Discrimination Of Speech Sounds Research Articles

Atypical central auditory speech-sound discrimination in children who stutter as indexed by the mismatch negativity

PurposeRecent theoretical conceptualizations suggest that disfluencies in stuttering may arise from several factors, one of them being atypical auditory processing. The main purpose of the present study was to investigate whether speech sound encoding and central auditory discrimination, are affected in children who stutter (CWS). MethodsParticipants were 10 CWS, and 12 typically developing children with fluent speech (TDC). Event-related potentials (ERPs) for syllables and syllable changes [consonant, vowel, vowel-duration, frequency (F0), and intensity changes], critical in speech perception and language development of CWS were compared to those of TDC. ResultsThere were no significant group differences in the amplitudes or latencies of the P1 or N2 responses elicited by the standard stimuli. However, the Mismatch Negativity (MMN) amplitude was significantly smaller in CWS than in TDC. For TDC all deviants of the linguistic multifeature paradigm elicited significant MMN amplitudes, comparable with the results found earlier with the same paradigm in 6-year-old children. In contrast, only the duration change elicited a significant MMN in CWS. ConclusionsThe results showed that central auditory speech-sound processing was typical at the level of sound encoding in CWS. In contrast, central speech-sound discrimination, as indexed by the MMN for multiple sound features (both phonetic and prosodic), was atypical in the group of CWS. Findings were linked to existing conceptualizations on stuttering etiology.Educational objectives: The reader will be able (a) to describe recent findings on central auditory speech-sound processing in individuals who stutter, (b) to describe the measurement of auditory reception and central auditory speech-sound discrimination, (c) to describe the findings of central auditory speech-sound discrimination, as indexed by the mismatch negativity (MMN), in children who stutter.

Computational validation of the motor contribution to speech perception.

Action perception and recognition are core abilities fundamental for human social interaction. A parieto-frontal network (the mirror neuron system) matches visually presented biological motion information onto observers' motor representations. This process of matching the actions of others onto our own sensorimotor repertoire is thought to be important for action recognition, providing a non-mediated "motor perception" based on a bidirectional flow of information along the mirror parieto-frontal circuits. State-of-the-art machine learning strategies for hand action identification have shown better performances when sensorimotor data, as opposed to visual information only, are available during learning. As speech is a particular type of action (with acoustic targets), it is expected to activate a mirror neuron mechanism. Indeed, in speech perception, motor centers have been shown to be causally involved in the discrimination of speech sounds. In this paper, we review recent neurophysiological and machine learning-based studies showing (a) the specific contribution of the motor system to speech perception and (b) that automatic phone recognition is significantly improved when motor data are used during training of classifiers (as opposed to learning from purely auditory data).

Speech sound processing deficits and training-induced neural plasticity in rats with dyslexia gene knockdown.

In utero RNAi of the dyslexia-associated gene Kiaa0319 in rats (KIA-) degrades cortical responses to speech sounds and increases trial-by-trial variability in onset latency. We tested the hypothesis that KIA- rats would be impaired at speech sound discrimination. KIA- rats needed twice as much training in quiet conditions to perform at control levels and remained impaired at several speech tasks. Focused training using truncated speech sounds was able to normalize speech discrimination in quiet and background noise conditions. Training also normalized trial-by-trial neural variability and temporal phase locking. Cortical activity from speech trained KIA- rats was sufficient to accurately discriminate between similar consonant sounds. These results provide the first direct evidence that assumed reduced expression of the dyslexia-associated gene KIAA0319 can cause phoneme processing impairments similar to those seen in dyslexia and that intensive behavioral therapy can eliminate these impairments.

Attention Fine-Tunes Auditory–Motor Processing of Speech Sounds

The earliest stages of cortical processing of speech sounds take place in the auditory cortex. Transcranial magnetic stimulation (TMS) studies have provided evidence that the human articulatory motor cortex contributes also to speech processing. For example, stimulation of the motor lip representation influences specifically discrimination of lip-articulated speech sounds. However, the timing of the neural mechanisms underlying these articulator-specific motor contributions to speech processing is unknown. Furthermore, it is unclear whether they depend on attention. Here, we used magnetoencephalography and TMS to investigate the effect of attention on specificity and timing of interactions between the auditory and motor cortex during processing of speech sounds. We found that TMS-induced disruption of the motor lip representation modulated specifically the early auditory-cortex responses to lip-articulated speech sounds when they were attended. These articulator-specific modulations were left-lateralized and remarkably early, occurring 60-100 ms after sound onset. When speech sounds were ignored, the effect of this motor disruption on auditory-cortex responses was nonspecific and bilateral, and it started later, 170 ms after sound onset. The findings indicate that articulatory motor cortex can contribute to auditory processing of speech sounds even in the absence of behavioral tasks and when the sounds are not in the focus of attention. Importantly, the findings also show that attention can selectively facilitate the interaction of the auditory cortex with specific articulator representations during speech processing.

