Speech Perception Learning Research Articles

Comparing lexically guided perceptual learning in younger and older listeners

Numerous studies have shown that younger adults engage in lexically guided perceptual learning in speech perception. Here, we investigated whether older listeners are also able to retune their phonetic category boundaries. More specifically, in this research we tried to answer two questions. First, do older adults show perceptual-learning effects of similar size to those of younger adults? Second, do differences in lexical behavior predict the strength of the perceptual-learning effect? An age group comparison revealed that older listeners do engage in lexically guided perceptual learning, but there were two age-related differences: Younger listeners had a stronger learning effect right after exposure than did older listeners, but the effect was more stable for older than for younger listeners. Moreover, a clear link was shown to exist between individuals' lexical-decision performance during exposure and the magnitude of their perceptual-learning effects. A subsequent analysis on the results of the older participants revealed that, even within the older participant group, with increasing age the perceptual retuning effect became smaller but also more stable, mirroring the age group comparison results. These results could not be explained by differences in hearing loss. The age effect may be accounted for by decreased flexibility in the adjustment of phoneme categories or by age-related changes in the dynamics of spoken-word recognition, with older adults being more affected by competition from similar-sounding lexical competitors, resulting in less lexical guidance for perceptual retuning. In conclusion, our results clearly show that the speech perception system remains flexible over the life span.

Optimal categorization of sounds varying on a single dimension

Listeners were randomly presented narrow-band filtered noise bursts that varied in filter center frequency from two overlapping, Gaussian-like distributions. Participants mapped these distributions of sounds onto creatures in a video game where they received visual and auditory feedback about their accuracy. Categorization boundaries for each participant were estimated using logistic regression and compared with the optimal boundary from an ideal observer model. The participants appeared to be able to establish near optimal boundaries rapidly and had a remarkable ability to shift these boundaries when the underlying distributions changed - even when these changes were not explicitly signaled. These results suggest that listeners maintain a rather detailed representation of distributional information that is continuously updated during the task. This interpretation is in line with the assumptions underlying many current models of perceptual (statistical) learning in speech perception. However, it is possible to get optimal-like behavior by maintaining a general distributional representation or by using simpler “local” strategies based on only a few of the most recently experienced exemplars. The results will be presented with multiple categorization models, which testify to the difficulty of interpreting claims of distributional learning in categorization. [Work supported by NIH-NIDCD.]

Stimulus uncertainty in auditory perceptual learning

Stimulus uncertainty produced by variations in a target stimulus to be detected or discriminated, impedes perceptual learning under some, but not all experimental conditions. To account for those discrepancies, it has been proposed that uncertainty is detrimental to learning when the interleaved stimuli or tasks are similar to each other but not when they are sufficiently distinct, or when it obstructs the downstream search required to gain access to fine-grained sensory information, as suggested by the Reverse Hierarchy Theory (RHT). The focus of the current review is on the effects of uncertainty on the perceptual learning of speech and non-speech auditory signals. Taken together, the findings from the auditory modality suggest that in addition to the accounts already described, uncertainty may contribute to learning when categorization of stimuli to phonological or acoustic categories is involved. Therefore, it appears that the differences reported between the learning of non-speech and speech-related parameters are not an outcome of inherent differences between those two domains, but rather due to the nature of the tasks often associated with those different stimuli.

Perceptual learning of co-articulation in speech

Four experiments investigated the novel issue of learning to accommodate the co-articulated nature of speech. Experiment 1 established a co-articulatory mismatch effect for a set of vowel–consonant (VC) syllables (reaction times were faster for co-articulation matching than for mismatching stimuli). A rhyme judgment training task on words (Experiment 2) or VC stimuli (Experiment 3) with mismatching information was followed by a phoneme monitoring task on a set of VC stimuli; training and test stimuli contained physically identical (same condition) or new (different condition) mismatching co-articulatory information (along with a set containing matching co-articulatory information). A third group received no training. A co-articulatory mismatch effect was found without training but not when the same mismatching tokens were used at training and test. Both word (Experiment 2) and syllable (Experiment 3) training stimuli eliminated the mismatch effect; overall reaction times were somewhat slower when the training stimuli were words. Perceptual learning generalized to new tokens only when the acoustic manifestation of the critical co-articulatory information in the training stimuli was sufficiently large (Experiments 3 and 4). The results are discussed in terms of speech processing and perceptual learning in speech perception.

Perceptual learning for speech

Adult language users have an enormous amount of experience with speech in their native language. As a result, they have very well-developed processes for categorizing the sounds of speech that they hear. Despite this very high level of experience, recent research has shown that listeners are capable of redeveloping their speech categorization to bring it into alignment with new variation in their speech input. This reorganization of phonetic space is a type of perceptual learning, or recalibration, of speech processes. In this article, we review several recent lines of research on perceptual learning for speech.

