Agrammatic patterns in Alzheimer's disease: Evidence from tense, agreement, and aspect

In this review we summarize the progress that has been made in the research on attentional and executive deficits in Alzheimer's disease. Like memory, attention is now recognized as consisting of subtypes that differ in their function and anatomical basis. We base our review upon a classification of three subtypes of attention: selective, sustained and divided. This model derives from lesion studies, animal electrophysiological recordings and functional imaging. We examine how these subcomponents of attention can be reconciled with neuropsychological models of attentional control, particularly the Supervisory Attentional System and the Central Executive System of Shallice and Baddeley, respectively. We also discuss the relationship of attention to the concept of executive function. Current evidence suggests that after an initial amnesic stage in Alzheimer's disease, attention is the first non-memory domain to be affected, before deficits in language and visuospatial functions. This is consistent with the possibility that difficulties with activities of daily living, which occur in even mildly demented patients, may be related to attentional deficits. It appears that divided attention and aspects of selective attention, such as set-shifting and response selection, are particularly vulnerable while sustained attention is relatively preserved in the early stages. The phenomenon of cognitive slowing in Alzheimer's disease and normal ageing emphasizes the need to discriminate quantitative changes in attention dysfunction from qualitative changes which may be specifically related to the disease process. The neuropathological basis of these attentional deficits remains unsettled, with two competing hypotheses: spread of pathology from the medial temporal to basal forebrain structures versus corticocortical tract disconnection. Finally we discuss the difficulties of comparing evidence across studies and look at the implications for the design of future studies and future directions that may be fruitful in the research on attention in Alzheimer's disease.

Functional neuroimaging studies have shown that different cognitive functions activate overlapping brain regions. An activation overlap may occur because a region is involved in operations tapped by different cognitive functions or because the activated area comprises subregions differentially involved in each of the functions. To investigate these issues, we directly compared brain activity during episodic retrieval (ER) and working memory (WM) using event-related functional MRI (fMRI). ER was investigated with a word recognition test, and WM was investigated with a word delayed-response test. Two-phase trials distinguished between retrieval mode and cue-specific aspects of ER, as well as between encoding/maintenance and retrieval aspects of WM. The results revealed a common fronto-parieto-cerebellar network for ER and WM, as well as subregions differentially involved in each function. Specifically, there were two main findings. First, the results differentiated common and specific subregions within the prefrontal cortex: (i) left dorsolateral areas were recruited by both functions, possibly reflecting monitoring operations; (ii) bilateral anterior and ventrolateral areas were more activated during ER than during WM, possibly reflecting retrieval mode and cue-specific ER operations, respectively; and (iii) left posterior/ventral (Broca's area) and bilateral posterior/dorsal areas were more activated during WM than during ER, possibly reflecting phonological and generic WM operations, respectively. Second, hippocampal and parahippocampal regions were activated not only for ER but also for WM. This result suggests that indexing operations mediated by the medial temporal lobes apply to both long-term and short-term memory traces. Overall, our results show that direct cross-function comparisons are critical to understand the role of different brain regions in various cognitive functions. 2002 Elsevier Science (USA)