Chapter 4 Sensation and Perception

Sensation : stimulation of sense organs Perception: selection, organization, and interpretation of sensory

input Psychophysics = the study of how physical stimuli are translated

into psychological experience

Sensation and Perception: The Distinction

Figure 4.1The distinction between sensation and perception. Sensation involves the stimulation of sensory organs, whereas perception involves the processing and interpretation of sensory input. The two processes merge at the point where sensory receptors convert physical energy into neural impulses.

Sensation begins with a detectable stimulus Fechner: the concept of the threshold

Absolute threshold: detected 50% of the time.Just noticeable difference (JND): smallest difference

detectable Weber’s law: size of JND proportional to size of initial

stimulus

Psychophysics: Basic Concepts

Figure 4.2The absolute threshold. If absolute thresholds were truly absolute, then at threshold intensity the probability of detecting a stimulus would jump from 0 to 100%, as graphed here in blue. In reality, the chances of detecting a stimulus increase gradually with stimulus intensity, as shown in red. Accordingly, an “absolute” threshold is defined as the intensity level at which the probability of detection is 50%.

Psychophysics: Concepts and Issues

Psychophysical scaling: Fechner’s Law Signal-detection Theory: Sensory processes + decision

processes Subliminal perception: Existence vs. practical effects Sensory Adaptation: Decline in sensitivity

Figure 4.3Possible outcomes in signal-detection theory. Four outcomes are possible in attempting to detect the presence of weak signals. The criterion you set for how confident you want to feel before reporting a signal will affect your responding. For example, if you require high confidence before reporting a signal, you will minimize false alarms, but you’ll be more likely to miss some signals.

Light = electromagnetic radiation Amplitude: perception of brightness Wavelength: perception of color

purity: mix of wavelengths perception of saturation, or richness of

colors.

Vision: The Stimulus

Figure 4.5Light, 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: housing and channeling Components:

Cornea: where light enters the eye Lens: focuses the light rays on the retina Iris: colored ring of muscle, constricts or dilates via

amount of light Pupil: regulates amount of light

The Eye: Converting Light Into Neural Impulses

Light and the Eye

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Retina: absorbs light, processes images, and sends information to the brain

Optic disk: where the optic nerve leaves the eye/ blind spot Receptor cells:

Rods: black and white/ low light vision Cones: color and daylight vision

Adaptation: becoming more or less sensitive to light as needed

The Retina: An Extension of the CNS

Figure 4.9The distribution of rods and cones. (a) The names for rods and cones are based on their shape. As you can see here, rods are a little more elongated and cones are stubbier. (b) This chart shows how rods and cones are distributed in the retina. The eye on top shows some of the locations (in degrees relative to the fovea) that are listed along the bottom of the chart. As the purple graph shows, most of the cones are concentrated in the fovea. The density of the rods, graphed in red, is greatest just outside each side of the fovea and then declines toward the periphery. There are no receptors of either type in the blind spot, which is why it is the blind spot. (Source: Adapted from Lindsay & Norman, 1977)

Figure 4.10The process of dark adaptation. The declining thresholds over time indicate that your visual sensitivity is improving, as less and less light is required for you to be able to see. Visual sensitivity improves markedly during the first 5 to 10 minutes after entering a dark room, as the eye’s bright-light receptors (the cones) rapidly adapt to low light levels. However, the cones’ adaptation, which is plotted in purple, soon reaches its limit, and further improvement comes from the rods’ adaptation, which is plotted in red. The rods adapt more slowly than the cones, but they are capable of far greater visual sensitivity in low levels of light.

Light -> rods and cones -> neural signals -> bipolar cells -> ganglion cells -> optic nerve -> optic chiasm -> opposite half brain ->

Main pathway: lateral geniculate nucleus (thalamus) -> primary visual cortex (occipital lobe) magnocellular: where parvocellular: what

Second pathway: superior colliculus ->thalamus -> primary visual cortex

The Retina and the Brain: Visual Information Processing

Figure 4.7The 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 closeup shows the several layers of cells in the retina. The cells closest to the back of the eye (the rods and cones) are the receptor cells that actually detect light. The intervening layers of cells receive signals from the rods and cones and form circuits that begin the process of analyzing incoming information before it is sent to the brain. These cells feed into many optic fibers, all of which head toward the “hole” in the retina where the optic nerve leaves the eye—the point known as the optic disk (which corresponds to the blind spot).

Figure 4.12Visual 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).

Figure 4.14Visual pathways from the primary visual cortex. Cortical processing of visual input is begun in the primary visual cortex. From there, signals are shuttled through the secondary visual cortex and onward to a variety of other areas in the cortex along a number of pathways. Two prominent pathways are highlighted here. The magnocellular, or “where pathway,” which processes information about motion and depth, moves on to areas of the parietal lobe. The parvocellular, or “what pathway,” which processes information about color, form, and texture, moves on to areas of the temporal lobe.

Early 1960’s: Hubel and Wiesel Microelectrode recording of axons in primary visual

cortex of animals Discovered feature detectors: neurons that respond

selectively to lines, edges, etc. Groundbreaking research: Nobel Prize in 1981.

