juvenile period, the ability to learn actually continues to develop through childhood and adolescence, suggesting that the neural mechanisms that support skill learning are slow to mature. To address this issue, we asked whether the rate and magnitude of perceptual learning varies as a function of age as male and female gerbils trained on an auditory task. Adolescents displayed a slower rate of perceptual learning compared with their young and mature counterparts. We recorded auditory cortical neuron activity from a subset of adolescent and adult gerbils as they underwent perceptual training. While training enhanced the sensitivity of most adult units, the sensitivity of many adolescent units remained unchanged, or even declined across training days. Therefore, the average rate of cortical improvement was significantly slower in adolescents compared with adults. Both smaller differences between sound-evoked response magnitudes and greater trial-to-trial response fluctuations contributed to the poorer sensitivity of individual adolescent neurons. Together, these findings suggest that elevated sensory neural variability limits adolescent skill learning.SIGNIFICANCE STATEMENT The ability to learn new skills emerges gradually as children age. This prolonged development, often lasting well into adolescence, suggests that children, teens, and adults may rely on distinct neural strategies to improve their sensory and motor capabilities. Here, we found that practice-based improvement on a sound detection task is slower in adolescent gerbils than in younger or older animals. Neural recordings made during training revealed that practice enhanced the sound sensitivity of adult cortical neurons, but had a weaker effect in adolescents. This latter finding was partially explained by the fact that adolescent neural responses were more variable than in adults. Our results suggest that one mechanistic basis of adult-like skill learning is a reduction in neural response variability.

Development of unfamiliar accent comprehension continues through adolescence

School-age children's understanding of unfamiliar accents is not adult-like and the age at which this ability fully matures is unknown. To address this gap, eight- to fifteen-year-old children's (n = 74) understanding of native- and non-native-accented sentences in quiet and noise was assessed. Children's performance was adult-like by eleven to twelve years for the native accent in noise and by fourteen to fifteen years for the non-native accent in quiet. However, fourteen- to fifteen-year old's performance was not adult-like for the non-native accent in noise. Thus, adult-like comprehension of unfamiliar accents may require greater exposure to linguistic variability or additional cognitive-linguistic growth.

Development of perception and perceptual learning for multi-timescale filtered speech

The perception of temporally changing auditory signals has a gradual developmental trajectory. Speech is a time-varying signal, and slow changes in speech (filtered at 0-4 Hz) are preferentially processed by the right hemisphere, while the left extracts faster changes (filtered at 22-40 Hz). This work examined the ability of 8- to 19-year-olds to both perceive and learn to perceive filtered speech presented diotically for each filter type (low vs high) and dichotically for preferred or non-preferred laterality. Across conditions, performance improved with increasing age, indicating that the ability to perceive filtered speech continues to develop into adolescence. Across age, performance was best when both bands were presented dichotically, but with no benefit for presentation to the preferred hemisphere. Listeners thus integrated slow and fast transitions between the two ears, benefitting from more signal information, but not in a hemisphere-specific manner. After accounting for potential ceiling effects, learning was greatest when both bands were presented dichotically. These results do not support the idea that cochlear implants could be improved by providing differentially filtered information to each ear. Listeners who started with poorer performance learned more, a factor which could contribute to the positive cochlear implant outcomes typically seen in younger children.

Comprehension of Degraded Speech Matures During Adolescence

PURPOSE

The aim of the study was to compare comprehension of spectrally degraded (noise-vocoded [NV]) speech and perceptual learning of NV speech between adolescents and young adults and examine the role of phonological processing and executive functions in this perception.

METHOD

Sixteen younger adolescents (11-13 years), 16 older adolescents (14-16 years), and 16 young adults (18-22 years) listened to 40 NV sentences and repeated back what they heard. They also completed tests assessing phonological processing and a variety of executive functions.

RESULTS

Word-report scores were generally poorer for younger adolescents than for the older age groups. Phonological processing also predicted initial word-report scores. Learning (i.e., improvement across training times) did not differ with age. Starting performance and processing speed predicted learning, with greater learning for those who started with the lowest scores and those with faster processing speed.

