Modality-Specific Perceptual Learning of Vocoded Auditory versus Lipread Speech: Different Effects of Prior Information.

Traditionally, speech perception training paradigms have not adequately taken into account the possibility that there may be modality-specific requirements for perceptual learning with auditory-only (AO) versus visual-only (VO) speech stimuli. The study reported here investigated the hypothesis that there are modality-specific differences in how prior information is used by normal-hearing participants during vocoded versus VO speech training. Two different experiments, one with vocoded AO speech (Experiment 1) and one with VO, lipread, speech (Experiment 2), investigated the effects of giving different types of prior information to trainees on each trial during training. The training was for four ~20 min sessions, during which participants learned to label novel visual images using novel spoken words. Participants were assigned to different types of prior information during training: Word Group trainees saw a printed version of each training word (e.g., "tethon"), and Consonant Group trainees saw only its consonants (e.g., "t_th_n"). Additional groups received no prior information (i.e., Experiment 1, AO Group; Experiment 2, VO Group) or a spoken version of the stimulus in a different modality from the training stimuli (Experiment 1, Lipread Group; Experiment 2, Vocoder Group). That is, in each experiment, there was a group that received prior information in the modality of the training stimuli from the other experiment. In both experiments, the Word Groups had difficulty retaining the novel words they attempted to learn during training. However, when the training stimuli were vocoded, the Word Group improved their phoneme identification. When the training stimuli were visual speech, the Consonant Group improved their phoneme identification and their open-set sentence lipreading. The results are considered in light of theoretical accounts of perceptual learning in relationship to perceptual modality.

Metamodal Coupling of Vibrotactile and Auditory Speech Processing Systems through Matched Stimulus Representations.

It has been postulated that the brain is organized by "metamodal," sensory-independent cortical modules capable of performing tasks (e.g., word recognition) in both "standard" and novel sensory modalities. Still, this theory has primarily been tested in sensory-deprived individuals, with mixed evidence in neurotypical subjects, thereby limiting its support as a general principle of brain organization. Critically, current theories of metamodal processing do not specify requirements for successful metamodal processing at the level of neural representations. Specification at this level may be particularly important in neurotypical individuals, where novel sensory modalities must interface with existing representations for the standard sense. Here we hypothesized that effective metamodal engagement of a cortical area requires congruence between stimulus representations in the standard and novel sensory modalities in that region. To test this, we first used fMRI to identify bilateral auditory speech representations. We then trained 20 human participants (12 female) to recognize vibrotactile versions of auditory words using one of two auditory-to-vibrotactile algorithms. The vocoded algorithm attempted to match the encoding scheme of auditory speech while the token-based algorithm did not. Crucially, using fMRI, we found that only in the vocoded group did trained-vibrotactile stimuli recruit speech representations in the superior temporal gyrus and lead to increased coupling between them and somatosensory areas. Our results advance our understanding of brain organization by providing new insight into unlocking the metamodal potential of the brain, thereby benefitting the design of novel sensory substitution devices that aim to tap into existing processing streams in the brain.SIGNIFICANCE STATEMENT It has been proposed that the brain is organized by "metamodal," sensory-independent modules specialized for performing certain tasks. This idea has inspired therapeutic applications, such as sensory substitution devices, for example, enabling blind individuals "to see" by transforming visual input into soundscapes. Yet, other studies have failed to demonstrate metamodal engagement. Here, we tested the hypothesis that metamodal engagement in neurotypical individuals requires matching the encoding schemes between stimuli from the novel and standard sensory modalities. We trained two groups of subjects to recognize words generated by one of two auditory-to-vibrotactile transformations. Critically, only vibrotactile stimuli that were matched to the neural encoding of auditory speech engaged auditory speech areas after training. This suggests that matching encoding schemes is critical to unlocking the brain's metamodal potential.

Lipreading: A Review of Its Continuing Importance for Speech Recognition With an Acquired Hearing Loss and Possibilities for Effective Training.

