Preservation of episodic memory in semantic dementia: The importance of regions beyond the medial temporal lobes

Abstract

Episodic memory impairment represents one of the hallmark clinical features of patients withAlzheimer's disease (AD) attributable to the degeneration of medial temporal and parietal regions of the brain. In contrast, a somewhat paradoxical profile of relatively intact episodic memory, particularly for non-verbal material, is observed in SD, despite marked atrophy of the hippocampus. This retrospective study investigated the neural substrates of episodic memory retrieval in 20 patients with a diagnosis of SD and 21 disease-matched cases of AD and compared their performance to that of 35 age-and education-matched healthy older were modality-specific occurring exclusively on the verbal task. Controlling for semantic processing ameliorated these deficits in SD, while memory impairments persisted in AD.Voxel-based morphometry analyses revealed significant overlap in the neural correlates of verbal episodic memory in AD and SD with predominantly anteromedial regions, including the bilateral hippocampus, strongly implicated. Controlling for semantic processing negated this effect in SD, however, a distributed network of frontal, medial temporal, and parietal regions was implicated in AD. Our study corroborates the view that episodic memory deficits in SD arise very largely as a consequence of the conceptual loading of traditional tasks. We propose that the functional integrity of frontal and parietal regions enables new learning to occur in SD in the face of significant hippocampal and anteromedial temporal lobe pathology, underscoring the inherent complexity of the episodic memory circuitry.

Distinct white matter injury associated with medial temporal lobe atrophy in Alzheimer's versus semantic dementia

“…To account for this apparent paradox, it has been proposed that the two neurodegenerative disorders may disrupt distinct brain networks related to the MTL [La Joie et al, ; Ranganath and Ritchey, ]. This is in line with the idea that cumulative insult in episodic memory‐related regions beyond the MTL (e.g., medial and lateral parietal cortex) might account for the different episodic memory profiles in AD versus SD [Irish et al, ; Nestor et al, ; Pleizier et al, ]. In support of the network degeneration hypothesis, a recent study showed that cortical regions more impaired in AD versus SD, or reversely, were all connected to the hippocampus in healthy controls [La Joie et al, ].…”

The Neuroanatomical, Neurophysiological and Psychological Basis of Memory: Current Models and Their Origins

“…Patients with semantic dementia is characterized by a loss of conceptual knowledge, and this loss may reflect the disruption of a central semantic hub or the degeneration of a temporosylvian language network for verbal concepts (Irish et al, 2014). These patients manifesting in striking alterations in naming and comprehension (Irish et al, 2016). These patients are “characterized by a clear dissociation between marked single-word comprehension” (Agosta et al, 2010), unable to retrieve the names of objects, irregular word reading deficits, identify the color the correct objects, and sparing of fluency, phonology, syntax and working memory (Binder and Desai, 2011).…”

An update on semantic dementia: genetics, imaging, and pathology

“…In contrast, recent studies have confirmed that episodic memory is relatively preserved in SD, particularly when tasks with minimal conceptual loading are employed [12, 13]. The intact performance on traditional non-conceptually loaded episodic memory tasks converges with the performance of SD patients on autobiographical memory tasks.…”

“…Taken together, these findings suggest that spared syntactic processing in SD depends on preserved functionality of left frontal, parietal and, to lesser degree, posterior temporal regions. The relative integrity of regions beyond the medial temporal cortices, such as the posterior cingulate and frontal cortices, also seems to sub-serve other cognitive functions such as the preserved episodic memory performance in SD patients [13, 55]. …”