Chapter 4 Sensation and Perception

Transcription

1 Chapter 4 Sensation and Perception Sensation is the conversion of energy from the environment into a pattern of response by the nervous system. Perception is the interpretation of that information. Sensing the world around us Stimuli are energies in the environment that affect what we do. Receptors are the specialized cells in our bodies that convert environmental energies into signals for the nervous system. The Detection of Light Light is the stimulus that the visual system is designed to detect. Visible light is just one very small portion of the electromagnetic spectrum, which is the continuum of all the frequencies of radiated energy. The human eye is designed to detect energy in the wavelengths from 400 to 700 nm. The Structure of the Eye The pupil is an adjustable opening in the eye through which light enters. The iris is the structure on the surface of the eye, surrounding the pupil, and containing the muscles that make the pupil dilate or constrict. The iris gives your eye its characteristic color, too. The cornea is a rigid, transparent structure on the very outer surface of the eyeball. It focuses light by directing it through the pupil. When the light goes through the pupil, it is directed to the lens. The lens is a flexible structure that can vary in thickness, enabling the eye to accommodate, adjusting its focus for objects at different distances. The lens directs the light through a clear, jellylike substance called the vitreous humor to the back of the eyeball. At the back of the eye is the retina, the structure containing the visual receptors. The Visual Receptors The retina contains two types of specialized neurons, the rods and the cones. Rods far outnumber cones in the human eye. About 5-10% of the visual receptors in the human retina are cones. The cones are utilized in color vision, daytime vision and detail vision. The rods are adapted for vision in dim light. Species that are active at night have few cones and many rods, giving them particularly good night vision. The fovea is the center of the human retina, and the location of the highest proportion of cones. It is the area of the eye with the greatest acuity. Rods are more plentiful in the periphery of the retina.

2 The Visual Pathway The visual receptors send their impulses away from the brain, toward the center of the eye. First the bipolar cells gather the impulses from the rods and cones. Then the bipolar cells make synaptic contacts with ganglion cells. The axons of the ganglion cells join together to form the optic nerve, which makes a U-turn and exits the eye. There are no photoreceptors at the point at which the nerve leaves the eye. This is called the blind spot. You are not aware of your blind spot because information from the retina of each eye fills in the blind spot in the other eye. This integration occurs in the visual cortex. At the optic chiasm, half of each optic nerve crosses to go to the opposite side of the brain. At this point the axons begin to separate, sending information to a number of locations in the brain. The greatest number of axons goes to the occipital lobe via the thalamus. The information from each retina is integrated in the visual cortex. Each cell in the cortex receives input from both the left and the right retinas. When the retinas are focused on the same point in space, the input from each side is easily integrated because the message from each is almost the same. If the images conflict with each other, cortical cells will be alternately stimulated and inhibited as they try to integrate the information. The alternation between seeing the conflicting information from each retina is called binocular rivalry. The brain activity of the visual cortex is crucial for the sense of vision. People with intact eyes but a damaged visual cortex loses the ability to imagine visual imagery. Color Vision Different wavelengths of electromagnetic energy correspond to different colors of light. There are three kinds of cones that respond to different wavelengths. Cells in the visual path process the information from these cones in terms of opposites. The three types of information are: Red vs Green Yellow vs Blue White vs Black The cells in the cerebral cortex integrate the input from the parts of the visual field to create a color experience for the objects that we see. The Young-Helmholtz theory This is also known as the trichromatic theory. It proposes that our receptors respond to three primary colors. Color vision depends on the relative rate of response by the three types of cones.

3 Each type of cone is most sensitive to a specific range of electromagnetic wavelengths. Short wavelengths are seen as blue. Medium wavelengths are seen as green. Long wavelengths are seen as red. Each wavelength induces different levels of activity in each type of cone. For example, light that stimulates the medium and long wavelength cones about equally will be perceived as yellow. Light that excites all three types equally is perceived as white. The Opponent-Process Theory Trichromatic theory does not account for some of the more complicated aspects of color perception. People experience four colors as primary red, green, blue and yellow. People also report seeing colored afterimages after staring at an object of one color. If you stare at a red object, you tend to see a green afterimage when you stop staring. Because of these facts, Ewald Hering proposed that we perceive color not in terms of separate categories but rather in a system of paired opposites. Red vs Green Yellow vs Blue White vs Black The negative afterimages that we experience after staring at objects are results of the alternating stimulation and inhibition of neurons in the visual system. A bipolar neuron that responds strongly to yellow will be inhibited by blue. After you ve stared at a yellow object, your fatigued bipolar cell will behave as if it s been inhibited, and yield a sensation of blue. The Retinex Theory The trichromatic and opponent-process theory don t account for our experience of color constancy. Color constancy is the tendency of an object to appear nearly the same color even though we see it in a variety of lighting conditions. Edwin Land proposed that we perceive color because the cerebral cortex compares various retinal patterns (thus the name retina + cortex = retinex. ) By comparing different patterns of light from different areas of the retina, cortical cells synthesize a color perception for each area. The fact that certain types of brain damage disrupt color constancy, causing for example an object to look orange under one level or type of lighting, and red, green, yellow or even white under other conditions, is considered to be strong evidence for the Retinex theory. Colorblindness Total inability to distinguish colors is very rare except as a result of brain damage. About 4% of all people are partly colorblind.

