Visual mismatch negativity highlights abnormal pre-attentive visual processing in mild cognitive impairment and Alzheimer's disease

Alzheimer's disease (AD) and mild cognitive impairment (MCI) are traditionally characterised in relation to abnormalities in higher-level brain function, particularly memory. However, in the study presented here we report significant abnormality in pre-attentive visual processing in both MCI and AD compared to healthy ageing. The functional integrity of the automatic change detection system was determined by measuring the visual mismatch negativity (vMMN) component of the N2 visual event-related potential (ERP) in 8 individuals with amnestic MCI, 10 individuals with AD and 10 healthy older adult controls. Compared to healthy ageing, both the MCI and AD groups displayed a significant abnormally increased vMMN over the 140-250 ms measurement epoch although such effects were not apparent over the later 250-400 ms epoch. These novel findings support the view that deficits in brain function in both AD and MCI are not exclusively high-level and indeed that abnormal output from 'low level' or pre-attentive processes (as evinced by the abnormal vMMN) is likely to contribute to the high-level processing deficits so characteristic of these clinical disorders.