Perceptual learning of speech processed by a Cochlear Implant simulator — An fMRI investigation

Perceptual learning of speech processed by a Cochlear Implant simulator — An fMRI investigation

Does perceptual learning in speech reflect changes in phonetic category representation or decision bias?

Recent studies show that perceptual boundaries between phonetic categories are changeable with training (Norris, McQueen, & Cutler, 2003). For example, Kraljic and Samuel (2005) exposed listeners in a lexical decision task to ambiguous /s-integral/ sounds in either s-word contexts (e.g., legacy) or integral-word contexts (e.g., parachute). In a subsequent /s/-/integral/ categorization test, listeners in the /s/ condition categorized more tokens as /s/ than did those in the /integral/ condition. The effect--termed perceptual learning in speech--is assumed to reflect a change in phonetic category representation. However, the result could be due to a decision bias resulting from the training task. In Experiment 1, we replicated the basic Kraljic and Samuel (2005) experiment and added an AXB discrimination test. In Experiment 2, we used a task that is less likely to induce a decision bias. Results of both experiments and signal detection analyses point to a true change in phonetic representation.

Enhancing learning on foreign-accent adaptation with a combination of active training and passive stimulus exposure

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.]

Selective attention and perceptual learning of speech

Phonetic experience can change the perceptual distance between speech sounds, increasing both within-category similarity and between-category distinctiveness. Such warping of perceptual space is frequently characterized in terms of changes in selective attention: Listeners are assumed to attend more strongly to category-differentiating features while ignoring less relevant ones. However, the link between the distribution of selective attention and categorization-related differences in perceptual distance has not been empirically demonstrated. To explore this relationship, listeners were given 6 h of training to categorize sounds according to one of two acoustic features while ignoring the other. The features were voice onset time and onset f0, which are normally correlated and can both serve as a cue to consonant voicing. Before and after training, listener’s performance on a Garner selective attention task was compared with assessment of the perceptual distance between tokens. Results suggest that training can induce both warping of perceptual space and changes in the distribution of selective attention, but the two phenomena are not necessarily related. Results are consistent with a two-stage model of perceptual learning, involving both preattentive adjustment of acoustic cue weighting and higher-level changes in the distribution of selective attention between acoustic cues. [Work supported by NIH-NIDCD 1R03DC006811.]

Perceptual learning in speech: Stability over time

Perceptual representations of phonemes are flexible and adapt rapidly to accommodate idiosyncratic articulation in the speech of a particular talker. This letter addresses whether such adjustments remain stable over time and under exposure to other talkers. During exposure to a story, listeners learned to interpret an ambiguous sound as [f] or [s]. Perceptual adjustments measured after 12 h were as robust as those measured immediately after learning. Equivalent effects were found when listeners heard speech from other talkers in the 12 h interval, and when they had the opportunity to consolidate learning during sleep.

Generalization in perceptual learning for speech

Lexical context strongly influences listeners' identification of ambiguous sounds. For example, a sound midway between /f/ and /s/ is reported as /f/ in "sheri_," but as /s/ in "Pari_." Norris, McQueen, and Cutler (2003) have demonstrated that after hearing such lexically determined phonemes, listeners expand their phonemic categories to include more ambiguous tokens than before. We tested whether listeners adjust their phonemic categories for a specific speaker. Do listeners learn a particular speaker's "accent"? Similarly, we examined whether perceptual learning is specific to the particular ambiguous phonemes that listeners hear, or whether the adjustments generalize to related sounds. Participants heard ambiguous /d/ or /t/ phonemes during a lexical decision task. They then categorized sounds on /d/-/t/ and /b/-/p/ continua, either in the same voice that they had heard for lexical decision, or in a different voice. Perceptual learning generalized across both speaker and test continua: Changes in perceptual representations are robust and broadly tuned.

Perceptual learning for speech: Is there a return to normal?

Recent work on perceptual learning shows that listeners’ phonemic representations dynamically adjust to reflect the speech they hear ( Norris, McQueen, & Cutler, 2003). We investigate how the perceptual system makes such adjustments, and what (if anything) causes the representations to return to their pre-perceptual learning settings. Listeners are exposed to a speaker whose pronunciation of a particular sound (either /s/ or /ʃ/) is ambiguous (e.g., halfway between /s/ and /ʃ/). After exposure, participants are tested for perceptual learning on two continua that range from /s/ to /ʃ/, one in the Same voice they heard during exposure, and one in a Different voice. To assess how representations revert to their prior settings, half of Experiment 1’s participants were tested immediately after exposure; the other half performed a 25-min silent intervening task. The perceptual learning effect was actually larger after such a delay, indicating that simply allowing time to pass does not cause learning to fade. The remaining experiments investigate different ways that the system might unlearn a person’s pronunciations: listeners hear the Same or a Different speaker for 25 min with either: no relevant (i.e., ‘good’) /s/ or /ʃ/ input (Experiment 2), one of the relevant inputs (Experiment 3), or both relevant inputs (Experiment 4). The results support a view of phonemic representations as dynamic and flexible, and suggest that they interact with both higher- (e.g., lexical) and lower-level (e.g., acoustic) information in important ways.

