Two distinct neural networks support the mapping of meaning to a novel word

Children can learn the meaning of a new word from context during normal reading or listening, without any explicit instruction. It is unclear how such meaning acquisition is supported and achieved in human brain. In this functional magnetic resonance imaging (fMRI) study we investigated neural networks supporting word learning with a functional connectivity approach. Participants were exposed to a new word presented in two successive sentences and needed to derive the meaning of the new word. We observed two neural networks involved in mapping the meaning to the new word. One network connected the left inferior frontal gyrus (LIFG) with the middle frontal gyrus (MFG), medial superior frontal gyrus, caudate nucleus, thalamus, and inferior parietal lobule. The other network connected the left middle temporal gyrus (LMTG) with the MFG, anterior and posterior cingulate cortex. The LIFG network showed stronger interregional interactions for new than real words, whereas the LMTG network showed similar connectivity patterns for new and real words. We proposed that these two networks support different functions during word learning. The LIFG network appears to select the most appropriate meaning from competing candidates and to map the selected meaning onto the new word. The LMTG network may be recruited to integrate the word into sentential context, regardless of whether the word is real or new. The LIFG and the LMTG networks share a common node, the MFG, suggesting that these two networks communicate in working memory. Hum Brain Mapp, 2011. © 2010 Wiley‐Liss, Inc.     

Experiments for isolated-word recognition with single- and two-layer perceptrons

Several design strategies for feed-forward networks are examined within the scope of pattern classification. Single- and two-layer perceptron models are adapted for experiments in isolated-word recognition. Direct (one-step) classification as well as several hierarchical (two-step) schemes have been considered. For a vocabulary of 20 English words spoken repeatedly by 11 speakers, the word classes are found to be separable by hyperplanes in the chosen feature space. Since for speaker-dependent word recognition the underlying data base contains only a small training set, an automatic expansion of the training material improves the generalization properties of the networks. This method accounts for a wide variety of observable temporal structures for each word and gives a better overall estimate of the network parameters which leads to a recognition rate of 99.5%. For speaker-independent word recognition, a hierarchical structure with pairwise training of two-class models is superior to a single uniform network (98% average recognition rate).     

Top-down influences on lexical selection during spoken word production: A 4T fMRI investigation of refractory effects in picture naming

Spoken word production is assumed to involve stages of processing in which activation spreads through layers of units comprising lexical-conceptual knowledge and their corresponding phonological word forms. Using high-field (4T) functional magnetic resonance imaging (fMRI), we assessed whether the relationship between these stages is strictly serial or involves cascaded-interactive processing, and whether central (decision/control) processing mechanisms are involved in lexical selection. Participants performed the competitor priming paradigm in which distractor words, named from a definition and semantically related to a subsequently presented target picture, slow picture-naming latency compared to that with unrelated words. The paradigm intersperses two trials between the definition and the picture to be named, temporally separating activation in the word perception and production networks. Priming semantic competitors of target picture names significantly increased activation in the left posterior temporal cortex, and to a lesser extent the left middle temporal cortex, consistent with the predictions of cascaded-interactive models of lexical access. In addition, extensive activation was detected in the anterior cingulate and pars orbitalis of the inferior frontal gyrus. The findings indicate that lexical selection during competitor priming is biased by top-down mechanisms to reverse associations between primed distractor words and target pictures to select words that meet the current goal of speech.     

Alpha activity marking word boundaries mediates speech segmentation

This study examined the neurophysiological mechanisms of speech segmentation, the process of parsing the continuous speech signal into isolated words. Individuals listened to sequences of two monosyllabic words (e.g. gas source) and non‐words (e.g. nas sorf). When these phrases are spoken, talkers usually produce one continuous s‐sound, not two distinct s‐sounds, making it unclear where one word ends and the next one begins. This ambiguity in the signal can also result in perceptual ambiguity, causing the sequence to be heard as one word (failed to segment) or two words (segmented). We compared listeners' electroencephalogram activity when they reported hearing one word or two words, and found that bursts of fronto‐central alpha activity (9–14 Hz), following the onset of the physical /s/ and end of phrase, indexed speech segmentation. Left‐lateralized beta activity (14–18 Hz) following the end of phrase distinguished word from non‐word segmentation. A hallmark of enhanced alpha activity is that it reflects inhibition of task‐irrelevant neural populations. Thus, the current results suggest that disengagement of neural processes that become irrelevant as the words unfold marks word boundaries in continuous speech, leading to segmentation. Beta activity is likely associated with unifying word representations into coherent phrases.     

