A method of testing a subject for impairment includes presenting the subject with a display of a smoothly moving object, repeatedly moving over a tracking path and, while presenting the display to the subject, measuring the subject's right eye positions and measuring the subject's left eye positions. The method further includes generating a disconjugacy metric by comparing the measured right eye positions with the measured left eye positions, comparing the disconjugacy metric with a predetermined baseline to determine whether the disconjugacy metric is indicative of an impairment, and generating a report based on the disconjugacy metric.