Sensation and Perception for AP Psychology (page 3)

Touch (Somatosensation)

Just as hearing is sensitivity to pressure on receptors in the cochlea, touch is sensitivity to pressure on the skin. Psychologists often use somatosensation as a general term for four classes of tactile sensations: touch/pressure, warmth, cold, and pain. Other tactile sensations result from simultaneous stimulation of more than one type of receptor. For example, burning results from stimulation of warmth, cold, and pain receptors. Itching results from repeated gentle stimulation of pain receptors, a tickle results from repeated stimulation of touch receptors, and the sensation of wetness results from simultaneous stimulation of adjacent cold and pressure receptors. Transduction of mechanical energy of pressure/touch and heat energy of warmth and cold occurs at sensory receptors distributed all over the body just below the skin's surface. Neural fibers generally carry the sensory information to your spinal cord. Information about touch usually travels quickly from your spinal cord to your medulla, where nerves criss-cross, to the thalamus, arriving at the opposite sides of your somatosensory cortex in your parietal lobes. Weber used a two-point discrimination test to determine that regions such as your lips and fingertips have a greater concentration of sensory receptors than your back. The amount of cortex devoted to each area of the body is related to the sensitivity of that area. Touch is necessary for normal development and promotes a sense of well-being.

Pathways for temperature and pain are slower and less defined. You probably have a harder time localizing where you sense warmth and pain on your skin than where you sense touch or pressure. Pain is often associated with secretion of substance P, and relief from pain is often associated with secretion of endorphins. Because the experience of pain is so variable, pain requires both a biological and psychological explanation. Ronald Melzack and Patrick Wall's gate-control theory attempts to explain the experience of pain. You experience pain only if the pain messages can pass through a gate in the spinal cord on their route to the brain. The gate is opened by small nerve fibers that carry pain signals. Conditions that keep the gate open are anxiety, depression, and focusing on the pain. The gate is closed by neural activity of larger nerve fibers, which conduct most other sensory signals, or by information coming from the brain. Massage, electrical stimulation, acupuncture, ice, and the natural release of endorphins can influence the closing of the gate. The experience of pain alerts you to injury and often prevents further damage.

Body Senses

The body senses of kinesthesis and the vestibular sense provide information about the position of your body parts and your body movements in your environment. Close your eyes and touch your nose with your index finger. Kinesthesis is the system that enables you to sense the position and movement of individual parts of your body. Sensory receptors for kinesthesis are nerve endings in your muscles, tendons, and joints.

Your vestibular sense is your sense of equilibrium or body orientation. Your inner ear has semicircular canals at right angles to each other. Hair-like receptor cells are stimulated by acceleration caused when you turn your head. The vestibular sacs respond to straightline accelerations with similar receptors. The combined activities of your vestibular sense, kinesthesis, and vision enable you to maintain your balance.

Chemical Senses

Gustation (taste) and olfaction (smell) are called chemical senses because the stimuli are molecules. Your chemical senses are important systems for warning and attraction. You won't eat rotten eggs or drink sour milk and you can smell smoke before a sensitive household smoke detector. Evolutionarily, these adaptations increased chances of survival.

Taste receptor cells are most concentrated on your tongue in taste buds embedded in tissue called fungiform papillae, but are also on the roof of your mouth and the opening of your throat. Tasters have an average number of taste buds, nontasters have fewer taste buds, and supertasters have the most. You can taste only molecules that dissolve in your saliva or a liquid you drink. Scientists have identified five types of taste receptors for sweet, salty, sour, bitter, and, most recently, umami or glutamate. Babies show a preference for sweet and salty, both necessary for survival; and disgust for bitter and sour, which are characteristic of poisonous and spoiled substances. Supertasters are more sensitive than others to bitter, spicy foods and alcohol, which they find unpleasant. Each receptor is sensitive to specific chemicals that initiate an action potential. The pathway for taste messages passes to the brainstem, thalamus, and primary gustatory cortex. Receptors for different tastes activate different regions of the primary taste cortex. Our tongues also have receptors for touch, pain, cold, and warmth. The sensory interaction of taste, temperature, texture, and olfaction determine flavor.

Odor molecules reach your moist olfactory epithelium high in your nasal cavity through the nostrils of your nose and the nasal pharynx linking your nose and mouth. Dissolved odor molecules bind to receptor sites of olfactory receptors, triggering an action potential. Research has not uncovered basic odors. Axons of olfactory sensory neurons pass directly into the olfactory bulbs of the brain. Sensory information about smell is transmitted to the hypothalamus and structures in the limbic system associated with memory and emotion as well as the primary cortex for olfaction on the underside of the frontal lobes, but not the thalamus. The primary olfactory cortex is necessary for making fine distinctions among odors and using those distinctions to consciously control behavior.

