Are the same phoneme and lexical layers used in speech production and comprehension? A case-series test of Foygel and Dell's (2000) model of aphasic speech production

Comprehension exposures to words in sentence contexts impact spoken word production

Comprehension or production of isolated words and production of words embedded in sentence contexts facilitated later production in previous research. The present study examined the extent to which contextualized comprehension exposures would impact later production. Two repetition priming experiments were conducted with Spanish-English bilingual participants. In Experiment 1 (N = 112), all encoding stimuli were presented visually, and in Experiment 2 (N = 112), all encoding stimuli were presented auditorily. After reading/listening or translating isolated words or words embedded in sentences at encoding, pictures corresponding to each target word were named aloud. Repetition priming relative to new items was measured in RT and accuracy. Relative to isolated encoding, sentence encoding reduced RT priming but not accuracy priming. In reading/listening encoding conditions, both isolated and embedded words elicited accuracy priming in picture naming, but only isolated words elicited RT priming. In translation encoding conditions, repetition priming effects in RT (but not accuracy) were stronger for lower-frequency words and with lower proficiency in the picture-naming response language. RT priming was strongest when the translation response at encoding was produced in the same language as final picture naming. In contrast, accuracy priming was strongest when the translation stimulus at encoding was comprehended in the same language as final picture naming. Thus, comprehension at encoding increased the rate of successful retrieval, whereas production at encoding speeded later production. Practice of comprehension may serve to gradually move less well-learned words from receptive to productive vocabulary.

How word comprehension exposures facilitate later spoken production: implications for lexical processing and repetition priming

Both comprehension and production exposures to words facilitate spoken production of the same words in picture-naming tasks performed several minutes later. Three experiments examined the mechanisms by which different types of comprehension exposures to words facilitate spoken production. Both overt and silent reading and listening tasks elicited substantial priming in picture naming at 10-minute but not 1-week retention intervals. Relative to silent conditions, encoding conditions that involved speaking the target word overtly elicited stronger priming effects in both RT and accuracy and larger frequency effects in RT. Frequency effects were not reliable in accuracy priming or silent-encoding RT priming. Articulatory suppression did not diminish priming effects relative to silent reading/listening, and priming effects did not depend on whether presentations at encoding were visual or auditory. Together, the results indicate that a common modality-general lemma representation is accessed in comprehension and production, that both lemma and phonological retrieval contribute to repetition priming in production, and that phonological retrieval is sensitive to word frequency. These results are consistent with a theory based on transfer-appropriate processing if word comprehension elicits top-down processing or feedback from the concept to the lemma.

Brain structures and cognitive abilities important for the self-monitoring of speech errors

The brain structures and cognitive abilities necessary for successful monitoring of one's own speech errors remain unknown. We aimed to inform self-monitoring models by examining the neural and behavioral correlates of phonological and semantic error detection in individuals with post-stroke aphasia. First, we determined whether detection related to other abilities proposed to contribute to monitoring according to various theories, including naming ability, fluency, word-level auditory comprehension, sentence-level auditory comprehension, and executive function. Regression analyses revealed that fluency and executive scores were independent predictors of phonological error detection, while a measure of word-level comprehension related to semantic error detection. Next, we used multivariate lesion-symptom mapping to determine lesion locations associated with reduced error detection. Reduced overall error detection related to damage to a region of frontal white matter extending into dorsolateral prefrontal cortex (DLPFC). Detection of phonological errors related to damage to the same areas, but the lesion-behavior association was stronger, suggesting the localization for overall error detection was driven primarily by phonological error detection. These findings demonstrate that monitoring of different error types relies on distinct cognitive functions, and provide causal evidence for the importance of frontal white matter tracts and DLPFC for self-monitoring of speech.

Friends and foes in the lexicon: homophone naming in aphasia

The study of homophones--words with different meanings that sound the same--has great potential to inform models of language production. Of particular relevance is a phenomenon termed frequency inheritance, where a low-frequency word (e.g., deer) is produced more fluently than would be expected based on its frequency characteristics, presumably because of shared phonology with a high-frequency homophone counterpart (e.g., dear). However, prior studies have been inconsistent in showing frequency inheritance. To explain this inconsistency, we propose a dual nature account of homophony: a high-frequency counterpart exerts 2 counterposing effects on a low-frequency homophone target during the 2 main stages of naming: (a) a detrimental impact during semantically driven lexical retrieval; (b) a beneficial impact during phonological retrieval. In a study of naming in participants with chronic aphasia followed by computational investigations, we find strong evidence for the dual nature account of homophony.

How damaged brains repeat words: a computational approach

Two routes have been proposed for auditory repetition: a lexical route which activates a lexical item and retrieves its phonology, and a nonlexical route which maps input phonology directly onto output phonology. But when is the nonlexical route recruited? In a sample of 103 aphasic patients, we use computational models to select patients who do and do not recruit the nonlexical route, and compare them in light of three hypotheses: 1 - Lexical-phonological hypothesis: when the lexical route is weak, the nonlexical route is recruited. 2 - Nonlexical hypothesis: when the nonlexical route is weak, it is abandoned. 3 - Semantic-access hypothesis: when access to meaning fails, the nonlexical route is recruited. In neurocognitive terms, hypotheses 1 and 2 identify different aspects of the intactness of the dorsal stream, while the third hypothesis focuses on the ventral stream. Our findings (and a subsequent meta-analysis of four studies) support hypotheses 2 and 3. Ultimately, we claim that the choice about whether to recruit the nonlexical route is guided, not by assessment of production abilities that support repetition, but instead by relying on accessible cues, namely whether the speaker understands the word, or can remember its sequence of phonemes.

Voxel-based lesion-parameter mapping: Identifying the neural correlates of a computational model of word production

The dual-route interactive two-step model explains the variation in the error patterns of aphasic speakers in picture naming, and word and nonword repetition tasks. The model has three parameters that can vary across individuals: the efficiency of the connections between semantic and lexical representations (s-weight), between lexical and phonological representations (p-weight), and between representations of auditory input and phonological representations (nl-weight). We determined these parameter values in 103 participants with chronic aphasia from left hemisphere stroke whose lesion locations had been determined. Then, using voxel-based lesion-parameter mapping, we mapped the parameters onto the brain, thus determining the neural correlates of the model's mechanisms. The maps and the behavioral findings supported the model's central claim that word repetition is affected by both the p and nl parameters. We propose that these two parameters constitute the model's analogue of the "dorsal stream" component of neurocognitive models of language processing.

Current tests and trends in single-case neuropsychology

In this issue of Cortex, Crawford, Garthwaite and Ryan publish bayesian statistical tests that will enable researchers to take account of covariates when comparing single patients to control samples. In this article, we provide some context for this development, from an audit of the Cortex archives. We suggest that single-case research is alive and well, and more rigorous than ever, and that current practice has been shaped considerably by Crawford and colleagues' statistical refinements over the past 12 years. However, there is scope for further tightening and standardisation of statistical methods and reporting standards. The advantages offered by the new bayesian tests should promote the even wider use of appropriate statistical methods, with benefits for the validity of individual studies, and for cross-comparability in the single-case literature.