Developmental dyslexia and discrimination in speech perception: a dynamic model study

At the behavioral level one of the primary disturbances involved in congenital dyslexia concerns phonological processing. At the neuroarchitectural level autopsies have revealed ectopies, e.g., a reduced number of neurons in the upper layers of the cortex and an increased number in the lower ones. In dynamic models of interacting neuronal populations the behavioral level can be related to the neurophysiological level. In this study an attempt is made to do so at the cortical level. The first focus of this model study are the results of a Finnish experiment assessing geminate stop perception in quasi speech stimuli by 6 month old infants using a head turning paradigm and evoked potentials. The second focus of this study are the results of a Dutch experiment assessing discrimination of transients in speech stimuli, by adult dyslexics and controls and 2 month old infants. There appears to be a difference in the phonemic perceptual boundaries of children at genetic risk for dyslexia and control children as revealed in the Finnish study. Assuming a lowered neuronal density in the 'dyslexic' model, reflecting ectopies, it may be postulated that less neuronal surface is available for synaptic connections resulting in a lowered synaptic density and thus a lowered amount of available neurotransmitter. A lowered synaptic density also implies a reduced amount of membrane surface available for neurotransmitter metabolism. By assuming both, a reduced upper bound of neurotransmitter and a reduced metabolic transmitter rate in the dynamic model, the Finnish experimental results can be approximated closely. This applies both to data from behavioral head turning and that of the evoked potential study. In the Dutch study adult dyslexics show poor performance in discriminating transients in the speech signal compared to the controls. The same stimuli were used in a a study comparing infants from dyslexic families and controls. Using the same transmitter parameters as in modeling the results of the Finnish study, also in this case the experimental results for adults and infants can be approximated closely. Simulation of behavioral and pharmaceutical interventions with the model provide predictions which can be put to the test in experiments.     

Towards a functional neuroanatomy of speech perception

The functional neuroanatomy of speech perception has been difficult to characterize. Part of the difficulty, we suggest, stems from the fact that the neural systems supporting 'speech perception' vary as a function of the task. Specifically, the set of cognitive and neural systems involved in performing traditional laboratory speech perception tasks, such as syllable discrimination or identification, only partially overlap those involved in speech perception as it occurs during natural language comprehension. In this review, we argue that cortical fields in the posterior-superior temporal lobe, bilaterally, constitute the primary substrate for constructing sound-based representations of speech, and that these sound-based representations interface with different supramodal systems in a task-dependent manner. Tasks that require access to the mental lexicon (i.e. accessing meaning-based representations) rely on auditory-to-meaning interface systems in the cortex in the vicinity of the left temporal-parietal-occipital junction. Tasks that require explicit access to speech segments rely on auditory-motor interface systems in the left frontal and parietal lobes. This auditory-motor interface system also appears to be recruited in phonological working memory.     

Testing between the TRACE model and the fuzzy logical model of speech perception

The TRACE model of speech perception (McClelland & Elman, 1986) is contrasted with a fuzzy logical model of perception (FLMP) (Oden & Massaro, 1978). The central question is how the models account for the influence of multiple sources of information on perceptual judgment. Although the two models can make somewhat similar predictions, the assumptions underlying the models are fundamentally different. The TRACE model is built around the concept of interactive activation, whereas the FLMP is structured in terms of the integration of independent sources of information. The models are tested against test results of an experiment involving the independent manipulation of bottom-up and top-down sources of information. Using a signal detection framework, sensitivity and bias measures of performance can be computed. The TRACE model predicts that top-down influences from the word level influence sensitivity at the phoneme level, whereas the FLMP does not. The empirical results of a study involving the influence of phonological context and segmental information on the perceptual recognition of a speech segment are best described without any assumed changes in sensitivity. To date, not only is a mechanism of interactive activation not necessary to describe speech perception, it is shown to be wrong when instantiated in the TRACE model.     

Evaluation and integration of visual and auditory information in speech perception

Three experiments were carried out to investigate the evaluation and integration of visual and auditory information in speech perception. In the first two experiments, subjects identified /ba/ or /da/ speech events consisting of high-quality synthetic syllables ranging from /ba/ to /da/ combined with a videotaped /ba/ or /da/ or neutral articulation. Although subjects were specifically instructed to report what they heard, visual articulation made a large contribution to identification. The tests of quantitative models provide evidence for the integration of continuous and independent, as opposed to discrete or nonindependent, sources of information. The reaction times for identification were primarily correlated with the perceived ambiguity of the speech event. In a third experiment, the speech events were identified with an unconstrained set of response alternatives. In addition to /ba/ and /da/ responses, the /bda/ and /tha/ responses were well described by a combination of continuous and independent features. This body of results provides strong evidence for a fuzzy logical model of perceptual recognition.     