CONCLUSIONS

Degraded (NV) speech comprehension is not mature even by early adolescence; however, like adults, adolescents are able to improve their comprehension of degraded speech with training. Thus, although adolescents may have initial difficulty in understanding degraded speech or speech as presented through hearing aids or cochlear implants, they are able to improve their perception with experience. Processing speed and phonological processing may play a role in degraded speech comprehension in these age groups.

Is the Role of External Feedback in Auditory Skill Learning Age Dependent?

PURPOSE

The purpose of this study is to investigate the role of external feedback in auditory perceptual learning of school-age children as compared with that of adults.

METHOD

Forty-eight children (7-9 years of age) and 64 adults (20-35 years of age) conducted a training session using an auditory frequency discrimination (difference limen for frequency) task, with external feedback (EF) provided for half of them.

RESULTS

Data supported the following findings: (a) Children learned the difference limen for frequency task only when EF was provided. (b) The ability of the children to benefit from EF was associated with better cognitive skills. (c) Adults showed significant learning whether EF was provided or not. (d) In children, within-session learning following training was dependent on the provision of feedback, whereas between-sessions learning occurred irrespective of feedback.

CONCLUSIONS

EF was found beneficial for auditory skill learning of 7-9-year-old children but not for young adults. The data support the supervised Hebbian model for auditory skill learning, suggesting combined bottom-up internal neural feedback controlled by top-down monitoring. In the case of immature executive functions, EF enhanced auditory skill learning. This study has implications for the design of training protocols in the auditory modality for different age groups, as well as for special populations.

Pitch and Duration Pattern Sequence Tests in 7- to 11-Year-Old Children: Results Depend on Response Mode

BACKGROUND

Pitch pattern sequence (PPS) and duration pattern sequence (DPS) tests are frequently used in the assessment of auditory processing disorder. Current recommendations suggest alternate, interchangeable modes for responding to stimuli.

PURPOSE

The objective of the study is to evaluate the influence of response mode (i.e., humming, pointing, and labeling) and age on PPS and DPS performance of 7- to 11-year-old children.

RESEARCH DESIGN

Laboratory-based testing of school children. Cross-sectional comparison of age, with repeated measures of age, test, ear, and response mode.

STUDY SAMPLE

From 452 children recruited, 228 right-handed children (109 girls) aged 7 years to 11 years 11 months (mean age 9 years 4 months) completed at least one test (PPS: 211, DPS: 198), and 181 children completed both tests. Audiology inclusion criteria include normal hearing thresholds (≤15 dB HL at octave frequencies 250-8000 Hz); word recognition in quiet ≥92%; tympanogram peak compensated static acoustic compliance 0.4-1.6 mmhos; and tympanometric peak pressure -100 to +50 daPa, all in both ears. Other inclusion criteria were Portuguese as first language; right handed; no musical training; no related, known, or observed phonological, learning, neurologic, psychiatric, or behavioral disorder; otologic history; and delayed neuropsychomotor or language development.

DATA COLLECTION AND ANALYSIS

PPS: 30 trials per ear and response condition of three consecutive 500 msec duration intermixed high (1430 Hz) or low (880 Hz) frequency tones presented monaurally at 50 dB HL. The first response condition was humming followed by labeling (naming: high or low). DPS: As per PPS except 1000 Hz tones of intermixed 500 (long) and 250 msec (short) duration. First response was pointing (at a symbolic object) followed by labeling. Trends across age and between tests were assessed using repeated measures generalized linear mixed models. Correlation coefficients were calculated to assess relations among test scores. The two-sided significance level was 0.05.

RESULTS

Older children performed better than younger children in all tasks. Humming the tone pattern (PPS humming) produced generally better performance than either articulating the attributes of the tones (labeling) or pointing to objects representing tone duration. PPS humming produced ceiling performance for many children of all ages. For both labeling tasks and DPS pointing, performance was better on the PPS than on the DPS, for stimulation of the right than the left ear, and in boys than girls. Individual performance on the two tasks was highly correlated.

CONCLUSIONS

Response mode does matter in the PPS and DPS. Results from humming should not be combined with or be a substitute for results obtained from a labeling response. Tasks that rely on labeling a tonal stimulus should be avoided in testing hearing in children or other special populations.

