Vestibular-neck interaction of human ocular counterroll

Compensatory rolling of the eye about the line of sight (ocular counterroll, OCR) was studied in four normal human subjects during 10 min of lateral tilt (20 degrees to the left/right) of body, head and trunk. OCR was objectively measured at 1-min intervals by determining the rotation of a circular sample of iris pattern using a videocamera and digital image processing. Body and head tilt caused symmetrical OCR (range 2.4 to 9.5 degrees) to either side, whereas trunk tilt induced no significant change in torsional eye position. No adaptation in OCR was found during the time spent in tilted orientation in neither paradigm though unsystematic torsional fluctuation was a regular finding in all subjects. These findings are taken as evidence that only the non-adapting otolith system, as opposed to the proprioceptive system of the neck, contributes to the generation and maintenance of OCR.