Case series methodology involves the systematic assessment of a sample of related patients, with the goal of understanding how and why they differ from one another. This method has become increasingly important in cognitive neuropsychology, which has long been identified with single-subject research. We review case series studies dealing with impaired semantic memory, reading, and language production, and draw attention to the affinity of this methodology for testing theories that are expressed as computational models and for addressing questions about neuroanatomy. It is concluded that case series methods usefully complement single-subject techniques.
case series; single-subject; cognitive neuropsychology; computational models; lexical access; semantic dementia; aphasia; semantic memory
Analysis of error types provides useful information about the stages and processes involved in normal and aphasic word production. In picture naming, semantic errors (horse for goat) generally result from something having gone awry in lexical access such that the right concept was mapped to the wrong word. This study used the new lesion analysis technique known as voxel-based lesion-symptom mapping to investigate the locus of lesions that give rise to semantic naming errors. Semantic errors were obtained from 64 individuals with post-stroke aphasia, who also underwent high-resolution structural brain scans. Whole brain voxel-based lesion-symptom mapping was carried out to determine where lesion status predicted semantic error rate. The strongest associations were found in the left anterior to mid middle temporal gyrus. This area also showed strong and significant effects in further analyses that statistically controlled for deficits in pre-lexical, conceptualization processes that might have contributed to semantic error production. This study is the first to demonstrate a specific and necessary role for the left anterior temporal lobe in mapping concepts to words in production. We hypothesize that this role consists in the conveyance of fine-grained semantic distinctions to the lexical system. Our results line up with evidence from semantic dementia, the convergence zone framework and meta-analyses of neuroimaging studies on word production. At the same time, they cast doubt on the classical linkage of semantic error production to lesions in and around Wernicke's area.
aphasia; voxel-based lesion-symptom mapping; naming; semantic; errors
Lexical access in language production, and particularly pathologies of lexical access, are often investigated by examining errors in picture naming and word repetition. In this article, we test a computational approach to lexical access, the two-step interactive model, by examining whether the model can quantitatively predict the repetition-error patterns of 65 aphasic subjects from their naming errors. The model’s characterizations of the subjects’ naming errors were taken from the companion paper to this one (Schwartz, Dell, N. Martin, Gahl & Sobel, 2006), and their repetition was predicted from the model on the assumption that naming involves two error prone steps, word and phonological retrieval, whereas repetition only creates errors in the second of these steps. A version of the model in which lexical-semantic and lexical-phonological connections could be independently lesioned was generally successful in predicting repetition for the aphasics. An analysis of the few cases in which model predictions were inaccurate revealed the role of input phonology in the repetition task.
When unimpaired participants name pictures quickly, they produce many perseverations that bear a semantic relation to the target, especially when the pictures are blocked by semantic category. These “semantic perseverations” have not shown the steep decay over lags (distance from prior occurrence) that typify the perseverations produced by people with aphasia on standard naming tasks (Cohen & Dehaene, 1998). To reconcile the discrepant findings, we studied semantic perseverations generated by participants with aphasia on a naming task that featured semantic blocking [Schnur, T. T., Schwartz, M. F., Brecher, A., & Hodgson, C. (2006). Semantic interference during blocked-cyclic naming: Evidence from aphasia. Journal of Memory and Language, 54, 199–227]. The temporal properties of these perseverations were investigated by analyzing their lag function and the influence of time (response-stimulus interval) on this function. To separate out the influence of chance on the observed lag distributions, chance data sets were created for individual participants by reshuffling whole trials (i.e., stimulus-response pairs) in a manner that preserved unique features of the blocking design. Analyses of chance-corrected lag functions revealed the expected recency bias, i.e., higher perseveration frequencies at short lags. Importantly, there was no difference between the lag functions for perseverations generated with a 5 s, compared to 1 s, response-stimulus interval. This combination of recency and insensitivity to elapsed time indicates that the perseveratory impetus in a named response does not passively decay with time but rather is diminished by interference from related trials. We offer an incremental learning account of these findings.
