Aphasic disorder in patients with closed head injury.
Quantitative assessment of 50 patients with closed head injury disclosed that anomic errors and word finding difficulty were prominent sequelae as nearly half of the series had defective scores on tests of naming and/or word association. Aphasic disturbance was associated with severity of brain injury as reflected by prolonged coma and injury of the brain stem. (+info)
Increased discrimination of "false memories" in autism spectrum disorder.
Individuals with autism spectrum disorder (ASD) have impaired ability to use context, which may manifest as alterations of relatedness within the semantic network. However, impairment in context use may be more difficult to detect in high-functioning adults with ASD. To test context use in this population, we examined the influence of context on memory by using the "false memory" test. In the false memory task, lists of words were presented to high-functioning subjects with ASD and matched controls. Each list consists of words highly related to an index word not on the list. Subjects are then given a recognition test. Positive responses to the index words represent false memories. We found that individuals with ASD are able to discriminate false memory items from true items significantly better than are control subjects. Memory in patients with ASD may be more accurate than in normal individuals under certain conditions. These results also suggest that semantic representations comprise a less distributed network in high-functioning adults with ASD. Furthermore, these results may be related to the unusually high memory capacities found in some individuals with ASD. Research directed at defining the range of tasks performed superiorly by high-functioning individuals with ASD will be important for optimal vocational rehabilitation. (+info)
Evidence for the transient nature of a neural system supporting goal-directed action.
Disruption of a neural system supporting goal-directed action gives rise to lapses of intention in healthy individuals and disorganized behavior in patients with prefrontal lesions. Evidence from behavioral studies indicates that the occurrence of lapses in selective attention, working memory and prospective memory tasks is transient in nature. In the current study, we used event-related brain potentials to demonstrate that lapses are associated with a slow wave over the frontal region that begins well before stimulus onset and lasts for several hundred milliseconds. The magnitude of this slow wave was modulated by task demands, indicating that attentional processes can be flexibly allocated in the service of goal-directed action. Together the findings of these experiments indicate that lapses result from a transient inability to bring to bear the goals of the individual upon the action selection system. (+info)
Functional magnetic resonance imaging (fMRI) activity in the hippocampal region during recognition memory.
Neuroimaging studies have often failed to observe activity in the hippocampal region during memory retrieval. Recently, two functional magnetic resonance imaging studies reported activity in the hippocampal region associated with recollective success. In both, participants studied pictures of objects and were given a recognition memory test with words that either did or did not name the studied objects. The recognition test was therefore cross-modal or associative in nature. These findings raise the question of what circumstances are required to observe activity in the hippocampal region during memory retrieval. Here, we report that robust hippocampal activity for targets relative to foils occurred during retrieval in a recognition memory task when single words were used at both study and test, as well as when pictures of single nameable objects were used at both study and test. The hippocampal region is involved not just in overtly associative tasks but more broadly in the recollection of recently occurring facts and events. (+info)
Separate neural subsystems within 'Wernicke's area'.
Over time, both the functional and anatomical boundaries of 'Wernicke's area' have become so broad as to be meaningless. We have re-analysed four functional neuroimaging (PET) studies, three previously published and one unpublished, to identify anatomically separable, functional subsystems in the left superior temporal cortex posterior to primary auditory cortex. From the results we identified a posterior stream of auditory processing. One part, directed along the supratemporal cortical plane, responded to both non-speech and speech sounds, including the sound of the speaker's own voice. Activity in its most posterior and medial part, at the junction with the inferior parietal lobe, was linked to speech production rather than perception. The second, more lateral and ventral part lay in the posterior left superior temporal sulcus, a region that responded to an external source of speech. In addition, this region was activated by the recall of lists of words during verbal fluency tasks. The results are compatible with an hypothesis that the posterior superior temporal cortex is specialized for processes involved in the mimicry of sounds, including repetition, the specific role of the posterior left superior temporal sulcus being to transiently represent phonetic sequences, whether heard or internally generated and rehearsed. These processes are central to the acquisition of long- term lexical memories of novel words. (+info)
Prefrontal regions supporting spontaneous and directed application of verbal learning strategies: evidence from PET.
The prefrontal cortex has been implicated in strategic memory processes, including the ability to use semantic organizational strategies to facilitate episodic learning. An important feature of these strategies is the way they are applied in novel or ambiguous situations-failure to initiate effective strategies spontaneously in unstructured settings is a central cognitive deficit in patients with frontal lobe disorders. The current study examined strategic memory with PET and a verbal encoding paradigm that manipulated semantic organization in three encoding conditions: spontaneous, directed and unrelated. During the spontaneous condition, subjects heard 24 words that were related in four categories but presented in mixed order, and they were not informed of this structure beforehand. Any semantic reorganization was, therefore, initiated spontaneously by the subject. In the directed condition, subjects were given a different list of 24 related words and explicitly instructed to notice relationships and mentally group related words together to improve memory. The unrelated list consisted of 24 unrelated words. Behavioural measures included semantic clustering, which assessed active regrouping of words into semantic categories during free recall. In graded PET contrasts (directed > spontaneous > unrelated), two distinct activations were found in left inferior prefrontal cortex (inferior frontal gyrus) and left dorsolateral prefrontal cortex (middle frontal gyrus), corresponding to levels of semantic clustering observed in the behavioural data. Additional covariate analyses in the first spontaneous condition indicated that blood flow in orbitofrontal cortex (OFC) was strongly correlated with semantic clustering scores during immediate free recall. Thus, blood flow in OFC during encoding predicted which subjects would spontaneously initiate effective strategies during free recall. Our findings indicate that OFC performs an important, and previously unappreciated, role in strategic memory by supporting the early mobilization of effective behavioural strategies in novel or ambiguous situations. Once initiated, lateral regions of left prefrontal cortex control verbal semantic organization. (+info)
Cognitive association formation in human memory revealed by spatiotemporal brain imaging.
Cognitive theory posits association by juxtaposition or by fusion. We employed the measurement of event-related brain potentials (ERPs) to a concept fusion task in order to explore memory encoding of these two types of associations between word pairs, followed by a memory test for original pair order. Encoding processes were isolated by subtracting fusion task ERPs corresponding to pairs later retrieved quickly from ERPs corresponding to pairs later retrieved slowly, separately for pairs fused successfully and unsuccessfully (i.e., juxtaposed). Analyses revealed that the encoding of these two types of associations yields different ERP voltage polarities, scalp topographies, and brain sources extending over the entire time course of processing. (+info)
The neural bases of sentence comprehension: a fMRI examination of syntactic and lexical processing.
One of the challenges to functional neuroimaging is to understand how the component processes of reading comprehension emerge from the neural activity in a network of brain regions. In this study, functional magnetic resonance imaging (fMRI) was used to examine lexical and syntactic processing in reading comprehension by independently manipulating the cognitive demand on each of the two processes of interest. After establishing a consistency with earlier research showing the involvement of the left perisylvian language areas in both lexical access and syntactic processing, the study produced new findings that are surprising in two ways: (i) the lexical and syntactic factors each impact not just individual areas, but they affect the activation in a network of left-hemisphere areas, suggesting that changing the computational load imposed by a given process produces a cascade of effects in a number of collaborating areas; and (ii) the lexical and syntactic factors usually interact in determining the amount of activation in each affected area, suggesting that comprehension processes that operate on different levels of language may nevertheless draw on a shared infrastructure of cortical resources. The results suggest that many processes in sentence comprehension involve multiple brain regions, and that many brain regions contribute to more than one comprehension process. The implication is that the language network consists of brain areas which each have multiple relative specializations and which engage in extensive interarea collaborations. (+info)