Figure 4.1 Light, the physical stimulus for vision. (a) Light waves vary in amplitude and wavelength. (b) Within the spectrum of visible light, amplitude (corresponding to physical intensity) affects mainly the experience of brightness. Wavelength affects mainly the experience of color, and purity is the key determinant of saturation. (c) If white light (such as sunlight) passes through a prism, the prism separates the light into its component wavelengths, creating a rainbow of colors. However, visible light is only the narrow band of wavelengths to which human eyes happen to be sensitive.

The Eye: A Living Optical Instrument

Figure 4.2 The human eye. Light passes through the cornea, pupil, and lens and falls on the light-sensitive surface of the retina, where images of objects are reflected upside down. The lens adjusts its curvature to focus the images falling on the retina. The pupil regulates the amount of light passing into the rear chamber of the eye.

Figure 4.3 Nearsightedness and farsightedness. The pictures on the right simulate how a scene might look to nearsighted and farsighted people. Nearsightedness occurs because light from distant objects focuses in front of the retina. Farsightedness is due to the opposite situation—light from close objects focuses behind the retina.
Photos: Craig McClain

The Retina: The Brain’s Envoy in the Eye

Slide 9

Figure 4.7 Visual pathways through the brain. (a) Input from the right half of the visual field strikes the left side of each retina and is transmitted to the left hemisphere (shown in red). Input from the left half of the visual field strikes the right side of each retina and is transmitted to the right hemisphere (shown in green). The nerve fibers from each eye meet at the optic chiasm, where fibers from the inside half of each retina cross over to the opposite side of the brain. After reaching the optic chiasm, the major visual pathway projects through the lateral geniculate nucleus in the thalamus and onto the primary visual cortex (shown with solid lines). A second pathway detours through the superior colliculus and then projects through the thalamus and onto the primary visual cortex (shown with dotted lines). (b) This inset shows a vertical view of how the optic pathways project through the thalamus and onto the visual cortex in the back of the brain [the two pathways mapped out in diagram (a) are virtually indistinguishable from this angle].

Vision and the Brain: Information Processing

Figure 4.8 Hubel and Wiesel’s procedure for studying the activity of neurons in the visual cortex. As the cat is shown various stimuli, a microelectrode records the firing of a neuron in the cat’s visual cortex. The figure shows the electrical responses of a visual cell apparently “programmed” to respond to lines oriented vertically.

Viewing the World in Color

Figure 4.12 The color circle and complementary colors. Colors opposite each other on this color circle are complements, or “opposites.” Additively, mixing complementary colors produces gray. Opponent process principles help explain this effect as well as the other peculiarities of complementary colors noted in the text.

Theories of Color Vision

Figure 4.14 Reconciling theories of color vision. Contemporary explanations of color vision include aspects of both the trichromatic and opponent process theories. As predicted by trichromatic theory, there are three types of receptors for color—cones sensitive to short, medium, and long wavelengths. However, these cones are organized into receptive fields that excite or inhibit the firing of higher-level visual cells in the retina, thalamus, and cortex. As predicted by opponent process theory, some of these cells respond in antagonistic ways to blue versus yellow, red versus green, and black versus white.

Perceiving Forms, Patterns, and Objects

Each of these figures can be seen as two different things. Can you identify them?

A pretty boring video clip

Figure 4.17 Feature analysis in form perception. One vigorously debated theory of form perception is that the brain has cells that respond to specific aspects or features of stimuli, such as lines and angles. Neurons functioning as higher-level analyzers then respond to input from these “feature detectors.” The more input each analyzer receives, the more active it becomes. Finally, other neurons weigh signals from these analyzers and make a “decision” about the stimulus. In this way perception of a form is arrived at by assembling elements from the bottom up.

Figure 4.18 Bottom-up versus top-down processing. As explained in these diagrams, bottom-up processing progresses from individual elements to whole elements, whereas top-down processing progresses from the whole to the individual elements.

Figure 4.19 The principle of figure and ground. Whether you see two faces or a vase depends on which part of this drawing you see as figure and which as background. Although this reversible drawing allows you to switch back and forth between two ways of organizing your perception, you can’t perceive the drawing both ways at once.

More on Figure-Ground

Perceiving Depth or Distance

Principles of Perception

Figure 4.20 Gestalt principles of perceptual organization. Gestalt principles help explain some of the factors that influence form perception. (a) Proximity: These dots might well be organized in vertical columns rather than horizontal rows, but because of proximity (the dots are closer together horizontally), they tend to be perceived in rows. (b) Closure: Even though the figures are incomplete, you fill in the blanks and see a circle and a dog. (c) Similarity: Because of similarity of color, you see dots organized into the number 2 instead of a random array. If you did not group similar elements, you wouldn’t see the number 2 here. (d) Simplicity: You could view this as a complicated 11-sided figure, but given the preference for simplicity, you are more likely to see it as an overlapping rectangle and triangle. (e) Continuity: You tend to group these dots in a way that produces a smooth path rather than an abrupt shift in direction.

