The Dichoptic Contrast Ordering Test: A method for measuring the depth of binocular imbalance

In binocular vision, the relative strength of the input from the two eyes can have significant functional impact. These inputs are typically balanced, however in some conditions (e.g. amblyopia) one eye will dominate over the other. To quantify imbalances in binocular vision, we have developed the Dichoptic Contrast Ordering Test (DiCOT). Implemented on a tablet device, the program uses rankings of perceived contrast (of dichoptically-presented stimuli) to find a scaling factor that balances the two eyes. We measured how physical interventions (applied to one eye) affect the DiCOT measurement. These were: i) neutral density filters, ii) Bangerter filters, and iii) optical blur introduced by a +3 D lens. The DiCOT results were compared to those from the Dichoptic Letter Test (DLT). Both the DiCOT and the DLT showed excellent test-retest reliability, however the magnitude of the imbalances introduced by the interventions was greater in the DLT. To find consistency between the methods, rescaling the DiCOT results from individual conditions gave good results. However, the adjustments needed for the +3 D lens condition were quite different from those for the ND and Bangerter filter. Our results indicate that the DiCOT and DLT measure partially separate aspects of binocular imbalance. This supports the simultaneous use of both measures in future studies.