BACKGROUND Visual illusions provide insight into visual perception processes. We examined the McCollough effects (ME) and watercolor effects (WE) in patients with early Alzheimer disease (AD) and cognitively healthy older adults, and evaluated the influence of acetylcholinesterase inhibitors in the AD mild dementia (MD) stage. MATERIAL AND METHODS We included 28, 27, and 26 patients with MD, amnestic mild cognitive impairment (MCI), and normal cognition (control group), respectively. Participants completed the CDR, MMSE, ADAS-Cog 13, Ishihara test, and ME and WE evaluations. ME was evaluated by identifying chromatic changes in vertical, horizontal black, and white line patterns. WE was evaluated by identifying white or colored sections. RESULTS Regarding ME, white vertical lines appeared red, with no significant differences between groups (H=0.834, P=0.659). Differences were observed in perception of white horizontal lines as green (H=10.27, P=0.006). All in the control group, 25 of 27 in MCI group, and 22 of 28 in MD group reported seeing WE (Fisher exact 6.66, P=0.024). In binary logistic regression, cognitive tests and Ishihara results predicted perception of WE. Regarding MD, no significant differences were reported between patients taking or not taking acetylcholinesterase inhibitors (chi-square 0.749, P=0.38; P=0.19, P=1.00, respectively). CONCLUSIONS Perceptions of ME and WE differed significantly between cognitively normal participants and those with early AD, offering insights into the functional alterations of the visual system and ongoing neurodegeneration. The ME after-effect of red horizontal lines might represent very early AD changes, which could aid in a better understanding of AD visual perception.