14. Visual Processing: Eye and Retina
Part 1 of 7

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

In this chapter you will learn about how the visual system initiates the processing of external stimuli. The chapter will familiarize you with measures of visual sensation by discussing the basis of form perception, visual acuity, visual field representation, binocular fusion, and depth perception.  An important aspect is the regional differences in our visual perception:  the central visual field is color-sensitive, has high acuity vision, operates at high levels of illumination whereas the periphery is more sensitive at low levels of illumination, is relatively color insensitive, and has poor visual acuity. You will learn that the image is first projected onto a flattened sheet of photoreceptor cells that lie on the inner surface of the eye (retina). The information gathered by millions of receptor cells is projected next onto millions of bipolar cells, which, in turn, send projects to retinal ganglion cells. These cells encode different aspects of the visual stimulus, and thus carry independent, parallel, streams of information about stimulus size, color, and movement to the visual thalamus.

The condition of the visual system can be determined by examining various aspects of visual sensation. For example, the ability to detect and identify small objects (i.e., visual acuity) can be affected by disorders in the transparent media of the eye and/or visual nervous system.   The inability to detect objects in specific areas of space (i.e., visual field defects) is often related to neural damage.

Spatial Orientation and the Visual Field

The visual field is that area in space perceived when the eyes are in a fixed, static position looking straight ahead.

The monocular visual field  (Figure 14.1)

• is that area of space visible to one eye
• can be mapped parametrically
  •  Perimetry testing provides a detailed map of the visual field.  The potential visual field is described as a hemisphere.  However, it does not form a perfect hemisphere as the brow, nose and cheekbones obscure the view - most prominently in the nasal hemisphere
• is subdivided into two halves, the hemifields (Figure 14.1  Inset).
  • A horizontal line drawn from 0° to 180° through center of the field defines the superior & inferior hemifields. 
  • A vertical line drawn from 90° to 270° through center point defines the left & right hemifields, which are often termed the nasal and temporal hemifields.
• may be further subdivided into quadrants:
  • the superior and inferior nasal quadrants
  • the superior and inferior temporal quadrants.
• contains a blind spot,
  • a small area in which objects cannot be viewed
  • which is located within the temporal hemifield.

The monocular visual field (Figure 14.1) is determined with one eye covered. The area of overlap of the visual field of one eye with that of the opposite eye is called the binocular field (Figure 14.2).  All areas of the binocular visual field are “seen” by both eyes.

The ability to locate objects in space and the ability to orient ourselves with respect to external objects are dependent upon the representation of visual space within the nervous system.  The clinical examination of the visual fields most commonly used is the confrontation field test.  It defines the outer limits of our subjective visual space.  Neurological disorders of the visual system can often be localized based on the area of blindness within the visual field. 

Visual Acuity

Visual acuity is the ability to detect and recognize small objects visually depends on the refractory (focusing) power of the eye's lens system and the cytoarchitecture of the retina. 

Visual acuity is

• measured under high illumination 
• the smallest size of a dark object in a light background that can be correctly identified

In the clinical setting, an eye chart 

• is used to measure the patient’s visual acuity.
• consists of rows of black letters on a bright white background.
• is used to measure visual acuity at a distance of 20 ft from the chart.
• reports visual acuity as the ratio of the eye chart distance (i.e., 20 ft) to the “normal distance” of the lowest row of letters correctly identified by the patient (e.g., row 3, which is 70 ft).

Color Vision

Color vision is the ability to detect differences in the wavelengths of light is called color vision.  Clinically it may be tested with an Ishihara chart:  a chart with spots of different colors that are spatially organized to form numbers that differ for ``normal” and color-blind eyes.

As mentioned above, the human has a trichromatic visual system, whereby visible colors can be created by a mixture of red, green and blue lights.  The most common form of color blindness results in a confusion of red and green shades (i.e., red-green color blindness).  Most cases of color blindness result from an absent or defective gene responsible for producing the red or green photopigment (protanopia, the lack of red; and deuteranopia, the lack of green).  As these genes are located on the X chromosome, color blindness is more common in males than in females.

Regional differences: There are regional differences in color sensation, visual acuity and low-illumination sensitivity within the visual field (Figure 14.3).

A small “blindspot” is

• located in the temporal hemifield (Figure 14.3 Left)
• where objects cannot be seen.

Binocular Fusion and Depth Perception

When a pencil is held an arm’s length away with both eyes open, most individuals will see a single object and recognize it as a pencil.  However, if one rapidly closes each eye alternately (i.e., left eye closed, right eye opened, then right eye opened and left eye closed); you should see the pencil “jumping” from left to right as you alternate the eye closure.  This is so because the image in each eye is slightly different (disparate):  Notice that because each eye is located on either side of the nose, the viewing angle of each eye is slightly different - especially when viewing near objects (Figure 14.4). 

Although the area in space defined by the binocular visual field (Figure 14.4) represents corresponding areas of the monocular visual fields, the angle at which this space is viewed by each eye is slightly different.  Consequently, the images of the corresponding (binocular) space are slightly different in each eye.  The nervous system fuses these disparate binocular images to produce a single image (e.g., of the pencil located an arm’s length away).  The process of producing a single image from the two disparate monocular images is called binocular fusion.

Clinically, binocular fusion is tested by holding up one or two fingers in front of the patient and asking the patient (who should be wearing corrective lenses if they are normally worn) how many fingers they see.  If the patient reports seeing four fingers when only two are presented, the patient is unable to produce binocular fusion. 

Binocular fusion permits the perception a single clear image and also provides extra cues for depth perception.  That is, the binocular disparity between the two images is used by the nervous system to allow the perception of a three-dimensional world where the approximate distance of an object can be determined.  The nervous system cannot fuse disparate binocular images when the disparity is too great.  When corresponding areas of the normal binocular visual field are not in alignment (e.g., in strabismus where one eye deviates from the normal position and/or is paralyzed), the nervous system cannot fuse the disparate images and gradually adapts by “ignoring” the image from the deviant eye.  In fact, strabismus at birth, if uncorrected, may result in a form of central blindness, amblyopia, where the image from the deviant eye is no longer represented at cortical levels of the nervous system.  The uncorrected, long-term amblyope is functionally blind in one eye and has poor depth perception. 

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