PURPOSE: The goal of this review article is to reinvigorate interest in lipreading and lipreading training for adults with acquired hearing loss. Most adults benefit from being able to see the talker when speech is degraded; however, the effect size is related to their lipreading ability, which is typically poor in adults who have experienced normal hearing through most of their lives. Lipreading training has been viewed as a possible avenue for rehabilitation of adults with an acquired hearing loss, but most training approaches have not been particularly successful. Here, we describe lipreading and theoretically motivated approaches to its training, as well as examples of successful training paradigms. We discuss some extensions to auditory-only (AO) and audiovisual (AV) speech recognition. METHOD: Visual speech perception and word recognition are described. Traditional and contemporary views of training and perceptual learning are outlined. We focus on the roles of external and internal feedback and the training task in perceptual learning, and we describe results of lipreading training experiments. RESULTS: Lipreading is commonly characterized as limited to viseme perception. However, evidence demonstrates subvisemic perception of visual phonetic information. Lipreading words also relies on lexical constraints, not unlike auditory spoken word recognition. Lipreading has been shown to be difficult to improve through training, but under specific feedback and task conditions, training can be successful, and learning can generalize to untrained materials, including AV sentence stimuli in noise. The results on lipreading have implications for AO and AV training and for use of acoustically processed speech in face-to-face communication. CONCLUSION: Given its importance for speech recognition with a hearing loss, we suggest that the research and clinical communities integrate lipreading in their efforts to improve speech recognition in adults with acquired hearing loss.

During Lipreading Training With Sentence Stimuli, Feedback Controls Learning and Generalization to Audiovisual Speech in Noise.

PURPOSE: This study investigated the effects of external feedback on perceptual learning of visual speech during lipreading training with sentence stimuli. The goal was to improve visual-only (VO) speech recognition and increase accuracy of audiovisual (AV) speech recognition in noise. The rationale was that spoken word recognition depends on the accuracy of sublexical (phonemic/phonetic) speech perception; effective feedback during training must support sublexical perceptual learning. METHOD: Normal-hearing (NH) adults were assigned to one of three types of feedback: Sentence feedback was the entire sentence printed after responding to the stimulus. Word feedback was the correct response words and perceptually near but incorrect response words. Consonant feedback was correct response words and consonants in incorrect but perceptually near response words. Six training sessions were given. Pre- and posttraining testing included an untrained control group. Test stimuli were disyllable nonsense words for forced-choice consonant identification, and isolated words and sentences for open-set identification. Words and sentences were VO, AV, and audio-only (AO) with the audio in speech-shaped noise. RESULTS: Lipreading accuracy increased during training. Pre- and posttraining tests of consonant identification showed no improvement beyond test-retest increases obtained by untrained controls. Isolated word recognition with a talker not seen during training showed that the control group improved more than the sentence group. Tests of untrained sentences showed that the consonant group significantly improved in all of the stimulus conditions (VO, AO, and AV). Its mean words correct scores increased by 9.2 percentage points for VO, 3.4 percentage points for AO, and 9.8 percentage points for AV stimuli. CONCLUSIONS: Consonant feedback during training with sentences stimuli significantly increased perceptual learning. The training generalized to untrained VO, AO, and AV sentence stimuli. Lipreading training has potential to significantly improve adults' face-to-face communication in noisy settings in which the talker can be seen.

Errors on a Speech-in-Babble Sentence Recognition Test Reveal Individual Differences in Acoustic Phonetic Perception and Babble Misallocations.

