Anna M. Woollams

Making sense of progressive non-fluent aphasia: an analysis of conversational speech

The speech of patients with progressive non-fluent aphasia (PNFA) has often been described clinically, but these descriptions lack support from quantitative data. The clinical classification of the progressive aphasic syndromes is also debated. This study selected 15 patients with progressive aphasia on broad criteria, excluding only those with clear semantic dementia. It aimed to provide a detailed quantitative description of their conversational speech, along with cognitive testing and visual rating of structural brain imaging, and to examine which, if any features were consistently present throughout the group; as well as looking for sub-syndromic associations between these features. A consistent increase in grammatical and speech sound errors and a simplification of spoken syntax relative to age-matched controls were observed, though telegraphic speech was rare; slow speech was common but not universal. Almost all patients showed impairments in picture naming, syntactic comprehension and executive function. The degree to which speech was affected was independent of the severity of the other cognitive deficits. A partial dissociation was also observed between slow speech with simplified grammar on the one hand, and grammatical and speech sound errors on the other. Overlap between these sets of impairments was however, the rule rather than the exception, producing continuous variation within a single consistent syndrome. The distribution of atrophy was remarkably variable, with frontal, temporal and medial temporal areas affected, either symmetrically or asymmetrically. The study suggests that PNFA is a coherent, well-defined syndrome and that varieties such as logopaenic progressive aphasia and progressive apraxia of speech may be seen as points in a space of continuous variation within progressive non-fluent aphasia.

Capturing multidimensionality in stroke aphasia: mapping principal behavioural components to neural structures

Butler et al. relate behavioural deficits in 31 patients with chronic stroke aphasia to underlying neural structures. Using principal components analysis, they reduce a neuropsychological battery to three independent dimensions: phonological, semantic and executive-cognition. Phonological and semantic processing are linked to dorsal and ventral pathway integrity, respectively

Word or word-like? dissociating orthographic typicality from lexicality in the left occipito-temporal cortex

Prior lesion and functional imaging studies have highlighted the importance of the left ventral occipito-temporal (LvOT) cortex for visual word recognition. Within this area, there is a posterior–anterior hierarchy of subregions that are specialized for different stages of orthographic processing. The aim of the present fMRI study was to dissociate the effects of subword orthographic typicality (e.g., cider [high] vs. cynic [low]) from the effect of lexicality (e.g., pollen [word] vs. pillen [pseudoword]). We therefore orthogonally manipulated the orthographic typicality of written words and pseudowords (nonwords and pseudohomophones) in a visual lexical decision task. Consistent with previous studies, we identified greater activation for pseudowords than words (i.e., an effect of lexicality) in posterior LvOT cortex. In addition, we revealed higher activation for atypical than typical strings, irrespective of lexicality, in a left inferior occipital region that is posterior to LvOT cortex. When lexical decisions were made more difficult in the context of pseudohomophone foils, left anterior temporal activation also increased for atypical relative to typical strings. The latter finding agrees with the behavior of patients with progressive anterior temporal lobe degeneration, who have particular difficulty recognizing words with atypical orthography. The most novel outcome of this study is that, within a distributed network of regions supporting orthographic processing, we have identified a left inferior occipital region that is particularly sensitive to the typicality of subword orthographic patterns.

What's in a word? A parametric study of semantic influences on visual word recognition.

To what extent does semantic information play a functional role in visual word recognition? Theories of word recognition vary in the importance assigned to semantic information in visual lexical decision, with past research suggesting that the nature of the foils is a crucial determinant of semantic reliance. Here, we explored the conditions under which semantic variables influence lexical decision. Normal readers performed visual lexical decision tasks in which imageability and semantic priming were manipulated, with nonword foils varying systematically in their orthographic and phonological similarity to the real words. The effects of imageability and semantic priming increased in magnitude as nonword foils became progressively more wordlike. These findings provide a clear illustration of the flexible use of semantic information to support normal visual word recognition.

