Question
If you ask a patient to move his head in response to the pendular tracking while keeping his eyes on the light, does this give any valid information regarding the Vestibular Ocular Reflex (VOR)? I would expect a straight line in perfect coordination but a lagging of eye movement if the coordination is poor.
Answer
The test procedure you are describing requires interaction between VOR and tracking mechanisms. However, depending on the frequency of the target and head motion, one of the mechanisms dominates the response. For target or head frequencies above 1 Hz, the tracking mechanism does not function and the response is entirely mediated by the VOR. For frequencies below 1 Hz, the response is a combination of VOR and tracking.
You are right that theoretically, the response will resemble a flat line. That is, the eyes should stay approximately in the same position with respect to the head. However, remember there is neural delay from the time target movements are detected to the time the eyes are moved. For predictable target movements, we can send the eyes ahead to eliminate this neural delay. The predictive mechanisms do not work well when you combine the target movements with eye movements. So in practice, you see low amplitude eye movements that are seemingly unrelated to either the head or target movements. For that reason, the procedure is not considered a high-yield test. It is easier to measure the VOR and tracking independently.
A similar procedure is used during the active head rotation test or Vestibular Autorotation Test (VAT) in which the patient is asked to fixate on a stationary target while moving the head side to side. The difference is that the movements are usually above 1 Hz and the VOR dominates the response.
More information on this topic can be found in Dr. Barin's course, Correlations in Vestibular Test Findings: /ceus/recordedcoursedetails.asp?class_id=13040
Kamran Barin, Ph.D. is the Director of Balance Disorders Clinic at the Ohio State University Medical Center and Assistant Professor, Department of Otolaryngology, Department of Speech and Hearing Sciences, and Biomedical Engineering Program. He received his Master's and Doctorate degrees in Electrical/Biomedical Engineering from The Ohio State University. He has taught national and international courses and seminars in different areas of vestibular assessment and rehabilitation. He has served on NIH and NASA panels and was an appointed member of the Equilibrium Subcommittee of the American Neurotology Society. He works as a consultant to GN Otometrics and provides courses and other educational material to the company.
