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first-degree fusion

First-degree fusion is the lowest of the three grades of binocular vision, and it is also called simultaneous perception. Each eye is shown a different picture, and the patient is asked to place one on the other, such as a bird into a cage on the synoptophore.1

The test shows only that both eyes are seeing at the same moment, because the two pictures never need to merge and the task says nothing about depth. If the bird disappears whenever the cage appears, one eye is being suppressed.1

The grades build on one another: second-degree fusion is flat fusion of two nearly identical pictures, and third-degree fusion is stereopsis.1

More detail

Vision therapy uses first-degree targets in the early stages of treating suppression, because two unlike pictures are hard to suppress and the patient can learn to hold both in view at once.2 Some texts name the three levels bifoveal fixation, fusion and stereopsis instead.

Sources (6)
  1. Kanski, Clinical Ophthalmology (2024), ch. 18, Grades of binocular vision
  2. Scheiman and Wick, Clinical Management of Binocular Vision, ch. 6
  3. Rutstein, R., Daum, K. (1997). Anomalies of Binocular Vision: Diagnosis & Management, Mosby.
  4. Steinbach, M. J., & Nawrot, M. (2021). Binocular vision. In Neuroscience (pp. 1-18). Academic Press.
  5. Levi, D. M., Knill, D. C., & Bavelier, D. (2015). Stereopsis and amblyopia: A mini-review. Vision research, 114, 17-30.
  6. Westheimer, G. (2009). The third dimension: From binocular vision to stereopsis. In The Senses: A Comprehensive Reference (pp. 775-786). Academic Press.

Reviewed October 2026

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