The Eye: III. Central Neurophysiology of Vision

Visual Pathways

Figure 52-1 shows the principal visual pathways from the two retinas to the visual cortex . The visual nerve signals leave the retinas through the optic nerves . At the optic chiasm , the optic nerve fibers from the nasal halves of the retinas cross to the opposite sides, where they join the fibers from the opposite temporal retinas to form the optic tracts . The fibers of each optic tract then synapse in the dorsal lateral geniculate nucleus of the thalamus and, from there, geniculocalcarine fibers pass via the optic radiation (also called the geniculocalcarine tract ) to the primary visual cortex in the calcarine fissure area of the medial occipital lobe.

Visual fibers also pass to several older areas of the brain: (1) from the optic tracts to the suprachiasmatic nucleus of the hypothalamus , presumably to control circadian rhythms that synchronize various physiological changes of the body with night and day; (2) into the pretectal nuclei in the midbrain to elicit reflex movements of the eyes to focus on objects of importance and activate the pupillary light reflex; (3) into the superior colliculus to control rapid directional movements of the two eyes; and (4) into the ventral lateral geniculate nucleus of the thalamus and surrounding basal regions of the brain, presumably to help control some of the body’s behavioral functions.

Thus, the visual pathways can be divided roughly into an old system to the midbrain and base of the forebrain and a new system for direct transmission of visual signals into the visual cortex located in the occipital lobes. In humans, the new system is responsible for perception of virtually all aspects of visual form, colors, and other conscious vision. In many primitive animals, however, even visual form is detected by the older system, using the superior colliculus in the same manner that the visual cortex is used in mammals.

Function of the Dorsal Lateral Geniculate Nucleus of the Thalamus

The optic nerve fibers of the new visual system terminate in the dorsal lateral geniculate nucleus , located at the dorsal end of the thalamus and also called the lateral geniculate body , as shown in Figure 52-1 . The dorsal lateral geniculate nucleus serves two principal functions. First, it relays visual information from the optic tract to the visual cortex by way of the optic radiation. This relay function is so accurate that there is exact point to point transmission with a high degree of spatial fidelity all the way from the retina to the visual cortex.

After passing the optic chiasm, half the fibers in each optic tract are derived from one eye and half are derived from the other eye, representing corresponding points on the two retinas. However, the signals from the two eyes are kept apart in the dorsal lateral geniculate nucleus. This nucleus is composed of six nuclear layers. Layers II, III, and V (from ventral to dorsal) receive signals from the lateral half of the ipsilateral retina, whereas layers I, IV, and VI receive signals from the medial half of the retina of the opposite eye. The respective retinal areas of the two eyes connect with neurons that are superimposed over one another in the paired layers, and similar parallel transmission is preserved all the way to the visual cortex.

The second major function of the dorsal lateral geniculate nucleus is to “gate” the transmission of signals to the visual cortex—that is, to control how much of the signal is allowed to pass to the cortex. The nucleus receives gating control signals from two major sources: (1) corticofugal fibers returning in a backward direction from the primary visual cortex to the lateral geniculate nucleus; and (2) reticular areas of the mesencephalon . Both of these sources are inhibitory and, when stimulated, can turn off transmission through selected portions of the dorsal lateral geniculate nucleus. Both of these gating circuits help highlight the visual information that is allowed to pass.

Finally, the dorsal lateral geniculate nucleus is divided in another way:

1.
Layers I and II are called magnocellular layers because they contain large neurons. These neurons receive their input almost entirely from the large type M retinal ganglion cells. This magnocellular system provides a rapidly conducting pathway to the visual cortex. However, this system is color blind, transmitting only black-and-white information. Also, its point to point transmission is poor because there are not many M ganglion cells, and their dendrites spread widely in the retina.
2.
Layers III through VI are called parvocellular layers because they contain large numbers of small to medium-sized neurons. These neurons receive their input almost entirely from the type P retinal ganglion cells that transmit color and convey accurate point to point spatial information, but at only a moderate velocity of conduction rather than at high velocity.

Organization and Function of the Visual Cortex

Figures 52-2 and 52-3 show the visual cortex , which is located primarily on the medial aspect of the occipital lobes. Like the cortical representations of the other sensory systems, the visual cortex is divided into a primary visual cortex and secondary visual areas .

Figure 52-2, The visual cortex in the calcarine fissure area of the medial occipital cortex.

Primary Visual Cortex

The primary visual cortex (see Figure 52-2 ) lies in the calcarine fissure area , extending forward from the occipital pole on the medial aspect of each occipital cortex. This area is the terminus of direct visual signals from the eyes. Signals from the macular area of the retina terminate near the occipital pole, as shown in Figure 52-2 , whereas signals from the more peripheral retina terminate at or in concentric half-circles anterior to the pole but still along the calcarine fissure on the medial occipital lobe. The upper portion of the retina is represented superiorly, and the lower portion is represented inferiorly.

Note in the figure the large area that represents the macula. It is to this region that the retinal fovea transmits its signals. The fovea is responsible for the highest degree of visual acuity. Based on retinal area, the fovea has several hundred times as much representation in the primary visual cortex as do the most peripheral portions of the retina.

The primary visual cortex is also called visual area I or the striate cortex because this area has a grossly striated appearance.

Secondary Visual Areas of the Cortex

The secondary visual areas, also called visual association areas , lie lateral, anterior, superior, and inferior to the primary visual cortex. Most of these areas also fold outward over the lateral surfaces of the occipital and parietal cortex, as shown in Figure 52-3 . Secondary signals are transmitted to these areas for analysis of visual meanings. For example, on all sides of the primary visual cortex is Brodmann’s area 18 (see Figure 52-3 ), which is where virtually all signals from the primary visual cortex pass next. Therefore, Brodmann’s area 18 is called visual area II , or simply V-2. The other, more distant secondary visual areas have specific designations—V-3, V-4, and so forth—up to more than a dozen areas. The importance of all these areas is that various aspects of the visual image are progressively dissected and analyzed.