Figure 4.21 Perceptual hypotheses. The images projected on the retina are often distorted, shifting representations of stimuli in the real world, requiring ongoing perceptual hypotheses about what form could be responsible for a particular pattern of sensory stimulation. For example, if you look directly down at a small, square piece of paper on a desk (a), the stimulus (the paper) and the image projected on your retina will both be square. But as you move the paper away on the desktop, as shown in (b) and (c), the square stimulus projects an increasingly trapezoidal image on your retina.

Figure 4.22 A famous reversible figure. What do you see? Consult the text to learn what the two possible interpretations of this figure are.

Figure 4.24 Context effects. The context in which a stimulus is seen can affect your perceptual hypotheses. The middle character in the word on the left is assumed to be an “H,” whereas in the word on the right the same character is assumed to be an “A.” In addition to showing the potential influence of context, this example shows the power of expectations and top-down processing.

Stability in the Perceptual World: Perceptual Constancies

Optical Illusions: The Power of Misleading Cues

Slide 32

Figure 4.27 The Müller-Lyer illusion. Go ahead, measure them: the two vertical lines are of equal length.

Figure 4.29 Four geometric illusions. Ponzo: The horizontal lines are the same length. Poggendorff: The two diagonal segments lie on the same straight line. Upside-down T: The vertical and horizontal lines are the same length. Zollner: The long diagonals are all parallel (try covering up some of the short diagonal lines if you don’t believe it).

Figure 4.31 Three classic impossible figures. The figures are impossible, yet they clearly exist—on the page. What makes them impossible is that they appear to be three-dimensional representations yet are drawn in a way that frustrates mental attempts to “assemble” their features into possible objects. It’s difficult to see the drawings simply as lines lying in a plane—even though this perceptual hypothesis is the only one that resolves the contradiction.

Our Sense of Hearing: The Auditory System

Figure 4.32 Sound, the physical stimulus for hearing. (a) Like light, sound travels in waves—in this case, waves of air pressure. A smooth curve would represent a pure tone, such as that produced by a tuning fork. Most sounds, however, are complex. For example, the wave shown here is for middle C played on a piano. The sound wave for the same note played on a violin would have the same wavelength (or frequency) as this one, but the “wrinkles” in the wave would be different, corresponding to the differences in timbre between the two sounds. (b) The table shows the main relations between objective aspects of sound and subjective perceptions.

The Auditory System

Sensory Processing in the Ear

The Auditory Pathway

Figure 4.35 The basilar membrane. This graphic shows how the cochlea might look if it was unwound and cut open to reveal the basilar membrane, which is covered with thousands of hair cells (the auditory receptors). Pressure waves in the fluid filling the cochlea cause oscillations to travel in waves down the basilar membrane, stimulating the hair cells to fire. Although the entire membrane vibrates, as predicted by frequency theory, the point along the membrane where the wave peaks depends on the frequency of the sound stimulus, as suggested by place theory.

Auditory Perception: Theories of Hearing

Taste: The Gustatory System

Figure 4.36 The tongue and taste. Taste buds are clustered around tiny bumps on the tongue called papillae. There are three types of papillae, which are distributed as shown here. The taste buds found in each type of papillae show slightly different sensitivities to the four basic tastes, as mapped out in the graph at the top. Thus, sensitivity to the primary tastes varies across the tongue, but these variations are small and all four primary tastes can be detected wherever there are taste receptors
Source: Adapted from Bartoshuk, L. M. (1993). Genetic and pathological taste variation: What can we learn from animal models and human disease? In D. Chadwick, J. Marsh, & J. Goode (Eds.), The molecular basis of smell and taste transduction (pp. 251–267). New York: Wiley.

The Chemical Senses: Smell

Figure 4.37 The olfactory system. Odor molecules travel through the nasal passages and stimulate olfactory cilia. An enlargement of these hairlike olfactory receptors is shown in the inset. The olfactory nerves transmit neural impulses through the olfactory bulb to the brain.

Touch: Sensory Systems in the Skin

Figure 4.38 Receptive field for touch. A receptive field for touch is an area on the skin surface that, when stimulated, affects the firing of a cell that responds to pressure on the skin. Shown here is a center-surround receptive field for a cell in the thalamus of a monkey, originally described by Mountcastle and Powell (1959). Receptive fields for touch come in a variety of sizes and functional arrangements.

Figure 4.39 Pathways for pain signals. Pain signals are sent inward from receptors to the brain along the two ascending pathways depicted here in red and black. The fast pathway, shown in red, and the slow pathway, shown in black, depend on different types of nerve fibers and are routed through different parts of the thalamus. The gate control mechanism hypothesized by Melzack and Wall (1965) apparently depends on signals in a descending pathway (shown in green) that originates in an area of the midbrain.