Brain Potentials to Speech and Acoustic Sound Discrimination Uncover the Origin of Individual Differences in Perceiving the Sounds of A Second Language

Brain Potentials to Speech and Acoustic Sound Discrimination Uncover the Origin of Individual Differences in Perceiving the Sounds of A Second Language

Hearing Matters

Figure: Neural response consistency improves after one year of FM use in children with dyslexia, with the average r-value increasing from 0.606 to 0.743. (Adapted from Proc Natl Acad Sci U S A 2012;109[41]:16731-16736.)In our September columnhttp://journals.lww.com/thehearingjournal/Fulltext/2013/09000/Hearing_Matters___For_Reading_Development,.4.aspx, we discussed the impact of impaired auditory processing on reading development, which suggests that auditory training might be a viable management strategy for children who have dyslexia or are at risk of developing reading problems. But is there any evidence to support treatment efficacy for this kind of disorder? Let's first look at evidence supporting remediation efforts. Even a short, three-hour training session on phonological awareness can improve reading-related skills and neural discrimination of speech sounds (mismatch negativity; MMN) in preschool children (Brain Res 2012;1448:42-55http://www.sciencedirect.com/science/article/pii/S0006899312001941). Several studies have shown benefits over longer time periods typical of commercial training programs. For example, children with dyslexia who participated in 35 to 40 sessions of Earobics, a popular auditory training program, had better auditory processing following training, as demonstrated by better speech-evoked auditory brainstem responses (ABRs) in noise and better performance on reading-related tests of cognitive ability (Behav Brain Res 2005;-156[1]:95-103http://www.sciencedirect.com/science/article/pii/S0166432804001809).Figure: Nina Kraus, PhDIn these children, ABR latencies prior to training predicted improvement in syllable discrimination, suggesting that the speech-evoked ABR can be useful in determining who is most likely to benefit from training. One example of an environmental modification used to enhance access to the acoustic signal is a frequency modulation (FM) system.There is now evidence for the benefits of using FM systems for improving both reading performance and neural response consistency (Proc Natl Acad Sci U S A 2012;109[41]:16731-16736http://www.pnas.org/content/109/41/16731.abstract). In the FM study, children with dyslexia (age 8 to 14) wore Phonak EduLink systems during lecture-based classes, approximately four hours per day, for the nine-month academic year. Another group of children with dyslexia attending the same classes did not use the assistive listening devices and served as the matched control group. In the summers before and after the school year, both groups, along with a group of typically developing children, were tested with ABRs.Figure: Samira Anderson, AuD, PhDAfter one year, the children using FM systems had improved neural response consistency and greater gains in reading compared with the matched control or typically developing groups. Importantly, response consistency in the FM group increased to levels equivalent to those of typically developing children. Notably, the children with dyslexia with good phonological skills prior to using the FM systems did not show improvements in either phonological skills or response consistency, suggesting that it is important to identify which children are going to benefit from FM use based on pretest measures. Electrophysiologic measures have demonstrated that both auditory training programs and FM use can improve auditory processing and reading outcomes. Furthermore, it may be possible to identify the children who are most likely to benefit from one or both of these treatment strategies.

Comparative morphology of the hominin and African ape hyoid bone, a possible marker of the evolution of speech.