Electrophysiological evidence for prelinguistic infants' word recognition in continuous speech

Children begin to talk at about age one. The vocabulary they need to do so must be built on perceptual evidence and, indeed, infants begin to recognize spoken words long before they talk. Most of the utterances infants hear, however, are continuous, without pauses between words, so constructing a vocabulary requires them to decompose continuous speech in order to extract the individual words. Here, we present electrophysiological evidence that 10-month-old infants recognize two-syllable words they have previously heard only in isolation when these words are presented anew in continuous speech. Moreover, they only need roughly the first syllable of the word to begin doing this. Thus, prelinguistic infants command a highly efficient procedure for segmentation and recognition of spoken words in the absence of an existing vocabulary, allowing them to tackle effectively the problem of bootstrapping a lexicon out of the highly variable, continuous speech signals in their environment.     

Neural correlates of lexical access during visual word recognition

People can discriminate real words from nonwords even when the latter are orthographically and phonologically word-like, presumably because words activate specific lexical and/or semantic information. We investigated the neural correlates of this identification process using event-related functional magnetic resonance imaging (fMRI). Participants performed a visual lexical decision task under conditions that encouraged specific word identification: Nonwords were matched to words on orthographic and phonologic characteristics, and accuracy was emphasized over speed. To identify neural responses associated with activation of nonsemantic lexical information, processing of words and nonwords with many lexical neighbors was contrasted with processing of items with no neighbors. The fMRI data showed robust differences in activation by words and word-like nonwords, with stronger word activation occurring in a distributed, left hemisphere network previously associated with semantic processing, and stronger nonword activation occurring in a posterior inferior frontal area previously associated with grapheme-to-phoneme mapping. Contrary to lexicon-based models of word recognition, there were no brain areas in which activation increased with neighborhood size. For words, activation in the left prefrontal, angular gyrus, and ventrolateral temporal areas was stronger for items without neighbors, probably because accurate responses to these items were more dependent on activation of semantic information. The results show neural correlates of access to specific word information. The absence of facilitatory lexical neighborhood effects on activation in these brain regions argues for an interpretation in terms of semantic access. Because subjects performed the same task throughout, the results are unlikely to be due to task-specific attentional, strategic, or expectancy effects.     

Facilitating and disrupting speech perception in word deafness

Background: Word deafness is a rare condition where pathologically degraded speech perception results in impaired repetition and comprehension but otherwise intact linguistic skills. Although impaired linguistic systems in aphasia resulting from damage to the neural language system (here termed central impairments) have consistently been shown to be amenable to external influences such as linguistic or contextual information (e.g., cueing effects in naming), it is not known whether similar influences can be shown for aphasia arising from damage to a perceptual system (here termed peripheral impairments).

Aims: This study aimed to investigate the extent to which pathologically degraded speech perception could be facilitated or disrupted by providing visual as well as auditory information.

Methods & Procedures: In three word repetition tasks, the participant with word deafness (AB) repeated words under different conditions: words were repeated in the context of a pictorial or written target, a distractor (semantic, unrelated, rhyme, or phonological neighbour) or a blank page (nothing). Accuracy and error types were analysed.

Outcomes & Results: AB was impaired at repetition in the blank condition, confirming her degraded speech perception. Repetition was significantly facilitated when accompanied by a picture or written example of the word and significantly impaired by the presence of a written rhyme. Errors in the blank condition were primarily formal, whereas errors in the rhyme condition were primarily miscues (saying the distractor word rather than the target).

Conclusions: Cross-modal input can both facilitate and further disrupt repetition in word deafness. The cognitive mechanisms behind these findings are discussed. Both top-down influence from the lexical layer on perceptual processes and intra-lexical competition within the lexical layer may play a role.     

