Some of the early symptoms seen in AD are commonly seen as part of normal aging as well, making the distinction of early AD from age-related normal cognitive decline particularly challenging (Killiany et al., 2000; Spaan et al., 2003). A related diagnostic category that emerged in this context is that of Mild Cognitive Impairment (MCI), sometimes referred to as Questionable Dementia (Fowler et al., 2002) or prodromal AD (Blackwell et al., 2004; Rivas-Vazquez, Mendez, Rey, & Carrazana, 2004). While the term MCI was used for several decades in different contexts, in the last few years there is a growing consensus that amnestic MCI (Petersen et al., 2001) reflects a transitional phase between normal aging and early AD (Collie & Maruff, 2000; Petersen et al., 2001; Rivas-Vazquez et al., 2004). Current criteria for amnestic MCI are: subjective memory complaints, objective memory deficit, unaffected overall cognition, normal capacity to perform activities of daily living, and that dementia criteria are not met ( Petersen et al., 2001; Rivas-Vazquez et al., 2004). The rate of progression of individuals with amnestic MCI to AD is approximately 12% per year, compared with a conversion rate of about 1-2% per year among normal elderly people (Petersen et al., 2001).
Meeting criteria for a diagnosis of AD is based on impairment of memory and at least two other domains of cognition (McKhann et al., 1984), but by then the disease can be quite advanced, with considerable neuronal loss. Thus, understanding the specific neurocognitive changes implicated in the pre-clinical phase of AD enables the development of screening procedures that show sensitivity to these early stages.