Later research: cells specific to faces in the temporal lobes of monkeys and humans

Hubel and Wiesel: Feature Detectors and the Nobel Prize

Figure 4.13Hubel 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 simple cell apparently “programmed” to respond to lines oriented vertically.

Wavelength determines color Longer = red / shorter = violet

Amplitude determines brightness Purity determines saturation

Basics of Color Vision

Figure 4.6Saturation. Variations in saturation are difficult to describe, but you can see examples for two colors here.

Figure 4.17Additive versus subtractive color mixing. Lights mix additively because all the wavelengths contained in each light reach the eye. If red, blue, and green lights are projected onto a white screen, they produce the colors shown on the left, with white at the intersection of all three lights. If paints of the same three colors were combined in the same way, the subtractive mixture would produce the colors shown on the right, with black at the intersection of all three colors.

Figure 4.16Subtractive color mixing. Paint pigments selectively reflect specific wavelengths that give rise to particular colors, as you can see here for blue and yellow, which both also reflect back a little green. When we mix blue and yellow paint, the mixture absorbs all the colors that blue and yellow absorbed individually. The mixture is subtractive because more wavelengths are removed than by each paint alone. The yellow paint in the mixture absorbs the wavelengths associated with blue and the blue paint in the mixture absorbs the wavelengths associated with yellow. The only wavelengths left to be reflected back are some of those associated with green, so the mixture is seen as green.

Figure 4.18Complementary 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.

Trichromatic theory - Young and Helmholtz Receptors for red, green, blue – color mixing

Opponent Process theory – Hering 3 pairs of antagonistic colors

red/green, blue/yellow, black/white Current perspective: both theories necessary

Theories of Color Vision

Figure 4.11Receptive fields in the retina and lateral antagonism. Visual cells’ receptive fields in the retina are often circular with a center-surround arrangement, so that light striking the center of the field produces the opposite result of light striking the surround. In the receptive field depicted here, light in the center produces excitatory effects (symbolized by blue at the synapse) and increased firing in the visual cell, whereas light in the surround produces inhibitory effects (symbolized by red at the synapse) and decreased firing. However, the arrangement in other receptive fields may be just the opposite. Note that no light (a) and light in both center and surround (d) produce similar baseline rates of firing. This visual cell is more sensitive to contrast than to absolute levels of light. In (b) and (c) there is a contrast between the light falling on the center versus the surround, producing increased or decreased activity in the visual cell.

Figure 4.19Demonstration of a complementary afterimage. Stare at the dot in the center of the flower for at least 60 seconds, then quickly shift your gaze to the dot in the white rectangle. You should see an afterimage of the flower—but in complementary colors.

Figure 4.20Three types of cones. Research has identified three types of cones that show varied sensitivity to different wavelengths of light. As the graph shows, these three types of cones correspond only roughly to the red, green, and blue receptors predicted by trichromatic theory, so it is more accurate to refer to them as cones sensitive to short, medium, and long wavelengths.

Figure 4.21Explaining color perception. Contemporary theories 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 when lights of these colors stimulate their receptive fields.

Reversible figures and perceptual sets Feature detection theory - bottom-up processing. Form perception - top-down processing

Gestalt psychologists: the whole is more than the sum of its parts

Reversible figures and perceptual sets demonstrate that the same visual stimulus can result in very different perceptions

Perceiving Forms, Patterns, and Objects

Figure 4.23Feature 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 4.24Bottom-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.

Gestalt principles of form perception: figure-ground, proximity, similarity , continuity, closure,

and simplicity Recent research:

Distal (stimuli outside the body) vs. proximal (stimulus energies impinging on sensory receptors) stimuli.

Perceptual hypotheses Context

Principles of Perception

Figure 4.22A poster for a trained seal act. Or is it? The picture is an ambiguous figure, which can be interpreted as either of two scenes, as explained in the text.

Figure 4.25The 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.

Figure 4.26Gestalt principles of perceptual organization. Gestalt principles help explain how people subjectively organize 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 a rectangle and a triangle. (e) Continuity: You tend to group these dots in a way that produces a smooth path rather than an abrupt shift in direction. (f) Common Region: Although all eight dots shown here share a variety of similarities, they are grouped in pairs that share regions. (g) Connectedness: Although all eight dots shown here are similar, they are grouped in pairs that are connected.

Figure 4.27Distal and proximal stimuli. Proximal stimuli are often distorted, shifting representations of distal stimuli in the real world. If you look directly down at a small, square piece of paper on a desk (a), the distal stimulus (the paper) and the proximal stimulus (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 distal stimulus projects an increasingly trapezoidal image on your retina, making the proximal stimulus more and more distorted. Nevertheless, you continue to perceive a square.

Figure 4.28A famous reversible figure. What do you see?

Figure 4.29Unambiguous drawings of the reversible figure. These versions of the reversible figure in Figure 4.28 have been redrawn slightly to make the young woman more apparent on the left and the old woman more apparent on the right.

Figure 4.31Context effects. The context in which a stimulus is seen can affect your perceptual hypotheses.

