The eye is a complex organ responsible for collecting, focusing, and processing light. Modern genetic analysis indicates that there are many different classes of ganglion cells in the retina that act to initially select and segregate visual information. However, it is the central visual pathways that continue to segregate, combine, and ultimately interpret this information. Distinct pathways are responsible for a range of functions including conscious visual perception, sensory-motor integration, eye movements, and circadian rhythms. The central visual pathways refer broadly to all the regions of the brain conveying or receiving either direct or indirect retinal input. The system begins via the projection targets of the retina, comprised of several midbrain structures with diverse functions. However, certain targets such as the thalamic lateral geniculate nucleus (LGN) participate in a far more expansive and complex elaboration than others. This is the critical system for conscious perception and cognition that is so highly developed in primates and humans. In this overview, we will first introduce the central visual pathways with a brief review of the targets of the retinal projections ( Figs. 27.1 and 27.2 ). Second, by reviewing the types of conscious vision loss that occur after damage along the major visual pathways we provide a concise summary of their basic organization ( Fig. 27.3 ). We conclude the chapter with a brief mention of the relatively new neuroimaging techniques that have greatly expanded our ability to study the human visual system directly ( Fig. 27.4 ). Whereas Chapter 26 outlined predominantly subconscious, nonimage-forming visual pathways in the brain that receive input from melanopsin-expressing intrinsically photosensitive retinal ganglion cells, the subsequent chapters consider the conscious visual system in more detail. Also, in Chapter 28 , the optic nerve that carries retinal ganglion cell axons toward their targets is fully discussed in terms of development, structure, and potential injury. The LGN, described in detail in Chapter 29 , is an important relay in the visual pathway involved with form vision. The precision and specificity of the retinogeniculate connections are crucial to subsequent stages. From the LGN, the visual pathway proceeds to the primary visual cortex, V1. It is at the level of V1 that visual signals undergo more complex processing, which is detailed in Chapter 30 . Visual processing of a combination of stimulus features such as shape, color, contrast, motion, texture, and depth is further enhanced by over 30 extrastriate cortical areas, some of which are described in Chapter 31 .
Targets of the retinal projections
We begin with a brief review of the intricate but well described functional anatomy of the retinal projections ( Fig. 27.1 ). Although there are systematic variations in the size of optic nerves, the LGN, and primary visual cortex in humans, the basic pattern is the same. Axons from the output cells of the retina (ganglion cells) are specifically directed to the area of the blind spot and exit to form the optic nerve. Ganglion cells representing both the nasal and temporal sides of the retina all exit at this juncture but axons retain their spatial positions with respect to axons from neighboring regions. This precise preservation of the neighborhood relations (or “map”) of the retina is the basis for the retinotopic organization that defines these projections. This organization is of profound importance for understanding the visual system and is elaborated upon repeatedly in the chapters that follow. When optic nerve axons reach the optic chiasm (located above the pituitary gland), only those representing the temporal visual field (those located in the nasal side of the retina) cross contralaterally. These crossing fibers from nasal retina are known to slightly outnumber those that do not cross (53%–47%). Incidentally, some fibers from the nasal side representing the far edge of peripheral vision (in the temporal visual field) are known to deviate slightly anterior and form a structure referred to as Wilbrand’s knee (see Fig. 27.3 for the clinical relevance). The remaining axons from the temporal retina remain segregated to comprise the ipsilateral optic tract (along with fibers from the nasal retinal from the other side). Hence the optic tract and all subsequent connections receive information from the contralateral visual field. In other words, both sides of the brain receive input from both the left and right eye (although this information may not be truly integrated by single neurons until later stages, such as primary visual cortex).
The major targets of the retinal ganglion cell axons are (1) the LGN, (2) the superior colliculus (SC), (3) the pretectum, and (4) the pulvinar. There is a weaker projection to several small hypothalamic nuclei including (5) the suprachiasmatic, supraoptic, paraventricular nuclei, and to (6) the accessory optic system (AOS), including the nucleus of the optic tract (NOT) and the dorsal, medial, and terminal nuclei ( Fig. 27.1 ; Box 27.1 ; and see Fig. 9.8).