Localising semantic and syntactic processing in spoken and written language comprehension: An Activation Likelihood Estimation meta-analysis

We conducted an Activation Likelihood Estimation (ALE) meta-analysis to identify brain regions that are recruited by linguistic stimuli requiring relatively demanding semantic or syntactic processing. We included 54 functional MRI studies that explicitly varied the semantic or syntactic processing load, while holding constant demands on earlier stages of processing. We included studies that introduced a syntactic/semantic ambiguity or anomaly, used a priming manipulation that specifically reduced the load on semantic/syntactic processing, or varied the level of syntactic complexity. The results confirmed the critical role of the posterior left Inferior Frontal Gyrus (LIFG) in semantic and syntactic processing. These results challenge models of sentence comprehension highlighting the role of anterior LIFG for semantic processing. In addition, the results emphasise the posterior (but not anterior) temporal lobe for both semantic and syntactic processing.

Using principal component analysis to capture individual differences within a unified neuropsychological model of chronic post-stroke aphasia: Revealing the unique neural correlates of speech fluency, phonology and semantics

Individual differences in the performance profiles of neuropsychologically-impaired patients are pervasive yet there is still no resolution on the best way to model and account for the variation in their behavioural impairments and the associated neural correlates. To date, researchers have generally taken one of three different approaches: a single-case study methodology in which each case is considered separately; a case-series design in which all individual patients from a small coherent group are examined and directly compared; or, group studies, in which a sample of cases are investigated as one group with the assumption that they are drawn from a homogenous category and that performance differences are of no interest. In recent research, we have developed a complementary alternative through the use of principal component analysis (PCA) of individual data from large patient cohorts. This data-driven approach not only generates a single unified model for the group as a whole (expressed in terms of the emergent principal components) but is also able to capture the individual differences between patients (in terms of their relative positions along the principal behavioural axes). We demonstrate the use of this approach by considering speech fluency, phonology and semantics in aphasia diagnosis and classification, as well as their unique neural correlates. PCA of the behavioural data from 31 patients with chronic post-stroke aphasia resulted in four statistically-independent behavioural components reflecting phonological, semantic, executive–cognitive and fluency abilities. Even after accounting for lesion volume, entering the four behavioural components simultaneously into a voxel-based correlational methodology (VBCM) analysis revealed that speech fluency (speech quanta) was uniquely correlated with left motor cortex and underlying white matter (including the anterior section of the arcuate fasciculus and the frontal aslant tract), phonological skills with regions in the superior temporal gyrus and pars opercularis, and semantics with the anterior temporal stem.

Triangulation of the neurocomputational architecture underpinning reading aloud.

Significance Reading is a critical linguistic skill, but understanding of its cognitive and neural bases is incomplete. Using functional MRI, we found reading-related activation in two areas of anterior temporal cortex, an area not previously associated with reading. Activation profiles of these sites were consistent with the predictions of computational reading models that ascribe a key role to semantic knowledge in reading words with irregular spellings. We also found individual differences in the reading neural network that were predicted by an independent measure of semantic reliance in reading. Such individual differences have been hypothesized on theoretical grounds to explain variation in reading abilities among neurodegenerative patients. Here we provide the first direct evidence, to our knowledge, for their existence in the healthy brain. The goal of cognitive neuroscience is to integrate cognitive models with knowledge about underlying neural machinery. This significant challenge was explored in relation to word reading, where sophisticated computational-cognitive models exist but have made limited contact with neural data. Using distortion-corrected functional MRI and dynamic causal modeling, we investigated the interactions between brain regions dedicated to orthographic, semantic, and phonological processing while participants read words aloud. We found that the lateral anterior temporal lobe exhibited increased activation when participants read words with irregular spellings. This area is implicated in semantic processing but has not previously been considered part of the reading network. We also found meaningful individual differences in the activation of this region: Activity was predicted by an independent measure of the degree to which participants use semantic knowledge to read. These characteristics are predicted by the connectionist Triangle Model of reading and indicate a key role for semantic knowledge in reading aloud. Premotor regions associated with phonological processing displayed the reverse characteristics. Changes in the functional connectivity of the reading network during irregular word reading also were consistent with semantic recruitment. These data support the view that reading aloud is underpinned by the joint operation of two neural pathways. They reveal that (i) the ATL is an important element of the ventral semantic pathway and (ii) the division of labor between the two routes varies according to both the properties of the words being read and individual differences in the degree to which participants rely on each route.