Perseveration; semantic blocking; aphasia; naming; priming; incremental learning
Despite the existence of speech errors, verbal communication is successful because speakers can detect (and correct) their errors. The standard theory of speech-error detection, the perceptual-loop account, posits that the comprehension system monitors production output for errors. Such a comprehension-based monitor, however, cannot explain the double dissociation between comprehension and error-detection ability observed in the aphasic patients. We propose a new theory of speech-error detection which is instead based on the production process itself. The theory borrows from studies of forced-choice-response tasks the notion that error detection is accomplished by monitoring response conflict via a frontal brain structure, such as the anterior cingulate cortex. We adapt this idea to the two-step model of word production, and test the model-derived predictions on a sample of aphasic patients. Our results show a strong correlation between patients’ error-detection ability and the model’s characterization of their production skills, and no significant correlation between error detection and comprehension measures, thus supporting a production-based monitor, generally, and the implemented conflict-based monitor in particular. The successful application of the conflict-based theory to error-detection in linguistic, as well as non-linguistic domains points to a domain-general monitoring system.
Speech monitoring; Speech errors; Error detection; Aphasia; Computational models
We explored the neural basis of reversible sentence comprehension in a large group of aphasic patients (N=79). Voxel-based lesion-symptom mapping revealed a significant association between damage in temporoparietal cortex and impaired sentence comprehension. This association remained after we controlled for phonological working memory. We hypothesize that this region plays an important role in the thematic or what-where processing of sentences. In contrast, we detected weak or no association between reversible sentence comprehension and the ventrolateral prefrontal cortex, which includes Broca’s area, even for syntactically complex sentences. This casts doubt on theories that presuppose a critical role for this region in syntactic computations.
Broca’s area; Prefrontal; Temporoparietal; Thematic Roles; Syntax
Many research questions in aphasia can only be answered through access to substantial numbers of patients and to their responses on individual test items. Since such data are often unavailable to individual researchers and institutions, we have developed and made available the Moss Aphasia Psycholinguistics Project Database: a large, searchable, web-based database of patient performance on psycholinguistic and neuropsychological tests. The database contains data from over 170 patients covering a wide range of aphasia subtypes and severity, some of whom were tested multiple times. The core of the archive consists of a detailed record of individual-trial performance on the Philadelphia (picture) Naming Test. The database also contains basic demographic information about the patients and patients' overall performance on neuropsychological assessments as well as tests of speech perception, semantics, short-term memory, and sentence comprehension. The database is available at http://www.mappd.org/.
aphasia; database; picture naming; language
Semantic errors in aphasia (e.g., naming a horse as “dog”) frequently arise from faulty mapping of concepts onto lexical items. A recent study by our group used voxel-based lesion-symptom mapping (VLSM) methods with 64 patients with chronic aphasia to identify voxels that carry an association with semantic errors. The strongest associations were found in the left anterior temporal lobe (L-ATL), in the mid- to anterior MTG region. The absence of findings in Wernicke’s area was surprising, as were indications that ATL voxels made an essential contribution to the post-semantic stage of lexical access. In this follow-up study, we sought to validate these results by re-defining semantic errors in a manner that was less theory dependent and more consistent with prior lesion studies. As this change also increased the robustness of the dependent variable, it made it possible to perform additional statistical analyses that further refined the interpretation. The results strengthen the evidence for a causal relationship between ATL damage and lexically-based semantic errors in naming and lend confidence to the conclusion that chronic lesions in Wernicke’s area are not causally implicated in semantic error production.
aphasia; voxel-based lesion-symptom mapping; naming; semantic; errors
This paper investigates the cognitive processes underlying picture naming and auditory word repetition. In the 2-step model of lexical access, both the semantic and phonological steps are involved in naming, but the former has no role in repetition. Assuming recognition of the to-be-repeated word, repetition could consist of retrieving the word’s output phonemes from the lexicon (the lexical-route model), retrieving the output phonology directly from input phonology (the nonlexical-route model) or employing both routes together (the summation dual-route model). We tested these accounts by comparing the size of the word frequency effect (an index of lexical retrieval) in naming and repetition data from 59 aphasic patients with simulations of naming and repetition models. The magnitude of the frequency effect (and the influence of other lexical variables) was found to be comparable in naming and repetition, and equally large for both the lexical and summation dual-route models. However, only the dual-route model was fully consistent with data from patients, suggesting that nonlexical input is added on top of a fully-utilized lexical route.
