5 Steps to a 5 AP Psychology, 2010-2011 Edition (24 page)

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 8.1
).

You are said to be
near-sighted
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
far-sighted
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 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 criss-cross, 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, etc., simultaneously. Simultaneous processing of stimulus elements is called
parallel processing
. David Hubel and Torsten Weisel (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.

Figure 8.1 The eye
.

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.

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, dichro-mats; 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 colorblind 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, then cones stimulate ganglion cells in a pattern that translates the trichromatic code into an opponent-process code further processed in the thalamus.

In the dark, without visual stimuli that capture your attention, you can appreciate your sense of hearing, or
audition
. Evolutionarily, being able to hear approaching predators or prey in the dark, or behind one’s back, helped increase chances of survival. Hearing is the primary sensory modality for human language. How do you hear? Sound waves result from the mechanical vibration of molecules from a sound source such as your vocal cords or the strings of a musical instrument. The vibrations move in a medium, such as air, outward from the source, first compressing molecules, then letting them move apart. This compression and expansion is called one cycle of a sound wave. The greater the compression, the larger the
amplitude
or height of the sound wave and the louder the sound. The amplitude is measured in logarithmic units of pressure called decibels (dB). Established by Fechner, every increase of 10 dB corresponds to a 10-fold increase in volume. The absolute threshold for hearing is 0 dB. Normal conversations measure about 60 dB. Differences in the
frequency
of the cycles, the number of complete wavelengths that pass a point in a second (hertz or Hz), determine the highness or lowness of the sound called the
pitch
. The shorter the wavelength, the higher the frequency and the higher the pitch. The longer the wavelength, the lower the frequency and the lower the pitch. People are sensitive to frequencies between about 20 and 20,000 Hz. You are best able to hear sounds with frequencies within the range that corresponds to the human voice. You can tell the difference between the notes of the same pitch and loudness played on a flute and on a violin because of a difference in the purity of the wave form or mixture of the sound waves, a difference in
timbre
.

Your ear is well adapted for converting sound waves of vocalizations to the neural impulses you perceive as language (see
Figure 8.2
). Your outer ear consists of the pinna, which is the visible portion of the ear; the auditory canal, which is the opening into the head; and the eardrum or tympanum. Your outer ear channels sound waves to the eardrum that vibrates with the sound waves. This causes the three tiny bones called the ossicles (the hammer, anvil, and stirrup) of your middle ear to vibrate. The vibrating stirrup pushes against the oval window of the cochlea in the inner ear. Inside the cochlea is a basilar membrane with hair cells that are bent by the vibrations and transduce this mechanical energy to the electrochemical energy of neural impulses. Hair cells synapse with auditory neurons whose axons form the auditory nerve. The auditory nerve transmits sound messages through your medulla, pons, and thalamus to the auditory cortex of the temporal lobes. Crossing of most auditory nerve fibers occurs in the medulla and pons so that your auditory cortex receives input from both ears, but contralateral input dominates.

Figure 8.2 The ear
.

How do you know where a sound is coming from? With ears on both sides of your head, you can locate a sound source. The process by which you determine the location of a sound is called
sound localization
. If your friend calls to you from your left side, your left ear hears a louder sound than your right ear. Using parallel processing, your brain processes both intensity differences and timing differences to determine where your friend is. The location of a sound source directly in front, behind, above, or below you is harder for you to pinpoint by hearing alone because both of your ears hear the sound simultaneously at the same intensity. You need to move your head to cause a slight offset in the sound message to your brain from each ear.