Apples are not the only fruit: The effects of concept typicality on semantic representation in the anterior temporal lobe

Intuitively, an apple seems a fairly good example of a fruit, whereas an avocado seems less so. The extent to which an exemplar is representative of its category, referred to here as concept typicality, has long been thought to be a key dimension determining semantic representation. Concept typicality is, however, correlated with a number of other variables, in particular age of acquisition (AoA) and name frequency. Consideration of picture naming accuracy from a large case-series of semantic dementia (SD) patients demonstrated strong effects of concept typicality that were maximal in the moderately impaired patients, over and above the impact of AoA and name frequency. Induction of a temporary virtual lesion to the left anterior temporal lobe, the region most commonly affected in SD, via repetitive Transcranial Magnetic Stimulation produced an enhanced effect of concept typicality in the picture naming of normal participants, but did not affect the magnitude of the AoA or name frequency effects. These results indicate that concept typicality exerts its influence on semantic representations themselves, as opposed to the strength of connections outside the semantic system. To date, there has been little direct exploration of the dimension of concept typicality within connectionist models of intact and impaired conceptual representation, and these findings provide a target for future computational simulation.

The role of plasticity-related functional reorganization in the explanation of central dyslexias

This investigation explored the hypothesis that patterns of acquired dyslexia may reflect, in part, plasticity-driven relearning that dynamically alters the division of labour (DOL) between the direct, orthography → phonology (O → P) pathway and the semantically mediated, orthography → semantics → phonology (O → S → P) pathway. Three simulations were conducted using a variant of the triangle model of reading. The model demonstrated core characteristics of normal reading behaviour in its undamaged state. When damage was followed by reoptimization (mimicking spontaneous recovery), the model reproduced the deficits observed in the central dyslexias—acute phonological damage combined with recovery matched data taken from a series of 12 phonological dyslexic patients—whilst progressive semantic damage interspersed with recovery reproduced data taken from 100 observations of semantic dementia patients. The severely phonologically damaged model also produced symptoms of deep dyslexia (imageability effects, production of semantic and mixed semantic/visual errors). In all cases, the DOL changed significantly in the recovery period, suggesting that postmorbid functional reorganization is important in understanding behaviour in chronic-stage patients.

L is for tiger: Effects of phonological (mis)cueing on picture naming in semantic aphasia

Semantic aphasia (SA) refers to a subset of aphasic patients who exhibit multimodal semantic deficits (Jefferies, E., & Lambon Ralph, M. (2006). Semantic impairment in stroke aphasia vs semantic dementia: a case-series comparison. Brain, 129(8), 2132-2147). Consistent with their underlying semantic control deficit, SA picture naming accuracy can be improved considerably with a correct phonological cue. The performance of normal individuals in the tempo picture naming paradigm reveals an increased impact of both correct and incorrect phonological cueing, and it has been suggested that this technique reduces resources available for semantic control in neurologically-intact participants (Hodgson, C.. & Lambon Ralph, M. (2008). Mimicking aphasic semantic naming errors in normal speech production: Evidence from a novel experimental paradigm. Brain and Language, 104(1). 89-101.). We tested this hypothesis by considering the impact of both correct and incorrect phonological cues on picture naming in a case series of SA patients, using exactly the same items as those presented to normal participants for tempo naming. The results confirmed the positive effect of correct cues and revealed for the first time the negative effects of category co-ordinate miscues amongst these patients in both overall accuracy and semantic error rates. The implications of our results for current speech production models are considered