4 Colorblindness can result from the absence of one of the three types of cones. Colorblindness can also result when one of the three types of cones is less responsive than the other two. The color that stimulates that type of cone is seen as almost gray. Red-green colorblindness is the most common type. There are two forms protanopia, in which the afflicted person lacks long-wavelength cones, and deuteranopia, in which the person lacks medium-wavelength cones. Yellow-blue colorblindness (known as trianopia) is very rare. How We See Before animals could see color, there was no color. What you see is in your brain. Not an exact representation of the world around you, but a construction and interpretation of many stimuli. Sensation seems simple, but it is perhaps one of the most challenging areas of this science. Hearing The ear is designed to detect and transmit sound waves to the brain. Sound waves are vibrations in the air or other medium. Sound waves vary according to frequency and amplitude. Frequency is measured by the number of vibrations or cycles of the sound wave per second, referred to as hertz (Hz.) The perception of frequency is referred to as pitch. We perceive a high-frequency sound wave as high-pitched, and a low-frequency wave as low-pitched. Amplitude is intensity of sound waves and is perceived as loudness. Pitch and loudness are psychological experiences, and the perception of these qualities does not solely depend on frequency and amplitude. The Ear The ear is a complex organ. It converts weak sound waves into waves of pressure that can be transported by sensory neurons and interpreted by the brain. The cochlea is the location of the hearing receptors. It is a spiral-shaped organ with canals containing fluid. Sound waves strike the tympanic membrane, or eardrum. The vibrations of the eardrum cause three very tiny bones, the malleus, the incus, and the stapes, (literally the hammer, anvil and stirrup) work to make the sound waves become stronger signals. The stirrup causes the cochlea to vibrate. This vibration displaces hair cells along the basilar membrane within the cochlea.

5 The hair cells are connected to neurons of the auditory nerve. The auditory nerve transmits the impulses from the cochlea to the cerebral cortex. Hearing Loss There are two common forms of deafness. Conduction deafness results when the three special bones in the ear fail to transmit sound waves properly to the cochlea. Nerve deafness results from damage to the structures that receive and transmit the impulses - the cochlea, hair cells or auditory nerve. Pitch Perception Adult humans can hear sound waves approximately between 15 and 20,000 Hz. How we hear pitch depends in part on the frequency to which we are listening. At low frequency (up to about 100 Hz), we hear by the workings of the frequency principle. Sound waves passing through the fluid in the cochlea cause all the hair cells to vibrate, producing action potentials that are synchronized with the sound waves. At about Hz, we hear by the workings of the volley principle. Fewer hair cells can fire at this pace, but those that do respond in groups, called volleys, and produce action potentials. Volleys are the chief mechanism for transmitting most speech and music to the brain. Beyond 4000 Hz, we hear by the workings of the place principle. The place principle states that the location of the hair cells stimulated by the sound waves depends on their frequency. The highest frequency sounds vibrate hair cells near the stirrup. Between 100 and 4000 Hz, we are hearing due to the combined effects of the volley and place principles. Localization of Sounds How does the auditory system determine the source of a sound? To estimate the approximate location of origin of a sound, the auditory system compares the messages received by the two ears. The sound waves will arrive at the closer ear slightly sooner (if coming from the right, it arrives at the right ear just a little before it arrives at the left ear.) The distance of a sound can be estimated based on loudness and pitch. A sound that is growing louder is interpreted as approaching. A higher frequency sound is interpreted as nearer than a low frequency sound; a sound that is increasing in pitch is interpreted as approaching. The only cue for absolute distance is the amount of reverberation experienced by the listener. The Vestibular Sense

