The Nature, Testing, And Variables Influencing Fixation Disparity: Role Of The Fusion Lock.

A fixation disparity is a small angular measurement of the misalignment of the two eyes which can occur while still permitting single, fused binocular vision. It represents a small error in the aiming of the eyes which occurs without diplopia being detected. Fixation disparity is allowed because of the slight "slippage" provided to the fusional system through the existence of Panum's fusional areas. So long as binocular alignment is precise enough
to place the two retinal images of a single object within corresponding Panum's areas, the final perception is likely to be single and fused. Sensory fusion thus occurs in spite of a small error in motor fusion.