Single-word intelligibility testing in aphasia: Alternate forms reliability,phonetic complexity and word frequency

Background: We recently constructed a single-word speech intelligibility test intended to quantify magnitude of speech sound impairment in individuals with aphasia. To minimise listener learning and strategising, the test included a large set of possible (alternate) test forms and was constructed with high and diverse phonologic similarity among candidate words. Although the corpus was limited in phonetic complexity to single syllables, and although criteria for minimal word frequency were applied, it is possible that more fine-grained differences in phonetic complexity and/or word frequency may introduce varying levels of difficulty across alternate test forms and that reliability, therefore, may be compromised.

Aims: The dual purposes of this study were to evaluate alternate forms reliability for the new intelligibility test and to determine whether word frequency and/or phonetic complexity affected word identification scores.

Methods & Procedures: All words in the 600-word test corpus were coded for overall phonetic complexity and for frequency of occurrence in spoken English. Fifty-one versions of the target 50-word test were generated from this corpus by following designated pseudo-random selection procedures. Speech samples were collected from 13 speakers with aphasia, who each repeated three or four of these 50-word sets. Ten normal-hearing listeners were asked to indicate the words they thought the speakers were trying to say. Per cent accuracy was computed for each speaker, test form and target word.

Outcomes & Results: The intra-class correlation within speakers was 0.97, indicating that the scores across alternate forms were highly reproducible. Analyses at the word level showed that both phonetic complexity and word frequency affected identification accuracy. The effects were seen in almost all participants whose intelligibility scores were in the impaired range.

Conclusions: High frequency of occurrence and low phonetic complexity increase repetition accuracy for individuals with moderate to severe sound production difficulties and aphasia. However, the pseudo-random word selection in the examined word intelligibility test was sufficient to ensure strong alternate forms reliability. Further constraints on the word selection process for this test are not warranted.     

Lexical and child-related factors in word variability and accuracy in infants

The present study investigated the effects of lexical age of acquisition (AoA), phonological complexity, age and expressive vocabulary on spoken word variability and accuracy in typically developing infants, aged 1;9–3;1. It was hypothesized that later-acquired words and those with more complex speech sounds would be produced more variably and less accurately than earlier-acquired words and those with less complex speech sounds. It was also hypothesized that word variability would decrease and word accuracy would increase with increasing age and vocabulary knowledge. Participants' productions of 20 target words, experimentally controlled for AoA and phonological complexity, were audio-recorded during a play session. Results revealed a nonsignificant effect of AoA on variability and accuracy, a significant effect of phonological complexity on variability and accuracy, a significant effect of age on variability and accuracy and a significant effect of vocabulary on variability. Theoretical and clinical implications are discussed.     

Predictability affects early perceptual processing of word onsets in continuous speech

Event-related potential (ERP) evidence indicates that listeners selectively attend to word onsets in continuous speech, but the reason for this preferential processing is unknown. The current study measured ERPs elicited by syllable onsets in an artificial language to test the hypothesis that listeners direct attention to word onsets because their identity is unpredictable. Both before and after recognition training, participants listened to a continuous stream of six nonsense words arranged in pairs, such that the second word in each pair was completely predictable. After training, first words in pairs elicited a larger negativity beginning around 100 ms after onset. This effect was not evident for the completely predictable second words in pairs. These results suggest that listeners are most likely to attend to the segments in speech that they are least able to predict.     

The effects of lexical stress in aphasic word production

This paper investigates the effects of lexical stress pattern on speech production by aphasic subjects. The subjects are more accurate at producing words with first syllable stress than those with stress on the second syllable in both word and non-word repetition and picture naming. The subjects' accuracy in repetition is unaffected by the stress pattern of a preceding word in a phrase, but strongly affected by the phrase's syllable length. With three-syllable words, subjects are most accurate in both repetition and naming with words whose primary stress falls on either the first or the third syllable. Explanations of these results in terms of the vulnerability of initial unstressed syllables, or simple frequency-dependent stress assignment strategies, are rejected. Instead it is argued that an utterance's segmental and metrical structure are computed in parallel, but the computation requires resources that are limited in subjects who make phonological errors in speech production.     

