Phonological–lexical activation: A lexical component or an output buffer? Evidence from aphasic errors

The role of input vs. output phonological working memory in narrative production: Evidence from case series and case study approaches

Separable input and output phonological working memory (WM) capacities have been proposed, with the input capacity supporting speech recognition and the output capacity supporting production. We examined the role of input vs. output phonological WM in narrative production, examining speech rate and pronoun ratio - two measures with prior evidence of a relation to phonological WM. For speech rate, a case series approach with individuals with aphasia found no significant independent contribution of input or output phonological WM capacity after controlling for single-word production. For pronoun ratio, there was some suggestion of a role for input phonological WM. Thus, neither finding supported a specific role for an output phonological buffer in speech production. In contrast, two cases demonstrating dissociations between input and output phonological WM capacities provided suggestive evidence of predicted differences in narrative production, though follow-up research is needed. Implications for case series vs. case study approaches are discussed.

A tale of two lexica: Investigating computational pressures on word representation with neural networks

Introduction

The notion of a single localized store of word representations has become increasingly less plausible as evidence has accumulated for the widely distributed neural representation of wordform grounded in motor, perceptual, and conceptual processes. Here, we attempt to combine machine learning methods and neurobiological frameworks to propose a computational model of brain systems potentially responsible for wordform representation. We tested the hypothesis that the functional specialization of word representation in the brain is driven partly by computational optimization. This hypothesis directly addresses the unique problem of mapping sound and articulation vs. mapping sound and meaning.

Results

We found that artificial neural networks trained on the mapping between sound and articulation performed poorly in recognizing the mapping between sound and meaning and vice versa. Moreover, a network trained on both tasks simultaneously could not discover the features required for efficient mapping between sound and higher-level cognitive states compared to the other two models. Furthermore, these networks developed internal representations reflecting specialized task-optimized functions without explicit training.

Discussion

Together, these findings demonstrate that different task-directed representations lead to more focused responses and better performance of a machine or algorithm and, hypothetically, the brain. Thus, we imply that the functional specialization of word representation mirrors a computational optimization strategy given the nature of the tasks that the human brain faces.

Contributions of semantic and phonological working memory to narrative language independent of single word production: Evidence from acute stroke

ABSTRACTNeuropsychological case studies have provided evidence that individuals with semantic, but not phonological, working memory (WM) deficits have difficulty producing phrases containing several content words. These findings supported the claim of a phrasal scope of planning at the grammatical formulation stage of production, where semantic WM supports the maintenance of lexical-semantic representations as they are inserted into slots in phrasal constituents. Recent narrative production results for individuals at the acute stage of stroke supported the role for semantic WM in phrasal elaboration while suggesting a role for phonological WM at a subsequent phonological encoding stage in supporting fluent, rapid speech. In the present study, we employed a larger participant sample while controlling for single word production abilities at semantic and phonological levels. Results confirmed the relations between semantic WM and phrasal elaboration whereas the relation between phonological WM and speech rate was eliminated. There was, however, evidence that both impaired phonological retrieval and restricted phonological WM were related to the tendency to produce short, highly frequent words such as pronouns rather than longer, less frequent words. Future research is needed to address whether the results relating to phonological WM depend on damage specific to an output rather than an input phonological capacity.

An access deficit or a deficit in the phonological representations themselves: What can we learn from naming errors?

Anomic aphasia is characterized by good comprehension and non-word repetition but poor naming. Two sub-types of deficits might be hypothesized: faulty access to preserved phonological representations or preserved access to impaired representations. Phonological errors may occur only when representations are impaired or in post-lexical deficits (conduction aphasia). We analysed the incidence of phonological naming errors of 30 individuals, 25 with anomic aphasia based on poor naming but good repetition and comprehension, and five with conduction aphasia based on poor naming and poor repetition. Individuals with anomic aphasia produced very few phonological errors compared to individuals with conduction aphasia (0-19.1% versus 42-66%). However, six individuals with anomia produced more than 11% phonological errors, suggesting two patterns of deficit: either impaired lexical representations or impaired access to them. The lack of phonological errors in most individuals with anomic aphasia suggests that access to the phonological output lexicon is semantically, not phonologically driven.

