Postoperative Delirium After Dexmedetomidine versus Propofol Sedation in Older Adults Undergoing Hip Fracture Surgery with Spinal Anesthesia: A Randomized Controlled Trial.

While impaired episodic memory is a well-established hallmark of Alzheimer's disease (AD), semantic memory impairments are also present in the earliest stages of the disease and in amnestic mild cognitive impairment (MCI), a clinically heterogeneous condition associated with an increased risk of progression to AD , . Deficits occur across a range of semantic tasks including object naming , , , , , , , and category fluency , , , , , both of which have been linked to degradation of the semantic network , , . Importantly, recent studies suggest that semantic memory deficits may even precede episodic memory loss in AD, raising the possibility that disruptions in semantic processing may be detectable early in the disease course , , . Semantic cognition relies on distributed networks within frontal, temporal, parietal, and occipital regions that support both the storage and retrieval of conceptual knowledge , , . The semantic memory impairments exhibited by AD patients may arise from multiple disruptions within these networks. First, early pathological changes to the perirhinal and entorhinal cortices may degrade object-level semantic representations. These regions support the integration of cross-modal features that uniquely define individual objects, and their integrity is critical for maintaining semantic distinctions , , , . In healthy adults, the perirhinal cortex is implicated in object-specific semantic encoding , , and performance on object recognition tasks associated with perirhinal and entorhinal function has also been shown to predict conversion from MCI to AD . Second, impaired semantic memory in AD likely arises from disruptions to large-scale corticocortical interactions that support the integration of semantic information across distributed cortical networks . Neuropathological studies reveal systematic AD-related disruption of long-range projections linking functionally related cortical regions , , , , , and neuroimaging studies similarly demonstrate functional and structural connectivity alterations beginning in prodromal and even preclinical stages of the disease , , , , , , , , . These network-level disruptions affect multiple cortical regions implicated in semantic processing , , and are related to disease progression , , . Behavioral evidence further suggests that such network disruption may affect audiovisual integration differently in healthy aging and MCI. Using a McGurk paradigm involving both visual and auditory speech information, Festa et al. found that healthy older adults showed enhanced visual influence when bottom-up audiovisual support was weak, whereas participants with amnestic MCI showed reduced audiovisual integration and greater reliance on top-down lexical information under the same conditions. This dissociation suggests that MCI-related audiovisual deficits are not simply an amplification of normal age-related changes, but emerge when cross-modal integration tasks place greater demands on effective interactions among distributed cortical regions.