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Chapter 4

Chapter 4 - Sensation and Perception.odt

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Steve Joordens

Chapter 4 – Sensation and Perception Some unusual perceptional events are varieties of synesthesia, the perceptual experience of one sense that is evoked by another sense. For some synesthetes, musical notes evoke the visual sensation of color. Other people with synesthesia see printed letters or numbers in specific, consistent colors (always seeing the digit 2 as pink and 3 as green, for example). Still others experience specific tastes when certain sounds are heard. So, synesthesia is neither an isolated curiosity nor the result of faking. In fact, it may indicate that in some people, the brain is “wired” differently than in most, so that brain regions for different sensory modalities cross-activate one another. Psychologists know, however, that sensation and perception are two separate activities. Sensation is simple stimulation of a sense organ. It is the basic registration of light, sound, pressure, odor, or taste as parts of your body interact with the physical world.After a sensation registers in your central nervous system, perception takes place at the level of your brain: It is the organization, identification, and interpretation of a sensation in order to form a mental representation. We all know that we have five senses: vision, hearing, touch, taste, and smell.Arguably, we possess several more senses besides these five. Touch, for example, encompasses distinct body senses, including sensitivity to pain and temperature, joint position and balance, and even the state of the gut. Despite the variety of our senses, they all depend on the process of transduction, which occurs when many sensors in the body convert physical signals from the environment into encoded neural signals sent to the central nervous system. In vision, light reflected from surfaces provides the eyes with information about the shape, color, and position of objects. In audition, vibrations (from vocal cords or a guitar string, perhaps) cause changes in air pressure that propagate through space to a listener’s ears. In touch, the pressure of a surface against the skin signals its shape, texture, and temperature. In taste and smell, molecules dispersed in the air or dissolved in saliva reveal the identity of substances that we may or may not want to eat. In each case physical energy from the world is converted to neural energy inside the central nervous system. Given that perception is different for each of us, how could we ever hope to measure it? This question was answered in the mid-1800s by the German scientist and philosopher Gustav Fechner (1801–87). Fechner developed an approach to measuring sensation and perception called psychophysics: methods that measure the strength of a stimulus and the observer’s sensitivity to that stimulus. The simplest quantitative measurement in psychophysics is the absolute threshold, the minimal intensity needed to just barely detect a stimulus. A threshold is a boundary. As a way of measuring this difference threshold, Fechner proposed the just noticeable difference, or JND, the minimal change in a stimulus that can just barely be detected. The JND is not a fixed quantity; rather, it depends on how intense the stimuli being measured are and on the particular sense being measured. Consider measuring the JND for a bright light.An observer in a dark room is shown a light of fixed intensity, called the standard (S), next to a comparison light that is slightly brighter or dimmer than the standard. When S is very dim, observers can see even a very small difference in brightness between the two lights: The JND is small. But if S is bright, a much larger increment is needed to detect the difference: The JND is larger. Fechner applied Weber’s insight directly to psychophysics, resulting in a formal relationship called Weber’s law, which states that the just noticeable difference of a stimulus is a constant proportion despite variations in intensity. When calculating a difference threshold, it is the proportion between stimuli that is important; the measured size of the difference, whether in brightness, loudness, or weight, is irrelevant. An approach to psychophysics called signal detection theory holds that the response to a stimulus depends both on a person’s sensitivity to the stimulus in the presence of noise and on a person’s decision criterion. Observers in a signal detection experiment must decide whether they saw the light or not. If the light is presented and the observer correctly responds, “Yes,” the outcome is a hit. If the light is presented and the observer says, “No,” the result is a miss. However, if the light is not presented and the observer nonetheless says it was, a false alarm has occurred. Finally, if the light is not presented and the observer responds, “No,” a correct rejection has occurred: The observer accurately detected the absence of the stimulus. Signal detection theory proposes a way to measure perceptual sensitivity—how effectively the perceptual system represents sensory events—separately from the observer’s decision-making strategy. These are all examples of sensory adaptation, the observation that sensitivity to prolonged stimulation tends to decline over time as an organism adapts to current conditions. Imagine that while you are studying in a quiet room, your neighbor in the apartment next door turns on the stereo. That gets your attention, but after a few minutes the sounds fade from your awareness as you continue your studies. But remember that our perceptual systems emphasize change in responding to sensory events: When the music stops, you notice. 