Learning to decipher time-compressed speech: Robust acquisition with a slight difficulty in generalization among young adults with developmental dyslexia

Learning to decipher acoustically distorted speech serves as a test case for the study of language-related skill acquisition in persons with developmental dyslexia (DD). Deciphering this type of input is rarely learned explicitly and does not yield conscious insights. Problems in implicit and procedural skill learning have been proposed as possible causes of DD. Here we examined the learning of time-compressed (accelerated) speech and its generalization to novel materials among young adults with DD compared to typical readers (TD). All participants completed a training session that involved judging the semantic plausibility of sentences, during which the level of time-compression was changed using an adaptive (staircase) procedure according to each participant's performance. In the test, phase learning (test on same items) and generalization (test on new items and same items spoken by a new speaker) were assessed. Both groups showed robust gains after training. Moreover, after training, the initial disadvantage of the DD group was no longer significant. After training, both groups experienced relative difficulties in deciphering learned tokens spoken by a different voice, though participants with DD were less able to generalize the gains to deciphering new tokens. Thus, DD individuals benefited from repeated experience with time-compressed speech no less than typical readers, but their evolving skill was apparently more dependent on the specific characteristics of the tokens. Atypical generalization, which indicates that perceptual learning is contingent on lower-level features of the input though does not necessarily point to impaired learning potential per se, may explain some of the contradictory findings in published studies of speech perception in DD.

Auditory working memory predicts individual differences in absolute pitch learning

Absolute pitch (AP) is typically defined as the ability to label an isolated tone as a musical note in the absence of a reference tone. At first glance the acquisition of AP note categories seems like a perceptual learning task, since individuals must assign a category label to a stimulus based on a single perceptual dimension (pitch) while ignoring other perceptual dimensions (e.g., loudness, octave, instrument). AP, however, is rarely discussed in terms of domain-general perceptual learning mechanisms. This is because AP is typically assumed to depend on a critical period of development, in which early exposure to pitches and musical labels is thought to be necessary for the development of AP precluding the possibility of adult acquisition of AP. Despite this view of AP, several previous studies have found evidence that absolute pitch category learning is, to an extent, trainable in a post-critical period adult population, even if the performance typically achieved by this population is below the performance of a "true" AP possessor. The current studies attempt to understand the individual differences in learning to categorize notes using absolute pitch cues by testing a specific prediction regarding cognitive capacity related to categorization - to what extent does an individual's general auditory working memory capacity (WMC) predict the success of absolute pitch category acquisition. Since WMC has been shown to predict performance on a wide variety of other perceptual and category learning tasks, we predict that individuals with higher WMC should be better at learning absolute pitch note categories than individuals with lower WMC. Across two studies, we demonstrate that auditory WMC predicts the efficacy of learning absolute pitch note categories. These results suggest that a higher general auditory WMC might underlie the formation of absolute pitch categories for post-critical period adults. Implications for understanding the mechanisms that underlie the phenomenon of AP are also discussed.

Relationship between perceptual learning in speech and statistical learning in younger and older adults

Within a few sentences, listeners learn to understand severely degraded speech such as noise-vocoded speech. However, individuals vary in the amount of such perceptual learning and it is unclear what underlies these differences. The present study investigates whether perceptual learning in speech relates to statistical learning, as sensitivity to probabilistic information may aid identification of relevant cues in novel speech input. If statistical learning and perceptual learning (partly) draw on the same general mechanisms, then statistical learning in a non-auditory modality using non-linguistic sequences should predict adaptation to degraded speech. In the present study, 73 older adults (aged over 60 years) and 60 younger adults (aged between 18 and 30 years) performed a visual artificial grammar learning task and were presented with 60 meaningful noise-vocoded sentences in an auditory recall task. Within age groups, sentence recognition performance over exposure was analyzed as a function of statistical learning performance, and other variables that may predict learning (i.e., hearing, vocabulary, attention switching control, working memory, and processing speed). Younger and older adults showed similar amounts of perceptual learning, but only younger adults showed significant statistical learning. In older adults, improvement in understanding noise-vocoded speech was constrained by age. In younger adults, amount of adaptation was associated with lexical knowledge and with statistical learning ability. Thus, individual differences in general cognitive abilities explain listeners' variability in adapting to noise-vocoded speech. Results suggest that perceptual and statistical learning share mechanisms of implicit regularity detection, but that the ability to detect statistical regularities is impaired in older adults if visual sequences are presented quickly.

Perceptual learning: top to bottom

Perceptual learning has traditionally been portrayed as a bottom-up phenomenon that improves encoding or decoding of the trained stimulus. Cognitive skills such as attention and memory are thought to drive, guide and modulate learning but are, with notable exceptions, not generally considered to undergo changes themselves as a result of training with simple perceptual tasks. Moreover, shifts in threshold are interpreted as shifts in perceptual sensitivity, with no consideration for non-sensory factors (such as response bias) that may contribute to these changes. Accumulating evidence from our own research and others shows that perceptual learning is a conglomeration of effects, with training-induced changes ranging from the lowest (noise reduction in the phase locking of auditory signals) to the highest (working memory capacity) level of processing, and includes contributions from non-sensory factors that affect decision making even on a "simple" auditory task such as frequency discrimination. We discuss our emerging view of learning as a process that increases the signal-to-noise ratio associated with perceptual tasks by tackling noise sources and inefficiencies that cause performance bottlenecks, and present some implications for training populations other than young, smart, attentive and highly-motivated college students.