This study examines the morphology of the hyoid in three closely related species, Homo sapiens, Pan troglodytes, and Gorilla gorilla. Differences and similarities between the hyoids of these species are characterized and used to interpret the morphology and affi nities of the Dikika A. afarensis, Kebara 2 Neanderthal, and other fossil hominin hyoid bones. Humans and African apes are found to have distinct hyoid morphologies. In humans the maximum width across the distal tips of the articulated greater horns is usually slightly greater than the maximum length (distal greater horn tip to most anterior point of the hyoid body in the midline). A different pattern is usually found in the African ape hyoids, which have much greater maximum lengths. In humans, the hyoid body is also much more anteroposteriorly shallow in proportion to its height and width, and this is true for all age classes. The Dikika australopithecine hyoid body proportions are chimpanzeelike. A discriminant function analysis, using a larger subadult sample from the three extant species than that reported by Alemseged et al. (2006), confirms this finding. The Kebara hyoid dimensions (body alone, and articulated body and greater horns) are almost all within the observed range for human hyoids. Discriminant functions clearly distinguish human from African ape hyoids and classify the Kebara 2 hyoid as human (confirming the finding of Arensburg et al. 1989). Our virtual dissection of a chimpanzee air sac system shows its subhyoid extension into the dorsal hyoid body. Following Alemseged et al. (2006), the expanded bulla characteristic of the African ape and australopithecine hyoid body is therefore interpreted as refl ecting the presence of such a laryngeal air sac extension. Its absence in the human, Neanderthal, and H. heidelbergensis (Atapuerca SH) hyoids implicates the loss of the laryngeal air sacs as a derived Neanderthal and modern human trait, which evolved no later than the middle Pleistocene. If, as has been argued by de Boer (2012), the loss of the air sac helped to enhance perceptual discrimination of speech sounds, then this derived hyoid morphology can be added to the list of fossil markers of the capacity for speech.

Hearing Matters

Figure: Top panel: These single-trial responses to a speech syllable illustrate trial-to-trial variability in the ABR of a child with dyslexia. Bottom panel: Good readers have better neural response consistency than average or poor readers. ***p < 0.005, *p < 0.05, ~p < 0.10. (Adapted from J Neurosci 2013;33[8]:3500-3504. Adaptations are themselves works protected by copyright. In order to publish this adaptation, authorization must be obtained both from the copyright owner in the original work and from the copyright owner in the translation or adaptation.)As audiologists, we understand the important role hearing sensitivity plays in the development of reading skills. What we don't always consider, however, is the effect of central auditory processing. The accurate representation of speech sounds is an important aspect of learning to link sounds with orthographic symbols, a fundamental building block of reading (Proc Nat Acad Sci U S A 2010;107[17]:7939-7944 ). Furthermore, if a child is not making sound-to-meaning connections in language, then efficient, automatic representation of sound by the auditory system will not develop. It stands to reason that the evaluation of auditory-processing abilities, in addition to basic hearing thresholds, is an important part of the assessment of children who are having difficulty reading. RESPONSE CONSISTENCYFigure: Nina Kraus, PhDInpairments in frequency modulation, speech perception, and phonological awareness in kindergarteners and first graders predict a dyslexia diagnosis in third grade (Res Dev Disabil 2011;32[2]:560-570 ). Speech perception in children with dyslexia is particularly problematic in noise (Dev Sci 2009;12[5]:732-745 ), and temporal-processing deficits appear to underlie at least some types of dyslexia, with training leading to improvements (Science 1996;271[5245]:77-81 ).Figure: Samira Anderson, AuD, PhDOne of the mechanisms influencing the development of auditory skills and the quality of sound representation is the trial-to-trial consistency of neural responses. If there is jitter or asynchrony, the response will be degraded, resulting in an absent auditory brainstem response (ABR). Asynchronous firing may degrade the response to the extent that it interferes with reading. In fact, this effect was demonstrated in a rat model of dyslexia (Cereb Cort 2013; doi:10.1093/cercor/bht028 ). Rats were genetically engineered to have reduced expression of KIAA0319, since mutations of this gene are associated with dyslexia. The rats had lower trial-to-trial consistency and reduced discrimination of speech sounds, providing evidence that decreased expression of the KIAA0319 gene can lead to impaired phoneme processing in the cerebral cortex. The importance of neural response consistency was also recently demonstrated in children who had a wide range of reading abilities. Those with good reading scores had higher trial-to-trial consistency in the auditory brainstem response to a speech syllable than did children with poor reading scores (J Neurosci 2013;33[8]:3500-3504 ). Overall, these results provide convincing objective evidence for underlying auditory-processing deficits in at least some children with reading disorders. This study has important implications. Response consistency—an objective measure of auditory processing that reflects neural synchrony—appears to be an important factor in the development of successful reading. Because response consistency is an ABR measure, it would be feasible to incorporate it into the audiological battery. But let's take this a step further. Are there efficacious treatments for individuals with biological evidence of auditory-processing disorders? There is now support for the benefits of using FM systems for improving both reading performance and neural response consistency (Proc Natl Acad Sci U S A 2012;109[41]:16731-16736 ). This and other studies evaluating treatment strategies for children with language-based learning impairments will be discussed in the November issue of The Hearing Journal.

