P4‐079: Visual Object Pattern Separation: A Paradigm for Studying the Role of the Dentate Gyrus in Memory Disorders

The dentate gyrus (DG) plays a crucial role in associative memory, and degenerative changes which compromise DG neurogenisis are believed to contribute to memory disturbances in normal ageing and early AD (Ohm, 2007; Tatebayashi, 2003). Research in this field would benefit from memory tasks known to be sensitive to DG function. Toner et al (2009) used a visual object pattern separation task, known from fMRI work to be sensitive to DG activity, and found that 20 elderly non-demented individuals were more likely than younger ones to misidentify pictures as previously presented which were closely similar to previously presented pictures (‘lures'), whereas they were not impaired in identifying the previously presented pictures. The CDR System picture recognition task involves an initial presentation of 20 pictures of everyday scenes and objects, followed later by the same 20 pictures mixed with 20 very similar ones (lures). Examination of a database of 3067 healthy individuals aged 18 to 87 years confirmed and extended Toner et al's findings. Compared to individuals aged 71 to 87, those aged 18 to 30 showed a highly statistically significant advantage in correctly identifying lures (88.2% v 77.6%), whereas the elderly were slightly superior in identifying original pictures (82.8% v 84.9%). Further, the ability to detect lures declines decade by decade from the 20s to the 80s, whereas the ability to identify original pictures increases slightly (Interaction between type of picture and age, F=57.1, p < 0.0001). This pattern was not observed in a word recognition task, confirming the specificity of this phenomenon to pattern separation and directly supporting the DG involvement. Data from 73 amnestic MCI patients showed this pattern to be further exaggerated, the ability to detect lures being poorer than age-matched controls (66.5% v 75.7% p < 0.0001) but not that to identify original pictures. The picture recognition task has over 50 parallel versions for repeated administration in long-term population studies as well as therapeutic trials; offering the ability to monitor the role of the DG in various conditions associated with normal and pathological ageing, as well to assess therapies designed to promote neurogenisis in the DG.