Perceptual Processes

What you perceive is an active construction of reality. Perception results from the interaction of many neuron systems, each performing a simple task. Natural selection favors a perceptual system that is very efficient at picking up information needed for survival in a three-dimensional world in which there are predators, prey, competitors, and limited resources. According to the nativist direct-perception theory of James Gibson, inborn brain mechanisms enable even babies to create perceptions directly from information supplied by the sense organs. For visual perception, your visual cortex transmits information to association areas of your parietal and temporal lobes that integrate all the pieces of information to make an image you recognize. Your brain looks for constancies and simplicity, making a huge number of perceptual decisions, often without your conscious awareness, in essentially two different ways of processing. The particular stimuli you select to process greatly affect your perceptions.

Attention

Attention is the set of processes by which you choose from among the various stimuli bombarding your senses at any instant, allowing some to be further processed by your senses and brain. You focus your awareness on only a limited aspect of all you are capable of experiencing, which is selective attention. In data-driven bottom-up processing, your sensory receptors detect external stimulation and send these raw data to the brain for analysis. Hubel and Weisel's feature-detector theory assumes that you construct perceptions of stimuli from activity in neurons of the brain that are sensitive to specific features of those stimuli, such as lines, angles, even a letter or face. In his constructionist theory, Hermann von Helmholtz maintained that we learn through experience to convert sensations into accurate perceptions. Anne Treisman's feature-integration theory proposes that detection of individual features of stimuli and integration into a whole occur sequentially in two different stages. First, detection of features involves bottom-up parallel processing; and second, integration of features involves less automatic, partially top-down serial processing. Concept-driven topdown processing takes what you already know about particular stimulation, what you remember about the context in which it usually appears, and how you label and classify it, to give meaning to your perceptions. Your expectations, previous experiences, interests, and biases give rise to different perceptions. Where you perceive a conflict among senses, vision usually dominates, which is called visual capture. That accounts for why you think the voice is coming from a ventriloquist's wooden pal when the puppet's mouth moves.

Gestalt Organizing Principles of Form Perception

Max Wertheimer, Kurt Koffka, and Wolfgang Kohler studied how the mind organizes sensations into perceptions of meaningful patterns or forms, called a gestalt in German. These Gestalt psychologists concluded that in perception, the whole is different from, and can be greater than, the sum of its parts. Unlike structuralists of the early 1900s, they thought that forms are perceived not as combinations of features, but as wholes.

This is exemplified by the phi phenomenon, which is the illusion of movement created by presenting visual stimuli in rapid succession. Videos consist of slightly different frames projected rapidly one after another, giving the illusion of movement. Gestaltists also noted that we see objects as distinct from their surroundings. The figure is the dominant object, and the ground is the natural and formless setting for the figure. This is called the figure–ground relationship. Gestaltists claimed that the nervous system is innately predisposed to respond to patterns of stimuli according to rules or principles. Their most general principle was the law of Pragnanz, or good form, which claimed that we tend to organize patterns in the simplest way possible. Other principles of organization include proximity, closure, similarity and continuity or continuation. Consider the following: DEMON DAY BREAK FAST. Looking at the groups of letters, you probably read the four words demon, day, break, and fast, rather than Monday, daybreak, or breakfast. Proximity, the nearness of objects to each other, is an organizing principle. We perceive objects that are close together as parts of the same pattern. Do you know someone who writes letters with- out quite closing the letter "o" or crossing the "t"? You probably still know what the letter is. The principle of closure states that we tend to fill in gaps in patterns. The closure principle is not limited to vision. For example, if someone started singing, "Happy Birthday to…," you might finish it in your mind. The principle of similarity states that like stimuli tend to be perceived as parts of the same pattern. The principle of continuity or continuation states that we tend to group stimuli into forms that follow continuous lines or patterns.

Optical or visual illusions are discrepancies between the appearance of a visual stimulus and its physical reality. Visual illusions, such as reversible figures, illustrate the mind's tendency to separate figure and ground in the absence of sufficient cues for deciding which is which. Illusory contours illustrate the tendency of the perceptual system to fill in missing elements to perceive whole patterns.

Depth Perception

Survival in a three-dimensional world requires adaptations for determining the distances of objects around you. Depth perception is the ability to judge the distance of objects. You interpret visual cues that tell you how near or far away objects are. Cues are either monocular or binocular. Monocular cues are clues about distance based on the image of one eye, whereas binocular cues are clues about distance requiring two eyes.

