Digital Fusion Range Test Quantifies Horizontal Fusional Reserves

An automated digital fusion range test can obtain automated horizontal fusional reserve measurements without the need for subjective patient responses or clinician observations, according to findings published in Clinical and Experimental Optometry.

Researchers included 29 individuals (mean age, 22.9 years; 22 women) with a best-corrected visual acuity of 6/9.5 in the study and evaluated horizontal fusional reserves with a digital fusion range test using consumer-grade infrared eye tracker software, a dichoptic display, and custom analysis software programed with a numeric computing platform. 

Participants viewed a target at a distance of 55 cm and indicated whether it appeared as a single or double image. Horizontal fusional reserves were measured at 3 speed conditions (0.5, 1.0, 2.0 Δ/s) in both divergence and convergence conditions. A 2-minute rest period occurred between conditions to minimize fatigue. Outcomes were compared between objective measurements obtained by the eye tracker and subjective responses given by the participants.

Agreement between objective and subjective measures of break and recovery demonstrated the automated test’s ability to reliably measure horizontal fusional reserves, according to the report. On average, patients responded 0.91 seconds after the objective break and recovery times. Subjective or objective response or stimulus speed did not significantly affect convergence breaks (P =.26). Objective divergence breaks were lower than subjective breaks at 1.0 and 2.0 Δ/s (F_[1,21], 7.53; P <.012) and objective results were higher than subjective results at all speeds for convergence (F_[1,21], 7.66; P =.01) and divergence (F_[1,21], 71.6; P <.001) conditions.

Intraclass correlations between objective and subjective tests ranged between 0.964 and 0.994 with no substantial variation in reliability across test types or speeds.

“This paper proves that in-principle it is possible to obtain reliable, objective, and clinically useful measures of horizontal fusional reserves using consumer-grade eyetracking and dichoptic displays,” according to the study authors. “Eyetracking is a promising avenue for use in improving objective measures of binocular status, providing tools and treatment options for vergence disorders in clinical and community settings.”

Study limitations include the exclusion of individuals with moderate to high myopia who were unwilling to wear contact lenses and the use of convenience sampling.