Figure 4.30The Necker cube. The tinted surface of this reversible figure can become either the front or the back of the cube.

Binocular cues – clues from both eyes together retinal disparity convergence

Monocular cues – clues from a single eye motion parallax accommodation pictorial depth cues

Depth and Distance Perception

Figure 4.25Convergence and Depth Perception. One binocular depth cue is convergence. The more you have to converge your eyes together to focus on an object, the closer the object must be.

Perceptual constancies – stable perceptions amid changing stimuli Size Shape Brightness Hue Location in space

Stability in the Perceptual World: Perceptual Constancies

Optical Illusions - discrepancy between visual appearance and physical reality.

Famous optical illusions: Muller-Lyer Illusion, Ponzo Illusion, Poggendorf Illusion, Upside-down T illusion, and Zollner illusion, the Ames Room, and impossible figures

Cultural differences: Perceptual hypotheses at work

Optical Illusions: The Power of Misleading Cues

Figure 4.37The Müller-Lyer illusion. Go ahead, measure them: the two vertical lines are of equal length.

Figure 4.39Four 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 4.42Three 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.

Figure 4.43Another impossible figure. This impossible figure, drawn by Shepard (1990), seems even more perplexing than the classic impossible figure that it is based on (the one seen in the middle of Figure 4.42).

Visual Illusion

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Stimulus = sound waves (vibrations of molecules traveling in air) Amplitude (loudness) Wavelength (pitch) Purity (timbre)

Wavelength described in terms of frequency: measured in cycles per second (Hz) Frequency increase = pitch increase

Hearing: The Auditory System

Figure 4.44Sound, 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.

External ear (pinna): collects sound. Middle ear: the ossicles (hammer, anvil, stirrup) Inner ear: the cochlea

a fluid-filled, coiled tunnel contains the hair cells, the auditory receptorslined up on the basilar membrane

The Ear: Three Divisions

Figure 4.46The human ear. Converting sound pressure to information processed by the nervous system involves a complex relay of stimuli: Waves of air pressure create vibrations in the eardrum, which in turn cause oscillations in the tiny bones in the inner ear (the hammer, anvil, and stirrup). As they are relayed from one bone to the next, the oscillations are magnified and then transformed into pressure waves moving through a liquid medium in the cochlea. These waves cause the basilar membrane to oscillate, stimulating the hair cells that are the actual auditory receptors (see Figure 4.47).

Sound waves vibrate bones of the middle ear Stirrup hits against the oval window of cochlea Sets the fluid inside in motion Hair cells are stimulated with the movement of the basilar

membrane Physical stimulation converted into neural impulses Sent through the thalamus to the auditory cortex (temporal

lobes)

The Auditory Pathway

Figure 4.47The basilar membrane. The figure shows the cochlea 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.

Hermann von Helmholtz (1863) Place theory

Other researchers (Rutherford, 1886) Frequency theory

Georg von Bekesy (1947) Traveling wave theory

Theories of Hearing: Place or Frequency?

Two cues critical: Intensity (loudness) Timing of sounds arriving at each ear

Head as “shadow” or partial sound barrier Timing differences as small as 1/100,000 of a second

Auditory Localization: Where Did that Sound Come From?

Figure 4.48Cues in auditory localization. A sound coming from the left reaches the left ear sooner than the right. When the sound reaches the right ear, it is also less intense because it has traveled a greater distance and because it is in the sound shadow produced by the listener’s head. These cues are used to localize the sources of sound in space.

Taste (gustation) Physical stimulus: soluble chemical substances

Receptor cells found in taste buds Pathway: taste buds -> neural impulse -> thalamus -> cortex

Four primary tastes: sweet, sour, bitter, and salty Taste: learned and social processes

The Chemical Senses: Taste

Smell (Olfaction) Physical stimuli: substances carried in the air

dissolved in fluid, the mucus in the nose Olfactory receptors = olfactory cilia

Pathway: Olfactory cilia -> neural impulse -> olfactory nerve -> olfactory bulb (brain) Does not go through thalamus

The Chemical Senses: Smell

Figure 4.51The 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.

Physical stimuli = touch are mechanical, thermal, and chemical energy impinging on the skin.

Sensory receptors -> the spinal column -> brainstem -> cross to opposite side of brain -> thalamus -> somatosensory (parietal lobe)

Temperature: free nerve endings in the skin Pain receptors: also free nerve endings

Two pain pathways: fast vs. slow

Skin Senses: Touch

Figure 4.52Receptive 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.

Figure 4.53The two pathways for pain signals. Pain signals are sent from receptors to the brain along the two pathways depicted here. 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 posited by Melzack and Wall (1965) apparently depends on descending signals originating in an area of the midbrain (the pathway shown in green).

Kinesthesis - knowing the position of the various parts of the body Receptors in joints/muscles

Vestibular - equilibrium/balance

Other Senses: Kinesthetic and Vestibular

Figure 4.54The vestibular system. The semicircular canals in the inner ear (which are shown disproportionately large here) are the sensory organ for balance and head movement. Fluid movements in these canals stimulate neural impulses that travel along the vestibular nerve to the brain.