Visual Perception Human Body Systems © 2014 Project Lead The Way, Inc.

Color Vision The retina of the eye possesses two special types of nerve cells known as photoreceptors. Rods function in dim light and perceive shades of gray. Cones function in bright light and provide sharp, colorful images. Impulses from rods and cones pass through nerve cells to the optic nerve.

Color Vision There are three different types of cones: Red cones Blue cones Green cones Each type of cone is sensitive to a different range of wavelengths of light. Different types of cones function together to interpret colors other than blue, red, and green. If any of the cones malfunction, color deficiency or color blindness occurs.

Depth Perception Depth perception is the ability to judge the relative distances between objects in three dimensions. With one eye, the field of vision appears two-dimensional. With two eyes, the eyes see and the brain processes different views of the same object.

Accommodation Two parts of the eye – the cornea and the lens – focus light on the retina. The cornea does most of the work, but it cannot change shape. Fine adjustments are carried out by the lens. Accommodation is the combination of reflex actions by which the lens of the eye changes to keep the focal length, the distance between the center of the lens and its focal point, constant. Ciliary muscles in the eye assist adjustment of the lens.

Astigmatism Astigmatism is a condition in which the cornea or the lens is irregularly shaped. This shape change causes incoming light rays to refract and converge improperly. The light rays do not focus at a specific point on the retina, resulting in a blurry or distorted image. Astigmatism may be corrected with eyeglasses, contact lenses, or refractive surgery.

Blind Spot The optic nerve exits the eye at the retina on its way to the brain. Since this area of the retina does not have receptors that respond to light, it is referred to as the blind spot. An image that falls on this area can not be seen. Normally people do not recognize the blind spot because the eyes are always on the move and the brain ignores this “hole” in visual input.

Peripheral Vision Peripheral vision is the ability to see things that fall outside of the direct line of vision. Due to their proximity to the edge of the retina, rods are responsible for this aspect of vision. Peripheral vision is better for detecting movement than for processing sharp images, and is most often stronger in the dark.

Optical Illusions Optical illusions are visual tricks that actually take place in the brain rather than the eye. The visual cortex of the brain deciphers images sent from the eye, however surrounding objects, intense colors, distortions of expected patterns, and preconceptions can cause the mind to “see” and interpret an image differently.

Afterimages Afterimages are optical illusions that occur when looking away after staring intently at a fixed image or color. The constant light stimulating the retina causes the cones in that area to become fatigued. After looking away from the image, the less-stimulated cones, which are not fatigued, still function. The resulting image lasts briefly and because it comes only from the less-fatigued cones, is perceived as a negative image.