Integrity of input verbal short-term memory ability predicts naming accuracy in aphasia

Background

Contemporary models of aphasia predominantly attribute lexical retrieval deficits to impaired access and/or maintenance of semantic, lexical, and phonological representations of words. A central hypothesis of language-emergent models of verbal short-term memory (STM) is that temporary storage and maintenance of verbal information arises from activation of linguistic representations in long-term memory. This close relationship between short-term retention and linguistic representations has prompted accounts of aphasia that include impairments to both these components.

Aims

We investigated associations between measures of input semantic and phonological verbal STM and corresponding output processing measures. We hypothesised that both input and output functions of verbal STM rely on a common substrate (i.e., temporary activation and maintenance of long-term linguistic representations).

Methods & Procedure

Twenty adults with aphasia completed a series of semantic and phonological probe spans. Results were compared with naming performance in immediate and delayed conditions. The data were analysed using correlations, principal components analysis and linear regressions.

Results & Discussion

Input semantic and phonological verbal STM abilities were predictive of naming accuracy. Greater input semantic and phonological STM spans were associated with fewer semantic and phonological naming errors. Latent factors identified by principal components analysis of probe span data were consistent with a two-step interactive model of word retrieval. Probe spans measuring access to semantic and initial consonant-vowel representations aligned with lexical-semantic abilities (lexical-semantic factor). Probe spans assessing access to the rhyme component of a word measured lexical-phonological abilities (lexical-phonological factor). The principal components analysis indicated that stronger lexical-semantic abilities were associated with fewer semantic and nonword errors, and stronger lexical phonological abilities were associated with fewer formal and unrelated errors. In addition, our results were consistent with models that postulate serial access to phonology, proceeding from initial to final phonemes. The span measuring access to initial consonant-vowel was associated with lexical selection, while the span measuring access to rhyme information was associated with phonological selection.

Conclusion

Performance on input semantic and phonological tasks predicts accuracy of picture naming performance and types of errors made by people with aphasia. These results indicate overlap in input and output semantic and phonological processing, which must be accounted for in models of lexical processing. These findings also have implications for approaches to diagnosis and treatments for lexical comprehension and production that capitalise on the overlap of input and output processing.

Disentangling the Psycholinguistic Loci of Anomia with Cognitive Psychometric Models

This article reviews advanced statistical techniques for measuring impairments in object naming, particularly in the context of stroke-induced aphasia. Traditional testing strategies can be challenged by the multifaceted nature of impairments that arise due to the complex relationships between localized brain damage and disruption to the cognitive processes required for successful object naming. Cognitive psychometric models can combine response-type analysis with item-response theory to yield accurate estimates of multiple abilities using data collected from a single task. The models also provide insights about how the test items can be challenging in different ways. Although more work is needed to fully optimize their clinical utility in practice, these formal concepts can guide thoughtful selection of stimuli used in treatment or assessment, as well as providing a framework to interpret response-type data.

Professional or Amateur? The Phonological Output Buffer as a Working Memory Operator

The Phonological Output Buffer (POB) is thought to be the stage in language production where phonemes are held in working memory and assembled into words. The neural implementation of the POB remains unclear despite a wealth of phenomenological data. Individuals with POB impairment make phonological errors when they produce words and non-words, including phoneme omissions, insertions, transpositions, substitutions and perseverations. Errors can apply to different kinds and sizes of units, such as phonemes, number words, morphological affixes, and function words, and evidence from POB impairments suggests that units tend to substituted with units of the same kind—e.g., numbers with numbers and whole morphological affixes with other affixes. This suggests that different units are processed and stored in the POB in the same stage, but perhaps separately in different mini-stores. Further, similar impairments can affect the buffer used to produce Sign Language, which raises the question of whether it is instantiated in a distinct device with the same design. However, what appear as separate buffers may be distinct regions in the activity space of a single extended POB network, connected with a lexicon network. The self-consistency of this idea can be assessed by studying an autoassociative Potts network, as a model of memory storage distributed over several cortical areas, and testing whether the network can represent both units of word and signs, reflecting the types and patterns of errors made by individuals with POB impairment.

