ERPs reveal the temporal dynamics of auditory word recognition in specific language impairment.

We used event-related potentials (ERPs) to compare auditory word recognition in children with specific language impairment (SLI group; N=14) to a group of typically developing children (TD group; N=14). Subjects were presented with pictures of items and heard auditory words that either matched or mismatched the pictures. Mismatches overlapped expected words in word-onset (cohort mismatches; see: DOLL, hear: dog), rhyme (CONE -bone), or were unrelated (SHELL -mug). In match trials, the SLI group showed a different pattern of N100 responses to auditory stimuli compared to the TD group, indicative of early auditory processing differences in SLI. However, the phonological mapping negativity (PMN) response to mismatching items was comparable across groups, suggesting that just like TD children, children with SLI are capable of establishing phonological expectations and detecting violations of these expectations in an online fashion. Perhaps most importantly, we observed a lack of attenuation of the N400 for rhyming words in the SLI group, which suggests that either these children were not as sensitive to rhyme similarity as their typically developing peers, or did not suppress lexical alternatives to the same extent. These findings help shed light on the underlying deficits responsible for SLI.

Electrophysiological indices of phonological impairments in dyslexia.

PURPOSE: A range of studies have shown difficulties in perceiving acoustic and phonetic information in dyslexia; however, much less is known about how such difficulties relate to the perception of individual words. The authors present data from event-related potentials (ERPs) examining the hypothesis that children with dyslexia have difficulties with processing phonemic information within spoken words compared to age-matched readers with typical development. METHOD: The authors monitored ERPs to auditory words during a simple picture-word matching task. The key manipulation was the inclusion of both matching stimuli and three types of mismatches (cohort, CONE-comb; rhyme, CONE-bone; and unrelated, CONE-fox). RESULTS: Children with dyslexia showed atypical N400 ERP waveforms to both types of phonological mismatches, but not to phonologically unrelated mismatches, reflecting a relative insensitivity to phonological overlap among auditory words. CONCLUSION: The data suggest that children with dyslexia have impairments in integrating phonological information into word-level representations. The results suggest that speech perception difficulties in dyslexia might have consequences for processing auditory words.

Modulation of brain regions involved in word recognition by homophonous stimuli: an fMRI study.

We used rapid event-related fMRI to explore factors modulating the activation of orthographic and phonological representations of print during a visual lexical decision task. Stimuli included homophonous word and nonword stimuli (MAID, BRANE), which have been shown behaviorally to produce longer response times due to phonological mediation effects. We also manipulated participants' reliance on orthography by varying the extent to which nonword foils were orthographically typical (wordlike context) or atypical (non-wordlike context) of real words. Key findings showed that reading low-frequency homophones in the wordlike context produced activation in regions associated with phonological processing (i.e., opercular region of the left inferior frontal gyrus [IFG; BA 44]), the integration of orthography and phonology (i.e., the inferior parietal lobule (IPL), and lexicosemantic processing (i.e., left middle temporal gyrus, [MTG]). Pseudohomophones in the wordlike context produced greater activity relative to other nonword trials in regions engaged during both phonological processing (i.e., left IFG/precentral gyrus; BA 6/9]), and semantic processing (triangular region of the left IFG; BA 47). Homophonous effects in the non-wordlike context were primarily isolated to medial extrastriate regions, hypothesized to be involved in low level visual processing and not reading-related processing per se. These findings demonstrate that the degree to which phonological and orthographic representations of print are activated depends not only on homophony, but also on the word-likeness of nonword stimuli. Implications for models of visual word recognition are discussed.

Investigating the time course of spoken word recognition: electrophysiological evidence for the influences of phonological similarity.

Behavioral and modeling evidence suggests that words compete for recognition during auditory word identification, and that phonological similarity is a driving factor in this competition. The present study used event-related potentials (ERPs) to examine the temporal dynamics of different types of phonological competition (i.e., cohort and rhyme). ERPs were recorded during a novel picture-word matching task, where a target picture was followed by an auditory word that either matched the target (CONE-cone), or mismatched in one of three ways: rhyme (CONE-bone), cohort (CONE-comb), and unrelated (CONE-fox). Rhymes and cohorts differentially modulated two distinct ERP components, the phonological mismatch negativity and the N400, revealing the influences of prelexical and lexical processing components in speech recognition. Cohort mismatches resulted in late increased negativity in the N400, reflecting disambiguation of the later point of miscue and the combined influences of top-down expectations and misleading bottom-up phonological information on processing. In contrast, we observed a reduction in the N400 for rhyme mismatches, reflecting lexical activation of rhyme competitors. Moreover, the observed rhyme effects suggest that there is an interaction between phoneme-level and lexical-level information in the recognition of spoken words. The results support the theory that both levels of information are engaged in parallel during auditory word recognition in a way that permits both bottom-up and top-down competition effects.

Mismatch negativity reflects sensory and phonetic speech processing.

We examined phonetic and sensory processes in speech perception using mismatch negativity, an event-related potential component congruent with discrimination, but which occurs for unattended stimuli. Adult listeners (N=16) heard a repeated standard (the syllable 'da') that was interrupted infrequently by a phonetically different 'deviant' syllable ('ba'). The acoustic difference between standard and deviant was manipulated to create both acoustically Strong and Weak deviant stimuli. Mismatch negativities in response to the Strong deviant were significantly greater than those for the Weak deviant, in spite of the fact that both represented stable instances of the phonetic category. The data suggest that the mismatch negativity component can be strongly influenced by sensory factors beyond what is predicted by overt categorization and discrimination judgments.

Determining the role of phonology in silent reading using event-related brain potentials.

The goal of the present study was to delineate phonology's role in silent reading using event-related brain potential (ERP) techniques. Terminal endings of high cloze sentences were manipulated in four conditions in which the terminal word was: (1) the high cloze ending and thus orthographically, phonologically and semantically congruent (e.g., The gambler had a streak of bad luck.); (2) a pseudohomophone that was orthographically incongruent, but was phonologically congruent to the anticipated ending (e.g., The ship disappeared into the thick phog [fog].); (3) a word that was orthographically, phonologically and semantically incongruent to expectations (e.g., The dog chased the cat up the Queen [tree].); or (4) a nonword and consequently orthographically, phonologically and semantically incongruent to expectations (e.g., The gas station is about two miles down the bole [road].). A N270 was elicited by orthographically incongruent words and nonwords (conditions 2, 3 and 4), likely reflecting violations of orthographic form expectations, while the presence of the N400 to semantically incongruent words and nonwords (conditions 3 and 4) reflected violations of semantic expectations. The relative absence of the N400 response to pseudohomophones (condition 3) indicates that integrating word meaning with sentential context is influenced by the phonological representation of the presented letter string. The implication of these results for theories of word recognition is discussed.