The case against a speech mode of perception

On the basis of evidence from six areas of speech research, it is argued that there is no reason to assume that speech stimuli are processed by structures that are inherently different from structures used for other auditory stimuli. It is concluded that speech and non-speech auditory stimuli are probably perceived in the same way.     

The perception of alternated speech

When speech is rapidly alternated between the two ears, intelligibility declines as rates approach 3–5 switching cycles/sec and then paradoxically returns to a good level beyond that point. The present study examines previous explanations of the phenomenon by comparing intelligibility of alternated speech with that for presentation of an interrupted message to a single ear. Results favor one of two possible explanations, and a theoretical model to account for the effect is proposed.     

Chronometric analysis of speech perception

The relationship between the phonological properties of speech sounds and the corresponding semantic entries was studied in two experiments using response time measures. Monosyllabic words and nonsense words were used in both experiments. In Experiment I. Ss were each presented with individual items and were required, in three different conditions, to respond positively if (1) the item contained a particular final consonant, (2) the item was a real word, (3) the item contained either a particular consonant or was a real word. Latencies indicated that separate retrieval of phonological and lexical information took about the same time. but that their combined retrieval was longer, indicating a serial or overlapping process. In Experiment II, Ss were presented with pairs of items, and they responded positively if (1) the two items were physically identical, (2) the two items were lexically identical (both real words or both nonsense words). Response latencies were longer for lexical than for physical matches. Lexical matches were’ significantly slower than physical matches even on the same pair of items. The results imply differential accessibility to separate loci of phonological and semantic features.     

Prenatal maternal speech influences newborns' perception of speech sounds

Pregnant women recited a particular speech passage aloud each day during their last 6 weeks of pregnancy. Their newborns were tested with an operant-choice procedure to determine whether the sounds of the recited passage were more reinforcing than the sounds of a novel passage. The previously recited passage was more reinforcing. The reinforcing value of the two passages did not differ for a matched group of control subjects. Thus, third-trimester fetuses experience their mothers' speech sounds and that prenatal auditory experience can influence postnatal auditory preferences.     

Phonological context in speech perception

Speech perception can be viewed in terms of the listener’s integration of two sources of information: the acoustic features transduced by the auditory receptor system and the context of the linguistic message. The present research asked how these sources were evaluated and integrated in the identification of synthetic speech. A speech continuum between the glide-vowel syllables /ri/ and /li/ was generated by varying the onset frequency of the third formant. Each sound along the continuum was placed in a consonant-cluster vowel syllable after an initial consonant /p/, /t/, /s/, and /v/. In English, both /r/ and /l/ are phonologically admissible following /p/ but are not admissible following /v/. Only /l/ is admissible following /s/ and only /r/ is admissible following /t/. A third experiment used synthetic consonant-cluster vowel syllables in which the first consonant varied between /b/ and /d and the second consonant varied between /l/ and /r/. Identification of synthetic speech varying in both acoustic featural information and phonological context allowed quantitative tests of various models of how these two sources of information are evaluated and integrated in speech perception.     

Audio-visual speech perception is special

In face-to-face conversation speech is perceived by ear and eye. We studied the prerequisites of audio-visual speech perception by using perceptually ambiguous sine wave replicas of natural speech as auditory stimuli. When the subjects were not aware that the auditory stimuli were speech, they showed only negligible integration of auditory and visual stimuli. When the same subjects learned to perceive the same auditory stimuli as speech, they integrated the auditory and visual stimuli in a similar manner as natural speech. These results demonstrate the existence of a multisensory speech-specific mode of perception.     

Within-category discriminations in speech perception

The assumption that listeners are unaware of the highly encoded acoustic properties which lead to phoneme identification is questioned in the present study. It was found that some subjects can make use of small differences in voice onset time when making within-category discriminations. Subjects who can use these auditory features do so both implicitly (in a phonetic match task) and deliberately (in a physical match task). Results also indicate that some type of parallel process model is needed to account for the processing of auditory and phonetic information.     

