Human Organ Systems Course

Special Senses — Sensory Visual System

Michael Wall, M.D. and Randy Kardon, M.D., Ph.D.

January 21, 1998

Reading Assignment: Guyton pp. 623-661 

Key Principles

• The main function of the cornea and lens is to focus images on the retina.
• The iris constricts to protect our retinas from bright light and to aid in focus. It dilates to enhance vision in dim illumination.
• The very central visual field is optimized for perception of fine detail, color and depth.
• The peripheral visual field is primarily concerned with motion perception.
• Two parallel pathways comprise the functional architecture of the sensory visual system; one pathway is for "object vision or what is it?" — color, texture, form, stereopsis and contrast, the other is for perception of transients — motion and flickering lights to tell us "where is it?"
• Damage to nerve fibers at different locations in the sensory visual system results in predictable and anatomically localizing visual deficits.

Key Terms

visual field, photoreceptors, retinal ganglion cells, optic nerve, optic chiasm, optic tract, occipital lobe, parvocellular pathway, magnocellular pathway, color vision , motion perception, optics

1.  
2. The Eye
   1. Cornea (Figure 1)
   2. Iris
      1. the iris constricts to:
         1. protect our retinas from bright light
         2. to aid in focus
      2. the iris dilates to enhance vision in dim illumination
      3. anatomy
      4. innervation
         1. parasympathetic
         2. sympathetic
      5. the pupillary light reflex – Figure 2.
   3. Anterior chamber, canal of Schlemm and intraocular pressure
   4. Lens - aging produces cataract; pseudophakia = intraocular artificial lens placement after cataract surgery
   5. Vitreous- posterior vitreous detachment causes "floaters"; retinal traction can cause tears and subsequent retinal detachment
   6. principles of optics 
      1. refractive index of a transparent substance is the ratio of the velocity of light in air to the velocity in the substance

          

      2. refraction is the bending of light rays at an angulated surface; the degree of refraction increases based on:
         1. refractive indices of the two transparent media
         2. degree of angulation between the interface and the light waves (changing this is how refractive surgery corrects vision)

          

      3. focal point the point through which all light rays pass after passing through a lens (concave lens diverges rays, convex lens converges rays)
   7. Myopia, Hyperopia, Astigmatism and Presbyopia
      1. short eye = hyperopia (farsightedness)
      2. long eye = myopia (nearsightedness)
      3. unequal refractive power in 2 planes = astigmatism
      4. reduced ability of lens to change shape with age, causing objects at near distance to be under-focused "behind" the retina = presbyopia; reading glasses which further converge the light rays may be needed at about age 40. 
3. Central Visual Pathways (Figures 3,4) this outline is organized anatomically; for information on visual acuity and perimetry, see the end of the outline
   1. Prechiasmal visual system
      1. Retina (Figures 5,6) - e.g. branch retinal artery occlusion, retinitis pigmentosa
         1. retinal circulation
            1. central retinal circulation
            2. posterior ciliary circulation
         2. retinal pigment epithelium
         3. photoreceptors
            1. cones - color vision (Figure 7, 3 cones x 106 per retina)
               1. blue - short wavelength
               2. green - middle wavelength
               3. red - long wavelength (addition of these cones allowed discrimination of ripe fruit from foliage).
            2. rods
               1. 100 x 106 per retina
               2. 30 - 300 times more sensitive than cones
               3. up to 200 rods converge on a ganglion cell
         4. color vision disorders
            1. 8% of men have red green color deficiency
            2. this is usually a deficiency of green cones (5%) and is only for foveal vision
            3. blue/yellow color deficiency is rare
         5. retinal cell types
            1. ganglion cell -- 1.6 x 106 per optic nerve
            2. ganglion cell types (named after layer of LGN where they synapse
               1. M cell (magnocellular layer of LGN)
               2. P cell (parvocellular layer of LGN)
               3. W cell (function is speculative in human)
            3. ganglion cells respond best to light, contrast boarders and colored stimuli
            4. properties of ganglion cells

               Property	M Cell	P Cell
               Retinal distribution	Periphery	fovea
               Response to contrast	Transient	sustained
               Axonal velocity	Fast	slow
               Axon size	Large	small
               LGN cell layer	Magno	parvo
               Receptive field size	Large	small
               Motion discharge rate	High	low
               Spatial resolution	Low	high
               Temporal resolution	High	low
               Contrast gain	Low	high
               % of total M/P cells	10	90