6 What we generally call balance is the VESTIBULAR sense. The vestibule is a structure in the inner ear on each side of the head. Changes in the position of the vestibule cause receptors to be stimulated. These receptors tell the brain the direction of tilt, amount of acceleration and position of the head with respect to gravity. The vestibular sense plays a crucial role in maintaining balance and posture. The structure of the vestibular system Three semicircular canals are oriented in three directions. These canals contain a jellylike substance and are lined with hair cells. Acceleration causes the jellylike substance to move the hair cells, stimulating them. Hair cells are also contained in two otolith organs. The otoliths are calcium carbonate particles. These particles stimulate different sets of hair cells, depending on which way the head tilts. They are telling your brain which way is up. The Cutaneous Senses Touch is actually considered to be several independent senses: Pressure Warmth and Cold Pain Vibration Movement and Stretch of Skin These sensations depend on several different kinds of receptors. These are most noticeable in our skin, but we do have the same receptors in our internal organs, allowing us to feel internal pain, pressure, and temperature changes. Therefore we also refer to these senses as comprising the somatosensory system. The primary somatosensory cortex In certain areas, such as the fingertips and lips, there are proportionally many more cutaneous receptors. These areas also are allotted more tissue in the parietal lobes of the human cerebral cortex. Most humans with no impairment in these areas are very good at identifying familiar objects by touch alone. Pain Pain receptors are simple nerve endings that travel to the spinal cord. The perception of pain is a complex mixture of sensation and perception that is in part mediated by emotion. Two different areas of the brain govern sensory and emotional interpretations. This is one reason that at least some people can be distracted or use self-hypnosis to manage reactions to pain.

7 The Gate Theory of Pain Just seeking treatment or believing that one has been treated can result in a reduction of symptoms. The effectiveness of placebos in reducing the experience of pain has been well supported by experimental studies. A variety of processes can increase or decrease pain to injured areas of the body. On the basis of these observations, Metzack and Wall (1965) proposed the gate theory of pain. This is the theory that pain messages must pass through a gate, thought to be in the spinal cord. This gate can block the messages. Neurotransmitters and pain Substance P is a neurotransmitter that the nervous system releases for intense pains. Reactions to painful stimuli are reduced in animals that lack substance P. Glutamate is released in response to all pains. Endorphins, which are chemically identical to opiates, are released by the nervous system in response to the release of substance P. They effectively weaken pain sensations. Endorphin release can also be induced by sensory experiences such as listening to music or sexual activity. Capsaicin is the chemical that is present in hot peppers. It stimulates receptors that respond to painful heat. It causes the release of substance P and depletes supply of it in the nervous system. Creams containing capsaicin can be used to relieve muscle pain. The Chemical Senses Taste and smell are jointly referred to as the chemical senses. Many invertebrates rely almost entirely on these senses; other mammals use them much more heavily than do humans. Taste The sense of taste detects chemicals on the tongue. Its major function is to control and motivate our eating and drinking. The taste buds are located in the folds on the surface of the tongue. They contain the vast majority of human taste receptors. Taste Receptors Traditionally the view from Western medicine has held that there are four primary tastes sweet, sour, salty and bitter. The flavor of Monosodium glutamate (MSG), a common ingredient in Asian cooking, may represent a fifth. Researchers are using the word umami for this fifth type of taste receptor. Olfaction Olfaction is another term for the sense of smell.

8 The receptors for smell are located in the mucous membranes in the rear air passages of the nose. They detect the presence of airborne molecules of chemicals. We are aware now that there are at least hundreds of types of olfactory receptors (contrast this with the number of types of visual receptors.) Other mammals have far more than this. Each type of olfactory receptor is extremely specialized for one small group of closely related chemicals. Smell is vital for food selection. Neurons in the prefrontal cortex receive both taste and olfactory input, and combine them to produce the perception of flavor. The olfactory tract also bypasses the relay system in the thalamus. It travels to the olfactory bulb, a structure in the base of the brain that is directly in contact with the limbic system Especially in nonhuman mammals, olfaction plays a vital social role. These animals rely heavily on pheromones, chemicals that they release into the environment. Pheromones are important for sexual communication, acting upon the vomeronasal organ to send messages to other individuals regarding fertility and sexual receptivity. Humans prefer not to rely upon the social influences of pheromones and olfaction. But there is some evidence that they play a role anyway. In one study, it was shown that female college students who room together tend to have synchronized menstrual cycles. Sensory Systems The world that is sensed by a cat, a snail, or a bat is very different that the world that is sensed by you and me. The function of our senses is to give us the information that we need most to survive and thrive in our environment. Perception of Minimal Stimuli Thresholds Early psychological researchers thought it would be relatively simple to determine the weakest possible stimuli that humans could detect. They were wrong. It was soon discovered that no sharp line exists between stimuli that a person can detect and those that they cannot. Therefore, a sensory threshold was defined as intensity at which a given individual can detect a stimulus 50% of the time. There are no guarantees however that an individual will report all the stimuli above the threshold, or fail to report all those below it. The environment (i.e. lighting conditions) will also influence the individual s thresholds.