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.     

Metrical perception of trisyllabic speech rhythms

The perception of duration-based syllabic rhythm was examined within a metrical framework. Participants assessed the duration patterns of four-syllable phrases set within the stress structure XxxX (an Abercrombian trisyllabic foot). Using on-screen sliders, participants created percussive sequences that imitated speech rhythms and analogous non-speech monotone rhythms. There was a tendency to equalize the interval durations for speech stimuli but not for non-speech. Despite the perceptual regularization of syllable durations, different speech phrases were conceived in various rhythmic configurations, pointing to a diversity of perceived meters in speech. In addition, imitations of speech stimuli showed more variability than those of non-speech. Rhythmically skilled listeners exhibited lower variability and were more consistent with vowel-centric estimates when assessing speech stimuli. These findings enable new connections between meter- and duration-based models of speech rhythm perception.     

Connectionist modeling of speech perception.

Connectionist models of perception and cognition, including the process of deducing meaningful messages from patterns of acoustic waves emitted by vocal tracts, are developed and refined as human understanding of brain function, psychological processes, and the properties of massively parallel architectures advances. The present article presents several important contributions from diverse points of view in the area of connectionist modeling of speech perception and discusses their relative merits with respect to specific theoretical issues and empirical findings. TRACE, the Elman/Norris net, and Adaptive Resonance Theory constitute pivotal points exemplifying overall modeling success, progress in temporal representation, and plausible modeling of learning, respectively. Other modeling efforts are presented for the specific insights they offer, and the article concludes with a discussion of computational versus dynamic modeling of phonological processes.     

Significance of pauses for speech perception

Pauses can be used to facilitate certain operations involved in the production and in the perception of speech. In the case of speech perception, pauses have been found to improve the accuracy of detection and the recall of lists of digits and letters. The aim of the present experiments was to examine the effects of pause time on the perception of sentences. In experiment I, a semantic categorization task was used and in experiment II a sentence recall task. The results indicated that in sentences containing pauses between clauses, words were categorized more rapidly (experiment I) and propositions were recalled more accurately (experiment II) than in sentences containing pauses, within the clause. The results are interpreted in the context of existing models of speech processing, and the significane of pause time for cognitive activity is discussed.     

Levels of processing in speech perception

Four experiments are reported investigating previous findings that speech perception interferes with concurrent verbal memory but difficult nonverbal perceptual tasks do not, to any great degree. The forgetting produced by processing noisy speech could not be attributed to task difficulty, since equally difficult nonspeech tasks did not produce forgetting, and the extent of forgetting produced by speech could be manipulated independently of task difficulty. The forgetting could not be attributed to similarity between memory material and speech stimuli, since clear speech, analyzed in a simple and probably acoustically mediated discrimination task, produced little forgetting. The forgetting could not be attributed to a combination of similarity and difficutly since a very easy speech task involving clear speech produced as much forgetting as noisy speech tasks, as long as overt reproduction of the stimuli was required. By assuming that noisy speech and overtly reproduced speech are processed at a phonetic level but that clear, repetitive speech can be processed at a purely acoustic level, the forgetting produced by speech perception could be entirely attributed to the level at which the speech was processed. In a final experiment, results were obtained which suggest that if prior set induces processing of noisy and clear speech at comparable levels, the difference between the effects of noisy speech processing and clear speech processing on concurrent memory is completely eliminated.     

Segmentation of coarticulated speech in perception

The research investigates how listeners segment the acoustic speech signal into phonetic segments and explores implications that the segmentation strategy may have for their perception of the (apparently) context-sensitive allophones of a phoneme. Two manners of segmentation are contrasted. In one, listeners segment the signal into temporally discrete, context-sensitive segments. In the other, which may be consistent with the talker’s production of the segments, they partition the signal into separate, but overlapping, segments freed of their contextual influences. Two complementary predictions of the second hypothesis are tested. First, listeners will use anticipatory coarticulatory information for a segment as information for the forthcoming segment. Second, subjects will not hear anticipatory coarticulatory information as part of the phonetic segment with which it co-occurs in time. The first hypothesis is supported by findings on a choice reaction time procedure; the second is supported by findings on a 4IAX discrimination test. Implications of the findings for theories of speech production, perception, and of the relation between the two are considered.