1. A method of testing a subject for cognitive impairment, comprising: presenting the subject with a display of a smoothly moving object, repeatedly moving over a tracking path; measuring the subject's eye movements during the presenting; comparing the measured eye movements with movements of the smoothly moving object to generate an error signal, the error signal corresponding to at least one of timing error, position error and velocity error of the subject's eye movements during visual pursuit of the display of the smoothly moving object; analyzing variability of the error signal during visual pursuit of the display of the smoothly moving object to generate a variability metric, wherein the variability metric corresponds to how accurately and how consistently the subject visually tracks movement of the object; and presenting a report, based on at least the variability metric, that indicates whether the subject has a cognitive impairment.