How biofeedback works

In a biofeedback exercise the camera measures the eye deviation every frame (in prism dioptres, PD), and the target — the moving video the child follows — reacts to it.

The idea

As long as the eyes hold fusion (deviation stays below the threshold), the target advances smoothly along its path. When fusion breaks, the target does not react immediately — it first allows a few seconds, because a brief spike is usually just measurement noise (a blink, for instance, lifts the deviation for a moment). Only when the break persists does it step in, in stages.

What happens when fusion breaks

• Deviation crosses the threshold → the system remembers where it started (point P0) and starts a tolerance window. The target keeps going for now.
• If the eyes realign before the window ends → it was a false alarm, nothing happens.
• If the break persists past the window → the target returns to P0 (where it started) and waits a moment.
• If it still doesn't help → the target retreats a step further and waits again. The longer the break lasts, the further back it goes.
• As soon as fusion returns → the target moves forward again.

What the sliders mean

• Strabismus threshold (PD) — how large a deviation counts as a fusion break. Higher = more lenient, triggers only at a larger deviation.
• Release threshold (PD) — how far the deviation must drop before the target moves forward again. Kept below the strabismus threshold; the gap between them stops the target from flickering back and forth right at the edge.
• Trigger axis — which component of the deviation to watch: horizontal, vertical, or total. A child with a vertical deviation needs the "vertical" option.
• Tolerance window (s) — how many seconds to wait before taking the break seriously. Filters out brief spikes like blinks or twitches. Default 2.5 s.
• Step back (%) — how much of the path the target retreats at each further stage while the break continues. Default 15 %.
• Retreat speed — how fast the target moves back (it glides, it doesn't jump). Default 1.5×.
• Retreat vs. freeze — what to do after a confirmed break: retreat = step progressively backward; freeze = just stop at P0 and wait, without retreating.