Spectral discrimination of consonants for English and Chinese Listeners

Previous work in our laboratory showed that Chinese listeners had significantly higher thresholds for vowel formant discrimination than American English listeners, possibly due to a sparse vowel system in Chinese but a crowded vowel space in American English. Considering that the two languages have similar consonant densities (i.e., similar numbers of consonants), we hypothesized that English and Chinese listeners might have similar thresholds of spectral discrimination of consonant stimuli. Thresholds of spectral shift in the English consonant, /s/, were measured in an isolated form and in a VCV context for both English and Chinese listeners. Preliminary results showed that there was no significant difference in consonant discrimination thresholds between the two groups of listeners for either isolated or VCV consonants. These findings support that phonemic density may play an important role in spectral discrimination of speech sounds.

Infants’ brain responses for speech sound changes in fast multifeature MMN paradigm

ObjectiveWe investigated whether newborn speech-sound discrimination can be studied in 40min using fast multifeature mismatch negativity (MMN) paradigm and do the results differ from those obtained with the traditional oddball paradigm. MethodsNewborns’ MMN responses to five types of changes (consonant identity, F0, intensity, vowel duration and vowel identity) were recorded in the multifeature group (N=15) and vowel duration and vowel identity changes in the oddball group (N=13), after which the MMNs from both groups were compared with each others. ResultsStatistically significant MMNs in the 190–600ms time range from the stimulus onset were found for most change types in both paradigms. Newborn MMN responses were predominantly positive but a small number of participants elicited negative MMNs instead. MMN amplitudes did not differ between the multifeature and oddball groups. ConclusionsNewborn speech-sound discrimination can be assessed in a short recording time using the fast multifeature paradigm. SignificanceThe paradigm presented here can be used to record extensive auditory discrimination profiles in newborns and assess development of speech-sound discrimination and its difficulties.

Children’s brain responses to sound changes in pseudo words in a multifeature paradigm

ObjectiveThe multifeature mismatch negativity (MMN) paradigm has previously been used to study MMN responses to changes in tones or isolated syllables. We tested 4–12year old children’s MMNs to changes in a naturally produced pseudo word context. MethodsWe studied preschoolers’ (under the age of 7years, N=15, mean age 5years 4months) and school childrens’ (over the age of 7years, N=15, mean age 9years 3months) MMNs to five types of changes (vowel duration, fundamental frequency, gap, intensity, vowel identity) in the middle syllable of a pseudo word [tɑtɑtɑ] using a multifeature paradigm. ResultsVowel duration and gap changes elicited larger frontocentral MMN responses than other change types and the vowel identity change also produced an early positive p-MMR. The presence of the MMN was also determined at the individual level, and it was found that vowel duration and gap deviants produced MMNs in most of the participants. ConclusionsThe current study shows that children’s neural speech sound discrimination can be assessed in a word context in a short recording time (30min) by using the multifeature paradigm. SignificanceA paradigm which can be used to investigate the discrimination of several change types of speech-sounds in a natural context can be useful for investigating speech development and deficits.

The naïve language expert: introduction to the research topic

The naïve language expert: introduction to the research topic

Individual Differences in the Discrimination of Novel Speech Sounds: Effects of Sex, Temporal Processing, Musical and Cognitive Abilities

This study examined whether rapid temporal auditory processing, verbal working memory capacity, non-verbal intelligence, executive functioning, musical ability and prior foreign language experience predicted how well native English speakers (N = 120) discriminated Norwegian tonal and vowel contrasts as well as a non-speech analogue of the tonal contrast and a native vowel contrast presented over noise. Results confirmed a male advantage for temporal and tonal processing, and also revealed that temporal processing was associated with both non-verbal intelligence and speech processing. In contrast, effects of musical ability on non-native speech-sound processing and of inhibitory control on vowel discrimination were not mediated by temporal processing. These results suggest that individual differences in non-native speech-sound processing are to some extent determined by temporal auditory processing ability, in which males perform better, but are also determined by a host of other abilities that are deployed flexibly depending on the characteristics of the target sounds.

