The clinical diagnosis of AD remains a process of excluding other possible conditions with similar clinical symptoms, and because a definite diagnosis of Alzheimer's Disease can be only determined post-mortem, people believed to be suffering from it are referred to as suffering from Dementia of the Alzheimer's Type (DAT), or Probable Alzheimer's Disease based on a clinical evaluation of their functional abilities (Forstl & Kurz, 1999; Maccuspie-Moore, 2001).
Currently, the clinical diagnosis of DAT is based on the criteria of the National Institute of Neurological and Communicative Disorders and Stroke-Alzheimer's Disease and Related disorders Association (NINCDS-ADRDA; (McKhann et al., 1984)). These criteria include a gradual onset memory impairment plus impairment on two other cognitive domains. Laboratory and neuroimaging investigations are carried out to exclude other causes (McKhann et al., 1984). Unfortunately, this criteria are usually fulfilled after substantial neuropathology has already developed (Fowler et al., 2002).
Present and future therapeutic developments make the detection of dementia as early as possible a major clinical priority (Blackwell et al., 2004; Fowler et al., 2002; Lin et al., 2003; Perry & Hodges, 2000). Accordingly, there is a growing body of research that focuses on the detection of the earliest, preferably pre-clinical, neurocognitive changes in DAT. Previously published early screening protocols lack a sound theoretical background, and do not necessarily take advantage of demonstrated impairment in specific neurocognitive systems (Chiu et al., 2004; Estevez-Gonzalez et al., 2004; Mundt et al., 2000; Robert et al., 2003; Schneider & Levenson, 2002; Solomon et al., 1998). Addressing this difficulty involves the need for the identification and development of simpler approaches to evaluation that will show sensitivity to the specific symptoms associated with early stages of dementia, and will be minimally demanding for the patient. In other words, an ideal screening procedure will identify simple tas ks in which DAT patients, compared to any other population, will show greater difficulty. In the following section, two tasks that have shown very high sensitivity to early dementing processes and that are intimately related to the earliest neurocognitive staging observed in DAT are discussed.