Analysis of Contrasts in the Visual Image. If a person looks at a blank wall, only a few neurons in the primary visual cortex will be stimulated, regardless of whether the illumination of the wall is bright or weak. Therefore, what does the primary visual cortex detect? To answer this question, let us now place on the wall a large solid cross, as shown to the left in Figure 1. To the right is shown the spatial pattern of the most excited neurons in the visual cortex. Note that the areas of maximum excitation occur along the sharp borders of the visual pattern. Thus, the visual signal in the primary visual cortex is concerned mainly with contrasts in the visual scene, rather than with noncontrasting areas. We noted in Chapter 51 that this is also true of most of the retinal ganglion because equally stimulated adjacent retinal receptors mutually inhibit one another. However, at any border in the visual scene where there is a change from dark to light or light to dark, mutual inhibition does not occur, and the intensity of stimulation of most neurons is proportional to the gradient of contrast—that is, the greater the sharpness of contrast and the greater the intensity difference between light and dark areas, the greater the degree of stimulation.
Fig1. Pattern of excitation that occurs in the visual cortex in response to a retinal image of a dark cross.
Visual Cortex Also Detects Orientation of Lines and Borders—“Simple” Cells. The visual cortex detects not only the existence of lines and borders in the different areas of the retinal image but also the direction of orientation of each line or border—that is, whether it is vertical or horizontal or lies at some degree of inclination. This capability is believed to result from linear organizations of mutually inhibiting cells that excite second order neurons when inhibition occurs all along a line of cells where there is a contrast edge. Thus, for each such orientation of a line, specific neuronal cells are stimulated. A line oriented in a different direction excites a different set of cells. These neuronal cells are called simple cells. They are found mainly in layer IV of the primary visual cortex.
Detection of Line Orientation When a Line Is Displaced Laterally or Vertically in the Visual Field— “Complex” Cells. As the visual signal progresses farther away from layer IV, some neurons respond to lines that are oriented in the same direction but are not position specific. That is, even if a line is displaced moderate distances laterally or vertically in the field, the same few neurons will still be stimulated if the line has the same direction. These cells are called complex cells.
Detection of Lines of Specific Lengths, Angles, or Other Shapes. Some neurons in the outer layers of the primary visual columns, as well as neurons in some secondary visual areas, are stimulated only by lines or borders of specific lengths, by specific angulated shapes, or by images that have other characteristics. That is, these neurons detect still higher orders of information from the visual scene. Thus, as one goes farther into the analytical pathway of the visual cortex, progressively more characteristics of each visual scene are deciphered.
DETECTION OF COLOR
Color is detected in much the same way that lines are detected: by means of color contrast. For instance, a red area is often contrasted against a green area, a blue area against a red area, or a green area against a yellow area. All these colors can also be contrasted against a white area within the visual scene. In fact, this contrasting against white is believed to be mainly responsible for the phenomenon called “color constancy”—that is, when the color of an illuminating light changes, the color of the “white” changes with the light, and appropriate computation in the brain allows red to be interpreted as red even though the illuminating light has changed the color entering the eyes.
The mechanism of color contrast analysis depends on the fact that contrasting colors, called “opponent colors,” excite specific neuronal cells. It is presumed that the initial details of color contrast are detected by simple cells, whereas more complex contrasts are detected by complex and hypercomplex cells.
Effect of Removing the Primary Visual Cortex
Removal of the primary visual cortex in the human being causes loss of conscious vision—that is, blindness. However, psychological studies demonstrate that such “blind” people can still, at times, react subconsciously to changes in light intensity, to movement in the visual scene, or, rarely, even to some gross patterns of vision. These reactions include turning the eyes, turning the head, and avoidance. This vision is believed to be subserved by neuronal pathways that pass from the optic tracts mainly into the superior colliculi and other portions of the older visual system.
Fields of Vision; Perimetry
The field of vision is the visual area seen by an eye at a given instant. The area seen to the nasal side is called the nasal field of vision, and the area seen to the lateral side is called the temporal field of vision.
To diagnose blindness in specific portions of the retina, one charts the field of vision for each eye by a process called perimetry. This charting is performed by having the subject look with one eye toward a central spot directly in front of the eye; the other eye is closed. A small dot of light or a small object is then moved back and forth in all areas of the field of vision, and the subject indicates when the spot of light or object can and cannot be seen. The field of vision for the left eye is plotted as shown in Figure 2. In all perimetry charts, a blind spot caused by lack of rods and cones in the retina over the optic disc is found about 15 degrees lateral to the central point of vision, as shown in the figure.
Fig2. A perimetry chart showing the field of vision for the left eye. The red circle shows the blind spot.
Abnormalities in the Fields of Vision. Occasionally, blind spots are found in portions of the field of vision other than the optic disc area. Such blind spots, called scotomata, are frequently caused by damage to the optic nerve resulting from glaucoma (too much fluid pressure in the eyeball), allergic reactions in the retina, or toxic conditions such as lead poisoning or excessive use of tobacco.
Another condition that can be diagnosed by perimetry is retinitis pigmentosa. In this disease, portions of the retina degenerate, and excessive melanin pigment is deposited in the degenerated areas. Retinitis pigmentosa usually causes blindness in the peripheral field of vision first and then gradually encroaches on the central areas.
Effect of Lesions in the Optic Pathway on the Fields of Vision. Destruction of an entire optic nerve causes blind ness of the affected eye.
Destruction of the optic chiasm prevents the crossing of impulses from the nasal half of each retina to the oppo site optic tract. Therefore, the nasal half of each retina is blinded, which means that the person is blind in the temporal field of vision for each eye because the image of the field of vision is inverted on the retina by the optical system of the eye; this condition is called bitemporal hemianopsia. Such lesions frequently result from tumors of the pituitary gland pressing upward from the sella turcica on the bottom of the optic chiasm.
Interruption of an optic tract denervates the corresponding half of each retina on the same side as the lesion; as a result, neither eye can see objects to the opposite side of the head. This condition is known as homonymous hemianopsia.