The Roles of Retrieval Practice Versus Errorless Learning in Strengthening Lexical Access in Aphasia

Purpose

The purpose of this study was to determine how 2 methods known to improve naming impairment in aphasia (i.e., retrieval practice and errorless learning) affect lexical access. We hypothesized that instances of naming during retrieval practice use and strengthen item-specific connections in each of 2 stages of lexical access: Stage 1, meaning-to-word connections, and Stage 2, word-to-phonology connections. In contrast, errorless learning prioritizes opportunities for repeating words, which we expect to primarily strengthen item-specific connections in Stage 2 because repetition circumvents the need for semantically driven word retrieval.

Method

We tested the outcomes of retrieval practice versus errorless learning training for items that were selected because the naming errors they elicited suggested weakened connections at Stage 1 or at Stage 2 of lexical access for each of 10 individuals with chronic aphasia. Each participant's Stage 1 items and Stage 2 items were divided evenly between the 2 training conditions. Naming tests were administered 1 day and 1 week after training to assess retention of training gains. We also examined whether the participants' pretraining naming error profiles were associated with the relative efficacy of retrieval practice versus errorless learning.

Results

The posttraining naming tests showed an advantage of retrieval practice over errorless learning for Stage 1 items and an advantage of errorless learning over retrieval practice for Stage 2 items. In addition, greater percentages of phonological error naming responses prior to training were associated with greater posttraining accuracy in the errorless learning condition relative to the retrieval practice condition.

Conclusions

The findings suggest that the advantage of retrieval practice for naming impairment in aphasia largely results from greater strengthening of practiced semantic-lexical connections compared with errorless learning, which prioritizes repetition and, therefore, mainly confers strengthening of practiced lexical-phonological connections. Understanding how specific training conditions improve naming can help predict the relative efficacy of each method for individuals with aphasia.

Word repetition and retrieval practice effects in aphasia: Evidence for use-dependent learning in lexical access

This study tested the hypothesis that a use-dependent learning mechanism operates at each of two stages of lexical access: retrieval of a word from semantics ("Stage 1"), followed by retrieval of the word's constituent phonemes ("Stage 2"). Two participants with aphasia were selected due to their contrasting types of naming impairment (Stage 1 versus Stage 2 difficulty). For each participant, items were assigned to naming training that involved retrieval practice (retrieval of the name from semantics) or repetition practice (hear the name and orally repeat it). Naming tests were administered one day and one week after training. The results supported the predicted training effects: (a) Because successful naming via retrieval practice requires both Stage 1 and Stage 2, this technique uses and strengthens item-specific connections in both stages. (b) Because word repetition circumvents semantically driven retrieval, this technique primarily uses and strengthens item-specific connections in Stage 2.

Comparing phoneme frequency, age of acquisition, and loss in aphasia: Implications for phonological universals

Phonological complexity may be central to the nature of human language. It may shape the distribution of phonemes and phoneme sequences within languages, but also determine age of acquisition and susceptibility to loss in aphasia. We evaluated this claim using frequency statistics derived from a corpus of phonologically transcribed Italian words (phonitalia, available at phonitalia,org), rankings of phoneme age of acquisition (AoA) and rate of phoneme errors in patients with apraxia of speech (AoS) as an indication of articulatory complexity. These measures were related to cross-linguistically derived markedness rankings. We found strong correspondences. AoA, however, was predicted by both apraxic errors and frequency, suggesting independent contributions of these variables. Our results support the reality of universal principles of complexity. In addition they suggest that these complexity principles have articulatory underpinnings since they modulate the production of patients with AoS, but not the production of patients with more central phonological difficulties.

Understanding semantic and phonological processing deficits in adults with aphasia: Effects of category and typicality

BACKGROUND

Semantic and phonological processing deficits are often present in aphasia. The degree of interdependence between the deficits has been widely studied with variable findings. Semantic variables such as category and typicality have been found to influence semantic processing in healthy individuals and persons with aphasia but their influence on phonological processing is unknown.

