Stereothresholds during Voluntary Head Movement and Disconjugate Image Motion

SIGNIFICANCE Stereothresholds increase in the presence of disconjugate image motion, whether this motion results from vergence errors that occur during active head movements or is imposed externally. PURPOSE During rapid voluntary oscillations of the head, vergence eye position has been reported to vary with a peak-to-peak amplitude of about 0.5 degrees-a considerably greater amplitude than when the head is still. Concurrently, stereopsis was reported to be unaffected by voluntary head motion. In the present study, we measured stereothresholds during voluntary side-to-side head movements and during imposed disconjugate image motion with the head stationary, to simulate that produced during active head movement. METHODS Stereothresholds were measured for a pair of 30-arcmin bright vertical lines presented on an oscilloscope and viewed through a custom mirror haploscope. Data were obtained from four normal observers during voluntary side-to-side head movements at temporal frequencies up to 1.5 Hz and also while the head remained still. In addition, stereothresholds were measured with the head stationary when opposite rotations of the galvanometer-driven mirrors in each channel of the haploscope created disconjugate image motion to simulate vergence variability during active head movement. RESULTS During head motion, average stereothresholds increased from about 10 to about 14 arcsec. With imposed disconjugate image motion, stereothresholds rose systematically to about 35 arcsec when the peak-to-peak motion amplitude was 0.5 degrees. Stereothresholds depend primarily on the amplitude of imposed motion and only marginally on variations of the disjunctive-motion wave form. CONCLUSIONS Stereothresholds are elevated modestly during active head movements. The results obtained with imposed disjunctive image motion are consistent with a previously proposal that stereothresholds vary according to the unsigned, time-averaged deviation of the stereotarget from the plane of the horopter.