Disruption of Perceptual Learning by a Brief Practice Break

Some forms of associative learning require only a single experience to create a lasting memory [1, 2]. In contrast, perceptual learning often requires extensive practice within a day for performance to improve across days [3, 4]. This suggests that the requisite practice for durable perceptual learning is integrated throughout each day. If the total amount of daily practice is the only important variable, then a practice break within a day should not disrupt across-day improvement. To test this idea, we trained human listeners on an auditory frequency-discrimination task over multiple days and compared the performance of those who engaged in a single continuous practice session each day [4] with those who were given a 30-min break halfway through each practice session. Continuous practice yielded significant perceptual learning [4]. In contrast, practice with a rest break led to no improvement, indicating that the integration process had decayed within 30 min. In a separate experiment, a 30-min practice break also disrupted durable learning on a non-native phonetic classification task. These results suggest that practice trials are integrated up to a learning threshold within a transient memory store before they are sent en masse into a memory that lasts across days. Thus, the oft cited benefits of distributed over massed training [5, 6] may arise from different mechanisms depending on whether the breaks occur before or after a learning threshold has been reached. Trial integration could serve as an early gatekeeper to plasticity, helping to ensure that longer-lasting changes are only made when deemed worthwhile.

Transient sex differences during adolescence on auditory perceptual tasks

Many perceptual abilities differ between the sexes. Because these sex differences have been documented almost exclusively in adults, they have been attributed to sex-specific neural circuitry that emerges during development and is maintained in the mature perceptual system. To investigate whether behavioral sex differences in perception can also have other origins, we compared performance between males and females ranging in age from 8 to 30 years on auditory temporal-interval discrimination and tone-in-noise detection tasks on which there are no sex differences in adults. If sex differences in perception arise only from the establishment and subsequent maintenance of sex-specific neural circuitry, there should be no sex differences during development on these tasks. In contrast, sex differences emerged in adolescence but resolved by adulthood on two of the six conditions, with signs of a similar pattern on a third condition. In each case, males reached mature performance earlier than females, resulting in a sex difference in the interim. These results suggest that sex differences in perception may arise from differences in the maturational timing of common circuitry used by both sexes. They also imply that sex differences in perceptual abilities may be more prevalent than previously thought based on adult data alone.

Rapid auditory learning of temporal gap detection

The rapid initial phase of training-induced improvement has been shown to reflect a genuine sensory change in perception. Several features of early and rapid learning, such as generalization and stability, remain to be characterized. The present study demonstrated that learning effects from brief training on a temporal gap detection task using spectrally similar narrowband noise markers defining the gap (within-channel task), transfer across ears, however, not across spectrally dissimilar markers (between-channel task). The learning effects associated with brief training on a gap detection task were found to be stable for at least a day. These initial findings have significant implications for characterizing early and rapid learning effects.

Benefits of Stimulus Exposure: Developmental Learning Independent of Task Performance

Perceptual learning (training-induced performance improvement) can be elicited by task-irrelevant stimulus exposure in humans. In contrast, task-irrelevant stimulus exposure in animals typically disrupts perception in juveniles while causing little to no effect in adults. This may be due to the extent of exposure, which is brief in humans while chronic in animals. Here we assessed the effects of short bouts of passive stimulus exposure on learning during development in gerbils, compared with non-passive stimulus exposure (i.e., during testing). We used prepulse inhibition of the acoustic startle response, a method that can be applied at any age, to measure gap detection thresholds across four age groups, spanning development. First, we showed that both gap detection thresholds and gap detection learning across sessions displayed a long developmental trajectory, improving throughout the juvenile period. Additionally, we demonstrated larger within- and across-animal performance variability in younger animals. These results are generally consistent with results in humans, where there are extended developmental trajectories for both the perception of temporally-varying signals, and the effects of perceptual training, as well as increased variability and poorer performance consistency in children. We then chose an age (mid-juveniles) that displayed clear learning over sessions in order to assess effects of brief passive stimulus exposure on this learning. We compared learning in mid-juveniles exposed to either gap detection testing (gaps paired with startles) or equivalent gap exposure without testing (gaps alone) for three sessions. Learning was equivalent in both these groups and better than both naïve age-matched animals and controls receiving no gap exposure but only startle testing. Thus, short bouts of exposure to gaps independent of task performance is sufficient to induce learning at this age, and is as effective as gap detection testing.