The role of multimodal sensory feedback in the perceptual learning of speech under a novel acoustic transformation

This experiment examined spoken word recognition during and after perceptual learning of an unusual acoustic transformation of speech. Previous research has found that adults and children receive more benefit from cochlear implants when auditory information is effectively integrated with visual lipreading information. Because cochlear implant users have little, if any, preoperative experience with auditory information, these findings suggest that perceivers are able to capitalize on already-obtained lipreading skills in order to effectively learn to perceive new acoustic information about speech. This may be due to the fact that auditory and visual speech are lawfully related to the same physical event, a spoken utterance. Alternatively, it is possible that the addition of any relevant stimulus, regardless of its lawful relations with auditory speech, will aid in the process of learning how to deal with new perceptual information. To test these hypotheses, normal-hearing participants were trained with three different conditions of perceptual input while learning to perceive frequency-inverted speech: auditory-alone, auditory with orthographic visual stimulation, and auditory with visual information about the lips and face. The results of this study showed interesting patterns of perceptual learning and are discussed in terms of their implication for current theories of perceptual integration.

Perceptual learning in speech

This study demonstrates that listeners use lexical knowledge in perceptual learning of speech sounds. Dutch listeners first made lexical decisions on Dutch words and nonwords. The final fricative of 20 critical words had been replaced by an ambiguous sound, between [f] and [s]. One group of listeners heard ambiguous [f]-final words (e.g., [ witlo?], from witlof, chicory) and unambiguous [s]-final words (e.g., naaldbos, pine forest). Another group heard the reverse (e.g., ambiguous [na:ldbo?], unambiguous witlof). Listeners who had heard [?] in [f]-final words were subsequently more likely to categorize ambiguous sounds on an [f]–[s] continuum as [f] than those who heard [?] in [s]-final words. Control conditions ruled out alternative explanations based on selective adaptation and contrast. Lexical information can thus be used to train categorization of speech. This use of lexical information differs from the on-line lexical feedback embodied in interactive models of speech perception. In contrast to on-line feedback, lexical feedback for learning is of benefit to spoken word recognition (e.g., in adapting to a newly encountered dialect).

Talker-specific learning in speech perception.

The effects of perceptual learning of talker identity on the recognition of spoken words and sentences were investigated in three experiments. In each experiment, listeners were trained to learn a set of 10 talkers' voices and were then given an intelligibility test to assess the influence of learning the voices on the processing of the linguistic content of speech. In the first experiment, listeners learned voices from isolated words and were then tested with novel isolated words mixed in noise. The results showed that listeners who were given words produced by familiar talkers at test showed better identification performance than did listeners who were given words produced by unfamiliar talkers. In the second experiment, listeners learned novel voices from sentence-length utterances and were then presented with isolated words. The results showed that learning a talker's voice from sentences did not generalize well to identification of novel isolated words. In the third experiment, listeners learned voices from sentence-length utterances and were then given sentence-length utterances produced by familiar and unfamiliar talkers at test. We found that perceptual learning of novel voices from sentence-length utterances improved speech intelligibility for words in sentences. Generalization and transfer from voice learning to linguistic processing was found to be sensitive to the talker-specific information available during learning and test. These findings demonstrate that increased sensitivity to talker-specific information affects the perception of the linguistic properties of speech in isolated words and sentences.

Perceptual space and learning in speech perception.

When listeners are given training on synthetic speech, intelligibility improves and listeners become more efficient in using cognitive resources for the perception of speech [L. Lee and H. C. Nusbaum, J. Acoust. Soc. Am. Suppl. 1 85, S125 (1989)]. The present study investigated one possible account of the connection between changes in intelligibility and changes in efficiency in using cognitive capacity: Learning may shift cognitive resources away from the analysis of acoustic properties of synthetic speech that are not informative about phonetic structure to more informative acoustic properties. If learning changes the distribution of attention to the acoustic signal, a change should occur in the structure of the perceptual space used in recognizing synthetic speech. Multidimensional scaling was performed on consonant confusions before and after training in order to determine if and how the structure of perceptual space is affected by learning. The results are consistent with the hypothesis that perceptual learning of synthetic speech changes the way listeners focus attention on the acoustic properties of the speech signal. [Work supported by NIDCD.]