Lexical access; Aphasia; Repetition; Picture naming; Computational models; Case-series; Word frequency
We investigated the influence of phonological neighborhood density (PND) on the performance of aphasic speakers whose naming impairments differentially implicate phonological or semantic stages of lexical access. A word comes from a dense phonological neighborhood if many words sound like it. Limited evidence suggests that higher density facilitates naming in aphasic speakers, as it does in healthy speakers. Using well controlled stimuli, Experiment 1 confirmed the influence of PND on accuracy and phonological error rates in two aphasic speakers with phonological processing deficits. In Experiments 2 and 3, we extended the investigation to an aphasic speaker who is prone to semantic errors, indicating a semantic deficit and/or a deficit in the mapping from semantics to words. This individual had higher accuracy, and fewer semantic errors, in naming targets from high versus low density neighborhoods. It is argued that the results provide strong support for interactive approaches to lexical access, where reverberatory feedback between word- and phoneme-level lexical representations not only facilitates phonological level processes but also privileges the selection of a target word over its semantic competitors.
phonology; neighborhood density; aphasia; interactivity; lexical access
Patients with damage involving left ventrolateral prefrontal cortex (left VLPFC) often show syntactic deficits. They also show exaggerated interference effects during a variety of non-syntactic tasks, including picture naming and working memory. Conceivably, both deficits could arise from inadequate biasing of competitive interactions during language production. To test this hypothesis, we manipulated “positional” interference during multiword naming by priming one of the nouns in the same or different position. Experimental case studies of four left VLPFC patients revealed that two of the patients showed exaggerated positional interference, greater number of errors, including omissions during multiword production, increased production difficulty when the order of nouns did not match the predominant English pattern as well as impaired comprehension of non-canonical reversible sentences. These results suggest that these two patients had an impairment in “selection for position”. Different from the other two, their lesions included a subregion of frontal cortex (BA 44/6) that has been shown in neuroimaging studies to pay a role in sequencing.
aphasia; language; production; syntax; sequencing; interference; prefrontal
Semantic short-term memory (STM) deficits have been traditionally defined as an inability to maintain semantic representations over a delay (R. Martin, Shelton & Yaffee, 1994). Yet some patients with semantic STM deficits make numerous intrusions of items from previously presented lists, thus presenting an interesting paradox: Why should an inability to maintain semantic representations produce an increase in intrusions from earlier lists? In this study, we investigated the relationship between maintenance deficits and susceptibility to interference in a group of 20 aphasic patients characterized with weak semantic or weak phonological STM. Patients and matched control participants performed a modified item-recognition task designed to elicit semantic or phonological interference from list items located one, two, or three trials back (Hamilton & R. Martin, 2007). Controls demonstrated significant effects of interference in both versions of the task. Interference in patients was predicted by the type and severity of their STM deficit; that is, shorter semantic spans were associated with greater semantic interference and shorter phonological spans were associated with greater phonological interference. We interpret these results through a new perspective, the reactivation hypothesis, and we discuss their importance for accounts emphasizing the contribution of maintenance mechanisms for STM impairments in aphasia as well as susceptibility to interference.
WORKING-MEMORY; RECENT-NEGATIVES; INHIBITION; SEMANTIC; PHONOLOGICAL
Naming a picture of a dog primes the subsequent naming of a picture of a dog (repetition priming) and interferes with the subsequent naming of a picture of a cat (semantic interference). Behavioral studies suggest that these effects derive from persistent changes in the way that words are activated and selected for production, and some have claimed that the findings are only understandable by positing a competitive mechanism for lexical selection. We present a simple model of lexical retrieval in speech production that applies error-driven learning to its lexical activation network. This model naturally produces repetition priming and semantic interference effects. It predicts the major findings from several published experiments, demonstrating that these effects may arise from incremental learning. Furthermore, analysis of the model suggests that competition during lexical selection is not necessary for semantic interference if the learning process is itself competitive.
Some theories of lexical access in production locate the effect of lexical frequency at the retrieval of a word’s phonological characteristics, as opposed to the prior retrieval of a holistic representation of the word from its meaning. Yet there is evidence from both normal and aphasic individuals that frequency may influence both of these retrieval processes. This inconsistency is especially relevant in light of recent attempts to determine the representation of another lexical property, age of acquisition or AoA, whose effect is similar to that of frequency. To further explore the representations of these lexical variables in the word retrieval system, we performed hierarchical, multinomial logistic regression analyses of 50 aphasic patients’ picture-naming responses. While both log frequency and AoA had a significant influence on patient accuracy and led to fewer phonologically related errors and omissions, only log frequency had an effect on semantically related errors. These results provide evidence for a lexical access process sensitive to frequency at all stages, but with AoA having a more limited effect.
