second-degree fusion

Second-degree fusion is the second of three grades of binocular vision, the ability to merge two similar pictures, one seen by each eye, into a single composite picture.¹ Each picture is incomplete in one small detail that the other picture supplies, so the patient sees the whole picture only when both eyes contribute.¹

The grades build on one another.¹ First-grade simultaneous perception requires only that two dissimilar pictures, such as a bird and a cage, be seen at once.¹ Third-grade stereopsis adds an impression of depth from two pictures of the same object taken from slightly different angles.¹ On the synoptophore, the classic second-grade slides show two rabbits, one lacking a tail and the other lacking a bunch of flowers, and a patient with fusion sees one rabbit with both.¹

Moving the synoptophore arms then forces the eyes to converge and diverge to hold the fused picture, and this measures the range of motor fusion.¹ Simple fusion without any range is of little value in everyday life.¹ In vision therapy, second-degree or flat fusion targets are identical pictures with suppression checks built into their design.² A simple version draws a vertical line on one copy of a picture and a horizontal line on the other, and the patient who fuses the pair sees a cross.³