Effects of earth-fixed vs head-fixed targets on static ocular counterroll.

OBJECTIVE To investigate whether static ocular counterroll (OCR) gain is reduced during viewing of an earth-fixed vs a head-fixed target. METHODS Twelve healthy individuals were recruited. The target consisted of a red fixation cross against a grid pattern at a viewing distance of 33 cm. The target was mounted on a wall (earth fixed) or was coupled to the head (head fixed). Changes in mean torsional eye position were plotted as a function of head position steps (0 degrees +/- 25 degrees in 5 degrees steps), and sigmoidal fits were performed. Mean static OCR gain was calculated by taking the derivative of the fitted functions. RESULTS Mean static OCR gain was 40% lower with a head-fixed target (-0.084) than with an earth-fixed target (-0.141) (P < .001). CONCLUSION The reduction in static OCR gain during viewing of a head-fixed target indicates that static OCR is partially negated when a target moves with the head.