The anterior temporal lobe semantic hub is a part of the language neural network: selective disruption of irregular past tense verbs by rTMS

There is growing evidence from patient and neuroimaging studies that the anterior temporal lobe (ATL) should be considered a crucial part of the neural network that underpins language. Specifically, this region supports semantic representations that play a key role in various aspects of language processing. In this study, we tested the critical importance of this region for language processing in normal participants by applying repetitive transcranial magnetic stimulation over the left ATL semantic region. The ability to generate the past tense of English verbs has often been used as a test case for neurocognitive models of language. Accordingly, we used this aspect of language to investigate the impact of repetitive transcranial magnetic stimulation (rTMS) over the left ATL. As predicted by single mechanism accounts of past-tense generation, ATL rTMS had a selective impact on participants' ability to generate the past tense of irregular verbs. When combined with other evidence, these results confirm that the ATL semantic hub is a key component of the neural network for language

Category-specific deficits: Insights from semantic dementia and Alzheimer's disease

Recent investigations and theorising about category-specific deficits have begun to focus upon patients with progressive brain disease such as semantic dementia and Alzheimer's disease. In this commentary we briefly review what insights have been gained from studying patients of this type. We concentrate on four specific issues: the sensory/functional distinction, correlation between features, neuroanatomical considerations, and confounding factors

Deficits of knowledge versus executive control in semantic cognition: Insights from cued naming

Deficits of semantic cognition in semantic dementia and in aphasia consequent on CVA (stroke) are qualitatively different. Patients with semantic dementia are characterised by progressive degradation of central semantic representations, whereas multimodal semantic deficits in stroke aphasia reflect impairment of executive processes that help to direct and control semantic activation in a task-appropriate fashion [Jefferies, E., & Lambon Ralph, M. A. (2006). Semantic impairment in stroke aphasia vs. semantic dementia: A case-series comparison. Brain 129, 2132-2147]. We explored interactions between these two aspects of semantic cognition by examining the effects of cumulative phonemic cueing on picture naming in case series of these two types of patient. The stroke aphasic patients with multimodal semantic deficits cued very readily and demonstrated near-perfect name retrieval when cumulative phonemic cues reached or exceeded the target name's uniqueness point. Therefore, knowledge of the picture names was largely intact for the aphasic patients, but they were unable to retrieve this information without cues that helped to direct activation towards the target response. Equivalent phonemic cues engendered significant but much more limited benefit to the semantic dementia patients: their naming was still severely impaired even when most of the word had been provided. In contrast to the pattern in the stroke aphasia group, successful cueing was mainly confined to the more familiar un-named pictures. We propose that this limited cueing effect in semantic dementia follows from the fact that concepts deteriorate in a graded fashion [Rogers, T. T., Lambon Ralph, M. A., Garrard, P., Bozeat, S., McClelland, J. L., & Hodges, J. R., et al. (2004). The structure and deterioration of semantic memory: A neuropsychological and computational investigation. Psychological Review 111, 205-235]. For partially degraded items, the residual conceptual knowledge may be insufficient to drive speech production to completion but these items might reach threshold when they are bolstered by cues. (C) 2007 Elsevier Ltd. All rights reserved