Binocular rivalry: what happens when each eye sees something different

In healthy vision the brain merges the images from both eyes. So what happens if each eye is given images that cannot possibly be merged? Horizontal stripes to one eye, vertical stripes to the other, for example.

You might expect a blend of the two, but that is not what happens. Perception shows only the horizontal stripes for a while, then switches on its own to the vertical ones, then switches back. This is binocular rivalry, one of the most studied phenomena in vision science.

The images reaching the eyes never change; the only thing that changes is perception. That points straight at a conclusion: what you consciously see is determined not by the data your eyes deliver but by what the brain does with that data.

This is why vision research is so fond of rivalry: it is one of the rare situations where conscious perception shifts while the input stays fixed.

In both cases the brain chooses between two conflicting inputs. The difference lies in the nature of the choice. In a healthy visual system rivalry alternates: one wins for a while, then the other, and both eyes have a say.

In amblyopia the balance has tipped one way. The weaker eye loses continuously; suppression has stopped alternating and hardened into a habit.

Suppression in amblyopia is therefore not some strange malfunction but a one-directional version of a well-known mechanism.

Rivalry research has a well-established finding: how long each image dominates perception depends on the contrast difference between them. Lower one image's contrast and the other holds perception for longer.

That explains where the most basic adjustment in dichoptic treatment comes from. Reducing the contrast of what the strong eye receives makes room in perception for the weaker eye's signal, so the two eyes' contributions become more balanced.

Closing that contrast gap as progress builds follows the same logic: the goal is both eyes working together on equal terms.

Binocular rivalry looks like laboratory curiosity but connects directly to treatment. The next article takes on its permanent form in amblyopia, suppression, in detail.