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Chapter 4

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Wilfrid Laurier University
Mamdouh Shoukri

Lesson 4 1) Psychophysics: basic concepts and issues. Sensation is the stimulation of sense organs. Perception is the selection, organization, and interpretation of sensory input. Sensation involves the absorption of energy, such as light or sound waves, by sensory organs, such as the ears and eyes. Perception involves organizing and translating sensory input into something meaningful, such as your best friend’s face or other environmental stimuli. Psychophysics: the study of how physical stimuli are translated into psychological experience. a. Thresholds: looking for limits i. Sensation begins with a stimulus, any detectable input from the environment. ii. Fechner: Threshold is a dividing point between energy levels that do and do not have a detectable effect. iii. Absolute threshold for a specific type of sensory input is the minimum amount of stimulation that an organism can detect. It defines the boundaries of an organism’s sensory capabilities. It is anything but absolute. Researchers had to arbitrarily define the absolute threshold as the stimulus intensity detected 50% of the time. iv. Vision: a candle flame seen at 50 km on a dark clear night. Hearing: the tick of a watch under quiet conditions at 6 meters. Taste: one teaspoon (5ml) of sugar in 7.5 litters of water. Smell: one drop of perfume diffused into an entire volume of a six-room apartment. Touch: the wing of a fly falling on your cheek from a distance of 1 centimeter. b. Weighing the differences: the JND i. Just noticeable difference (JND) is the smallest difference in the amount of stimulation that a specific sense can detect. Absolute threshold is simply the JND from nothing (no stimulus input). ii. Weber’s law states that the size of a JND is a constant proportion of the size of the initial stimulus. This constant proportion is called the weber fraction. JND vary by sense, and the smallest detectable difference is a fairly stable proportion of the size of the original stimulus. His law not only apply to weight perception but to all of the senses. As stimuli increase in magnitude, the JND becomes larger. c. Psychophysical scaling i. Fechner’s law: states that the magnitude of a sensory experience is proportional to the number of JNDs that the stimulus causing the experience is above the absolute threshold. 1. An important ramification of Fechner’s law is that constant increments in stimulus intensity produce smaller and smaller increases in the perceived magnitude of sensation. 2. Perceptions can’t be measured on absolute scales. In the domain of sensory experience virtually everything is relative. ii. Signal-detection theory: proposes that the detection of stimuli involves decision processes as well as sensory processes, which are both influenced by a variety of factors besides stimulus intensity. 1. According to signal-detection theory, your performance will also depend on the level of “noise” in the system. Noise comes from all of the irrelevant stimuli in the environment and the neural activity they elicit. Variation in noise provides another reason why sensory thresholds depend on more than just the intensity of stimuli. iii. Subliminal perception: the registration of sensory input without conscious awareness. 1. Using diverse methodological and conceptual approaches, researchers examining a variety of phenomena, such as unconscious semantic priming, subliminal affective conditioning, and subliminal mere exposure effects. And subliminal psychodynamic activation, have found evidence that perception without awareness can take place. 2. Researchers have recently begun to use brain-imaging technology to study how the brain processes subliminal stimuli. iv. Sensory adaptation: a gradual decline in sensitivity due to prolonged stimulation. 1. In reality, the stimulus intensity of the odor is stable but with continued exposure, your sensitivity to it decreases. Sensory adaptation is a pervasive aspect of everyday life. 2. Sensory adaptation is an automatic, built-in process that keeps people tuned in to the changes rather than the constants in their sensory input. It allows people to ignore the obvious and focus on changes in their environment that may signal threats to safety. 3. Sensory adaptation also shows once again that there is no one to one correspondence between sensory input and sensory experience. 2) Our sense of sight: the visual system a. The stimulus: light i. Light: a form of electromagnetic radiation that travels as a wave, moving, naturally enough, at the speed of light. ii. Light waves vary in amplitude (height) and in wavelength (the distance between peaks). iii. Amplitude affects mainly the perception of brightness, while wavelength affects mainly the perception of color. iv. Light can also vary in its purity (how varied the mix is). Purity influences perception of the saturation, or richness, of color. v. Many insects can see shorter wavelengths than humans can see, in the ultraviolet spectrum, whereas many fish and reptiles can see longer wavelengths in the infrared spectrum. b. The eye: a living optical instrument i. Two main purposes: they channel light to the neural tissue that receives it, called the retina, and they house that tissue. ii. Components: 1. Light enters the eye through a transparent “window” at the front, the cornea. The cornea and the crystalline lens, located behind it, form an upside down image of objects on the retina. But the brain knows the rule for relating position on the retina to the corresponding positions in the world. 