are seen as lying in 2 visual directions, resulting in physiologic diplopia.

Fusion

Fusion is the cortical unification of visual objects from the 2 eyes into a single percept. For retinal images to be fused, they must be similar in size and shape. Fusion in areas near the fovea (central fusion) tolerates very little dissimilarity between the images in each eye before diplopia is elicited, because of the small receptive fields in this region. More 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

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 each eye are combined to form a single visual percept.

Motor fusion

Motor fusion is a vergence movement that causes similar retinal images to be maintained on corresponding retinal areas despite the tendency of natural (eg, heterophorias) or artificial causes to induce disparities. For example, if 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, fusional convergence movements maintain similar retinal images on corresponding retinal areas, and the eyes are observed to converge. This response is called fusional convergence. Measurement of fusional vergence amplitudes is discussed in Chapter 7.

Stereopsis

Stereopsis occurs when horizontal retinal disparity between the 2 eyes produces a subjective ordering of visual 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 object. 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 also contribute to depth perception. These cues include object overlap, relative object size, highlights and shadows, motion parallax, and perspective. Stereopsis is a binocular sensation of relative depth caused by horizontal disparity of retinal images. Although stereopsis operates at distances over 2000 ft, depth perception relies increasingly on monocular cues beyond 10–20 ft.

Selected Aspects of the Neurophysiology of Vision

The decussation of the optic nerves at the chiasm is essential for the development of binocular vision and stereopsis. With decussation, visual information from corresponding retinal areas in each eye runs through the lateral geniculate body (LGB) and optic tracts to the visual cortex, where the information from both eyes is commingled and modified by the integration of various inputs.

The magnocellular (M), parvocellular (P), and koniocellular (K) systems are parallel systems for

processing visual information in the retinogeniculocortical pathway. The M system is represented predominantly in the peripheral retina and is especially sensitive to moving stimuli but not sensitive to stationary images. The M system is also relatively insensitive to color. The P system is represented predominantly in the fovea and gives a slow tonic response to visual stimulation, carries highresolution information about object borders and color contrast, and is important for shape perception and the ability to see objects in detail. The K system is less well understood but is thought to be concerned with aspects of color vision, especially the color blue.

This monocular separation of the pathways is maintained through the lateral geniculate laminae into the striate cortex (also called the primary visual cortex, or V1), where the geniculate axon terminals from the right and left eyes are segregated into a system of alternating parallel stripes called ocular dominance columns. From there, the paired right and left monocular cells converge on the first binocular cells in layers 2, 3, and 4B of V1. Binocular vision and binocular motor fusion are made possible by horizontal connections that enable information from these monocular columns to be shared. There is also a projection from the visual cortex back to the LGB; this feedback pathway is thought to play a modulatory role.

Visual Development

In the human retina, most of the ganglion cells are generated between 8 and 15 weeks’ gestation, reaching a plateau of 2.2–2.5 million by week 18. After week 30, the ganglion cell population decreases dramatically during a period of rapid cell death that lasts 6–8 weeks. Thereafter, cell death continues at a low rate into the first few postnatal months. The retinal ganglion cell population is reduced to a final count of approximately 1.0–1.5 million. The loss of about 1 million optic axons may serve to refine the topography and specificity of the retinogeniculate projection by eliminating inappropriate connections.

The continued development of visual function after birth is accompanied by major anatomical changes occurring simultaneously at all levels of the central visual pathways. The fovea is still covered by multiple cell layers and is sparsely packed with cones, which may account for the estimated visual acuity of 20/400 at birth. During the first years of life, the photoreceptors redistribute within the retina and foveal cone density increases fivefold to achieve the configuration found in the adult retina, improving visual acuity to 20/20. In newborns, the white matter of the visual pathways is not fully myelinated. Myelin sheaths enlarge rapidly for the first 2 years after birth and then more slowly through the first decade of life. At birth, the neurons of the LGB are only 60% of their average adult size. Their volume gradually increases until age 2 years. Refinement of synaptic connections in the striate cortex continues for many years after birth. The density of synapses declines by 40% over several years, attaining final adult levels at about age 10 years.