Oscillatory activity and EEG phase synchrony of concurrent word segmentation and meaning-mapping in 9-year-old children

When learning a new language, one must segment words from continuous speech and associate them with meanings. These complex processes can be boosted by attentional mechanisms triggered by multi-sensory information. Previous electrophysiological studies suggest that brain oscillations are sensitive to different hierarchical complexity levels of the input, making them a plausible neural substrate for speech parsing. Here, we investigated the functional role of brain oscillations during concurrent speech segmentation and meaning acquisition in sixty 9-year-old children. We collected EEG data during an audio-visual statistical learning task during which children were exposed to a learning condition with consistent word-picture associations and a random condition with inconsistent word-picture associations before being tested on their ability to recall words and word-picture associations. We capitalized on the brain dynamics to align neural activity to the same rate as an external rhythmic stimulus to explore modulations of neural synchronization and phase synchronization between electrodes during multi-sensory word learning. Results showed enhanced power at both word- and syllabic-rate and increased EEG phase synchronization between frontal and occipital regions in the learning compared to the random condition. These findings suggest that multi-sensory cueing and attentional mechanisms play an essential role in children's successful word learning.     

On pure word deafness, temporal processing, and the left hemisphere.

Pure word deafness (PWD) is a rare neurological syndrome characterized by severe difficulties in understanding and reproducing spoken language, with sparing of written language comprehension and speech production. The pathognomonic disturbance of auditory comprehension appears to be associated with a breakdown in processes involved in mapping auditory input to lexical representations of words, but the functional locus of this disturbance and the localization of the responsible lesion have long been disputed. We report here on a woman with PWD resulting from a circumscribed unilateral infarct involving the left superior temporal lobe who demonstrated significant problems processing transitional spectrotemporal cues in both speech and nonspeech sounds. On speech discrimination tasks, she exhibited poor differentiation of stop consonant-vowel syllables distinguished by voicing onset and brief formant frequency transitions. Isolated formant transitions could be reliably discriminated only at very long durations (> 200 ms). By contrast, click fusion threshold, which depends on millisecond-level resolution of brief auditory events, was normal. These results suggest that the problems with speech analysis in this case were not secondary to general constraints on auditory temporal resolution. Rather, they point to a disturbance of left hemisphere auditory mechanisms that preferentially analyze rapid spectrotemporal variations in frequency. The findings have important implications for our conceptualization of PWD and its subtypes.     

Sex differentiates the role of emotional prosody during word processing

The meaning of a speech stream is communicated by more than the particular words used by the speaker. For example, speech melody, referred to as prosody, also contributes to meaning. In a cross-modal priming study we investigated the influence of emotional prosody on the processing of visually presented positive and negative target words. The results indicate that emotional prosody modulates word processing and that the time-course of this modulation differs for males and females. Women show behavioural and electrophysiological priming effects already with a small interval between the prosodic prime and the visual target word. In men, however, similar effects of emotional prosody on word processing occur only for a longer interval between prime and target. This indicates that women make an earlier use of emotional prosody during word processing as compared to men.     

A contextual approach to facilitating word retrieval in agrammatic aphasia

Background: Although virtually all individuals with aphasia demonstrate problems with word retrieval, this symptom might arise for different reasons in individuals with different types of aphasia. A trade‐off of dependence on semantic and syntactic information is hypothesised to underlie dissociations in word retrieval shown by fluent anomic and non‐fluent agrammatic speakers. This division of labour predicts that strengthening semantic input will improve word retrieval for those with anomic aphasia, whereas strengthening syntactic input through contextual cues will improve word retrieval for those with agrammatic aphasia.

Aims: To explore the outcome of a new treatment approach that proposes to improve word retrieval in individuals with agrammatic aphasia by implicitly strengthening the links between target words and associated words which co‐occur in a connected speech context.

Methods & Procedures: The outcomes of two therapy approaches were examined in two participants with agrammatic aphasia. One therapy approach focused on explicitly training semantic features of target words; the other, more novel, approach focused on implicitly strengthening contextual associations through story telling and retelling. It was predicted that the latter approach would result in greater benefits for the two agrammatic participants.

