Methods are for determining whether a subject who does not present clinical symptoms associated with Alzheimer's disease (AD) or mild cognitive impairment (MCI) is in a process of developing AD or MCI, or at risk for developing post-anesthesia delirium or cognitive dysfunction, by detecting the presence of epileptiform spikes in the brain of the subject during diagnostic anesthesia. Additional methods are for preventing, reducing likelihood of, or treating AD or MCI, or post-anesthesia delirium or cognitive dysfunction.