2. Lens is the transparent eye structure that focus the light rays falling on the retina. a. Accommodation: the lens is made up of relatively soft tissue, capable of adjustments tat facilitate a process. It occurs when the curvature of the lens adjusts to alter visual focus. Close object, the lens gets fatter (rounder); distant objects, flattens out. b. Nearsightedness: close objects are seen clearly but distant objects appear blurry because the focus of light from distant objects falls a little short of the retina. This focusing problem occurs when the cornea or lens bends the light too much, or when the eyeball is too long. c. Farsightedness: distant objects are seen clearly but close objects appear blurry because the focus of light from close objects falls behind the retina. This focusing problem typically occurs when the eyeball is too short. 3. Iris: the colored ring of muscle surrounding the pupil, or black center of the eye. 4. Pupil: the opening in the center of the iris that helps regulate the amount of light passing into the rear chamber of the eye. a. When the pupil constricts, it lets less light into the eye but it sharpens the image falling on the retina. When the pupil dilates (opens), it lets more light in but the image is less sharp. In bright light, the pupils constrict to take advantage of the sharpened image. But in dim light, the pupils dilate; image sharpness is sacrificed to allow more light to fall on the retina so that more remains visible. b. The eye itself is constantly in motion, moving in ways that are typically imperceptible to us. When we are looking at something, our eyes are scanning the visual environment and making brief fixation at various parts of the stimuli. Eye movements are referred to as saccades. c. The retina: the brain’s envoy in the eye. i. Retina: the neural tissue lining the inside back surface of the eye; it absorbs light, processes images, and sends visual information to the brain. ii. Optic disk: a hole in the retina where the optic nerve fibers exit the eye. Blind spot: the optic disk is a hole in the retina; you cannot see the part of an image that falls on it. iii. Visual receptors: rods and cones. The retina contains millions of receptor cells that are sensitive to light. Light must pass through several layers of cells before it gets to the receptors that actually detect it. Only about 10% of the light arriving at the cornea reaches these receptors. The retina contains two types of receptors, rods and cones. Their names are based on their shapes, as rods are elongated and cones are stubbier. Rods outnumber cones by a huge margin, as humans have 100 million to 125 million rods, but only 5 million to 6.4 million cones. 1. Cones are specialized visual receptors that play a key role in daylight vision and color vision. However, cones do not respond well to dim light, which is why you don’t’ see color very well in low illumination. Cones provide better visual acuity, that is sharpness and precise detail than rods. Cones are concentrated most heavily in the center of the retina and quickly fall off in density toward its periphery. a. Fovea: a tiny spot in the center of the retina that contains only cones; visual acuity is greatest at this spot. 2. Rods are specialized visual receptors that play a key role in night vision and peripheral vision. Rods handle night vision because they are more sensitive than cones to dim light. They handle the lion’s share of peripheral vision because they greatly outnumber cones in the periphery of the retina. The density of the rods is greatest just outside the fovea and gradually decreases toward the periphery of the retina. When you want to see a faintly illuminated object in the dark, it’s best to look slightly above or below the place where the object should be. Averting your gaze this way moves the image from the cone-filled fovea, which requires more light, to the rod-dominated area just outside the fovea, which requires less light. iv. Dark and light adaptation 1. Dark adaptation: the process in which the eyes become more sensitive to light in low illumination. The declining absolute thresholds over time indicate that you require less and less light to see. Dark adaptation is virtually complete in about 30 minutes, with considerable progress occurring in the first 10 minutes. 2. Light adaptation: the process whereby the eyes become less sensitive to light in high illumination. Light adaptation improves your visual acuity under the prevailing circumstances. 3. Both types of adaptation are due in large part to chemical changes in the rods and cones, but neural changes in the receptors and elsewhere in the retina also contribute. 4. The declining thresholds over time indicate that your visual sensitivity is improving, as less and less light is required 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. v. Information processing in the retina 1. Optic nerve: a collection of axons that connect the eye with the brain. These axons, which depart from the eye through the optic disk, carry visual information, encoded as a stream of neural impulses, to the brain. 