A Developmental Perspective in Learning the Mirror-Drawing Task

Is there late maturation of skill learning? This notion has been raised to explain an adult advantage in learning a variety of tasks, such as auditory temporal-interval discrimination, locomotion adaptation, and drawing visually-distorted spatial patterns (mirror-drawing, MD). Here, we test this assertion by following the practice of the MD task in two 5 min daily sessions separated by a 10 min break, over the course of 2 days, in 5–6-year-old kindergarten children, 7–8-year-old second-graders, and young adults. In the MD task, participants were required to trace a square while looking at their hand only as a reflection in a mirror. Kindergarteners did not show learning of the visual-motor mapping, and on average, did not produce even one full side of a square correctly. Second-graders showed increased online movement control with longer strokes, and robust learning of the visual-motor mapping, resulting in a between-day increase in the number of correctly drawn sides with no loss in accuracy. Overall, kindergarteners and second-graders producing at least one correct polygon-side on Day 1 were more likely to improve their performance between days. Adults showed better performance with improvements in the number of correctly drawn sides between- and within-days, and in accuracy between days. It has been suggested that 5-year-olds cannot learn the task due to their inability to detect and encapsulate previously produced accurate movements. Our findings suggest, instead, that these children lacked initial, accurate performance that could be enhanced through training. Recently, it has been shown that in a simple grapho-motor task the three age-groups improved their speed of performance within a session and between-days, while maintaining accuracy scores. Taken together, these data suggest that children’s motor skill learning depends on the task’s characteristics and their adopting an efficient and mature performance strategy enabling initial success that can be improved through training.

The role of age and executive function in auditory category learning

Auditory categorization is a natural and adaptive process that allows for the organization of high-dimensional, continuous acoustic information into discrete representations. Studies in the visual domain have identified a rule-based learning system that learns and reasons via a hypothesis-testing process that requires working memory and executive attention. The rule-based learning system in vision shows a protracted development, reflecting the influence of maturing prefrontal function on visual categorization. The aim of the current study was twofold: (a) to examine the developmental trajectory of rule-based auditory category learning from childhood through adolescence and into early adulthood and (b) to examine the extent to which individual differences in rule-based category learning relate to individual differences in executive function. A sample of 60 participants with normal hearing-20 children (age range=7-12years), 21 adolescents (age range=13-19years), and 19 young adults (age range=20-23years)-learned to categorize novel dynamic "ripple" sounds using trial-by-trial feedback. The spectrotemporally modulated ripple sounds are considered the auditory equivalent of the well-studied "Gabor" patches in the visual domain. Results reveal that auditory categorization accuracy improved with age, with young adults outperforming children and adolescents. Computational modeling analyses indicated that the use of the task-optimal strategy (i.e., a conjunctive rule-based learning strategy) improved with age. Notably, individual differences in executive flexibility significantly predicted auditory category learning success. The current findings demonstrate a protracted development of rule-based auditory categorization. The results further suggest that executive flexibility coupled with perceptual processes play important roles in successful rule-based auditory category learning.

Auditory Learning in Children With Cochlear Implants

PURPOSE

The purpose of this study was to examine and characterize the training-induced changes in speech-in-noise perception in children with congenital deafness who have cochlear implants (CIs).

METHOD

Twenty-seven children with congenital deafness who have CIs were studied. Eleven children with CIs were trained on a speech-in-noise task, number recognition in white noise, at home for 5 weeks (total 40 hr). Speech recognition thresholds (SRTs) in the trained, partially trained (numbers in speech-shaped noise), and untrained (digit triplets in speech-shaped noise) conditions were measured before, immediately after, and 3 weeks after training completion. Data were also collected from children (n = 13) and adults (n = 5) with normal hearing for comparison.

RESULTS

Analyses indicated that following training, the performance of children with CIs improved for all speech-in-noise tasks (∆SRT was approximately 3 dB). Training-induced improvements in speech-in-noise performance were retained for 3 weeks following cessation of training. Untrained children with CIs showed no such improvements. The performance of children with CIs, even after intensive training, was significantly lower than children and adults with normal hearing.

CONCLUSIONS

Training enhances speech-in-noise performance for children with congenital deafness who have CIs. Learning effects were stable and generalized to similar but untrained conditions. Current findings are encouraging for the consideration of home-based auditory training to be included in the pediatric CI habilitation programs.