Outcomes & Results: Although both therapy approaches appeared to be effective, the predicted advantage of the contextually based approach was not found. The evolution of the error patterns throughout the treatment was examined to help understand the mechanisms underlying the improvements shown for each participant.

Conclusions: A novel treatment involving training words in a story context was shown to result in improved word retrieval for two participants with agrammatic aphasia. The merits of the approach are discussed, relative to more traditional explicit word retrieval therapy approaches.     

Syllable-frequency effects in visual word recognition: evidence from ERPs

A lexical decision experiment was conducted while event related potentials (ERPs) were recorded. The word frequency and the first syllable frequency of each word were manipulated. Results showed that, while high frequency words produced less negative amplitudes in the N400 time window than low frequency words, the inverse pattern was found for syllable frequency. Words containing high frequency syllables produced more negative amplitudes than words containing low frequency syllables. Importantly, a significant syllable frequency effect was also obtained at the P200 time window. The results are interpreted in the framework of an interactive activation model, in which high frequency syllables produce the initial activation of a larger number of lexical candidates during the analysis of orthographic or phonological representations, which have to be inhibited later to allow the identification of a unique word. These findings support the idea that, at least in languages with clear syllabic boundaries, syllables are functional sublexical units during visual word recognition.     

Learning word order at birth: A NIRS study

In language, the relative order of words in sentences carries important grammatical functions. However, the developmental origins and the neural correlates of the ability to track word order are to date poorly understood. The current study therefore investigates the origins of infants’ ability to learn about the sequential order of words, using near-infrared spectroscopy (NIRS) with newborn infants. We have conducted two experiments: one in which a word order change was implemented in 4-word sequences recorded with a list intonation (as if each word was a separate item in a list; list prosody condition, Experiment 1) and one in which the same 4-word sequences were recorded with a well-formed utterance-level prosodic contour (utterance prosody condition, Experiment 2). We found that newborns could detect the violation of the word order in the list prosody condition, but not in the utterance prosody condition. These results suggest that while newborns are already sensitive to word order in linguistic sequences, prosody appears to be a stronger cue than word order for the identification of linguistic units at birth.     

Ear differences of abstract and concrete word recognition

Two experiments investigated the use of a running memory span auditory recognition task for eliciting left-right differences for concrete and abstract words under conditions of varying competitive stimulation. Ear differences were not observed under monaural, white noise, reversed crowd noise or crowd noise interference conditions (Experiments 1 and 2). However, a right ear advantage for abstract words was obtained when the competitive stimulation was speech (Experiment 2). No ear differences were observed for concrete words. In both experiments it was found that, as the nature of the interference condition approximated speech, then abstract word performance declined when compared to concrete word performance. These findings lend support to Paivio's dual coding theory of verbal processing and are congruent with a hypothesis of differential lateralization of semantic and imagery processes.     

Frequency of normative word associations in the speech of individuals at familial high-risk for schizophrenia

The intrusion of associations into speech in schizophrenia disrupts coherence and comprehensibility, a feature of formal thought disorder referred to as loosened associations. We have previously proposed that loosened associations may result from hyperactivity in semantic association networks, leading to an increased frequency of associated words appearing in speech. Using Computed Associations in Sequential Text (CAST) software to quantify the frequency of such associations in speech, we have reported more frequent normative associations in language samples from patients with schizophrenia and in individuals with schizotypal characteristics. The present study further examined this deviance in schizophrenia by studying normative associations in those who share genes with an individual with schizophrenia, (i.e. first-degree relatives of probands with schizophrenia; HR) but who do not have an illness. Familial high-risk participants (n=22), and controls (n=24) provided verbal responses to cards from the Thematic Apperception Test. CAST analysis revealed that HR used more associated words in their speech compared to controls. Furthermore, the frequency of normative word associations was positively correlated with dimensional and total scores of schizotypy derived from ratings of the structured interview for schizotypy, which confirms past research showing a relationship between schizotypy and hyperassociations. Our results suggest that some language disturbances in schizophrenia likely arise from an underlying psychopathological mechanism, hyperactivity of semantic associations.