Boundary extension as a tool for detection of cognitive change among individuals with mild cognitive impairment: A preliminary study

•Boundary extension (BE) was elevated in normal aging and MCI.•Familiarity may support BE to a higher extent than does recollection.•Familiarity in BE was reduced in MCI.•Educational level was positively correlated with familiarity.•Effects of plasma tau on cognitive function were mediated by educational level. Recent neuropathological research suggests that recognition memory supported by familiarity rather than recollection may be the earliest cognitive change in course of Alzheimer's disease (AD). Nonetheless, the findings on the issue of familiarity capacity in the prodromal AD remain inconsistent. Boundary extension (BE), in which the view recollected by the subject covers a wider angle than was actually observed, is a form of false memory. Given that BE occurs implicitly and automatically, it may be a candidate for assessing familiarity functioning in cases of AD. This was the issue explored in the current study. : One-hundred and six participants comprising a younger adult group (YA, n = 40), a healthy older adult group (OA, n = 40), and a group of patients with mild cognitive impairment (MCI, n = 26) underwent testing for BE and neuropsychological functions. Parts of OA and MCI underwent analysis for plasma tau levels. Receiver-operating characteristic analysis was used to assess memory associated with familiarity and recollection among participants. : The OA and MCI groups could be differentiated by the degree of familiarity associated with BE, wherein the latter group displayed minimal familiarity. Among OAs, familiarity was positively associated with education level. We observed a correlation between plasma tau levels and various neuropsychological functions. Most of the associations between plasma tau levels and neuropsychological functions were mediated by education level. : Our findings indicate that BE could detect early decline in familiarity and assess preserved cognitive functions in aging