Learning of a simple grapho-motor task by young children and adults: similar acquisition but age-dependent retention

Many new skills are acquired during early childhood. Typical laboratory skill learning tasks are not applicable for developmental studies that involve children younger than 8 years of age. It is not clear whether young children and adults share a basic underlying skill learning mechanism. In the present study, the learning and retention of a simple grapho-motor pattern were studied in three age groups: 5-6, 7-8, and 19-29 years. Each block of the task consists of identical patterns arranged in a spaced writing array. Progression across the block involves on-page movements while producing the pattern, and off-page movements between patterns. The participants practiced the production of the pattern using a digitizing tablet and were tested at 24 h and 2 weeks post-practice. All age groups produced the task blocks more quickly with practice, and the learning rate was inversely related to the initial production time. All groups exhibited additional gains 24 h post-practice that were well-retained 2 weeks later. The accuracy of the participants was maintained throughout the 2-weeks period. These findings suggest that young children and young adults use a similar mechanism when learning the task. Nevertheless, the 6-years-old spent more time off-page during retention testing than when tested at 24 h post-practice, thus supporting the notion that an age advantage may exists in the long-term retention of skills due to planning-dependent aspects.

Sources of pathology underlying listening disorders in children

Some children referred to audiology and developmental disability services have listening difficulties, despite normal audiograms. These children may be tested for 'auditory processing disorder' (APD), a controversial construct suggesting that neural dysfunction in the central auditory system leads to impaired auditory perception. An important question, not currently tested in clinical evaluation, is whether listening difficulties result from problems with bottom-up auditory sensory processing or top-down modulating cognition. Perceptual variability and poor performance on standardized tests suggest that listening difficulties are primarily cognitive in origin. However, evidence for impaired olivocochlear function and temporal processing deficits may implicate peripheral or central auditory dysfunction in some cases. Wide-spread, top-down modulation of auditory cortical, brainstem and ear function suggests that afferent and efferent control systems may not be simple to segregate. During normal maturation, hearing appears to develop in proportion to the complexity of both stimuli and tasks. But some younger individuals have mature hearing, highlighting individual differences that suggest APD may be due to a generalized developmental delay. Recent studies have investigated specific hypotheses showing, for example, that spatial hearing and executive function are compromised in some children with listening difficulties. Using speech stimuli (e.g. consonant-vowel syllables) to examine auditory brainstem responses, and psychophysiological relations between dichotic hearing and cortical physiology, various effects of auditory experience and development point the way to promising approaches for further studies of APD. Newer technology, from genetic sequencing to MRI, may have the sensitivity to test whether and how frequently APD is associated with impaired processing in the auditory system.

Auditory training during development mitigates a hearing loss-induced perceptual deficit

Sensory experience during early development can shape the central nervous system and this is thought to influence adult perceptual skills. In the auditory system, early induction of conductive hearing loss (CHL) leads to deficits in central auditory coding properties in adult animals, and this is accompanied by diminished perceptual thresholds. In contrast, a brief regimen of auditory training during development can enhance the perceptual skills of animals when tested in adulthood. Here, we asked whether a brief period of training during development could compensate for the perceptual deficits displayed by adult animals reared with CHL. Juvenile gerbils with CHL, and age-matched controls, were trained on a frequency modulation (FM) detection task for 4 or 10 days. The performance of each group was subsequently assessed in adulthood, and compared to adults with normal hearing (NH) or adults raised with CHL that did not receive juvenile training. We show that as juveniles, both CHL and NH animals display similar FM detection thresholds that are not immediately impacted by the perceptual training. However, as adults, detection thresholds and psychometric function slopes of these animals were significantly improved. Importantly, CHL adults with juvenile training displayed thresholds that approached NH adults. Additionally, we found that hearing impaired animals trained for 10 days displayed adult thresholds closer to untrained adults than those trained for 4 days. Thus, a relatively brief period of auditory training may compensate for the deleterious impact of hearing deprivation on auditory perception on the trained task.

Learning, worsening, and generalization in response to auditory perceptual training during adolescence

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.