9 The absolute threshold has been defined as the sensory threshold at the time of maximum sensitivity; that is, when conditions would allow for the best possible receptivity to the stimulus. Difference threshold, is the minimum difference in stimulus needed to perceive a change. Subliminal Perception The concept of subliminal perception is well known to the general public. Subliminal perception is the idea that a stimulus can influence behavior even when it is so weak or brief that we do not perceive it consciously. There is concern that subliminal perception can powerfully manipulate human behavior. What does subliminal mean? When the term subliminal is used, it refers to the quality of being below the (sensory) threshold. Scientists use it to indicate that the stimulus was not consciously detected in a given presentation. Because the only way to know if a stimulus has been detected is to ask, it is very difficult to interpret the results of research on subliminal stimuli. What subliminal perception cannot do Claims that subliminal stimuli in advertisements can make people buy things are unsupportable. This claim has been tested repeatedly and no evidence has been found. Advertisements in American culture have little need of subliminal stimuli. They are overtly and effectively manipulative. Messages in music (recorded backwards or superimposed) cannot make people do anything, evil or otherwise. This claim has also been repeatedly tested under controlled conditions. No one listening to the messages can discern these messages. No one s behavior has been changed after listening to music containing messages. Subliminal audiotapes just don t work Claims that addictions can be overcome, self-esteem improved, and general self-improvement can be achieved through the use of subliminal audiotapes are also unsupported. Any results achieved through the use of these tapes can be attributed to the placebo effect or to the individual user s motivation to improve. What subliminal perception can do Some subtle effects on subsequent perception and emotion have been supported Priming individuals to see an object in subsequent presentations has been achieved through repeated presentations (Bar & Biederman, 1998) Emotional states can be influenced by subliminal presentation of messages that may be perceived as emotionally loaded (Masling et al., 1991) Subliminal perception

10 The fact that subliminal perception can influence behavior at all is interesting. But the effects overall are much smaller than people hope or fear. Perception and Recognition of Patterns Brightness contrast There are interesting fundamental questions to answer in the area of perception How does your brain decide how bright an object is? The apparent brightness of an object that you are looking at can be increased or decreased by the objects around it. This phenomenon is called brightness contrast. Face Recognition There are several interesting processes involved in face recognition To some extent, we use unusual characteristics to recognize faces. Most people recognize faces as a synthesized whole configuration of features. There seems to be a module in the brain devoted to face recognition. If this area is damaged, it is possible to lose the ability to recognize faces. Children who have been diagnosed with autism also are much poorer than average at face recognition. The Feature-Detector Approach One explanation for how we analyze complex stimuli suggests that we break them down into component parts We have feature detectors, specialized neurons that respond to the presence of certain simple features, such as angles and lines. For example, one feature detector might be stimulated only by the presence of vertical lines, or 90 angles. Feature detectors are essential for the first stages of analysis, but perception of complex stimuli requires other processes as well. Hubel & Wiesel s experiments Important evidence for the existence of feature detectors comes from the Nobel Prize winning research of Hubel and Wiesel (1981). They inserted thin electrodes into cells of the visual cortex in monkeys and cats and recorded activity of those cells when different light patterns were shown to the animals. The researchers were able to identify cells that fired only in the presence of vertical bars of light, and others that only fired for horizontal bars. In later experiments, they found cells that only fired in response to movement in particular directions. The waterfall illusion experienced by humans is evidence that humans do indeed have feature detectors.