20/20 refers to a measurement associated with a Snellen chart, named after Hermann Snellen (1834–1908), the Dutch ophthalmologist who developed it as a means of assessing visual acuity, the ability to see fine detail; it is the smallest line of letters that a typical person can read from a distance of 20 feet. - The length of a light wave determines its hue, or what humans perceive as color. - The intensity or amplitude of a light wave—how high the peaks are— determines what we perceive as the brightness of light. - Purity is the number of distinct wavelengths that make up the light. Purity corresponds to what humans perceive as saturation, or the richness of colors. Eyes have evolved as specialized organs to detect light. Light that reaches the eyes passes first through a clear, smooth outer tissue called the cornea, which bends the light wave and sends it through the pupil, a hole in the colored part of the eye. This colored part is the iris, which is a translucent, doughnut-shaped muscle that controls the size of the pupil and hence the amount of light that can enter the eye. Immediately behind the iris, muscles inside the eye control the shape of the lens to bend the light again and focus it onto the retina, light-sensitive tissue lining the back of the eyeball. The muscles change the shape of the lens to focus objects at different distances, making the lens flatter for objects that are far away or rounder for nearby objects. This is called accommodation, the process by which the eye maintains a clear image on the retina. If your eyeballs are a little too long or a little too short, the lens will not focus images properly on the retina. If the eyeball is too long, images are focused in front of the retina, leading to nearsightedness (myopia). If the eyeball is too short, images are focused behind the retina, and the result is farsightedness (hyperopia). How does a wavelength of light become a meaningful image? The retina is the interface between the world of light outside the body and the world of vision inside the central nervous system. Two types of photoreceptor cells in the retina contain light-sensitive pigments that transduce light into neural impulses. Cones detect color, operate under normal daylight conditions, and allow us to focus on fine detail. Rods become active under low-light conditions for night vision. Rods and cones differ in several other ways as well, most notably in their numbers.About 120 million rods are distributed more or less evenly around each retina except in the very center, the fovea, an area of the retina where vision is the clearest and there are no rods at all. The absence of rods in the fovea decreases the sharpness of vision in reduced light, but it can be overcome. The retina is thick with cells. The photoreceptor cells (rods and cones) form the innermost layer. The middle layer contains bipolar cells, which collect neural signals from the rods and cones and transmit them to the outermost layer of the retina, where neurons called retinal ganglion cells (RGCs) organize the signals and send them to the brain. The bundled RGC axons—about 1.5 million per eye—form the optic nerve, which leaves the eye through a hole in the retina. Because it contains neither rods nor cones and therefore has no mechanism to sense light, this hole in the retina creates a blind spot, which is a location in the visual field that produces no sensation on the retina. Each axon in the optic nerve originates in an individual retinal ganglion cell (RGC). Most RGCs respond to input not from a single retinal cone or rod but from an entire patch of adjacent photoreceptors lying side by side, or laterally, in the retina.Aparticular RGC will respond to light falling anywhere within that small patch, which is called its receptive field, the region of the sensory surface that, when stimulated, causes a change in the firing rate of that neuron. Agiven RGC responds to a spot of light projected anywhere within a small, roughly circular patch of retina (Kuffler, 1953). Most receptive fields contain either a central excitatory zone surrounded by a doughnut-shaped inhibitory zone, which is called an on-center cell, or a central inhibitory zone surrounded by an excitatory zone, which is called an off-center cell. Light striking the retina causes a specific pattern of response in the three cone types. One type responds best to short-wavelength (bluish) light, the second type to medium-wavelength (greenish) light, and the third type to long-wavelength (reddish) light. Researchers refer to them as S-cones, M- cones, and L-cones, respectively. This trichromatic color representation means that the pattern of responding across the three types of cones provides a unique code for each color. Researchers can “read out” the wavelength of the light entering the eye by working backward from the relative firing rates of the three types of cones.A genetic disorder in which one of the cone types is missing—and, in some very rare cases, two or all three—causes a color deficiency. This trait is sex-linked, affecting men much more often than women. Colour deficiency is often referred to as color blindness, but in fact, people missing only one type of cone can still distinguish many colors, just not as many as someone who has the full complement of three cone types. Staring too long at one color fatigues the cones that respond to that color, producing a form of sensory adaptation that results in a color afterimage. The explanation stems from the colour-opponent system, where pairs of visual neurons work in opposition: red-sensitive cells against green-sensitive and blue-sensitive cells against yellow- sensitive. Streams of action potentials containing information encoded by the retina travel to the brain along the optic nerve. Half of the axons in the optic nerve that leave each eye come from retinal ganglion cells that code information in the right visual field, whereas the other half code information in the left visual field. These two nerve bundles link to the left and right hemispheres of the brain, respectively. The optic nerve travels from each eye to the lateral geniculate nucleus (LGN), located in the thalamus. From there the visual signal travels to the back of the brain, to a location called area V1, the part of the occipital lobe that contains the primary visual cortex. Here the information is systematically mapped into a representation of the visual scene. There are about 30 to 50 brain areas specialized for vision, located mainly in the occipital lobe at the back of the brain and in the temporal lobes on the sides of the brain. Two functionally distinct pathways, or visual streams, project from the occipital cortex to visual areas in other parts of the brain: - The ventral (“below”) stream travels across the occipital lobe into the lower levels of the temporal lobes and includes brain areas that represent an object’s shape and identity—in other words, what it is. The damage