third-degree fusion

Third-degree fusion is the older name for stereopsis, the third grade of binocular vision in the classic grading on the synoptophore. On that instrument it is the ability to obtain an impression of depth from two pictures of the same object taken from slightly different angles, such as a bucket seen in three dimensions.¹

Stereopsis occurs when horizontally disparate retinal elements are stimulated at the same time, and vertical displacement produces no stereoscopic effect.² Stereopsis is the highest form of binocular cooperation and adds a new quality to vision.² The old name misleads, because it implies a higher form of fusion, whereas stereopsis is a response to disparate stimulation of the retinal elements.²

In vision therapy, patients with nonstrabismic binocular disorders generally do best with targets that carry stereopsis, or third-degree fusion, cues.³ For heterophoria the usual sequence runs from third-degree to second-degree to first-degree fusion tasks, and in most cases first-degree tasks are not even required.³