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Most contemporary theories of semantic memory assume that concepts are formed from the distillation of information arising in distinct sensory and verbal modalities. The neural basis of this distillation or convergence of information was the focus of this study. Specifically, we explored two commonly posed hypotheses: (a) that the human middle temporal gyrus (MTG) provides a crucial semantic interface given the fact that it interposes auditory and visual processing streams and (b) that the anterior temporal region--especially its ventral surface (vATL)--provides a critical region for the multimodal integration of information. By utilizing distortion-corrected fMRI and an established semantic association assessment (commonly used in neuropsychological investigations), we compared the activation patterns observed for both the verbal and nonverbal versions of the same task. The results are consistent with the two hypotheses simultaneously: Both MTG and vATL are activated in common for word and picture semantic processing. Additional planned, ROI analyses show that this result follows from two principal axes of convergence in the temporal lobe: both lateral (toward MTG) and longitudinal (toward the anterior temporal lobe). [ABSTRACT FROM AUTHOR]

To understand the meanings of words and objects, we need to have knowledge about these items themselves plus executive mechanisms that compute and manipulate semantic information in a task-appropriate way. The neural basis for semantic control remains controversial. Neuroimaging studies have focused on the role of the left inferior frontal gyrus (LIFG), whereas neuropsychological research suggests that damage to a widely distributed network elicits impairments of semantic control. There is also debate about the relationship between semantic and executive control more widely. We used TMS in healthy human volunteers to create "virtual lesions" in structures typically damaged in patients with semantic control deficits: LIFG, left posterior middle temporal gyrus (pMTG), and intraparietal sulcus (IPS). The influence of TMS on tasks varying in semantic and nonsemantic control demands was examined for each region within this hypothesized network to gain insights into (i) their functional specialization (i.e., involvement in semantic representation, controlled retrieval, or selection) and (ii) their domain dependence (i.e., semantic or cognitive control). The results revealed that LIFG and pMTG jointly support both the controlled retrieval and selection of semantic knowledge. IPS specifically participates in semantic selection and responds to manipulations of nonsemantic control demands. These observations are consistent with a large-scale semantic control network, as predicted by lesion data, that draws on semantic-specific (LIFG and pMTG) and domain-independent executive components (IPS). [ABSTRACT FROM AUTHOR]

The most selective disorder of central conceptual knowledge arises in semantic dementia, a degenerative condition associated with bilateral atrophy of the inferior and polar regions of the temporal lobes. Likewise, semantic impairment in both herpes simplex virus encephalitis and Alzheimer's disease is typically associated with bilateral, anterior temporal pathology. These findings suggest that conceptual representations are supported via an interconnected, bilateral, anterior temporal network and that it may take damage to both sides to produce an unequivocal deficit of central semantic memory. We tested and supported this hypothesis by investigating a case series of 20 patients with unilateral temporal damage (following vascular accident or resection for tumour or epilepsy), utilizing a test battery that is sensitive to semantic impairment in semantic dementia. Only 1/20 of the cases, with a unilateral left lesion, exhibited even a mild impairment on the receptive semantic measures. On the expressive semantic tests of naming and fluency, average performance was worse in the left- than right-unilateral cases, but even in this domain, only one left-lesion case had scores consistently more than two standard deviations below control means. These results fit with recent parallel explorations of semantic function using repetitive transcranial magnetic stimulation as well as functional imaging in stroke aphasic and neurologically intact participants. The evidence suggests that both left and right anterior temporal lobe regions contribute to the representation of semantic memory and together may form a relatively damage-resistant, robust system for this critical aspect of higher cognition. [ABSTRACT FROM PUBLISHER]