OBJECTIVES: The ability to recognize words in connected speech under noisy listening conditions is critical to everyday communication. Many processing levels contribute to the individual listener's ability to recognize words correctly against background speech, and there is clinical need for measures of individual differences at different levels. Typical listening tests of speech recognition in noise require a list of items to obtain a single threshold score. Diverse abilities measures could be obtained through mining various open-set recognition errors during multi-item tests. This study sought to demonstrate that an error mining approach using open-set responses from a clinical sentence-in-babble-noise test can be used to characterize abilities beyond signal-to-noise ratio (SNR) threshold. A stimulus-response phoneme-to-phoneme sequence alignment software system was used to achieve automatic, accurate quantitative error scores. The method was applied to a database of responses from normal-hearing (NH) adults. Relationships between two types of response errors and words correct scores were evaluated through use of mixed models regression. DESIGN: Two hundred thirty-three NH adults completed three lists of the Quick Speech in Noise test. Their individual open-set speech recognition responses were automatically phonemically transcribed and submitted to a phoneme-to-phoneme stimulus-response sequence alignment system. The computed alignments were mined for a measure of acoustic phonetic perception, a measure of response text that could not be attributed to the stimulus, and a count of words correct. The mined data were statistically analyzed to determine whether the response errors were significant factors beyond stimulus SNR in accounting for the number of words correct per response from each participant. This study addressed two hypotheses: (1) Individuals whose perceptual errors are less severe recognize more words correctly under difficult listening conditions due to babble masking and (2) Listeners who are better able to exclude incorrect speech information such as from background babble and filling in recognize more stimulus words correctly. RESULTS: Statistical analyses showed that acoustic phonetic accuracy and exclusion of babble background were significant factors, beyond the stimulus sentence SNR, in accounting for the number of words a participant recognized. There was also evidence that poorer acoustic phonetic accuracy could occur along with higher words correct scores. This paradoxical result came from a subset of listeners who had also performed subjective accuracy judgments. Their results suggested that they recognized more words while also misallocating acoustic cues from the background into the stimulus, without realizing their errors. Because the Quick Speech in Noise test stimuli are locked to their own babble sample, misallocations of whole words from babble into the responses could be investigated in detail. The high rate of common misallocation errors for some sentences supported the view that the functional stimulus was the combination of the target sentence and its babble. CONCLUSIONS: Individual differences among NH listeners arise both in terms of words accurately identified and errors committed during open-set recognition of sentences in babble maskers. Error mining to characterize individual listeners can be done automatically at the levels of acoustic phonetic perception and the misallocation of background babble words into open-set responses. Error mining can increase test information and the efficiency and accuracy of characterizing individual listeners.

Neural mechanisms of vibrotactile categorization.

The grouping of sensory stimuli into categories is fundamental to cognition. Previous research in the visual and auditory systems supports a two-stage processing hierarchy that underlies perceptual categorization: (a) a "bottom-up" perceptual stage in sensory cortices where neurons show selectivity for stimulus features and (b) a "top-down" second stage in higher level cortical areas that categorizes the stimulus-selective input from the first stage. In order to test the hypothesis that the two-stage model applies to the somatosensory system, 14 human participants were trained to categorize vibrotactile stimuli presented to their right forearm. Then, during an fMRI scan, participants actively categorized the stimuli. Representational similarity analysis revealed stimulus selectivity in areas including the left precentral and postcentral gyri, the supramarginal gyrus, and the posterior middle temporal gyrus. Crucially, we identified a single category-selective region in the left ventral precentral gyrus. Furthermore, an estimation of directed functional connectivity delivered evidence for robust top-down connectivity from the second to first stage. These results support the validity of the two-stage model of perceptual categorization for the somatosensory system, suggesting common computational principles and a unified theory of perceptual categorization across the visual, auditory, and somatosensory systems.

Multisensory training can promote or impede visual perceptual learning of speech stimuli: visual-tactile vs. visual-auditory training.

In a series of studies we have been investigating how multisensory training affects unisensory perceptual learning with speech stimuli. Previously, we reported that audiovisual (AV) training with speech stimuli can promote auditory-only (AO) perceptual learning in normal-hearing adults but can impede learning in congenitally deaf adults with late-acquired cochlear implants. Here, impeder and promoter effects were sought in normal-hearing adults who participated in lipreading training. In Experiment 1, visual-only (VO) training on paired associations between CVCVC nonsense word videos and nonsense pictures demonstrated that VO words could be learned to a high level of accuracy even by poor lipreaders. In Experiment 2, visual-auditory (VA) training in the same paradigm but with the addition of synchronous vocoded acoustic speech impeded VO learning of the stimuli in the paired-associates paradigm. In Experiment 3, the vocoded AO stimuli were shown to be less informative than the VO speech. Experiment 4 combined vibrotactile speech stimuli with the visual stimuli during training. Vibrotactile stimuli were shown to promote visual perceptual learning. In Experiment 5, no-training controls were used to show that training with visual speech carried over to consonant identification of untrained CVCVC stimuli but not to lipreading words in sentences. Across this and previous studies, multisensory training effects depended on the functional relationship between pathways engaged during training. Two principles are proposed to account for stimulus effects: (1) Stimuli presented to the trainee's primary perceptual pathway will impede learning by a lower-rank pathway. (2) Stimuli presented to the trainee's lower rank perceptual pathway will promote learning by a higher-rank pathway. The mechanisms supporting these principles are discussed in light of multisensory reverse hierarchy theory (RHT).

