The extreme capsule and aphasia: proof-of-concept of a new way relating structure to neurological symptoms

We present anatomy-based symptom-lesion mapping to assess the association between lesions of tracts in the extreme capsule and aphasia. The study cohort consisted of 123 patients with acute left-hemispheric stroke without a lesion of language-related cortical areas of the Stanford atlas of functional regions of interest. On templates generated through global fibre tractography, lesions of the extreme capsule and of the arcuate fascicle were quantified and correlated with the occurrence of aphasia (n = 18) as defined by the Token Test. More than 15% damage of the slice plane through the extreme capsule was a strong independent predictor of aphasia in stroke patients, odds ratio 16.37, 95% confidence interval: 3.11–86.16, P < 0.01. In contrast, stroke lesions of >15% in the arcuate fascicle were not associated with aphasia. Our results support the relevance of a ventral pathway in the language network running through the extreme capsule.

Voxel-wise and spatial modelling of binary lesion masks: A review and comparison of methods

ObjectivesWhite matter lesions are a very common finding on MRI in older adults and their presence increases the risk of stroke and dementia. Accurate and computationally efficient modelling methods are necessary to map the association of lesion incidence with risk factors, such as hypertension. However, there is no consensus in the brain mapping literature whether a voxel-wise modelling approach is better for binary lesion data than a more computationally intensive spatial modelling approach that accounts for voxel dependence.MethodsWe review three regression approaches for modelling binary lesion masks including massunivariate probit regression modelling with either maximum likelihood estimates, or mean bias-reduced estimates, and spatial Bayesian modelling, where the regression coefficients have a conditional autoregressive model prior to account for local spatial dependence. We design a novel simulation framework of artificial lesion maps to compare the three alternative lesion mapping methods. The age effect on lesion probability estimated from a reference data set (13,680 individuals from the UK Biobank) is used to simulate a realistic voxel-wise distribution of lesions across age. To mimic the real features of lesion masks, we suggest matching brain lesion summaries (total lesion volume, average lesion size and lesion count) across the reference data set and the simulated data sets. Thus, we allow for a fair comparison between the modelling approaches, under a realistic simulation setting.ResultsOur findings suggest that bias-reduced estimates for voxel-wise binary-response generalized linear models (GLMs) overcome the drawbacks of infinite and biased maximum likelihood estimates and scale well for large data sets because voxel-wise estimation can be performed in parallel across voxels. Contrary to the assumption of spatial dependence being key in lesion mapping, our results show that voxel-wise bias-reduction and spatial modelling result in largely similar estimates.ConclusionBias-reduced estimates for voxel-wise GLMs are not only accurate but also computationally efficient, which will become increasingly important as more biobank-scale neuroimaging data sets become available.

Auditory-Visual Speech Behaviors are Resilient to Left pSTS Damage

ABSTRACTThe ability to understand spoken language is essential for social, vocational, and emotional health, but can be disrupted by environmental noise, injury, or hearing loss. These auditory deficits can be ameliorated by visual speech signals that convey redundant or supplemental speech information, but the brain regions critically responsible for these audiovisual (AV) interactions remain poorly understood. Previous TMS and lesion-mapping studies suggest that the left posterior superior temporal sulcus (pSTS) is causally implicated in producing the McGurk effect, an AV illusion in which auditory and visual speech are perceptually “fused.” However, previous research suggests that the McGurk effect is neurally and behaviorally dissociable from other visual effects on speech perception and, therefore, may not provide a generalizable index of AV interactions in speech perception more broadly. To examine whether the left pSTS is critically responsible for AV speech integration more broadly, we measured the strength of the McGurk effect, AV facilitation effects, and AV conflict effects longitudinally over 2 years in patients undergoing surgery for intrinsic tumors in the left pSTS (n = 2) or frontal lobes (control; n = 14). Results demonstrated that left pSTS lesions impaired experience of the McGurk effect, but did not uniformly reduce visual influences on speech perception. Additionally, when multisensory behaviors were affected by a lesion, AV speech perception abilities could recover over time. Our results suggest a causal dissociation between perceptual benefits produced by congruent AV speech and perceptual modulations produced by incongruent AV speech (the McGurk effect).These data are consistent with models proposing that that the pSTS is only one of multiple critical areas necessary for AV speech interactions.

Gesture errors in left and right hemisphere damaged patients: a behavioural and anatomical study

Objective. Erroneous gesture execution is at the core of motor cognition difficulties in apraxia. While a taxonomy of errors may provide important information about the nature of the disorder, classifications are currently often inconsistent. This study aims to identify the error categories which distinguish apraxic from non-apraxic patients. Method. Two groups of mixed and bucco-facial apraxic patients were compared to non-apraxic, left and right hemisphere damaged patients in tasks tapping the ability to perform transitive and intransitive limb and mouth actions. The errors were analysed and classified into 6 categories relating to content, configuration or movement, spatial or temporal parameters and unrecognisable actions. Results. Although all these error typologies may be observed, the most indicative of mixed apraxia relate to content and configuration, while configuration and unrecognisable/destructured action errors seem to be typical of bucco-facial apraxia. Spatial errors are similar in both apraxic and right brain damaged, non-apraxic patients. A lesion mapping analysis of left-brain damaged patients demonstrates that the error categories (except for spatial errors) are all associated with the fronto-parietal network. Tellingly, content errors are also associated with fronto-insular lesions and movement errors with damage to the paracentral territory (precentral and postcentral gyri). Spatial errors more frequently involve ventral frontal lesions. Conclusions. Bucco-facial and mixed apraxic patients make different types of errors in different types of actions. Not all errors are equally indicative of apraxia. In addition, the various error categories are associated with at least partially different neural correlates.