14. Visual Processing: Eye and Retina
Part 7 of 7

Valentin Draogoi, Ph.D.
(Content adapted from material by Chiyeko Tsuchitani, Ph.D.).

The chemical and physical integrity of the retina is essential for normal visual function.  Abnormalities in the blood supply and retinal pigment epithelium result in retinal dysfunctions.

Vitamin A deficiency can cause permanent blindness.  An adequate supply of photopigments is necessary to sustain photoreceptors.  The supply of all-trans retinal as a photopigment breakdown product is insufficient to maintain adequate photopigment production.   Vitamin A can be oxidized into all-trans retinal, and is, therefore, critical in the synthesis of photopigment.  In the eye, it is the retinal pigment epithelium that stores vitamin A.  The retinal pigment epithelium is also the site of the oxidization of vitamin A into all-trans retinal and conversion of all-trans retinal into 11-cis-retinal.  Vitamin A cannot be synthesized by the body and must be ingested.  It is found in blood and stored in the liver and retinal pigment epithelium.  Vitamin A deficiency, which can result from liver damage (e.g., from alcoholism or hepatitis), produces degeneration of photoreceptors with visual symptoms first presenting as “night blindness” (i.e., extremely poor vision under low illumination).

Retinitis pigmentosa is an inherited disorder in which there is a gradual and progressive failure to maintain the receptor cells.  One form involves the production of defective opsin that normally combines with 11-cis retinal to form rhodopsin.  Consequently, the rods do not contain sufficient rhodopsin and do not function as the low illumination receptors.   A symptom of this condition is “night blindness” and loss of peripheral vision.  In this form of retinitis pigmentosa, the cones receptors function normally and central vision remains intact.  Other forms of retinitis pigmentosa that affect the cones may progress to destroy central vision.

Macular Degeneration.  The leading cause of blindness in the elderly is age-related macular degeneration.  The dry form of macular degeneration involves intraocular proliferation of cells in the macular area (i.e., in the fovea and the immediately surrounding retinal areas).  In the wet form of macular degeneration, the capillaries of the choroid coat invade the macular area and destroy receptor cells and neurons.  In both forms, the visual loss is in the central visual field and the patient will complain of blurred vision and difficulty reading.  Laser surgery is the most common treatment for the wet form but has the disadvantage of destroying normal retinal cells. It also may not be effective in preventing cell proliferation following treatment. 

Retinal detachment.  When the neural retina is torn away from the retinal pigment epithelium (e.g., by a blow to the eye), there is a loss of vision in the area of detachment.  The loss of vision results because the neural retina is dependent on the retinal pigment epithelium for 11-cis retinal, nutrients and photoreceptor integrity.  The retinal pigment epithelium supplies glucose and essential ions to the neural retina, helps support the photoreceptor cell outer segment, removes outer segment disks shed by the receptor cells, and converts retinol and stores vitamin A for photopigment resynthesis.  Lasers may be used to weld the detachment to prevent it from increasing in size.  However, the detached and welded areas are functionally blind.

Diabetic retinopathy.  The pathological process in diabetic retinopathy involves microaneurysms and punctate hemorrhages in the retina.  The tiny swollen blood vessels and/or bleeding in the underlying choroid coat damage the receptor cells and retinal neurons and result in blindness in the regions affected.  Lasers may be used to seal swollen and/or leaking blood vessels.

This chapter described the stimulus (light) properties that are important for the visual perception of our external environment, such as color, brightness, color and brightness contrasts (for form perception and visual acuity), visual field representation, binocular fusion and depth perception.  Remember that there are regional differences in visual perception:  the central visual field is color-sensitive, has high acuity vision and operates at high levels of illumination (i.e., operates with the photopic, light-adapted subsystem).  In contrast, the visual field periphery is more sensitive at low levels of illumination, is relatively color insensitive and has poor visual acuity (i.e., operates with the scotopic, dark-adapted, subsystem).  The chapter also described how the lens system of the eye produces an image on the retina of light emitted by or reflected off objects in space.  The image is a smaller, inverted, and reversed picture of the object.  Keep in mind that the image projected onto the retina is, in fact, projected onto a flattened sheet of receptor cells that line the inner surface of the eye.  The following chapter will describe the function of the visual receptors and other retinal neurons in converting the visual image into an array of neural activity.

The chapter also reviewed the retinal neurons and the laminar structure of the retina. The image projected onto the retina is distributed over a mosaic of photoreceptors.  Light energy projected onto each photoreceptor is converted into receptor membrane potential changes by a process that involves photosensitive pigments and cyclic nucleotide-gated ion channels in the photoreceptor outer segment.  The phototransduction process converts light energy into photoreceptor membrane potential changes that produce a chemical signal (the release of glutamate), which results in membrane potential changes in the postsynaptic bipolar and horizontal cells.  The receptor substrate for scotopic and photopic vision lies in differences between the rod and cone receptors.

In the primate eye, the information gathered by 125 million receptor cells converges on 10 million bipolar cells, which, in turn, converge on 1 million retinal ganglion cells.  The degree of convergence from receptors to bipolar cell and bipolar cells to ganglion cell differs regionally within the retina.  In the peripheral retina, the convergence can be fifty or more rod receptors to one bipolar cell, which increases the sensitivity to dim lights but decreases the spatial acuity of the peripheral bipolar cell.  In addition, these peripheral bipolar cells are color insensitive.  The M-ganglion cells receive input from many peripheral bipolar cells, have large receptive fields, are sensitive to small brightness contrasts and are color insensitive.  They also generate transient responses and are uniquely sensitive to changes in illumination levels and movement.  In contrast, the bipolar cells in the macula synapse with few foveal-cone receptors, which maintain the spatial resolution of the densely packed cones.  Such macular bipolar cells have small receptive field centers, are color sensitive but must operate at high illumination levels. Each P-ganglion cell synapses with few macular bipolar cells and is color sensitive, but less sensitive to dim “white” light and to small brightness contrasts.  The P ganglion cells have smaller receptive fields than the M ganglion cells and respond with sustained discharges to maintained stimuli.  As the M ganglion cells and P ganglion cells respond to different aspects of the visual stimulus, they are described to be encoding and carrying independent, parallel, streams (M-stream and P-stream) of information about stimulus size, color, and movement.

[1] Be cautious with exam questions.  Note whether the question is about visual fields (what is seen, e.g., hemifields or quadrants) or retinal, nuclear or cortical areas (anatomical regions) affected.

[2] Note that of all the retinal neurons, only the photoreceptors respond directly to light or dark or color.

[3] Calcium, which enters the receptor cell at the cGMP-gated channels, plays a role in regulating processes that inactivate photopigments, decrease cGMP production and decrease the sensitivity of cGMP-gated channels to cGMP.

[4] Note that whereas cones are color sensitive, they will also respond to white light as white light contains all the wavelengths of visible light.

[5] Recall that horizontal cells provide ‘’lateral” connections between receptor cells only

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