Audiovisual spoken word training can promote or impede auditory-only perceptual learning: prelingually deafened adults with late-acquired cochlear implants versus normal hearing adults.

Training with audiovisual (AV) speech has been shown to promote auditory perceptual learning of vocoded acoustic speech by adults with normal hearing. In Experiment 1, we investigated whether AV speech promotes auditory-only (AO) perceptual learning in prelingually deafened adults with late-acquired cochlear implants. Participants were assigned to learn associations between spoken disyllabic C(=consonant)V(=vowel)CVC non-sense words and non-sense pictures (fribbles), under AV and then AO (AV-AO; or counter-balanced AO then AV, AO-AV, during Periods 1 then 2) training conditions. After training on each list of paired-associates (PA), testing was carried out AO. Across all training, AO PA test scores improved (7.2 percentage points) as did identification of consonants in new untrained CVCVC stimuli (3.5 percentage points). However, there was evidence that AV training impeded immediate AO perceptual learning: During Period-1, training scores across AV and AO conditions were not different, but AO test scores were dramatically lower in the AV-trained participants. During Period-2 AO training, the AV-AO participants obtained significantly higher AO test scores, demonstrating their ability to learn the auditory speech. Across both orders of training, whenever training was AV, AO test scores were significantly lower than training scores. Experiment 2 repeated the procedures with vocoded speech and 43 normal-hearing adults. Following AV training, their AO test scores were as high as or higher than following AO training. Also, their CVCVC identification scores patterned differently than those of the cochlear implant users. In Experiment 1, initial consonants were most accurate, and in Experiment 2, medial consonants were most accurate. We suggest that our results are consistent with a multisensory reverse hierarchy theory, which predicts that, whenever possible, perceivers carry out perceptual tasks immediately based on the experience and biases they bring to the task. We point out that while AV training could be an impediment to immediate unisensory perceptual learning in cochlear implant patients, it was also associated with higher scores during training.

Auditory Perceptual Learning for Speech Perception Can be Enhanced by Audiovisual Training.

Speech perception under audiovisual (AV) conditions is well known to confer benefits to perception such as increased speed and accuracy. Here, we investigated how AV training might benefit or impede auditory perceptual learning of speech degraded by vocoding. In Experiments 1 and 3, participants learned paired associations between vocoded spoken nonsense words and nonsense pictures. In Experiment 1, paired-associates (PA) AV training of one group of participants was compared with audio-only (AO) training of another group. When tested under AO conditions, the AV-trained group was significantly more accurate than the AO-trained group. In addition, pre- and post-training AO forced-choice consonant identification with untrained nonsense words showed that AV-trained participants had learned significantly more than AO participants. The pattern of results pointed to their having learned at the level of the auditory phonetic features of the vocoded stimuli. Experiment 2, a no-training control with testing and re-testing on the AO consonant identification, showed that the controls were as accurate as the AO-trained participants in Experiment 1 but less accurate than the AV-trained participants. In Experiment 3, PA training alternated AV and AO conditions on a list-by-list basis within participants, and training was to criterion (92% correct). PA training with AO stimuli was reliably more effective than training with AV stimuli. We explain these discrepant results in terms of the so-called "reverse hierarchy theory" of perceptual learning and in terms of the diverse multisensory and unisensory processing resources available to speech perception. We propose that early AV speech integration can potentially impede auditory perceptual learning; but visual top-down access to relevant auditory features can promote auditory perceptual learning.