2. Receptive field of a visual cell: the retinal area that, when stimulated, affects the firing of that cell. a. Particularly common shapes and sizes of receptive fields are circular fields with a center-surround arrangement. In these receptive fields, light falling in the center has the opposite effect of light falling in the surrounding area. The rate of firing of a visual cell might be increased by light in the center of its receptive field and decreased by light in the surrounding area. Other visual cells may work in just the opposite way. Either way, when receptive fields are stimulated, retinal cells send signals both toward the brain and laterally (sideways) toward nearby visual cells. These lateral signals allow visual cells in the retina to have interactive effects on each other. 3. Lateral antagonism (lateral inhibition): the most basic of these interactive effects. Occurs when neural activity in a cell opposes activity in surrounding cells. It is responsible for the opposite effects that occur when light falls on the inner versus outer portions of center-surround receptive fields. It allows the retina to compare the light falling in a specific area against general lighting. This means that the visual system can compute the relative amount of light at a point instead of reacting to absolute levels of light. d. Vision and the brain i. Visual pathways to the brain 1. Optic chiasm: the point at which the optic nerves from the inside half of each eye cross over and then project to the opposite half of the brain. This arrangement ensures that signals from both eyes go to both hemispheres of the brain. 2. After reaching the optic chiasm, the optic nerves fibers diverge along two pathways. The main pathway projects into the thalamus, the brain’s major relay station. Here, about 90% of the axons from the retinas synapse in the lateral geniculate nucleus (LGN). Visual signals are processed in the LGN and then distributed to areas in the occipital lobe that make up the primary visual cortex. The second visual pathway leaving the optic chiasm branches off to an area in the midbrain called the superior colliculus before travelling through the thalamus and on to the occipital lobe. The principal function of the second pathway appears to be the coordination of visual input with other sensory input. 3. Lightrods and conesneural signalsbipolar cellsganglion cellsoptic nerveoptic chiasmopposite half brain a. Main pathway: optic chiasmlateral geniculate nucleus (thalamus)primary visual cortex (occipital lobe) i. Magnocellular: where ii. Parvocellular: what b. Second pathway: superior colliculus thalamusprimary visual cortex 4. The main visual pathway is subdivided into two more specialized pathways called the magnocellular and parvocellular channels (based on the layers of the LGN they synapse in). These channels engage in parallel processing, which involves simultaneously extracting different kinds of information from the same input. ii. Information processing in the visual cortex 1. Individual cells in the primary visual cortex don’t really response much to little spots—they are much more sensitive to lines, edges, and other more complicated stimuli. 2. Simple cells respond best to a line of the correct width, oriented at the correct angle, and located in the correct position in its receptive field. 3. Complex cells also care about width and orientation, but they respond to any position in their receptive fields. Some complex cells are most responsive if a line sweeps across their receptive field—but only if it’s moving in the “right” direction. 4. The key point of all of this is that the cells in the visual cortex seem to be highly specialized. They have been characterized as feature detectors, neurons that respond selectively to very specific features of more complex stimuli. 5. After visual input is processed in the primary visual cortex, it is often routed to other cortical areas for additional processing. These signals travel through two streams: a. Ventral stream: processes the details of what objects are out there (the perception of form and color) b. Dorsal stream: processes where the object are (the perception of motion and depth) 6. As signals move farther along in the visual processing system, neurons become even more specialized or fussy about what turns them on, and the stimuli that activate them become more and more complex. Ex: cells in the temporal lobe of monkeys and humans that are especially sensitive to pictures of face. These neurons respond even to pictures that merely suggest the form of faces. 7. Another dramatic finding in this area of research is that the neurons in the ventral stream pathway that are involved in perceiving faces can learn from experience. 8. 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 pathways”, which processes information about color, form, and texture, moves on to areas of the temporal lobe. iii. Multiple methods in vision research 1. fMRI, microelectrodes, observations of the performance. 2. Early 1960’s: Hubel and Wiesel a. Microelectrode recording of axons in primary visual cortex of animals. b. Discovered feature detector: neurons that respond selectively to lines, edges, etc. c. Groundbreaking research: Nobel Prize in 1981. 3. McCullough effect: a well-known afterimage phenomenon that differs from other color afterimage effects because it is contingent on both color and pattern/form. 4. Visual agnosia: an inability to recognize familiar objects. The area of the brain known as V1, or the primary visual cortex mediates the effect, and that it does not depend on conscious form perception. e. Viewing the world in color i. The stimulus for color 1. Light with the longest wavelengths appears red, whereas those with the shortest appear violet. 