Fig. 1.

Model of cognitive decline. The black line represents the trajectory of cognitive decline due to neurodegenerative disease. The yellow dashed line represents the threshold for clinical dementia, i.e., the inability to manage activities of daily living. The red line represents the impact of additional injuries to the brain, which can decrease brain reserve and cause a leftward shift of the trajectory of cognitive decline, leading patients to cross the threshold for clinical dementia earlier. The green line represents the effect of increased cognitive and brain reserve, which can cause a rightward shift in the trajectory of cognitive decline, leading patients to cross the threshold for clinical dementia later. These principles involving theoretical shifts in the trajectory of cognitive decline also apply to adults without neurodegenerative disease, though the initial downward trajectory is much less steep.