Abstract
Stereopsis is traditionally measured by using stereo tests that are presented upright while the observer views the test in a habitual head position. Here, we investigated the effect of head-and stimulus tilt on stereoacuity.
First, we measured stereoacuity using a 4AFC bandpass-filtered circles(size=1.25; sf=4cyl/) on a passive-3-D-monitor with enforced eye-and head positions. Head tilts were 0 or 20 pitch, roll, or yaw. Ten binocularly normal(Titmus 60’’ or better) adults fixated a central cross and clicked on the circle with a backwards depth for forty trials/condition. Stereoacuity thresholds(62.5% cumulative-Gaussian-fit) worsened with tilt, but did not significantly differ from the 0 condition[p>0.05].
Next, we measured stereoacuity while either the head(H), monitor(M), or both(B) were tilted 0, 22.5, or 45 roll in random order (eighty trials/condition). The stimulus size was 8, made of four quadrants, each containing 0.04 Gaussian dots(30dots/-2). Interocular disparity was Gaussian(=0.67), convex in three and concave in one quadrant. The task for eight participants was to click on the concave target. There was a significant interaction between conditions and tilts[p<0.05], which was due to decreasing stereoacuities with increasing tilt for conditions H and M[p<0.05], but not B[p>0.05]. Stereoacuities for M tended to be worse than for H, but this trend was not significant[p = 0.084]. The decrease in stereoacuity with tilt was greater for left- compared to right-roll(M=1.3 vs. 1.1”/[p=0.001]; H=1.1 vs. 0.8”/[p=0.01]; B=0.09 vs. 0.05”/[p=0.23], respectively).
Stereoacuities were unaffected by head tilt using sparse-circles, but significantly changed using dot stimuli, indicating a role for stimulus density in depth perception assessment. Cyclotorsions have a negligible effect on stereoacuity since there was no effect of tilt in B nor a significant difference between H and M. Rolling the head leftwards generates poorer stereoacuities than rightwards. The results may have implications for clinical screening in patients with abnormal head posture.