The neural basis of non-native speech perception in bilingual children

The goal of the present study is to reveal how the neural mechanisms underlying non-native speech perception change throughout childhood. In a pre-attentive listening fMRI task, English monolingual and Spanish-English bilingual children - divided into groups of younger (6-8yrs) and older children (9-10yrs) - were asked to watch a silent movie while several English syllable combinations played through a pair of headphones. Two additional groups of monolingual and bilingual adults were included in the analyses. Our results show that the neural mechanisms supporting speech perception throughout development differ in monolinguals and bilinguals. While monolinguals recruit perceptual areas (i.e., superior temporal gyrus) in early and late childhood to process native speech, bilinguals recruit perceptual areas (i.e., superior temporal gyrus) in early childhood and higher-order executive areas in late childhood (i.e., bilateral middle frontal gyrus and bilateral inferior parietal lobule, among others) to process non-native speech. The findings support the Perceptual Assimilation Model and the Speech Learning Model and suggest that the neural system processes phonological information differently depending on the stage of L2 speech learning.

Different patterns of perceptual learning on spectral modulation detection between older hearing-impaired and younger normal-hearing adults

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.

Perceptual acclimatization post nonlinear frequency compression hearing aid fitting in older children

PURPOSE

In this study, the authors evaluated the effect of frequency compression hearing aids on speech perception ability and the time course and magnitude of acclimatization-related changes.

METHOD

Participants included children ages 11-18 years. Speech perception ability was evaluated over well-controlled baseline, treatment, and withdrawal study phases. Study-worn hearing aids were individually fitted to all participants. The authors evaluated speech perception ability using outcomes of speech detection (/s/ and /[symbol in text]/ sounds), /s-[symbol in text]/ discrimination, and plural and consonant recognition.

RESULTS

Indices of change were discussed on a case-by-case basis across all study phases. Significant treatment effects were measured for all cases, on at least one measure, with some listeners displaying significant acclimatization trends following a trial of frequency compression.

CONCLUSION

Findings suggest that frequency compression provided varying outcomes, both in benefit and acclimatization, across listeners. For some, a period of acclimatization was necessary before change could be measured. For others, performance remained stable over the time course under evaluation, suggesting that some but not all children will experience improved speech recognition ability after a period of frequency compression hearing aid use.

Auditory sequence analysis and phonological skill

This work tests the relationship between auditory and phonological skill in a non-selected cohort of 238 school students (age 11) with the specific hypothesis that sound-sequence analysis would be more relevant to phonological skill than the analysis of basic, single sounds. Auditory processing was assessed across the domains of pitch, time and timbre; a combination of six standard tests of literacy and language ability was used to assess phonological skill. A significant correlation between general auditory and phonological skill was demonstrated, plus a significant, specific correlation between measures of phonological skill and the auditory analysis of short sequences in pitch and time. The data support a limited but significant link between auditory and phonological ability with a specific role for sound-sequence analysis, and provide a possible new focus for auditory training strategies to aid language development in early adolescence.

Delayed retention of new word-forms is better in children than adults regardless of language ability: a factorial two-way study

Background

Nonword repetition, the ability to retain and repeat unfamiliar sequences of phonemes is usually impaired in children with specific language impairment (SLI), but it is unclear whether this explains slow language learning. Traditional nonword repetition tests involve a single presentation of nonwords for immediate repetition. Here we considered whether rate of learning of novel phonological sequences was impaired when the same items were presented repeatedly.

Methodology/Principal Findings

Three complex nonwords were each presented for repetition five times in two sessions (A and B) separated by one hour. We studied both adults and children from (i) families with a child with SLI and (ii) families whose children did not have SLI. This gave a 2×2 design with familial SLI as one factor, and age (up to or above 18 years) as the other. Overall, participants from families with SLI were poorer at nonword repetition than their peers from typical-language families, and there was a trend for children with SLI to show less within-session learning than typically developing children. However, between-session retention, measured as the difference between the last trial from session 1 and the first trial of session 2, showed a significant age effect, η² = .139, p = .004, regardless of family SLI status. Adult participants showed a decrease in score from the last trial of session A to the first trial of session B, whereas children maintained their level of performance, regardless of whether or not they had SLI.

Conclusions/Significance

Poor nonword repetition in SLI appears to reflect inadequate encoding of phonological information, rather than problems retaining encoded information. Furthermore, the nonword learning task is consistent with the notion of a sensitive period in language learning: Children show better retention over a delay for new phonological sequences than adults, regardless of overall level of language ability.