11 In this illusion, a person first stares at a waterfall for one minute or more. If the person then looks at cliffs immediately after staring at the waterfall, the cliffs will appear to flow upward. This suggests that the cells that detect downward motion have become fatigued from the act of staring at the waterfall. Do feature detectors explain perception? Scientists believe that feature detectors are just a first step in a series of complex processes that create perception. Simple visual illusions such as the Necker cube suggest that we must also actively impose meaning on images that we see. There is a branch of psychology that specializes in explaining how humans arrive at the integrated whole images and make meaningful interpretations of the visual world. Gestalt Psychology Gestalt psychology focuses on the human ability to perceive overall patterns. The word Gestalt has no true English equivalent, but is close to synonymous with pattern or configuration. According to Gestalt psychologists, visual perception is an active creation, not merely the adding up of lines and movement. Principles of Gestalt Psychology When looking at an image, we make a distinction between figure and ground. This is a picture of a reversible figure a stimulus that can be perceived in more than one way. When we decide which side is the front of the object, then we will see it as a stable image. We are imposing order on an array, not just adding up small features. The principle of proximity states that humans tend to perceive objects close together as belonging to a group. The principle of similarity states that we perceive objects that resemble each other as forming a group. We may perceive continuation, and fill in gaps in lines, or closure of familiar figures. We tend to perceive a good figure, one that is simple and symmetrical. Gestalt visual principles have analogs in the perception of sound. Perception of Movement and Depth Visual constancy Visual constancy is our tendency to perceive objects as keeping their size, shape and color even though the image that strikes our retina changes from moment to moment. So an automobile that is driving away looks like it is moving away, not merely shrinking, even though the image on our two retinas is growing smaller.

12 Motionblindness can result from damage to a small area of the temporal lobe. This fact is further evidence that the visual system analyzes different aspects of an image via different pathways in the brain. Perception of Movement How do we distinguish between our own movement and the movement of objects? The vestibular system works to keep the visual system informed of the movements of your head. We see motion when an object is moving relative to the background. When an object is stationary and the background is moving, we may experience induced movement, a visual illusion in which we incorrectly perceive the object as moving. Stroboscopic movement is an illusion of movement created by a rapid succession of stationary images. Animation and motion pictures work by stroboscopic movement. The phi effect, in which your brain creates motion from rows of adjacent lights blinking on and off sequentially, is exploited by many a nightclub and motel owner. Depth Perception Our retinas are two-dimensional surfaces, but they give us very good depth perception our ability to perceive distance. There are several factors involved in creating our depth perception. Some are binocular cues (depending on both eyes) and others are monocular (needing only one eye.) Binocular cues One important contributor is retinal disparity, which is the difference in apparent position of an object seen by each retina. This discrepancy allows us to gauge distance. Convergence is the degree to which our eyes must turn in to allow us to focus on a very close object. Monocular cues Monocular cues allow a person to judge depth and distance accurately using only one eye. Object size can be used if we already have an idea of the approximate size of the objects. Linear perspective, as in the case of parallel lines that converge as they approach the horizon. Detail generally objects that are closer can be seen in greater detail than those that are farther away. Texture gradient refers to the fact that clusters of objects will seem more densely packed the farther away the clusters are. Interposition nearby objects will obstruct objects that are farther away. Shadows give clues to distance depending on size and position.

13 Accommodation as you will recall is how the lens changes shape to focus on objects, growing thinner to focus on far objects and thicker to focus on close things. Motion parallax is the principle that close objects will pass by faster than distant objects. Optical Illusions An optical illusion is a misinterpretation of a visual stimulus. Psychologists are attempting to find a parsimonious explanation for these misinterpretations. Many can be explained by considering the relationship between size perception and depth perception. When we misjudge distance, we misjudge size as well. For example, the Ames room illusion causes us to misjudge the heights of people standing in it using a powerfully misleading set of background cues. We see an immensely tall and a very short person, but once we remove all the misleading cues, we realize that they are people of similar height standing at different distances in relation to us. Even a two-dimensional drawing can contain cues that lead to the erroneous perception of depth. The drawings of M.C. Escher work by this principle. Vision plays a prominent role in some auditory illusions. Visual capture effect is the tendency to identify a sound as coming from a visually prominent source rather than its actual source. The inaccurate judgment of sound s distance leads us also to misjudge its intensity. Ventriloquism works using this auditory illusion. Cross-cultural influences It is thought that how an individual sees the Muller-Lyer illusion is partly influenced by cultural and other factors. The illusion is stronger for city dwellers and for children. This suggests that experience with buildings and with drawings of objects may have some impact on interpretation of two-dimensional images. The Moon Illusion To most people, the moon appears to be about 30% larger when it is close to the horizon. Measuring it with navigational equipment will prove to you that it is in fact the same size. It is hard to explain exactly why this illusion occurs, but it probably is influenced by our tendency to use background cues for reference in judging size. When the moon is at the horizon, we can compare it to the other familiar objects and the interposed terrain, so we judge it to be very large. When it is high in the sky, we have no basis to gauge its distance at all. We unconsciously judge the horizon moon to be more distant, therefore larger.

14 Visual Illusions and Perception The Moon Illusion and all that we are learning about visual perception and misperception reinforce an important point. What you are seeing is not out there it s in your brain. Vision is usually an accurate if complex reconstruction of the world around us, but we can be very, very mistaken about what we think we see.

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