            5. amacrine cell
            6. Muller’s cell
            7. bipolar cell
         6. ganglion cells from the macula project axons directly to the optic nerve (to the temporal margin of the optic disc); Extramacular retinal ganglion cells and fibers are divided by the temporal horizontal temporal raphe (Figure 12). Cells below this raphe send axons that course in the temporal arcades and curve around the papillomacular bundle to enter the inferotemporal part of the optic disc (vice versa for the superior fibers). There is no nasal raphe.
         7. since fibers from the nasal retina (subserving the temporal visual field) enter the optic disc on its superior and inferior poles, lesions of these nerve fiber bundles cause wedge or sector shaped areas of visual loss with the apex of the defect pointing toward the blind spot.
         8. as the nerve fibers course toward the optic chiasm the fibers subserving the central field congregate in the central portion of the optic nerve. These fibers are especially susceptible to injury, compression or metabolic dysfunction, hence visual loss termed the central scotoma is the hallmark of optic nerve disease. (A scotoma is an area of visual loss surrounded by an area of preserved vision, The blind spot is the scotoma in the temporal visual field due to absence of visual receptors at the site of the optic nerve head.)
         9. retinal-related visual field defects
            1. can respect the nasal horizontal meridian
            2. are monocular defects - corresponding to the retinal lesion
            3. the apex of defect often points toward blind spot
      2. Optic Nerve - e.g. glaucoma with magnocellular or large fiber loss, optic neuritis, giant cell arteritis
         1. types of axons (M and P, Figure 8 for M and P pathways)
         2. visual field defects
            1. cecocentral scotoma - core of nerve
            2. arcuate nerve fiber bundle defects - outer portion of nerve - like those seen with retinal disease
   2. Optic Chiasm - e.g. pituitary tumor
      1. 53% of fibers cross
      2. hallmark - bitemporal hemianopia (a hemianopia is loss of one half of a visual field, remember with hemianopias)
   3. Retrochiasmatic Optic Pathways – hallmark of damage is homonymous hemianopia, that is defects respect the vertical midline which splits function of the fovea not the optic nerve
      1. Optic tract - incongruous homonymous hemianopia
      2. Lateral geniculate nucleus (Figure 9)
         1. magnocellular layers (I,II)
         2. parvocellular layers (III - VI)
      3. Optic radiations
      4. Occipital lobe
         1. geniculocalcarine fibers terminate in layer 4
         2. cortical magnification factor - projections from the macula (central 10° of the visual field use 40% of the fibers and are subserved by about half of primary visual cortex.
         3. retinotopic organization - fibers synapse in the occipital lobe in a topographic (point to point) fashion. Fibers from the fovea synapse at the occipital tip and fibers from the temporal periphery in the anterior occipital lobe.
         4. occipital visual field defects are characterized by being exquisitely congruous (congruous implies the visual field maps of the two eyes are nearly identical)
         5. magnocellular (dorsolateral) pathway for "Where is it?"- or is it moving?
         6. parvocellular (ventromedial) pathway for "What is it?"
4. Visual acuity
   1.  
   2. Visual acuity is the ability to discriminate the fine details of an object or scene.
   3. It is always tested with best optical correction.
   4. What does 20/20 mean?
      1. average normal visual acuity (for all ages) is the ability to detect a 5 minute by 5 minute figure (60 minutes per degree) with a stroke width of 1 minute of arc at a distance of 20 feet.
      2. 20/200 then means that the patient can resolve at 20 feet what a normal observer can resolve at 200 feet.
      3. average acuity for age 20 - 40 is 20/15! It increases to about 20/30 at age 70.
5. Quantitation of the visual field: Perimetry (Figures 10-13)
   1.  
   2. Visual Field- the portion of space in which objects are visible at the same moment during steady fixation of gaze in one direction.

       

   3. Perimetry is the systematic quantitation of the visual field.
   4. The visual island - our visual function has been described by the metaphor of "an island of vision surrounded by a sea of blindness." We can define this "visual island" of sensitivity by a three coordinate system (Figure 11). The "x" and "y" coordinates indicate the location of the test stimulus at a given height or elevation of the island, i.e. the position on the visual island irrespective of height. The third axis, the "z" axis, corresponds to the visual sensitivity and to the height of the visual island at any location of the test stimulus in the visual field.
   5. With kinetic perimetry (the stimulus is moving), stimulus brightness is held constant and the x and y (position) coordinates of the visual threshold to the moving target are plotted or mapped. The visual island is thus measured at various levels of elevation. A large, bright target is used to determine the stimulus threshold at the base of the island (coastline or outer absolute limits of the visual field). Relatively smaller and dimmer targets are then used to map the middle and peak of the island. Kinetic perimetry, therefore, surveys the island in the horizontal plane at various heights and the group of x and y coordinates tested define the isopter, (a line drawn through points of equal visual sensitivity to a stimulus of a specific size, brightness and color).
   6. The visual island is not only surveyed in horizontal axial sections by kinetic perimetry but also saggitally (in the vertical plane) by static perimetry (Figure 11, middle). With static perimetry the x and y coordinates are now fixed and the position on the z axis, representing the visual sensitivity, is varied. This is accomplished by keeping the size and location of a target constant and varying the brightness, (this lends itself very well to automation).
   7. Maps of visual sensitivity, made by either of these methods are very important in diagnosing diseases of the visual system. A full lecture will be devoted to these maps in your Neuroscience course.