Vision

Vision

Different levels of vision correlate to the different types of eyes found in various species. The simplest eye receptor is that of planarians, a flatworm that abounds in ponds and streams. Planarians are moderately cephalized and have eye cups (or eyespots) located near the ganglia, a dense cluster of nerve cells with ventral nerve cords running along the length of the body.

The receptor cells within the cup are formed by layers of darkly pigmented cells that block light. When light is shined on the cup, stimulation of the photoreceptors occurs only through an opening on one side of the cup where there are no pigmented cells. As the mouth of one eye cup faces left and slightly forward, and the other faces right and forward, the light shining on one side can enter the eye cup only on that side.

This allows the ganglia to compare rates of nerve impulses from the two cups. The planarian will turn until the sensations from the two cups have reached an equilibrium and decreased. The observable behavior of the planarian is to turn away from the light source and seek a dark place under an object, an adaptation that protects it from predators.

As evolution progressed and cephalization increased, vison became more complex as well. There are true image-forming eyes of invertebrates : compound eyes and single-lens eyes . Compound eyes are found in insects and crustaceans (phylum Arthropoda) and in some polychaete worms (phylum Annelida). The compound eye has up to several thousand light detectors called ommatidia, each with its own cornea and lens. This allows each ommatidium to register light from a tiny part of the field of view. Differences in light intensity across the many ommatidia result in a mosaic image.

Although the image is not as sharp as that of a human eye, there is greater acuity at detecting movement—an important adaptation for catching flying insects or to avoid threats of predation. This ability to detect movement is partly due to the rapid recovery of the photoreceptors in the compound eye. Whereas the human eye can distinguish up to 50 flashes per second, the compound eye recovers from excitation rapidly enough to distinguish flashes of at the rate of 330 per second. Compound eyes also allow for excellent color vision, and some insects can even see into the UV range of the spectrum.

The second type of invertebrate eye, the single-lens eye, is found in jellyfish, polychaetes, spiders, and many mollusks . Its workings are similar to that of a camera. The single lens focuses light onto the retina, a bilayer of cells that are photosensitive, allowing for an image to be formed.

Vertebrate vision also uses single-lens vision, but it evolved independently and differs from the single-lens eyes of invertebrates. Vertebrate eyes have the ability to detect an almost countless variety of colors and can form images of objects that are miles away. It can also respond to as little as one photon of light. But as it is actually the brain that "sees," one must also have an understanding of how the eye generates sensations in the form of action potentials and how the signals travel to the visual centers of the brain.

Structure of the Vertebrate Eye

The globe of the eyeball is composed of the sclera, a tough, white outer layer of connective tissue , and of the choroid, a thin, pigmented inner layer. The sclera becomes transparent at the cornea, which is at the front of the eye where light can enter. The anterior choroid forms the iris, the colored part of the eye. By changing size, the iris regulates the amount of light entering the pupil, the hole in the center of the iris. Just inside the choroid is the retina, which forms the innermost layer of the eye and contains the photoreceptor cells.

Information from the photoreceptor cells of the retina passes through the optic disc, where the optic nerve attaches to the eye. The optic disc can be thought of as a blind spot in a vertebrate's field of vision because no photoreceptors are present in the disc. Any light that is focused on the lower outside part of the retina, the area of the optic disc, cannot be detected.

The eye is actually composed of two cavities, divided by the lens and the ciliary body. The anterior, smaller cavity is between the lens and the cornea and the posterior , larger one is behind the lens, within the eyeball itself.

The ciliary body is involved in constant production of the clear, watery aqueous humor that fills the anterior cavity of the eye. (Blockage of the ducts from which the aqueous humor drains can lead to glaucoma and eventually blindness, as the increased pressure compresses the retina.) The posterior cavity is lubricated by the vitreous humor, a jellylike substance that occupies most of the volume of the eye. Both humors function as liquid lenses that help focus light on the retina.