Formant discrimination of speech and non-speech sounds for English and Chinese listeners

Thresholds of formant discrimination of speech and non-speech sounds were compared for native listeners of English and Chinese. English listeners showed significantly better thresholds than Chinese listeners, not only for English vowels, but also for Chinese vowels. Thresholds for vowel-spectrum-shaped noise were comparable for the two groups. These results suggest that English listeners are more sensitive to formant frequency changes of vowel stimuli than Chinese listeners, possibly due to the denser vowel space for English than for Mandarin Chinese. However, the psychophysical capacity to discriminate formant frequency changes in non-speech sounds is similar for English and Chinese listeners.

Auditory-Motor Processing of Speech Sounds

The motor regions that control movements of the articulators activate during listening to speech and contribute to performance in demanding speech recognition and discrimination tasks. Whether the articulatory motor cortex modulates auditory processing of speech sounds is unknown. Here, we aimed to determine whether the articulatory motor cortex affects the auditory mechanisms underlying discrimination of speech sounds in the absence of demanding speech tasks. Using electroencephalography, we recorded responses to changes in sound sequences, while participants watched a silent video. We also disrupted the lip or the hand representation in left motor cortex using transcranial magnetic stimulation. Disruption of the lip representation suppressed responses to changes in speech sounds, but not piano tones. In contrast, disruption of the hand representation had no effect on responses to changes in speech sounds. These findings show that disruptions within, but not outside, the articulatory motor cortex impair automatic auditory discrimination of speech sounds. The findings provide evidence for the importance of auditory-motor processes in efficient neural analysis of speech sounds.

Neural evidence of allophonic perception in children at risk for dyslexia

Learning to read is a complex process that develops normally in the majority of children and requires the mapping of graphemes to their corresponding phonemes. Problems with the mapping process nevertheless occur in about 5% of the population and are typically attributed to poor phonological representations, which are – in turn – attributed to underlying speech processing difficulties. We examined auditory discrimination of speech sounds in 6-year-old beginning readers with a familial risk of dyslexia (n=31) and no such risk (n=30) using the mismatch negativity (MMN). MMNs were recorded for stimuli belonging to either the same phoneme category (acoustic variants of /bə/) or different phoneme categories (/bə/ vs. /də/). Stimuli from different phoneme categories elicited MMNs in both the control and at-risk children, but the MMN amplitude was clearly lower in the at-risk children. In contrast, the stimuli from the same phoneme category elicited an MMN in only the children at risk for dyslexia. These results show children at risk for dyslexia to be sensitive to acoustic properties that are irrelevant in their language. Our findings thus suggest a possible cause of dyslexia in that they show 6-year-old beginning readers with at least one parent diagnosed with dyslexia to have a neural sensitivity to speech contrasts that are irrelevant in the ambient language. This sensitivity clearly hampers the development of stable phonological representations and thus leads to significant reading impairment later in life.

Assessing toddlers’ speech-sound discrimination

ObjectiveValid and reliable methods for assessing speech perception in toddlers are lacking in the field, leading to conspicuous gaps in understanding how speech perception develops and limited clinical tools for assessing sensory aid benefit in toddlers. The objective of this investigation was to evaluate speech-sound discrimination in toddlers using modifications to the Change/No-Change procedure [1]. MethodsNormal-hearing 2- and 3-year-olds’ discrimination of acoustically dissimilar (“easy”) and similar (“hard”) speech-sound contrasts were evaluated in a combined repeated measures and factorial design. Performance was measured in d′. Effects of contrast difficulty and age were examined, as was test-retest reliability, using repeated measures ANOVAs, planned post hoc tests, and correlation analyses. ResultsThe easy contrast (M=2.53) was discriminated better than the hard contrast (M=1.72) across all ages (p<.0001). The oldest group of children (M=3.13) discriminated the contrasts better than youngest (M=1.04; p<.0001) and the mid-age children (M=2.20; p=.037), who in turn discriminated the contrasts better than the youngest children (p=.010). Test-retest reliability was excellent (r=.886, p<.0001). Almost 90% of the children met the teaching criterion. The vast majority demonstrated the ability to be tested with the modified procedure and discriminated the contrasts. The few who did not were 2.5 years of age and younger. ConclusionsThe modifications implemented resulted, at least preliminarily, in a procedure that is reliable and sensitive to contrast difficulty and age in this young group of children, suggesting that these modifications are appropriate for this age group. With further development, the procedure holds promise for use in clinical populations who are believed to have core deficits in rapid phonological encoding, such as children with hearing loss or specific language impairment, children who are struggling to read, and second-language learners.

