Distinct theta and alpha electrophysiological dynamics during audiovisual semantic processing in healthy aging and mild cognitive impairment.

Semantic memory decline is increasingly recognized as an early feature of Alzheimer's disease (AD) and amnestic mild cognitive impairment (MCI), yet the neural dynamics supporting automatic and controlled semantic retrieval in healthy aging remain poorly defined. This study examined task-evoked oscillatory activity during audiovisual object recognition in young adults (YA; N = 27), healthy older adults (OA; N = 33), and individuals with amnestic MCI (N = 21). Participants judged object orientation while viewing living and nonliving images paired with congruent or incongruent characteristic sounds, allowing semantic relationships to be manipulated under implicit retrieval demands. Accuracy was high across groups, although participants with MCI showed reduced performance under the more perceptually challenging inverted conditions. Reaction times were slower in OA than YA and further slowed in MCI, with group differences varying by object animacy and semantic congruency. Event-related spectral perturbation analyses revealed distinct group-related patterns. Healthy older adults showed reduced early and increased late theta activity in frontocentral and parieto-occipital regions, consistent with delayed recruitment of control-related and perceptual-attentional processes. In contrast, MCI participants showed elevated and less condition-sensitive posterior alpha power, together with task- and condition-specific differences in frontocentral theta. The principal pattern of delayed theta recruitment in healthy aging and elevated posterior alpha in MCI was also observed in a supplementary task requiring explicit audiovisual semantic judgments. These findings provide preliminary evidence that healthy aging and amnestic MCI are associated with partly distinct patterns of task-evoked oscillatory activity during audiovisual semantic processing.