AIMS

This study examined the nature of semantic and phonological access in aphasia by comparing adults with aphasia to healthy control participants. Semantic and phonological tasks were used to assess the difference in processing requirements between and within each group as well as examine the effects of category and typicality on different stages of semantic and phonological processing.

METHODS & PROCEDURES

Thirty-two persons with aphasia and ten neurologically healthy adults were administered nine tasks: Category Superordinate, Category Coordinate, Semantic Feature, Rhyme Judgment (No-Name), Syllable Judgment (No-Name), Phoneme Verification (No-Name), Rhyme Judgment (Name-Provided), Syllable Judgment (Name-Provided), and Phoneme Verification (Name-Provided). Accuracy and reaction time data were collected for each of these tasks and between-group and within-group differences were analyzed via MANOVA/MANCOVA and hierarchical clustering analyses.

OUTCOMES & RESULTS

Persons with aphasia performed with significantly lower accuracy than controls on phonological tasks but performed comparably on semantic tasks. Participants with aphasia were significantly slower than controls on all semantic and phonological tasks. Clustering of the nine tasks by accuracy revealed different processing requirements in the participants with aphasia compared to the control group while clustering by reaction time revealed similar trends in both groups in that phonological (no-name) items required the most processing time. Significant effects of category and typicality were noted in the semantic tasks but not in any of the phonological tasks.

CONCLUSIONS

Individuals with aphasia demonstrated overall impaired phonological processing with relatively preserved semantic processing as compared to controls. Per accuracy and reaction time measures, distinct trends in processing load for semantic tasks versus phonological tasks were seen in the individuals with aphasia whereas only speed of processing and not accuracy was impacted by phonological processing load in the control group. The results align most closely with discrete serial processing models of lexical processing as category and typicality effects were robust in the semantic tasks but not in any of the phonological tasks. Alternative explanations for these results also are discussed.

Phonological, lexical, and semantic errors produced by impairment at the output buffer in a Spanish aphasic patient

We present a single case of a right-handed female patient, RH, who was categorized as suffering from conduction aphasia. She presented no articulatory problems during spontaneous speech but made a significant number of phonological paraphasias in naming and repetition tasks. The number of errors increased for long words and pseudowords. This pattern of results points to damage in the "Phonological Output Buffer" (POB) as the basis of this disorder. However, this patient did not make mistakes when reading words and pseudowords aloud, even when we introduced a delay between the presentation of the word and its production to test the working memory resources of the phonological buffer. Furthermore, the patient's ability to name objects, repeat words, and write to dictation improved with her degree of familiarity with the items. The damage could be situated at the point where phonemes are selected and ordered to produce words. We posit that the deficits observed in this patient, and the differences encountered between her performance and that of others described in the literature, in particular in reading tasks, can be explained by considering POB damage to be gradual in nature. According to this explanation, the performance of patients with damage to the POB will depend on the amount of information provided by the stimulus (word/nonword), the language particularities (regular/irregular), and the nature of the task demands (repetition, writing, naming, or reading).

Encoding order and developmental dyslexia: a family of skills predicting different orthographic components

We investigated order encoding in developmental dyslexia using a task that presented nonalphanumeric visual characters either simultaneously or sequentially--to tap spatial and temporal order encoding, respectively--and asked participants to reproduce their order. Dyslexic participants performed poorly in the sequential condition, but normally in the simultaneous condition, except for positions most susceptible to interference. These results are novel in demonstrating a selective difficulty with temporal order encoding in a dyslexic group. We also tested the associations between our order reconstruction tasks and: (a) lexical learning and phonological tasks; and (b) different reading and spelling tasks. Correlations were extensive when the whole group of participants was considered together. When dyslexics and controls were considered separately, different patterns of association emerged between orthographic tasks on the one side and tasks tapping order encoding, phonological processing, and written learning on the other. These results indicate that different skills support different aspects of orthographic processing and are impaired to different degrees in individuals with dyslexia. Therefore, developmental dyslexia is not caused by a single impairment, but by a family of deficits loosely related to difficulties with order. Understanding the contribution of these different deficits will be crucial to deepen our understanding of this disorder.