The lens itself is a transparent protein disc that focuses images on the retina. Many fish focus by moving the lens forward and backward, camera-style. In humans and other mammals, focusing is achieved through accommodation, the changing of the shape of the lens. For viewing objects at a distance, the lens is flattened and when viewing objects up close, the lens is almost spherical.

Accommodation is controlled by the ciliary muscle, which contracts to pull the border of the choroid layer of the eye to the lens, causing suspensory ligaments to slacken. There is a decrease in tension and the lens becomes more elastic, allowing the lens to become rounder. For viewing at a distance, the ciliary muscle relaxes, allowing the choroid to expand, thus placing more tension on the suspensory ligament and pulling the lens flatter.

Signal Transduction

The photoreceptor cells in the retina are of two types: rods and cones. The rods are more sensitive to light but are not involved in distinguishing color. They function in night vision and then only in black and white. Cones require greater amounts of light to be stimulated and are, therefore, not initiated in night vision. However, they are involved in distinguishing colors.

The human retina has approximately 125 million rod cells and approximately 6 million cone cells. The rods and cones account for nearly 70 percent of all receptors in the body, emphasizing the importance of vision in a human's perception of the environment. The numbers of photoreceptors are partly correlated with nocturnal or diurnal habits of the species, with nocturnal mammals having a maximum number of rods.

In humans the highest density of rods is at the lateral regions of the retina. Rods are completely absent from the fovea, the center of the visual field. This is why it is harder to see a dim star at night if you look at it directly than if you look at the star at an angle, allowing the starlight to be focused onto rodpopulated regions of the retina. However, the sharpest day vision is achieved by looking directly at an object because the cones are most dense in the fovea, approximately 150 thousand cones per square millimeter. Some birds actually have more than one million cones per square millimeter, enabling species such as hawks to spot mice from very high altitudes.

Photoreceptor cells have an outer segment with folded membrane stacks with embedded visual pigments. Retinal is the light-absorbing molecule synthesized from vitamin A and bonded to opsin, a membrane protein in the photoreceptor. The opsins vary in structure from one type of photoreceptor to another. The light-absorbing ability of retinal is affected by the specific identity of the opsin partner.

The chemical response of retinal to light triggers a chain of metabolic events, which causes a change in membrane voltage of the photoreceptor cells. The light hyperpolarizes the membrane by decreasing its permeability to sodium ions, so there are fewer neurotransmitters being released by the cells in light than in dark. Therefore a decrease in chemical signals to cells with which photoreceptors synapse serves as a message that the photoreceptors have been stimulated by light.

The axons of rods and cones synapse with neurons , bipolar cells, which synapse with ganglion cells. The horizontal cells and amacrine cells help integrate information before it is transmitted to the brain.

The axons of the ganglion cells form optic nerves that meet at the optic chiasma near the center of the base of the cerebral cortex. The nerve tracts are arranged so that what is in the left field of view of both eyes is transmitted to the right side of the brain (and vice versa).

The signals from the rods and cones follow two pathways: the vertical pathway and the lateral pathway. In the vertical pathway, the information goes directly from receptor cells to the bipolar cells and then to the ganglion cells. In the lateral pathway, the horizontal cells carry signals from one photoreceptor to other receptor cells and several bipolar cells. When the rods or cones stimulate horizontal cells, these in turn stimulate nearby receptors but inhibit more distant receptor and bipolar cells that are not illuminated. This process, termed lateral inhibition , sharpens the edges of our field of vision and enhances contrasts in images.

The information received by the brain is highly distorted. Although the anatomy and physiology of vision has been extensively studied, there is still much to learn about how the brain can convert a coded set of spots, lines, and movements to perceptions and recognition of objects.

see also Nervous System; Growth and Differention of the Nervous System.

Danielle Schnur

Bibliography

Bradbury, Jack W., and Sandra L. Vehrencamp. Principles of Animal Communication. Sunderland, MA: Sinauer, 1998.

Campbell, Neil A., Lawrence G. Mitchell, and Jane B. Reece. Biology: Concepts and Connections, 3rd ed. Menlo Park, California: Benjamin/Cummings Publishing Company, 1993.