Motion Misperception in Anisomyopia Before and After Optical Correction

Abstract

Purpose: To investigate interocular delay in anisomyopes at different spatial frequencies.

Methods: Interocular delay (difference in processing speeds between eyes) was measured psychophysically in 21 anisomyopes (observers with a large refractive difference), 20 isomyopes, and 19 emmetropes at 0.5, 1, and 2 cycles per degree (c/deg). During the visual task, small Gabor elements with lateral movements were shown to both eyes. When interocular delay was present, the stimuli created an illusory percept of a cylinder rotating in depth (motion misperception) despite no depth cues. Anisomyopes and isomyopes were tested before and after optical correction; emmetropes were tested only before. Clinical differences between eyes in anisomyopes, including axial length, visual acuity, and spherical equivalent, were also measured.

Results: Anisomyopes showed interocular delay at 2 c/deg, with the more myopic eye faster before optical correction (Cohen's d = 0.48), correlating with clinical differences (P < 0.05). Optical correction abolished this delay at 2 c/deg. At 0.5 and 1 c/deg, anisomyopes showed no delay before optical correction, although there were spatial differences between the eyes. Surprisingly, they showed interocular delay after optical correction (more myopic eye faster) when the images of both eyes were spatially equal (P < 0.05). Isomyopes and emmetropes showed no interocular delay at any spatial frequency before and after optical correction.

Conclusions: Anisomyopes experience motion misperception at 2 c/deg before optical correction and at 0.5 and 1 c/deg after correction, suggesting optical and neural origins of interocular delay. Tailored interventions based on clinical characteristics may help improve visual function such as motion perception.

Figure 1.

Illustration of the visual task and data analysis. (A) Two eyes were dichoptically presented with horizontally moving Gabor elements, forming a rotating cylinder. (B) The interocular phase difference between the stimuli shown to the two eyes and interocular delay determined the perceived rotation of the cylinder. The interocular phase differences were −1.5°, −0.75°, −0.375°, −0.1875°, −0.0938°, −0.0469°, −0.0234°, 0°, 0.0234°, 0.0469°, 0.0938°, 0.1875°, 0.375°, 0.75°, and 1.5°. Subjects were given two response choices to report clockwise or anticlockwise rotation of the stimulus. (C) The interocular phase difference corresponding to 50% of clockwise responses (i.e., ambiguous perception) was defined as the PSE. A positive PSE indicates faster left eye (LE) processing, and a negative PSE indicates faster right eye (RE) processing. This graph shows data from one representative subject.

Figure 2.

Procedure for the experiment. Anisomyopes, isomyopes, and emmetropes were asked to undergo clinical assessment first, including subjective refraction, axial length, visual acuity, stereopsis, and corneal curvature measurements. Participants then completed a practice test block (150 trials) of the visual task to be used in the experiment. After familiarization with the task, anisomyopes and isomyopes participated in sessions both before and after optical correction with soft contact lenses, whereas emmetropes participated only in uncorrected conditions. Testing occurred randomly at three spatial frequencies (0.5, 1, and 2 c/deg), with each spatial frequency tested twice. A, anisomyopes (n = 21); I, isomyopes (n = 20); E, emmetropes (n = 19). In total, each emmetropic observer completed 1800 trials, and each anisomyopic and isomyopic observer completed 3600 trials.

Figure 3.

rPSE of the three groups at different spatial frequencies. We calculated rPSE from PSE based on the SE difference. When the SE difference was ≥0, rPSE = PSE; when the SE difference was <0, rPSE = −PSE. For anisomyopia, a positive rPSE indicates faster processing in the more myopic eye, whereas a negative rPSE indicates slower processing in the more myopic eye. The second axis (right side) translates rPSE values (deg) into interocular delay (ms). Error bars represent standard error. *P < 0.05, **P < 0.01 (one-sample t-tests or Wilcoxon signed-rank tests).

Figure 4.

Scatterplot of correlation between PSE and difference in visual acuity in uncorrected anisomyopia at each spatial frequency. The correlation method is Spearman correlation, with the one-tailed P-value reported. The shadowed area means the blurrier eye is faster. LE, left eye; RE, right eye.

Figure 5.

Schematic diagram illustrating the impact of motion misperception on driving safety in the corrected anisomyope, where the right eye was previously blurrier. (A) The target (0.5 c/deg) moves toward the previously focused eye (leftward). (B) The target (0.5 c/deg) moves toward the previously blurrier eye (rightward).