Previous studies of patients with phonological and surface alexia have demonstrated a double dissociation between the reading of pseudo words and words with atypical spelling-to-sound relationships. A corresponding double dissociation in the neuronal activation patterns for pseudo words and exception words has not, however, been consistently demonstrated in normal subjects. Motivated by the literature on acquired alexia, the present study contrasted pseudo words to exception words and explored how neuronal interactions within the reading system are inf luenced by word type. Functional magnetic resonance imaging was used to measure neuronal responses during reading in 22 healthy volunteers. The direct comparison of reading pseudo words and exception words revealed a double dissociation within the left frontal cortex. Pseudo words preferentially increased left dorsal premotor activation, whereas exception words preferentially increased left pars triangularis activation. Critically, these areas correspond to those previously associated with phonological and semantic processing, respectively. Word-type dependent interactions between brain areas were then investigated using dynamic causal modeling. This revealed that increased activation in the dorsal premotor cortex for pseudo words was associated with a selective increase in effective connectivity from the posterior fusiform gyrus. In contrast, increased activation in the pars triangularis for exception words was associated with a selective increase in effective connectivity from the anterior fusiform gyrus. The present investigation is the first to identify distinct neuronal mechanisms for semantic and phonological contributions to reading. [ABSTRACT FROM AUTHOR]

The disadvantage in producing the past tense of regular relative to irregular verbs shown by some patients with non-fluent aphasia has been alternatively attributed (a) to the failure of a specific rule-based morphological mechanism, or (b) to a more generalised phonological impairment that penalises regular verbs more than irregular owing to the on-average greater phonological complexity of regular past-tense forms. Guided by the second of these two accounts, the current study was designed to identify more specific aspects of phonological deficit that might be associated with the pattern of irregular > regular past-tense production. Nonfluent aphasic patients (N = 8) were tested on past-tense verb production tasks and assessed with regard to the impact of three main manipulations in other word-production tasks: (i) insertion of a delay between stimulus and response in repetition; (ii) presence/ number of consonant clusters in a target word in repetition; (iii) position of stress within a bi-syllabic word in repetition and picture naming. The performance of all patients deteriorated in delayed repetition; but the patients with the largest discrepancy between regular and irregular past-tense production showed greater sensitivity to the other two manipulations. The phonological nature of the factors that correlated with verb-inflection performance emphasises the role of a phonological deficit in the observed pattern of irregular > regular. [ABSTRACT FROM AUTHOR]

We report the case of a patient, JO, who showed intact perception and comprehension of spoken words but who was impaired at accessing the meanings of words she was required to read silently. Letter recognition and written lexical decision were both intact, as was her reading aloud of both words and nonwords. JO's visual comprehension deficit suggests an impairment in mapping between representations in the visual input lexicon and the semantic system. This appears to be the counterpart in reading of "word meaning deafness" (a disorder of spoken word recognition in which patients can perceive spoken words and make auditory lexical decisions but have problems comprehending heard words, despite good comprehension of written words). Hence we refer to this new form of acquired dyslexia as "word meaning blindness." JO's comprehension of words she read aloud was much better, presumably because recoding print into sound enabled her to use her preserved auditory comprehension processes to access meanings. She seemed unable, however, to use "inner speech" to access speech comprehension processes covertly, and further testing indicated a separate impairment of inner speech, which had the effect of making her word meaning blindness more apparent. [ABSTRACT FROM AUTHOR]

Abstract: This study examined the full range of effects associated with “semantic access impairment” – namely, refractory variables (semantic relatedness, speed of presentation and item repetition), inconsistency, the absence of frequency effects and facilitation by cues – in a series of stroke patients with multimodal semantically impairment. By investigating all of these factors in a group of patients who were not specifically selected to show “access” effects, we were able to establish (1) whether this pattern is a common consequence of infarcts that produce semantic impairment and (2) if these symptoms co-occur. All of the patients showed effects of cueing and an absence of frequency effects in comprehension. Patients whose brain damage included the left inferior prefrontal cortex (LIPC) also showed marked effects of refractory variables; in contrast, two patients with temporal–parietal but not frontal lesions were less sensitive to these variables. Parallel results were obtained for cyclical naming and word–picture matching tasks suggesting that the LIPC plays a role in semantic selection as well as lexical retrieval. Rapid presentation and item repetition is likely to have increased the selection demands in both of these tasks in a similar fashion. Unlike patients with classical “semantic access impairment”, our semantically impaired stroke patients showed significant test–retest consistency, indicating that their difficulties did not result from an unpredictable failure of semantic access—instead, their deficits were interpreted as arising from failures of semantic control. [Copyright &y& Elsevier]