7 Sensation and Perception
STEP 4 Review the Knowledge You Need to Score High
While psychologists study all sensory processes, a major focus is visual perception because most of us depend so much on sight. Initial visual sensation and perception take place in three areas: in the cones and rods of the retina located at the back inner surface of your eye; in the pathways through your brain and in your occipital lobes, also called the visual cortex. The image formed on your retina is upside down and incomplete. Your brain fills in information and straightens out the upside-down image almost immediately.
Millions of rods and cones are the photoreceptors that convert light energy to electrochemical neural impulses. Your eyeball is protected by an outer membrane composed of the sclera, tough, white, connective tissue that contains the opaque white of the eye, and the cornea, the transparent tissue in the front of your eye.
Rays of light entering your eye are bent first by the curved transparent cornea, pass through the liquid aqueous humor and the hole through your muscular iris called the pupil, are further bent by the lens, and pass through your transparent vitreous humor before focusing on the rods and cones in the back of your eye (see Figure 7.1).
Figure 7.1 The eye.
You are said to be nearsighted if too much curvature of the cornea and/or lens focuses an image in front of the retina so nearby objects are seen more clearly than distant objects. You are said to be farsighted if too little curvature of the cornea and/or lens focuses the image behind the retina so distant objects are seen more clearly than nearby ones. Astigmatism is caused by an irregularity in the shape of the cornea and/or the lens. This distorts and blurs the image at the retina.
The more abundant rods have a lower threshold than cones do and are sensitive to light and dark, as well as movement. Three different kinds of cones are each most sensitive to a different range of wavelengths of light, which provides the basis for color vision. When it suddenly becomes dark, your gradual increase in sensitivity to the low level of light, called dark adaptation, results from a shift from predominantly cone vision to predominantly rod vision. Rods and cones both synapse with a second layer of neurons in front of them in your retina, called bipolar cells. More rods synapse with one bipolar cell than do cones. Small amounts of stimulation from each rod to a bipolar cell can enable it to fire in low light. In bright light, just one cone can stimulate a bipolar cell sufficiently to fire, providing greater visual acuity or resolution. Bipolar cells transmit impulses to another layer of neurons in front of them in your retina, the ganglion cells. Axons of these cells converge to form the optic nerve of each eye. Where the optic nerve exits the retina, there aren’t any rods or cones, so the part of an image that falls on your retina in that area is missing—the blind spot. At the optic chiasm on the underside of your brain, half the axons of the optic nerve from each eye crisscross, sending impulses from the left half of each retina to the left side of your brain and from the right half of each retina to the right side of your brain. The thalamus then routes information to the primary visual cortex of your brain, where specific neurons called feature detectors respond only to specific features of visual stimuli, for example, a line in a particular orientation. Many different feature detectors can process the different elements of visual information, such as color, contours, orientation, and so forth, simultaneously. Simultaneous processing of stimulus elements is called parallel processing. David Hubel and Torsten Wiesel (1979) won a Nobel Prize for the discovery that most cells of the visual cortex respond only to particular features, such as the edge of a surface. More complex features trigger other detector cells, which respond only to complex patterns.
The colors of objects you see depend on the wavelengths of light reflected from those objects to your eyes. Light is the visible portion of the electromagnetic spectrum. Do you remember ROYGBIV? The letters stand for the colors red, orange, yellow, green, blue, indigo, and violet, which combine to produce white light. The colors vary in wavelength from the longest (red) to the shortest (violet). A wavelength is the distance from the top of one wave to the top of the next wave. The sun and most electric light bulbs essentially give off white light. When light hits an object, different wavelengths of light can be reflected, transmitted, or absorbed. Generally, the more lightwaves your eyes receive (the higher the amplitude of the wave), the brighter an object appears. The wavelengths of light that reach your eye from the object determine the color, or hue, the object appears to be. If an object absorbs all of the wavelengths, then none reach your eyes and the object appears black. If the object reflects all of the wavelengths, then all reach your eyes and the object appears white. If it absorbs some of the wavelengths and reflects others, the color you see results from the color(s) of the waves reflected. For example, a rose appears red when it absorbs orange, yellow, green, blue, indigo, and violet wavelengths and reflects the longer red wavelengths to your eyes.
“Information about colors of light is easy to remember: COlor receptors are COnes. ROYGBIV is a long acronym, so red is a long wave. Primary colors of light and the three kinds of cones are like my computer monitor and color TV—RGB.”
—Matt, AP student
What enables you to perceive color? In the 1800s, Thomas Young and Hermann von Helmholtz accounted for color vision with the trichromatic theory that three different types of photoreceptors are each most sensitive to a different range of wavelengths. People with three different types of cones are called trichromats; with two different types, dichromats; and with only one, monochromats. Cones are maximally sensitive to red, green, or blue. Each color you see results from a specific ratio of activation among the three types of receptors. For example, yellow results from stimulation of red and green cones. People who are color-blind lack a chemical usually produced by one or more types of cones. The most common type of color blindness is red—green color blindness resulting from a defective gene on the X chromosome, for a green cone chemical, or, less often, for a red cone chemical. Because it is a sex-linked recessive trait, males more frequently have this inability to distinguish colors in the red—orange—green range. Blue—yellow color blindness and total color blindness are rarer. Although trichromatic theory successfully accounts for how you can see any color in the spectrum, it cannot explain how mixing complementary colors produces the sensation of white, or why after staring at a red image, if you look at a white surface, you see green (a negative afterimage). According to Ewald Hering’s opponent-process theory, certain neurons can be either excited or inhibited, depending on the wavelength of light, and complementary wavelengths have opposite effects. For example, the ability to see reds and greens is mediated by red—green opponent cells, which are excited by wavelengths in the red area of the spectrum and inhibited by wavelengths in the green area of the spectrum, or vice versa. The ability to see blues and yellows is similar. Black—white opponent cells determine overall brightness. This explains why mixing complementary colors red and green or blue and yellow produces the perception of white and the appearance of negative afterimages. Colors in afterimages are the complements of those in the original images. Recent physiological research essentially confirms both the trichromatic and opponent-process theories. Three different types of cones produce different photochemicals, and then cones stimulate ganglion cells in a pattern that translates the trichromatic code into an opponent-process code further processed in the thalamus.