2. Although wavelength wields the greatest influence, perception of color depends on complex blends of all three properties of light. Wavelength is most closely related to hue, amplitude to brightness, and purity to saturation. 3. There are two kinds of color mixture: a. Subtractive color: mixing works by removing some wavelengths of light, leaving less light than was originally there. Paints yield subtractive mixing because pigments absorb most wavelengths, selectively reflecting specific wavelengths that give rise to particular colors. Subtractive color mixing can also be demonstrated by stacking color filters. b. Additive color: mixing works by superimposing lights, putting more light in the mixture than exists in any one light by itself. 4. Human processes of color perception parallel additive color mixing much more closely than subtractive mixing. ii. Trichromatic theory of color vision 1. Trichromatic theory of color vision holds that the human eye has three types of receptors with differing sensitivities to different light wavelengths. They are red, green, and blue. 2. Color-blindness encompasses a variety of deficiencies in the ability to distinguish among colors. More in males than females. Most people who are color-blind are dichromate: they make do with only two color channels. iii. Opponent process theory of color vision 1. Complementary colors are pairs of colors that produce grey tones when mixed together. 2. Afterimage: a visual image that persists after a stimulus is removed. 3. Trichromatic theory cannot account for the appearance of complementary afterimages. 4. Opponent process theory of color vision holds that color perception depends on receptors that make antagonistic responses to three pairs of color. Red versus green, yellow versus blue, and black versus white. 5. Grapheme-color synesthesia: when individuals perceive a letter or digit, they concurrently and unintentionally experience the perception of an associated color. iv. Reconciling theories of color vision 1. George Wald demonstrated that the eye has three types of cones, with each type being most sensitive to a different band of wavelengths. The three types of cones represent the three different color receptors predicted by trichromatic theory. 2. Cells in the retina, the LGN, and the visual cortex that respond in opposite ways to red versus green and blue versus yellow. There are ganglion cells in the retina that are excited by green and inhibited by red. Other ganglion cells in the retina work in just the opposite way, as predicted in opponent process theory. 3. Mapmakers have long known that a minimum of four colors is needed to create maps in which no two adjacent countries are the same color. Purves, Lotto and Polger argue that the human visual system evolved to solve a similar problem—ensuring that no two areas separated by a common boundary will look the same if they actually are different. f. Perceiving forms, patterns, and objects i. Word: 1. Reversible figure: a drawing that is compatible with two interpretations that can shift back and orth. 2. The key point is simply this: the same visual input can result in radically different perceptions. No one-to-one correspondence exists between sensory input and what you perceive. This is a principal reason that people’s experience of the world is subjective. 3. Perceptual set: a readiness to perceive a stimulus in a particular way. 4. Inattentional blindness: the failure to see fully visible objects or events in a visual display. ii. Feature analysis: assembling forms 1. Feature analysis: the process of detecting specific elements in visual input and assembling them into a more complex form. 2. Feature analysis assumes that form perception involves: a. Bottom-up processing, a progression from individual element to the whole. (Feature detection theory) i. Detect specific features of stimuluscombine specific features into more complex formsrecognize stimulus b. Top-down processing: a progression from the whole to the elements. (Form perception) i. Formulate perceptual hypothesis about the nature of the stimulus as a wholeselect and examine features to check hypothesisrecognize stimulus c. Subjective contours: the perception of contours where none actually exist. iii. Looking at the whole picture: gestalt principles 1. Gestalt psychologists: the whole is more than the sum of its parts. a. Reversible figures and perceptual sets demonstrate that the same visual stimulus can result in very different perceptions. 2. Phi phenomenon: the illusion of movement created by presenting visual stimuli in rapid succession. 3. Gestalt principles of form perception: a. Figure and ground: the figure is the thing being looked at, and the ground is the background against which it stands. b. Proximity: things that are close to one another seem to belong together. c. Closure: people often group elements to create a sense of closure, or completeness. d. Similarity: people also tend to group stimuli that are similar e. Simplicity: the Gestaltist’s most general principle was the law of Pragnanz. The idea is that people tend to group elements that combine to form a good figure.
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