A method of monitoring a patient includes obtaining a first image of an object, obtaining a second image of the object, determining a level of similarity between the first and second images, obtaining a third image of the object, determining a level of similarity between the first and third images, analyzing a time series of values that includes the determined level of similarity between the first and second images and the determined level of similarity between the first and third images, and determining a state of the patient based at least on a result of the act of analyzing.