Binocular cues include retinal disparity and convergence. Your principal binocular cue is retinal disparity, which is the slightly different view the two eyes have of the same object because the eyes are a few centimeters apart. You can experience retinal disparity by extending your arm directly in front of you with your thumb up. Close one eye while looking at your thumb with the other. Then close the open eye and open the closed eye. Your thumb appears to move with respect to the background. If you follow the same procedure with your thumb closer, you'll notice that your thumb appears to move more. The degree of retinal disparity decreases with distance. With both eyes open, your brain fuses the two images, resulting in perception of depth. Convergence is the inward turning of your eyes that occurs when you look at an object that is close to you; the closer an object, the more convergence. You can experience convergence by looking at the tip of your nose with both eyes. Convergence is a less important distance cue than retinal disparity and cannot be used for objects beyond about 8 meters (about 25 feet).

Monocular cues include motion parallax, accommodation, interposition or overlap, relative size, relative clarity, texture gradient, relative height or elevation, linear perspective, and relative brightness. Motion parallax and accommodation require active use of your eye in viewing, whereas the other monocular cues are pictorial depth cues that can be given in a flat picture. Motion parallax involves images of objects at different distances moving across the retina at different rates. Closer objects appear to move more than distant objects when you move your head. When riding in a moving vehicle, you see very close objects move rapidly in the opposite direction; more distant objects move more slowly past you; extremely far away objects, such as the moon, seem to move with you. Accommodation of the lens increases as an object gets closer.

Look outside your window to notice all of the pictorial cues.

• Interposition or overlap can be seen when a closer object cuts off the view of part or all of a more distant one.
• Relative size of familiar objects provides a cue to their distance when the closer of two same-size objects casts a larger image on your retina than the farther one.
• Relative clarity can be seen when closer objects appear sharper than more distant, hazy objects.
• Texture gradient provides a cue to distance when closer objects have a coarser, more distinct texture than far away objects that appear more densely packed or smooth.
• Relative height or elevation can be seen when the objects closest to the horizon appear to be the farthest from you. The lowest objects in our field of vision generally seem the closest.
• Linear perspective provides a cue to distance when parallel lines, such as edges of sidewalks, seem to converge in the distance.
• Relative brightness can be seen when the closer of two identical objects reflects more light to your eyes.
• Optical illusions, such as the Muller-Lyer illusion, and the Ponzo illusion, in which two identical horizontal bars seems to differ in length, may occur because distance cues lead one line to be judged as farther away than the other. Similarly, the moon illusion may occur because the moon when near the horizon is judged to be farther away than when it is high in the sky, although in both positions it casts the same image on the retina.

At the San Francisco Exploratorium website, you can see examples of visual illusions and link to other great sites. Go to www. exploratorium.org.

Perceptual Constancy

As a car approaches, you know that it's not growing in size, even though the image it casts on your retina gets larger, because you impose stability on the constantly changing sensations you experience. This phenomenon is called perceptual constancy. Three perceptual constancies are size constancy, by which an object appears to stay the same size despite changes in the size of the image it casts on the retina as it moves farther away or closer; shape constancy, by which an object appears to maintain its normal shape regardless of the angle from which it is viewed; and brightness constancy, by which an object maintains a particular level of brightness regardless of the amount of light reflected from it. The real shape, orientation, size, brightness, and color are perceived as remaining relatively constant even when there are significant variations in the image it projects. This enables you to identify objects no matter what your viewing angle is, how far away you are, or how dim the lights are.

Perceptual Adaptation and Perceptual Set

Have you ever looked through a periscope or displacement goggles and tried to reach for an object only to find it wasn't where you thought it was? If you repeated your actions, after a short period of time you were probably able to reach the item easily. You adapted to the changed visual input. Newly sighted people who had been blind from birth are immediately able to distinguish colors and to separate figure from ground, but only gradually become able to visually recognize shapes. Visual perception can also be influenced by cultural factors, assumptions, and beliefs. To make use of the cue of relative size, you need to be familiar with the object and have been exposed to viewing objects in the distance.

Culture and Experience

Your perceptual set or mental predisposition can influence what you perceive when you look at ambiguous stimuli. Your perceptual set is determined by the schemas you form as a result of your experiences. Schemas are concepts or frameworks that organize and interpret information. This can account for people's interpretations of unidentified flying objects (UFOs), the Loch Ness monster, or seeing a cloud of dust in a movie.

Extrasensory Perception

Parapsychologists study evidence for psychological phenomena that are currently inexplicable by science. They try to answer the question "Is there perception without sensation?" ESP (extrasensory perception) is the controversial claim that perception can occur apart from sensory input. Parapsychology, the study of paranormal events, investigates claims of ESP, including

• telepathy: mind-to-mind communication;
• clairvoyance: perception of remote events;
• precognition: perception of future events;
• telekinesis or psychokinesis: moving remote objects through mental processes.

In 1998, a National Research Council investigation on ESP concluded that the best available evidence at that time did not support the contention that these phenomena exist.

Practice questions for this study guide can be found at:

Sensation and Perception Review Questions for AP Psychology