To characterize the effects of Alzheimer's disease (AD) on cortical functional connectivity in perception, we analyzed interhemispheric lagged synchronization (ILS) in the source space of high-density EEG recorded in aged controls and patients with amnestic mild cognitive impairment (aMCI) or AD while they viewed collinear and noncollinear bilateral moving gratings. Beta-band ILS was lower in aMCI and AD compared with controls in a large region centered on BA39. As previously reported, in young adults, collinear iso-oriented gratings versus noncollinear gratings synchronizes EEG reflecting perceptual grouping. Only aged controls showed the expected beta-band ILS increase originating in the dorsal visual stream (BA18). The aMCI group only showed a theta-band increase in an adjacent region (BA19). In AD patients, there was no ILS increase. Regression analysis revealed that the posterior callosal area and EEG slowing predict reduction of beta but not emergence of theta ILS response. Considering that we found no between-group differences in resting ILS, perception-related EEG appears to be more sensitive to AD effects, including ILS signs of neurodegeneration and compensation.