Visual Acuity and Sensitivity

While reading or similarly viewing objects in daylight, each eye is oriented so that the image falls within a tiny area of the retina called the fovea centralis. The fovea is a pinhead-sized pit (fovea = pit) within a yellow area of the retina called the macula lutea. The pit is formed as a result of the displacement of neural layers around the periphery; therefore, light falls directly on photoreceptors in the center (fig. 10.41). Light falling on other areas, by contrast, must pass through several layers of neurons, as previously described.

There are approximately 120 million rods and 6 million cones in each retina, but only about 1.2 million nerve fibers enter the optic nerve of each eye. This gives an overall convergence ratio of photoreceptors on ganglion cells of about 105 to 1. This is misleading, however, because the degree of convergence is much lower for cones than for rods. In the fovea, the ratio is 1 to 1.

The photoreceptors are distributed in such a way that the fovea contains only cones, whereas more peripheral regions of the retina contain a mixture of rods and cones. Approximately 4,000 cones in the fovea provide input to approximately 4,000 ganglion cells; each ganglion cell in this region, therefore, has a private line to the visual field. Each ganglion cell in the fovea thus receives input from an area of retina corresponding to the diameter of one cone (about 2 |im). Peripheral to the fovea, however, many rods synapse with a single bipolar cell, and many bipolar cells synapse with a single ganglion cell. A single ganglion cell outside the fovea thus may receive input from large numbers of rods, corresponding to an area of about 1 mm2 on the retina (fig. 10.42).

Sensory Physiology 273

Sensory Physiology 273

Fovea Cones Rods

Rods Cones Rods in fovea

Rods Cones Rods in fovea

■ Figure 10.41 The fovea centralis. When the eyes "track" an object, the image is cast upon the fovea centralis of the retina. The fovea is literally a "pit" formed by parting of the neural layers. In this region, light thus falls directly on the photoreceptors (cones).

Convergence Bipolar Ganglion

■ Figure 10.42 Convergence in the retina and light sensitivity. Since bipolar cells receive input from the convergence of many rods (a), and since a number of such bipolar cells converge on a single ganglion cell, rods maximize sensitivity to low levels of light at the expense of visual acuity. By contrast, the 1:1:1 ratio of cones to bipolar cells to ganglion cells in the fovea (b) provides high visual acuity, but sensitivity to light is reduced.

Since each cone in the fovea has a private line to a ganglion cell, and since each ganglion cell receives input from only a tiny region of the retina, visual acuity is greatest and sensitivity to low light is poorest when light falls on the fovea. In dim light only the rods are activated, and vision is best out of the corners of the eye when the image falls away from the fovea. Under these conditions, the convergence of large numbers of rods on a single bipolar cell and the convergence of large numbers of bipolar cells on a single ganglion cell increase sensitivity to dim light at the expense of visual acuity. Night vision is therefore less distinct than day vision.

The difference in visual sensitivity between cones in the fovea centralis and rods in the periphery of the retina can be demonstrated easily using a technique called averted vision. If you go out on a clear night and stare hard at a very dim star, it will disappear. This is because the light falls on the fovea and is not sufficiently bright to activate the cones. If you then look slightly off to the side, the star will reappear because the light falls away from the fovea, on the rods.

The only part of the visual field that is actually seen clearly is that tiny part (about 1%) that falls on the fovea centralis. We are unaware of this because rapid eye movements shift different parts of the visual field onto the fovea. A common visual impairment, particularly in older people, is macular degeneration—degeneration of the macula lutea and its central fovea. People with macular degeneration lose the clarity of vision provided by the fovea in the central region of the visual field. In most cases, the damage is believed to be related to the loss of retinal pigment epithelium in this region. In some cases, the damage is made worse by growth of new blood vessels (neovascularization) from the choroid into the retina.

Point of fixation (eyes are focusing on a close object)

Optic nerve

Optic chiasma

Optic tract

Superior colliculus (visual reflexes)

Point of fixation (eyes are focusing on a close object)

Optic nerve

Visual Reflexes

Monocular field Binocular field Macular field

Lens

Retina

Macula lutea

Lateral geniculate nucleus Optic radiation

Monocular field Binocular field Macular field

Lens

Retina

Macula lutea

Lateral geniculate nucleus Optic radiation

- Occipital lobe of cerebrum (visual cortex)

■ Figure 10.43 The neural pathway for vision. The neural pathway leading from the retina to the lateral geniculate body, and then to the visual cortex, is needed for visual perception. As a result of the crossing of optic fibers, the visual cortex of each cerebral hemisphere receives input from the opposite (contralateral) visual field.

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