- •
Lateral geniculate nucleus
- •
Superior colliculus
- •
Pretectum
- •
Pulvinar
- •
Hypothalamic nuclei
- •
Accessory optic system
The most significant projection, in terms of the number of optic nerve fibers, is to the lateral LGN in the thalamus. The LGN is the major termination site of the retinal ganglion cells and plays an important role in the visual pathway leading to the primary visual cortex. Approximately 90% of all retinal ganglion cells project to the LGN, which is laminated. An important function of the LGN is to organize its retinal inputs by their receptive field properties. Although only a small fraction of the inputs to the LGN are from the retina, they create a strong excitatory drive, and are precisely retinotopic. Other LGN synapses reflect either local interneurons, inputs from other midbrain (e.g., SC) or brain stem sources, or feedback from visual cortex, all generally thought to provide modulatory input. Each LGN layer receives input from a specific eye and class of ganglion cell. Whereas electrophysiological studies suggest that the neuronal signals coming into and leaving the LGN are quite similar, the LGN appears to be involved in regulating information flow between the retina and primary visual cortex, the major projection target of the LGN (see Chapter 29 ). The left and right eye segregation of axons in the six major LGN layers synapse in layer 4 of V1, and form the basis for monocular dominance columns, which form the substrate for binocularity, as discussed in detail in Chapter 30 . Also noteworthy is that the magnification of the central visual field that originates in the retina, owing to a greater density of ganglion cells, is maintained in the LGN, and is exaggerated even further at the level of V1 (see Fig. 27.3 ).
The SC is a midbrain structure that, in conjunction with the cortical frontal eye fields and the brainstem reticular formation, is involved in the generation of visually guided saccadic eye movements (see Chapter 9 ). The SC is a laminated, retinotopically organized nucleus, and, as seen in the LGN, the retinal projection retains eye segregation with alternating columns of left and right eye terminals forming a banded pattern throughout the superficial layers. Approximately 10% of all retinal ganglion cells project to the SC. The majority of retinal axons that terminate in the SC are small caliber, originate from ganglion cells with small dendritic fields, and do not project to other retinal targets. The SC is an evolutionarily conserved sensorimotor structure that integrates visual and other sensory information to drive reflexive behaviors. Nevertheless, more modern studies also emphasize a role in complex behaviors such as attention and decision-making, even contributing to object selectivity in certain extrastriate cortical regions to which it projects. An interesting feature of the SC is a lack of input from S-cones, and clever experiments have thus simulated “SC lesions” by studying the effects of stimuli carried only by S-cone input.
The pretectal complex, a group of small midbrain nuclei, is just rostral to the SC. It receives signals from a group of small-diameter retinal ganglion cells with large receptive fields and is involved with the control of the pupillary light reflex (see Chapter 25 ) by means of a projection to the Edinger-Westphal nucleus of the oculomotor complex. The pupillary light reflex demonstrates a consensual response primarily resulting from crossed and uncrossed optic nerve fibers that enter each pretectal complex, which in turn sends a bilateral projection to the Edinger-Westphal nucleus (see Chapter 9 ).
Retinal ganglion cells also project to three of four major subdivisions of the pulvinar nucleus of the thalamus. The pulvinar is the largest nuclear mass in the primate thalamus and receives a projection from the small-caliber fibers from the optic nerve and the SC. It projects to several visual cortical areas, including V1, extrastriate, and parietal areas. Thus, the pulvinar represents a pathway that can bypass the LGN to get to V1 and may play a role in processing form vision. More recent studies point to a role for the pulvinar in the coding of the “importance” of visual stimuli (i.e., visual salience or “attention”). In some case studies, damage to the pulvinar has been reported to cause a visual neglect of the contralateral visual field (e.g., ). It has been shown that the pulvinar integrates neural signals associated with eye and hand and arm movements and may receive signals associated with saccadic eye movements, which suggests its role is also one of formulating reference frames for hand-eye coordination.
The AOS consists of several small nuclei, the lateral terminal nucleus (LTN), the medial terminal nucleus (MTN), and the dorsal terminal nucleus (DTN), as well as the NOT in the midbrain. The AOS plays an important role in optokinetic nystagmus (OKN) in which slow compensatory and pursuit-type eye movements alternate with fast saccadic-type eye movements in response to viewing prolonged large field motion (see Chapter 9 ). In primates, lesions of the NOT and the DTN have been shown to modify the OKN and reduce or abolish optokinetic after nystagmus (OKAN).