Brain activity underlying auditory perceptual learning during short period training: simultaneous fMRI and EEG recording

BACKGROUND

There is an accumulating body of evidence indicating that neuronal functional specificity to basic sensory stimulation is mutable and subject to experience. Although fMRI experiments have investigated changes in brain activity after relative to before perceptual learning, brain activity during perceptual learning has not been explored. This work investigated brain activity related to auditory frequency discrimination learning using a variational Bayesian approach for source localization, during simultaneous EEG and fMRI recording. We investigated whether the practice effects are determined solely by activity in stimulus-driven mechanisms or whether high-level attentional mechanisms, which are linked to the perceptual task, control the learning process.

RESULTS

The results of fMRI analyses revealed significant attention and learning related activity in left and right superior temporal gyrus STG as well as the left inferior frontal gyrus IFG. Current source localization of simultaneously recorded EEG data was estimated using a variational Bayesian method. Analysis of current localized to the left inferior frontal gyrus and the right superior temporal gyrus revealed gamma band activity correlated with behavioral performance.

CONCLUSIONS

Rapid improvement in task performance is accompanied by plastic changes in the sensory cortex as well as superior areas gated by selective attention. Together the fMRI and EEG results suggest that gamma band activity in the right STG and left IFG plays an important role during perceptual learning.

A universal approach to modeling visual word recognition and reading: Not only possible, but also inevitable

I have argued that orthographic processing cannot be understood and modeled without considering the manner in which orthographic structure represents phonological, semantic, and morphological information in a given writing system. A reading theory, therefore, must be a theory of the interaction of the reader with his/her linguistic environment. This outlines a novel approach to studying and modeling visual word recognition, an approach that focuses on the common cognitive principles involved in processing printed words across different writing systems. These claims were challenged by several commentaries that contested the merits of my general theoretical agenda, the relevance of the evolution of writing systems, and the plausibility of finding commonalities in reading across orthographies. Other commentaries extended the scope of the debate by bringing into the discussion additional perspectives. My response addresses all these issues. By considering the constraints of neurobiology on modeling reading, developmental data, and a large scope of cross-linguistic evidence, I argue that front-end implementations of orthographic processing that do not stem from a comprehensive theory of the complex information conveyed by writing systems do not present a viable approach for understanding reading. The common principles by which writing systems have evolved to represent orthographic, phonological, and semantic information in a language reveal the critical distributional characteristics of orthographic structure that govern reading behavior. Models of reading should thus be learning models, primarily constrained by cross-linguistic developmental evidence that describes how the statistical properties of writing systems shape the characteristics of orthographic processing. When this approach is adopted, a universal model of reading is possible.

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.

The effects of context and musical training on auditory temporal-interval discrimination

Non sensory factors such as stimulus context and musical experience are known to influence auditory frequency discrimination, but whether the context effect extends to auditory temporal processing remains unknown. Whether individual experiences such as musical training alter the context effect is also unknown. The goal of the present study was therefore to investigate the effects of stimulus context and musical experience on auditory temporal-interval discrimination. In experiment 1, temporal-interval discrimination was compared between fixed context conditions in which a single base temporal interval was presented repeatedly across all trials and variable context conditions in which one of two base intervals was randomly presented on each trial. Discrimination was significantly better in the fixed than in the variable context conditions. In experiment 2 temporal discrimination thresholds of musicians and non-musicians were compared across 3 conditions: a fixed context condition in which the target interval was presented repeatedly across trials, and two variable context conditions differing in the frequencies used for the tones marking the temporal intervals. Musicians outperformed non-musicians on all 3 conditions, but the effects of context were similar for the two groups. Overall, it appears that, like frequency discrimination, temporal-interval discrimination benefits from having a fixed reference. Musical experience, while improving performance, did not alter the context effect, suggesting that improved discrimination skills among musicians are probably not an outcome of more sensitive contextual facilitation or predictive coding mechanisms.

The effects of global and local stimulus context on auditory frequency discrimination

Frequency discrimination thresholds are influenced by various listener, stimulus and task related factors. Here we are interested in the effects of repetitions within the stimulus set used in the discrimination task, and more specifically in the effects of stimulus repetitions within trials (local context) and across trials (global context). In order to examine these aspects, twenty native Hebrew speaking students (10 males and 10 females, aged 20-28), with no known language, hearing, learning or other cognitive or functional problems, performed 4 different auditory frequency discrimination tests differing in the degrees of global and local context they provide. Our findings show that discrimination thresholds were affected by both global and local contexts. In addition, the discrimination thresholds in the condition containing both local and global context were significantly lower than in the condition containing only global context or only local context. There was however no interaction between the two effects. These findings suggest that the effects of local and global context are at least to some extent, distinct, with each making a separate contribution to performance. Furthermore our findings show that the effect of global context was larger than that of local context, which could suggest that each of these effects is related to a separate component of working memory.

On the importance of anchoring and the consequences of its impairment in dyslexia

One of the main impediments of individuals with reading difficulties and individuals with language difficulties is poor working memory. Typically measured using verbal stimuli, working memory deficits have often been considered as one aspect of the phonological difficulty putatively underlying dyslexia. Over the years it has been shown that a broad range of auditory discrimination abilities are also mildly impaired. Here we present evidence that a domain general, rather than a phonology specific, deficits in the ability to implicitly use contextual information, which we term anchoring, can account for both types of deficits. We propose that anchoring ability, which reflects a basic biological mechanism for replacing effortful mechanisms of explicit working memory with automatic mechanisms of implicit memory, and consequently boost performance in both perceptual and cognitive tasks, is a crucial factor in our ability to be expert users of oral and written language.