Cortical activity patterns predict robust speech discrimination ability in noise

The neural mechanisms that support speech discrimination in noisy conditions are poorly understood. In quiet conditions, spike timing information appears to be used in the discrimination of speech sounds. In this study, we evaluated the hypothesis that spike timing is also used to distinguish between speech sounds in noisy conditions that significantly degrade neural responses to speech sounds. We tested speech sound discrimination in rats and recorded primary auditory cortex (A1) responses to speech sounds in background noise of different intensities and spectral compositions. Our behavioral results indicate that rats, like humans, are able to accurately discriminate consonant sounds even in the presence of background noise that is as loud as the speech signal. Our neural recordings confirm that speech sounds evoke degraded but detectable responses in noise. Finally, we developed a novel neural classifier that mimics behavioral discrimination. The classifier discriminates between speech sounds by comparing the A1 spatiotemporal activity patterns evoked on single trials with the average spatiotemporal patterns evoked by known sounds. Unlike classifiers in most previous studies, this classifier is not provided with the stimulus onset time. Neural activity analyzed with the use of relative spike timing was well correlated with behavioral speech discrimination in quiet and in noise. Spike timing information integrated over longer intervals was required to accurately predict rat behavioral speech discrimination in noisy conditions. The similarity of neural and behavioral discrimination of speech in noise suggests that humans and rats may employ similar brain mechanisms to solve this problem.

The role of Broca’s area in speech perception: Evidence from aphasia revisited

Motor theories of speech perception have been re-vitalized as a consequence of the discovery of mirror neurons. Some authors have even promoted a strong version of the motor theory, arguing that the motor speech system is critical for perception. Part of the evidence that is cited in favor of this claim is the observation from the early 1980s that individuals with Broca’s aphasia, and therefore inferred damage to Broca’s area, can have deficits in speech sound discrimination. Here we re-examine this issue in 24 patients with radiologically confirmed lesions to Broca’s area and various degrees of associated non-fluent speech production. Patients performed two same-different discrimination tasks involving pairs of CV syllables, one in which both CVs were presented auditorily, and the other in which one syllable was auditorily presented and the other visually presented as an orthographic form; word comprehension was also assessed using word-to-picture matching tasks in both auditory and visual forms. Discrimination performance on the all-auditory task was four standard deviations above chance, as measured using d′, and was unrelated to the degree of non-fluency in the patients’ speech production. Performance on the auditory–visual task, however, was worse than, and not correlated with, the all-auditory task. The auditory–visual task was related to the degree of speech non-fluency. Word comprehension was at ceiling for the auditory version (97% accuracy) and near ceiling for the orthographic version (90% accuracy). We conclude that the motor speech system is not necessary for speech perception as measured both by discrimination and comprehension paradigms, but may play a role in orthographic decoding or in auditory–visual matching of phonological forms.

Enhancing Speech Discrimination Through Stimulus Repetition

To evaluate the effects of sequential and alternating repetition on speech-sound discrimination. Typically hearing adults' discrimination of 3 pairs of speech-sound contrasts was assessed at 3 signal-to-noise ratios using the change/no-change procedure. On change trials, the standard and comparison stimuli differ; on no-change trials, they are identical. Listeners were presented with 5 repetition conditions: 2 and 4 sequential repetitions of the standard followed by sequential repetitions of the comparison; 2 and 4 alternating presentations of the standard and comparison; and 1 repetition of the standard and comparison. Both sequential and alternating repetition improved discrimination of the fricative and liquid contrasts, but neither was clearly superior to the other across the conditions. The results support previous findings that increasing the number of fricative and liquid stimulus presentations improves discriminability and extends the findings to natural speech stimuli. Further, the effect of repetition is robust: Both sequential and alternating repetitions improve speech-sound discrimination, and few differences emerge between the two types of stimulus repetitions. The results have implications for evaluating the strength of the internal representation of speech stimuli in clinical populations believed to have a core deficit in phonological encoding, such as children with hearing loss.