Lack of generalization of auditory learning in typically developing children

PURPOSE

To understand the components of auditory learning in typically developing children by assessing generalization across stimuli, across modalities (i.e., hearing, vision), and to higher level language tasks.

METHOD

Eighty-six 8- to 10-year-old typically developing children were quasi-randomly assigned to 4 groups. Three of the groups received twelve 30-min training sessions on multiple standards using either an auditory frequency discrimination task (AFD group), auditory phonetic discrimination task (PD group), or visual frequency discrimination task (VFD group) over 4 weeks. The 4th group, which was the no-intervention control (NI) group, did not receive any training. Thresholds on all tasks (AFD, PD, and VFD) were assessed immediately before and after training, along with performance on a battery of language assessments.

RESULTS

Relative to the other groups, both the AFD group and the PD group, but not the VFD group, showed significant learning on the stimuli upon which they were trained. However, in those instances where learning was observed, it did not generalize to the nontrained stimuli or to the language assessments.

CONCLUSIONS

Nonspeech (AFD) or speech (PD) discrimination training can lead to auditory learning in typically developing children of this age range. However, this learning does not always generalize across stimuli or tasks, across modalities, or to higher level measures of language ability.

A behavioral framework to guide research on central auditory development and plasticity

The auditory CNS is influenced profoundly by sounds heard during development. Auditory deprivation and augmented sound exposure can each perturb the maturation of neural computations as well as their underlying synaptic properties. However, we have learned little about the emergence of perceptual skills in these same model systems, and especially how perception is influenced by early acoustic experience. Here, we argue that developmental studies must take greater advantage of behavioral benchmarks. We discuss quantitative measures of perceptual development and suggest how they can play a much larger role in guiding experimental design. Most importantly, including behavioral measures will allow us to establish empirical connections among environment, neural development, and perception.

The cost and benefit of juvenile training on adult perceptual skill

Sensory experience during development can modify the CNS and alter adult perceptual skills. While this principle draws support from deprivation or chronic stimulus exposure studies, the effect of training is addressed only in adults. Here, we asked whether a brief period of training during development can exert a unique impact on adult perceptual skills. Juvenile gerbils were trained to detect amplitude modulation (AM), a stimulus feature elemental to animal communication sounds. When the performance of these juvenile-trained animals was subsequently assessed in adulthood, it was superior to a control group that received an identical regimen of training as adults. The juvenile-trained animals displayed significantly better AM detection thresholds. This was not observed in an adult group that received only exposure to AM stimuli as juveniles. To determine whether enhanced adult performance was due solely to learning the conditioned avoidance procedure, juveniles were trained on frequency modulation (FM) detection, and subsequently assessed on AM detection as adults. These animals displayed significantly poorer AM detection thresholds than all other groups. Thus, training on a specific auditory task (AM detection) during development provided a benefit to performance on that task in adulthood, whereas an identical training regimen in adulthood did not bring about this enhancement. In contrast, there was a cost, in adulthood, following developmental training on a different task (FM detection). Together, the results demonstrate a period of heightened sensitivity in the developing CNS such that behavioral training in juveniles has a unique impact on adult behavioral capabilities.

Separable developmental trajectories for the abilities to detect auditory amplitude and frequency modulation

Amplitude modulation (AM) and frequency modulation (FM) are inherent components of most natural sounds. The ability to detect these modulations, considered critical for normal auditory and speech perception, improves over the course of development. However, the extent to which the development of AM and FM detection skills follow different trajectories, and therefore can be attributed to the maturation of separate processes, remains unclear. Here we explored the relationship between the developmental trajectories for the detection of sinusoidal AM and FM in a cross-sectional design employing children aged 8-10 and 11-12 years and adults. For FM of tonal carriers, both average performance (mean) and performance consistency (within-listener standard deviation) were adult-like in the 8-10 y/o. In contrast, in the same listeners, average performance for AM of wideband noise carriers was still not adult-like in the 11-12 y/o, though performance consistency was already mature in the 8-10 y/o. Among the children there were no significant correlations for either measure between the degrees of maturity for AM and FM detection. These differences in developmental trajectory between the two modulation cues and between average detection thresholds and performance consistency suggest that at least partially distinct processes may underlie the development of AM and FM detection as well as the abilities to detect modulation and to do so consistently.