Fig 1. Binocular imbalance test.

Prior to the test itself, a dichoptic nonius alignment screen is presented (bottom). The test consists of a series of trials, each one divided into three steps (from bottom to top): firstly, a fusion frame of a dichoptic nine square grid surrounded by a high contrast frame is presented to facilitate fixation. After the experimenter presses the spacebar, the computer displays two dichoptic letters, randomly selected and filtered to the target interocular contrast, for 200 msec. Finally, both letters are presented binocularly one next to the other with enhanced contrast and size. The observer’s task is to indicate which letter he/she has perceived (two-alternative forced choice). The computer adjusts the imbalance ratio following a 1 up/ 1 down staircase for the next trial, until a valid threshold is obtained.