1. A method of detecting and tracking an eye in a succession of video images of a human subject to assess and characterize a state of the human subject, the method comprising the steps of: acquiring a first video image in the succession of video images and processing it to detect the human subject's eye and designate a location of the detected eye in the first video image; acquiring a second video image after acquiring said first video image, and then: (a) extracting an image search patch from the second video image centered on the designated location of the eye in the first video image; (b) identifying eye candidates within said extracted image search patch and accepting or rejecting the identified eye candidates based on appearance; and (c) if none of the identified eye candidates are accepted based on appearance: (i) extracting a set of eye periphery image patches from the first video image that are clustered about the designated location of the eye; (ii) searching the image search patch extracted from the first video image to locate a set of image patches within the image search patch that match the eye periphery image patches extracted from the first video image; and (iii) if a matching set of image patches is located in the image search patch, determining a center of the matching set of image patches from the second video image, and designating the location of the eye in the second video image as the determined center; (d) wherein the forementioned steps are implemented by a processor.