Finally, we consider that several small hypothalamic nuclei receive a direct retinal projection ( Fig. 27.2 ). The suprachiasmatic nucleus receives a sparse projection from fibers that leave the dorsal surface of the optic chiasm and has been implicated in the synchronization of circadian rhythms. The paraventricular and supraoptic nuclei are likely also involved with the regulation of the light-dark cycle for neuroendocrine functions. It is worth noting that the superchiasmatic nucleus is one of the recipients of intrinsically light sensitive ganglion cells that contain a unique opsin (melanopsin) unlike the four opsins found in rods or cones. These ganglion cells are few in number but have very large dendritic trees. This is the basis for the photoentrainment of the circadian rhythm (see Chapter 26 ).
Retinotopic pathways and visual field lesions
Much of our early knowledge of the retinotopic organization of the human central visual system derived from the visual field defects associated with lesions along the major pathway leading to form vision, the retinogeniculocortical pathway. Fig. 27.3 illustrates several known anatomical lesions, from the retina to the occipital lobes, and their subsequent effect on the visual fields. In the first example shown, complete interruption of one optic nerve, which may occur with severe degenerative disease or injury, results in permanent blindness in the affected eye. Partial interruption of the nerve fibers results in a partial loss of the visual field and can occur with glaucoma, optic disc drusen, pits, infarcts, or optic neuritis. Regions of local blindness are often referred to as scotomata (derived from Greek meaning darkness).
It is of clinical relevance to note that interruption of the optic nerve closer to the junction of the optic chiasm (#2 in Fig. 27.3 ) can also include loss of vision in the far temporal periphery, owing to Wilbrand’s knee, an abnormality resulting from chronic optic neuropathy on that side. In humans, a sheet of inferonasal fibers of the optic nerve deviate slightly toward the contralateral optic nerve before crossing over to the opposite optic tract.
Slightly further along the visual pathway, interruption of the decussating optic nerve fibers in the optic chiasm (#3 in Fig. 27.3 ) results in loss of vision in the temporal visual hemifields of both eyes called bitemporal hemianopia. Damage of this type commonly occurs with pituitary tumors as they grow and compress the overlying optic chiasm. Rarely, if pressure is exerted on the lateral edge of the optic chiasm (#4 in Fig. 27.3 ) damage arises primarily in the uncrossed fibers, resulting in loss of vision in the nasal hemifield, or nasal hemianopia, ipsilateral to the compression.
Lesions that occur after the chiasm are characterized by visual field defects that involve the temporal hemifield of the contralateral eye and the nasal hemifield of the ipsilateral eye; this is because of the partial decussation of optic nerve fibers at the optic chiasm. In this type of lesion, visual field loss occurs on the side contralateral to the lesion. Complete interruption at the level of the optic tract (#5 in Fig. 27.3 ) or beyond results in this type of vision loss. Homonymous hemianopia is the clinical term used to describe such loss of the contralateral visual field. It is usually difficult to determine whether the site of the lesion is at the level of the optic tract, LGN, or visual cortex for most homonymous hemianopias. In cases in which the lesion is in the optic tract, the homonymous hemianopia may be accompanied by an afferent pupillary defect in the contralateral eye ( Box 27.2 ). The likely reason for this is because ganglion cells in the nasal retina outnumber those in the temporal retina (53%–47%), thus causing a greater loss of fibers for the pupillary response of the contralateral eye (however, also see 3 ). Lesions at the level of the brachium of the SC (#7 in Fig. 27.1 ) may result in an afferent pupillary defect in the contralateral eye, but with intact, normal visual fields because lesions at this level spare the retinogeniculate fibers. Lesions involving the LGN (#6 in Fig. 27.1 ) are often difficult to distinguish from other optic tract lesions, but usually present with a visual field defect and an intact contralateral afferent pupillary reflex because lesions at this level normally spare the retinal fibers terminating in the pretectum responsible for the afferent pupillary reflex.
