Fusion is the cortical unification of 2 images of an object, 1 from each eye, into a single percept. For retinal images to be fused, they must be similar in size and shape. For fusion of macular images (central fusion) to occur, there can be very little dissimilarity between the images in each eye, because of the small receptive fields in the area near the fovea; otherwise, diplopia results. More image dissimilarity is tolerated in the periphery (peripheral fusion), where the receptive fields are larger. Fusion has been artificially subdivided into sensory fusion, motor fusion, and stereopsis.
Sensory fusion is based on the innate, orderly topographic relationship between the retinas and the visual cortex, whereby images falling on corresponding (or nearly corresponding) retinal points in the 2 eyes are combined to form a single visual percept.
Motor fusion is a vergence movement that allows similar retinal images to be maintained on corresponding retinal areas despite natural conditions (eg, heterophorias) or artificial causes that induce disparities. For example, when a progressive base-out prism is introduced before both eyes while a target is viewed, the retinal images move temporally over both retinas if the eyes remain in fixed position. However, because of a response called fusional convergence (see Chapter 4), the eyes instead converge, repositioning so that similar retinal images are projected on corresponding retinal areas. Measurement of fusional vergence amplitudes is discussed in Chapter 7.
Stereopsis occurs when horizontal retinal disparity between the 2 eyes produces a subjective ordering of images of objects in depth, or 3 dimensions. It is the highest form of binocular cooperation and adds a unique quality to vision. The region of points with binocular disparities that result in stereopsis is slightly wider than Panum’s area, so stereopsis is not simply a by-product of combining the disparate images from a point into a single visual percept. The brain interprets nasal disparity between 2 similar retinal images of an object in the midline as indicating that the object is farther away from the fixation point, and temporal disparity as indicating that the object is nearer. Binasal or bitemporal images are not a requirement for stereopsis; objects not in the midline in front of or behind the horopter also elicit stereopsis, even though their images fall on the nasal retina in 1 eye and the temporal retina in the other.
Stereopsis and depth perception (bathopsis) are not synonymous. Monocular cues—which include object overlap, relative object size, highlights and shadows, motion parallax, and perspective—also contribute to depth perception. Monocular patients can have excellent depth perception using these cues. Stereopsis is a binocular sensation of relative depth caused by horizontal disparity of retinal images.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.