Optimal Stereoacuity Reveals More Than Critical Time in Patients With Intermittent Exotropia

Abstract

Synopsis: Both optimal stereoacuity and integration time to achieve that are impaired in patients with intermittent exotropia. The deterioration of stereoacuity is more revealing since it correlates well with exotropia control score.

Background: Despite the periodic misalignment of two eyes, some intermittent exotropia (IXT) patients exhibit normal stereoacuity, particularly when evaluated with static tests. It is not clear if the temporal integration process of stereopsis is altered in IXT patients, thus warranting further research.

Methods: IXT patients (n = 29) and age-matched normal controls (n = 36) were recruited. Static stereopsis was measured with the Titmus stereoacuity test. In computer-generated random dots tests, stereoacuity was measured with a stimuli presentation duration varying from 100 to 1,200 ms. And the relationship between stereoacuity and stimuli duration was fitted into a quadratic model. Optimal stereoacuity was achieved when fitted curve flattened and the critical integration time was the duration needed to achieve optimal stereoacuity.

Results: IXT patients were not found to differ significantly from control subjects under the Titmus test, while the Random Dots stereotest showed significantly worse optimal stereoacuity and significantly longer critical integration time. Multiple regression analysis showed that age (R = -4.83; P = 0.04) had statistically significant negative correlation on the critical integration time, age (R = -6.45; P = 0.047) and exotropia control scores (R = 60.71; P = 0.007) had statistically significant effects on optimal stereoacuity.

Conclusion: The temporal integration for stereopsis is impaired in IXT patients, requiring longer critical integration time to achieve elevated optimal stereoacuity.

Keywords: critical time; intermittent exotropia; optimal stereoacuity; stereopsis; temporal integration.

FIGURE 1

The Random Dot test. (A) Stereoacuity was measured with different stimulus presentation durations, including 50, 100, 200, 400, 600, 800, 1,000, and 1,200 ms. (B) For each stimulus presentation duration, the stereo threshold was measured with a staircase procedure. Open and closed red circles represent correct and incorrect responses, respectively. The dotted line represents the final stereo threshold averaged from the last four reversals.

FIGURE 2

Examples illustrating how stereoacuities change with stimulus presentation durations. (A) A control subject. (B) An IXT patient. The black arrow indicates the location of optimal stereoacuity (Dmin) and the critical integration time (Tmin) required to achieve it.

FIGURE 3

Population data on Dmin and Tmin. (A) Scatter plot showing the relationship between Tmin and Dmin in the control subjects (blue dots) and the IXT patients (red dots). (B) Histogram showing the distribution of Dmin in the control subjects (blue bars) and the IXT patients (red bars). (C) Histogram showing the distribution of Tmin in the control subjects (blue bars) and the IXT patients (red bars). Circles represent the mean values.

FIGURE 4

Comparisons between Dmin, Tmin, exotropia control score and ocular deviation. (A) Tmin vs. exotropia control score, (B) Dmin vs. exotropia control score, (C) Tmin vs. ocular alignment, (D) Dmin vs. ocular alignment. The blue marker represents the control subjects, and the red marker represents the IXT patients.