SentenceShaper® (SSR) is a computer program that is for speech what a word-processing program is for written text; it allows the user to record words and phrases, play them back, and manipulate them on-screen to build sentences and narratives. A recent study demonstrated that when listeners rated the informativeness of functional narratives produced by chronic aphasic speakers with and without the program they gave higher informativeness ratings to the language produced with the aid of the program (Bartlett, Fink, Schwartz, & Linebarger, 2007). Bartlett et al. (2007) also compared unaided (spontaneous) narratives produced before and after the aided version of the narrative was obtained. In a subset of comparisons, the sample created after was judged to be more informative; they called this “topic-specific carryover”.
(1) To determine whether differences in informativeness that Bartlett et al.’s listeners perceived are also revealed by Correct Information Unit (CIU) analysis (Nicholas & Brookshire, 1993)—a well studied, objective method for measuring informativeness—and (2) to demonstrate the usefulness of CIU analysis for samples of this type.
Methods & Procedures
A modified version of the CIU analysis was applied to the speech samples obtained by Bartlett et al. (2007). They had asked five individuals with chronic aphasia to create functional narratives on two topics, under three conditions: Unaided (“U”), Aided (“SSR”), & Post-SSR Unaided (“Post-U”). Here, these samples were analysed for differences in % CIUs across conditions. Linear associations between listener judgements and CIU measures were evaluated with bivariate correlations and multiple regression analysis.
Outcomes & Results
(1) The aided effect was confirmed: samples produced with SentenceShaper had higher % CIUs, in most cases exceeding 90%. (2) There was little
That the percentage of CIUs was higher in SSR-aided samples than in unaided samples confirms the central finding in Bartlett et al. (2007), based on subjective judgements, and thus extends the evidence that aided effects from SentenceShaper are demonstrable across a range of measures, stimuli and participants (cf. Linebarger, Schwartz, Romania, Kohn, & Stephens, 2000). The data also attest to the effectiveness of the CIU analysis for quantifying differences in the informativeness of aphasic speech with and without SentenceShaper; and they support prior studies that have shown that CIU measures correlate with the informativeness ratings of unfamiliar listeners.
Aphasia; Sentenceshaper; AAC; Informativeness; functional communication
SentenceShaper™ (SSR) is a computer program that supports spoken language production in aphasia by recording and storing the fragments that the user speaks into the microphone, making them available for playback and allowing them to be combined and integrated into larger structures (i.e., sentences and narratives). A prior study that measured utterance length and grammatical complexity in story-plot narratives produced with and without the aid of SentenceShaper demonstrated an “aided effect” in some speakers with aphasia, meaning an advantage for the narratives that were produced with the support of this communication aid (Linebarger, Schwartz, Romania, Kohn, & Stephens, 2000). The present study deviated from Linebarger et al.’s methods in key respects and again showed aided effects of SentenceShaper in persons with aphasia.
Aims were (1) to demonstrate aided effects in “functional narratives” conveying hypothetical real-life situations from a first person perspective; (2) for the first time, to submit aided and spontaneous speech samples to listener judgements of informativeness; and (3) to produce preliminary evidence on topic-specific carryover from SentenceShaper, i.e., carryover from an aided production to a subsequent unaided production on the same topic.
Methods & Procedures
Five individuals with chronic aphasia created narratives on two topics, under three conditions: Unaided (U), Aided (SSR), and Post-SSR Unaided (Post-U). The 30 samples (5 participants, 2 topics, 3 conditions) were randomised and judged for informativeness by graduate students in speech-language pathology. The method for rating was Direct Magnitude Estimation (DME).
Outcomes & Results
Repeated measures ANOVAs were performed on DME ratings for each participant on each topic. A main effect of Condition was present for four of the five participants, on one or both topics. Planned contrasts revealed that the aided effect (SSR >U) was significant in each of these cases. For two participants, there was also topic-specific carryover (Post-U >U).
Listeners judged functional narratives generated on SentenceShaper to be more informative than comparable narratives spoken spontaneously. This extends the evidence for aided effects of SentenceShaper. There was also evidence, albeit weaker, for topic-specific carryover, suggesting that the program might be used effectively to practise for upcoming face-to-face interactions.