Figure 52-3, Transmission of visual signals from the primary visual cortex into secondary visual areas on the lateral surfaces of the occipital and parietal cortices. Note that the signals representing form, third-dimensional (3D) position, and motion are transmitted mainly into the superior portions of the occipital lobe and posterior portions of the parietal lobe. By contrast, the signals for visual detail and color are transmitted mainly into the anteroventral portion of the occipital lobe and the ventral portion of the posterior temporal lobe.

The Primary Visual Cortex has six Major Layers

Like almost all other portions of the cerebral cortex, the primary visual cortex has six distinct layers, as shown in Figure 52-4 . Also, as is true for the other sensory systems, the geniculocalcarine fibers terminate mainly in layer IV, but this layer is also organized into subdivisions. The rapidly conducted signals from the M retinal ganglion cells terminate in layer IVcα, and from there they are relayed vertically, both outward toward the cortical surface and inward toward deeper levels.

Figure 52-4, Six layers of the primary visual cortex. The connections shown on the left side of the figure originate in the magnocellular layers of the lateral geniculate nucleus (LGN) and transmit rapidly changing black and white visual signals. The pathways to the right originate in the parvocellular layers (layers III–VI) of the LGN; they transmit signals that depict accurate spatial detail, as well as color. Note especially the areas of the visual cortex called color blobs, which are necessary for detection of color.

The visual signals from the medium-sized optic nerve fibers, derived from the P ganglion cells in the retina, also terminate in layer IV, but at points different from the M signals. They terminate in layers IVa and IVcβ, the shallowest and deepest portions of layer IV, shown to the right in Figure 52-4 . From there, these signals are transmitted vertically both toward the surface of the cortex and to deeper layers. It is these P ganglion pathways that transmit the accurate point to point type of vision, as well as color vision.

Vertical Neuronal Columns in the Visual Cortex

The visual cortex is organized structurally into several million vertical columns of neuronal cells, with each column having a diameter of 30 to 50 micrometers. The same vertical columnar organization is found throughout the cerebral cortex for the other senses as well (and also in the motor and analytical cortical regions). Each column represents a functional unit. One can roughly calculate that each of the visual vertical columns has perhaps 1000 or more neurons.

After the optic signals terminate in layer IV, they are further processed as they spread outward and inward along each vertical column unit. This processing is believed to decipher separate bits of visual information at successive stations along the pathway. The signals that pass outward to layers I, II, and III eventually transmit signals for short distances laterally in the cortex. The signals that pass inward to layers V and VI excite neurons that transmit signals over much greater distances.

“Color Blobs” in the Visual Cortex

Interspersed among the primary visual columns, as well as among the columns of some of the secondary visual areas, are special column-like areas called color blobs . They receive lateral signals from adjacent visual columns and are activated specifically by color signals. Therefore, these blobs are presumably the primary areas for deciphering color.

Interaction of Visual Signals From the Two Separate Eyes

Recall that visual signals from the two separate eyes are relayed through separate neuronal layers in the lateral geniculate nucleus. These signals remain separated from each other when they arrive in layer IV of the primary visual cortex. In fact, layer IV is interlaced with stripes of neuronal columns, with each stripe about 0.5 millimeter wide; the signals from one eye enter the columns of every other stripe, alternating with signals from the second eye. This cortical area deciphers whether the respective areas of the two visual images from the two separate eyes are “in register” with each other—that is, whether corresponding points from the two retinas fit with each other. In turn, the deciphered information is used to adjust the directional gaze of the separate eyes so that they will fuse with each other (i.e., be brought into “register”). The information observed about degree of register of images from the two eyes also allows a person to distinguish the distance of objects by the mechanism of stereopsis .

Two Major Pathways for Analysis of Visual Information: (1) The Fast “Position” and “Motion” Pathway and (2) the Accurate Color Pathway

Figure 52-3 shows that after leaving the primary visual cortex, the visual information is analyzed in two major pathways in the secondary visual areas.

1.
Analysis of Third-Dimensional Position, Gross Form, and Motion of Objects. One of the analytical pathways, demonstrated in Figure 52-3 by the black arrows, analyzes the third-dimensional positions of visual objects in the space around the body. This pathway also analyzes the gross physical form of the visual scene, as well as motion in the scene. This pathway reveals where every object is during each instant and whether it is moving. After leaving the primary visual cortex, the signals flow generally into the posterior midtemporal area and upward into the broad occipitoparietal cortex. At the anterior border of the parietal cortex, the signals overlap with signals from the posterior somatic association areas that analyze three-dimensional aspects of somatosensory signals. The signals transmitted in this position-form-motion pathway are mainly from the large M optic nerve fibers of the retinal M ganglion cells, transmitting rapid signals but depicting only black and white with no color.
2.
Analysis of Visual Detail and Color. The red arrows in Figure 52-3 , passing from the primary visual cortex into secondary visual areas of the inferior, ventral , and medial regions of the occipital and temporal cortex , show the principal pathway for analysis of visual detail. Separate portions of this pathway specifically dissect out color as well. Therefore, this pathway is concerned with such visual feats as recognizing letters, reading, determining the texture of surfaces, determining detailed colors of objects, and deciphering from all this information what the object is and what it means.

Neuronal Patterns of Stimulation During Analysis of Visual Images

Analysis of Contrasts in Visual Images

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 52-5 . 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.

Figure 52-5, 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.

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