Effects of Abnormal Visual Experience on the Retinogeniculocortical Pathway

Abnormal visual experience resulting from visual deprivation, anisometropia, or strabismus can powerfully affect retinogeniculocortical development. Single-eyelid suturing in baby macaque monkeys usually produces axial myopia but no other significant anatomical changes in the eye. The lateral geniculate laminae that receive input from the deprived eye experience minor shrinkage, but these cells respond rapidly to visual stimulation, implying that a defect in the LGB is not likely to account for amblyopia. In the striate cortex, monocular visual deprivation causes the regions of the visual cortex driven predominantly by the closed eye (ocular dominance columns) to radically narrow (Fig 6-2). This happens because the 2 eyes compete for synaptic contacts in the cortex. As a result, the deprived eye loses many of the connections already formed at birth with postsynaptic

cortical targets. The open eye profits by the sprouting of terminal arbors beyond their usual boundaries to occupy territory relinquished by the deprived eye (Fig 6-3). However, the benefit derived from invading the cortical territory of the deprived eye is unclear because visual acuity does not improve beyond normal. Positron emission tomography (PET) has shown that cortical blood flow and glucose metabolism are lower during stimulation of the amblyopic eye compared with the normal eye, suggesting the visual cortex as the primary site of amblyopia. Monocular deprivation also devastates binocularity because few cells can be driven by both eyes.

Figure 6-2 Change of ocular dominance columns in macaque visual cortex after monocular deprivation. Radioactive proline was injected into the normal eye and transported to the visual cortex to reveal the projections of that eye. In these sections, cut parallel to the cortical surface, white areas show labeled terminals in layer 4. A, Normal monkey. The stripes representing the injected eye (bright) and noninjected eye (dark) have roughly equal spacing. B, Monkey that had 1 eye sutured closed from birth for 18 months. The bright stripes (open, injected eye) are widened and the dark ones (closed eye) are greatly narrowed, showing the devastating physical effect of deprivation amblyopia. (Scale bar = 1 mm.) (Reproduced with permission

from Kaufman PL, Alm A. Adler’s Physiology of the Eye. 10th ed. St Louis: Mosby; 2002:699. Originally from Hubel DH, Wiesel TN, LeVay S. Plasticity of ocular dominance columns in monkey striate cortex. Philos Trans R Soc Lond B Biol Sci.1977;278(961):377–409.)

Figure 6-3 Anatomical and physiologic maturation of ocular dominance columns of the primary visual cortex in normal and deprived monkeys. Birth: Broad overlap of afferents from the lateral geniculate nucleus, hence little dominance by right eye (RE) versus left eye (LE). Normal 6-month-old: Regression of overlapping afferents from both eyes with distinct areas of monocular dominance. The bar graph shows the classic U-shaped distribution obtained by single-cell recordings from the visual cortex. About half the cells are driven predominantly by the right eye and the other half by the left eye. A small number are driven equally by the 2 eyes. 1 = driven only by right eye; 7 = driven only by left eye; 2–6 = driven binocularly. Strabismus: Effect of artificial eye misalignment in the neonatal period on ocular dominance. The monkey alternated fixation (no amblyopia) and lacked fusion. Lack of binocularity is evident as exaggerated segregation into dominance columns. The bar graph shows the results of single-cell recordings obtained from this animal after age 1 year. Almost all neurons are driven exclusively by the right or left eye, with little binocular activity. Amblyopia: Effect of suturing the left eyelid shut shortly after birth. Dominance columns of the normal right eye are much wider than those of the deprivationally amblyopic left eye. The bar graph shows markedly skewed ocular dominance and little binocular activity. (Modified with permission from Tychsen L. Binocular

vision. In: Hart WM, ed. Adler’s Physiology of the Eye: Clinical Application. 9th ed. St Louis: Mosby; 1992:810.)

There is a critical period in which visual development of the macaque eye is vulnerable to the effects of eyelid suturing. This critical period corresponds to that in which the wiring of the striate cortex is still vulnerable to the effects of visual deprivation. During the critical period, the deleterious effects of suturing the right eyelid, for example, are correctable by reversal—that is, opening the