Morphological-compound dysgraphia in an aphasic patient: "A wild write through the lexicon"

We describe the case of a dysgraphic aphasic individual--S.G.W.--who, in writing to dictation, produced high rates of formally related errors consisting of both lexical substitutions and what we call morphological-compound errors involving legal or illegal combinations of morphemes. These errors were produced in the context of a minimal number of semantic errors. We could exclude problems with phonological discrimination and phonological short-term memory. We also excluded rapid decay of lexical information and/or weak activation of word forms and letter representations since S.G.W.'s spelling showed no effect of delay and no consistent length effects, but, instead, paradoxical complexity effects with segmental, lexical, and morphological errors that were more complex than the target. The case of S.G.W. strongly resembles that of another dysgraphic individual reported in the literature--D.W.--suggesting that this pattern of errors can be replicated across patients. In particular, both patients show unusual errors resulting in the production of neologistic compounds (e.g., "bed button" in response to "bed"). These patterns can be explained if we accept two claims: (a) Brain damage can produce both a reduction and an increase in lexical activation; and (b) there are direct connections between phonological and orthographic lexical representations (a third spelling route). We suggest that both patients are suffering from a difficulty of lexical selection resulting from excessive activation of formally related lexical representations. This hypothesis is strongly supported by S.G.W.'s worse performance in spelling to dictation than in written naming, which shows that a phonological input, activating a cohort of formally related lexical representations, increases selection difficulties.

How can audiovisual pathways enhance the temporal resolution of time-compressed speech in blind subjects?

In blind people, the visual channel cannot assist face-to-face communication via lipreading or visual prosody. Nevertheless, the visual system may enhance the evaluation of auditory information due to its cross-links to (1) the auditory system, (2) supramodal representations, and (3) frontal action-related areas. Apart from feedback or top-down support of, for example, the processing of spatial or phonological representations, experimental data have shown that the visual system can impact auditory perception at more basic computational stages such as temporal signal resolution. For example, blind as compared to sighted subjects are more resistant against backward masking, and this ability appears to be associated with activity in visual cortex. Regarding the comprehension of continuous speech, blind subjects can learn to use accelerated text-to-speech systems for "reading" texts at ultra-fast speaking rates (>16 syllables/s), exceeding by far the normal range of 6 syllables/s. A functional magnetic resonance imaging study has shown that this ability, among other brain regions, significantly covaries with BOLD responses in bilateral pulvinar, right visual cortex, and left supplementary motor area. Furthermore, magnetoencephalographic measurements revealed a particular component in right occipital cortex phase-locked to the syllable onsets of accelerated speech. In sighted people, the "bottleneck" for understanding time-compressed speech seems related to higher demands for buffering phonological material and is, presumably, linked to frontal brain structures. On the other hand, the neurophysiological correlates of functions overcoming this bottleneck, seem to depend upon early visual cortex activity. The present Hypothesis and Theory paper outlines a model that aims at binding these data together, based on early cross-modal pathways that are already known from various audiovisual experiments on cross-modal adjustments during space, time, and object recognition.

The cortical organization of lexical knowledge: a dual lexicon model of spoken language processing

Current accounts of spoken language assume the existence of a lexicon where wordforms are stored and interact during spoken language perception, understanding and production. Despite the theoretical importance of the wordform lexicon, the exact localization and function of the lexicon in the broader context of language use is not well understood. This review draws on evidence from aphasia, functional imaging, neuroanatomy, laboratory phonology and behavioral results to argue for the existence of parallel lexica that facilitate different processes in the dorsal and ventral speech pathways. The dorsal lexicon, localized in the inferior parietal region including the supramarginal gyrus, serves as an interface between phonetic and articulatory representations. The ventral lexicon, localized in the posterior superior temporal sulcus and middle temporal gyrus, serves as an interface between phonetic and semantic representations. In addition to their interface roles, the two lexica contribute to the robustness of speech processing.