caused by Betty’s stroke that you read about in Chapter 3 interrupted this “what pathway” .As a result, Betty could not recognize familiar faces even though she could still see them. - The dorsal (“above”) stream travels up from the occipital lobe to the parietal lobes (including some of the middle and upper levels of the temporal lobes), connecting with brain areas that identify the location and motion of an object—in other words, where it is. Because the dorsal stream allows us to perceive spatial relations, researchers originally dubbed it the “where pathway”. Neuroscientists later argued that because the dorsal stream is crucial for guiding movements, such as aiming, reaching, or tracking with the eyes, the “where pathway” should more appropriately be called the “how pathway”. Alarge region of the lateral occipital cortex was destroyed, an area in the ventral stream that is very active when people recognize objects. D. F.’s ability to recognize objects by sight was greatly impaired, although her ability to recognize objects by touch was normal. This suggests that the visual representation of objects, and not D. F.’s memory for objects, was damaged. Like Betty’s inability to recognize familiar faces, D. F.’s brain damage belongs to a category called visual-form agnosia, the inability to recognize objects by sight. Other patients with brain damage to the parietal section of the dorsal stream have difficulty using vision to guide their reaching and grasping movements, a condition termed optic ataxia. These questions refer to what researchers call the binding problem in perception, which concerns how features are linked together so that we see unified objects in our visual world rather than free-floating or mis-combined features. In everyday life, we correctly combine features into unified objects so automatically and effortlessly that it may be difficult to appreciate that binding is ever a problem at all. However, researchers have discovered errors in binding that reveal important clues about how the process works. One such error is known as an illusory conjunction, a perceptual mistake where features from multiple objects are incorrectly combined. Why do illusory conjunctions occur? Treisman and her colleagues have tried to explain them by proposing a feature integration theory, which holds that focused attention is not required to detect the individual features that comprise a stimulus, such as the color, shape, size, and location of letters, but is required to bind those individual features together. From this perspective, attention provides the “glue” necessary to bind features together, and illusory conjunctions occur when it is difficult for participants to pay full attention to the features that need to be glued together. How do feature detectors help the visual system get from a spatial array of light hitting the eye to the accurate perception of an object in different circumstances, such as your friend’s face? Some researchers argue for a modular view: that specialized brain areas, or modules, detect and represent faces or houses or even body parts. This view suggests we not only have feature detectors to aid in visual perception but also “face detectors,” “building detectors,” and possibly other types of neurons specialized for particular types of object perception. Psychologists and researchers who argue for a more distributed representation of object categories challenge the modular view. Researchers have shown that although a subregion in the temporal lobes does respond more to faces than to any other category, parts of the brain outside this area may also be involved in face recognition. In this view, it is the pattern of activity across multiple brain regions that identifies any viewed object, including faces. Taken together, these experiments demonstrate the principle of perceptual constancy: Even as aspects of sensory signals change, perception remains consistent. Before object recognition can even kick in, the visual system must perform another important task: to group the image regions that belong together into a representation of an object. The idea that we tend to perceive a unified, whole object rather than a collection of separate parts is the foundation of Gestalt psychology. Gestalt principles characterize many aspects of human perception.Among the foremost are the Gestalt perceptual grouping rules, which govern how the features and regions of things fit together. Here’s a sampling: -Simplicity: Abasic rule in science is that the simplest explanation is usually the best. This is the idea behind the Gestalt grouping rule of Pragnanz, which translates as “good form.” When confronted with two or more possible interpretations of an object’s shape, the visual system tends to select the simplest or most likely interpretation. - Closure: We tend to fill in missing elements of a visual scene, allowing us to perceive edges that are separated by gaps as belonging to complete objects. - Continuity: Edges or contours that have the same orientation have what the Gestaltists called “good continuation,” and we tend to group them together perceptually - Similarity: Regions that are similar in color, lightness, shape, or texture are perceived as belonging to the same object. - Proximity: Objects that are close together tend to be grouped together. - Common fate: Elements of a visual image that move together are perceived as parts of a single moving object Researchers have proposed two broad explanations of object recognition, one based on the object as a whole and the other on its parts. Each set of theories has strengths and weaknesses, making object recognition an active area of study in psychology. -According to image-based object recognition theories, an object you have seen before is stored in memory as a template, a mental representation that can be directly compared to a viewed shape in the retinal image. Shape templates are stored along with name, category, and other associations to that object. Your memory compares its templates to the current retinal image and selects the template that most closely matches the current image. - Parts-based object recognition theories propose instead that the brain deconstructs viewed objects into a collection of parts. One important parts-based theory cont
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