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Chapter 6

Chapter 6- Sensation and Perception.docx

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Lawrence Murphy

Sensation and Perception  Sensation: the detection of physical energy emitted or reflected by physical objects o Occurs when energy in the external environment or the body stimulates receptors in the sense organs  Perception: process by which the brain organizes and interprets sensory information Separate Sensations?  Sense receptors: specialized cells that convert physical energy in the environment or the body to electrical energy that can be transmitted as nerve impulses to the brain o Dendrites of sensory neurons responsible for smell, pressure, pain, & temperature o Specialized cells for vision, hearing, taste  Doctrine of specific nerve energies (Müller): principle that different sensory modalities exist because signals received by the sense organs stimulate different nerve pathways leading to different areas of the brain o If possible, allows for sensory substitution o Sensory crossover also occurs in synesthesia where stimulation of one sense consistently evokes a sensation in another  Sensation begins with the sense receptors, which convert the energy of a stimulus into electrical impulses that travel along nerves to the brain. Separate sensations can be accounted for by anatomical codes (as set forth by the doctrine of specific nerve energies) and functional codes in the nervous system. In the sensory substitution, sensory crossover from one modality to another occurs and, in synesthesia, sensation in one modality evokes a sensation in another modality, but these experiences are rare  Psychologists specializing in psychophysics have studied sensory sensitivity by measuring absolute and difference thresholds. Signal-detection theory, however, holds that responses in a detection task consist of both a sensory process and a decision process and will vary with the person’s motivation, alertness, and expectations  Our senses are designed to respond to change and contrast in the environment. When stimulation is unchanging, sensory adaptation occurs. Too little stimulation can cause sensory deprivation, and too much stimulation can cause sensory overload. Selective attention provides overload and allows us to focus on what is important, but it also deprives us of sensory information we may need, as in inattentional blindness Measuring the Senses  Psychophysics: field concerned with how the physical properties of stimuli are related to our psychological experience of them o Commonly relies on measuring absolute threshold, difference threshold, and applying signal-detection theory  Absolute threshold: the smallest quantity of physical energy that can be reliably detected by an observer (50% of the time) o Senses are sharp, but only tuned into narrow band of physical energies  Difference threshold: the smallest difference in stimulation that can reliably be detected by an observer when two stimuli are compared o Also called just noticeable difference (JND) o Weber’s law: size of JND proportional to size of initial stimulus o JND for weight is 1/30  Signal detection theory: divides the detection of sensory signals into a sensory process and a decision process o False alarm is type 1 error (false positive) o Miss is type 2 error (false negative) Sensory Adaptation & Deprivation  Sensory adaptation: reduction or disappearance of sensory responsiveness when stimulation is unchanging or repetitious o Useful as it spares us from responding to unimportant information  Sensory deprivation: the absence of normal levels of sensory stimulation o Varied responses somewhat dependant on expectations & interpretations (eg. Hallucinations) Sensing Without Perceiving  Selective attention: focusing of attention on selected aspects of the environment and blocking out the others  Inattentional blindness: failure to consciously perceive something you are looking at because you are not attending to it Vision: the Stimulus  Light= electromagnetic radiation 1. Amplitude: perception of brightness 2. Wavelength: perception of colour 3. Purity: mix of wavelengths  Perception of saturation, or richness of colours  Light stimuli (waves) have physical characteristics that affect three psychological dimensions of our visual world 1. Hue: Dimension of visual experience specified by colour names  Related to the wavelength of light 2. Brightness: dimension of visual experience related to the amount of light emitted from or reflected by an object  Related to amplitude of wavelength 3. Saturation: dimension of visual experience related to the complexity of light waves  Vividness or purity of colour An Eye on the World  Cornea: protects eye and bends light toward lens  Lens: focuses on objects by changing shape  Iris: controls amount of light that gets into eye  Pupil: widens or dilates to let in more light Visual Receptors  Retina: neural issue lining the back of the eyeball’s interior, which contains the receptors for vision o Rods: visual receptors that respond to dim light  120-125 million, concentrated in the periphery of the retina  Highly sensitive to light (used for night vision) o Cones: visual receptors involved in colour vision  7-8 million, concentrated in the centre of the retina (fovea)  Low sensitivity (used for day vision)  Sensitive to colour Differences between Rods and Cones Rods Cones How many? 120-125 million 7-8 million Where most concentrated? Periphery of retina Centre (fovea) of retina How sensitive? High sensitivity Low sensitivity Sensitive to colour? No Yes The Retina  We experience chemical changes in rods & cones when our eyes adjust fully to dim illumination (called dark adaptation)  Retinal processing also involves ganglion cells o Neurons in the retina that gather information from receptor cells (by way of intermediary bipolar cells) o Axons form the optic nerve which leaves the eye at the optic disk (location of blind spot) Vision is Not like a Camera  Visual processing is an active process & involves many types of cells in different brain regions o Cortical cells respond to lines of specific orientations, others respond to properties of shapes & arrangements (eg. Spirals, faces, greebles)  Feature detector cells: cells in the visual cortex that are sensitive to specific features of the environment Constructing the Visual World  We rely on various Gestalt principles to organize visual input  Figure: item of interest that stands out from the rest of the environment  Ground: environment or background Depth and Distance Perception  Visual system relies on two types of cues to judge where an object is, and how far away from us it is o Binocular cues – used for objects that are fairly close to us o Monocular cues – used when objects are far away Binocular Cues  Binocular cues provide visual cues to depth or distance requiring two eyes o Convergence: the turning inward of the eyes, which occurs when they focus on a nearby object o Retinal disparity: the slight difference in lateral separation between two objects as seen by the left eye and the right eye Monocular Cues  Monocular cues are visual cues to depth or distance that can be used by one eye alone o Light and shadow, Interposition, Motion parallax, Linear perspective, Relative size, Relative clarity, and Texture gradients o (See pages 205 – 207 for descriptions and images)  Perception of distance also affected by emotional factors Visual Constancies  Another important perceptual skill is perceptual constancy o The accurate perception of objects as stable or unchanged despite changes in the sensory patterns they produce o Best studied are shape, location, size brightness, and colour constancies How we See Colour  Trichromatic theory: proposes three basic types of cones, each sensitive to a certain range of wavelengths (red, blue, green) o Interaction of activity in three cones assumed to produce all the different experiences of hue o Colour blindness: typically a genetic variation causing the absence of dysfunction of one or more of the cones  Most are “colour deficient” meaning the person is unable to distinguish red and green (see everything as blue, yellow, brown, and grey)  Occurs in 8% of white males and is rare in women  Opponent-process theory: assumes that the visual system treats pairs of colours as opposing or antagonistic o Occurs in the ganglion cells, and neurons in thalamus & visual cortex o Three opponent pairs: red/green; blue/yellow; black/white  Cells that are inhibited by one colours, produce burst of firing when opponent colour presented o Negative Afterimage Visual Illusions  Our systems are sometimes fooled when making sense of the world  Perceptual illusions give us information about perceptual strategies used by brain, and how misleading messages are interpreted  Many classic visual illusions  Our systems are sometimes fooled when making sense of the world  Perceptual illusions give us information about perceptual strategies used by brain, and how misleading messages are interpreted  Muller-Lyer images- two lines, corners of rooms, same size  Vision is affected by the wavelength, frequency, and complexity of light, which produce the psychological dimensions of visual experience-hue, brightness, and saturation- the visual receptors, rods and cones are located in the retina of the eye.  Specific aspects of the visual world, such as lines at various orientations, are detected by feature-detector cells in the visual areas on the brain. Some of these cells respond maximally to complex patterns. A heated debate is now going on about the existence of specialized “face modules” in the brain. In general, however, the brain takes on fragmentary information about lines, angles, shapes, motion, brightness, texture, and other features of what we see and comes up with a unified view of the world  The trichromatic and opponent-process theories of colour vision apply to different stages of processing. In the first stage, three types of cones in the retina respond selectively to different wavelengths of light. In the second, opponent-process cells in the retina and the thalamus respond in opposite fashion to short and long wavelengths of light  Perception involves the active construction of a model of the world from movement to movement. The Gestalt principles (ex. Figure and ground, proximity, closure, similarity, and continuity) describe visual strategies used by the brain to perceive forms  We localize objects in visual space by using both binocular and monocular cues to depth. Binocular cues include, among others, interposition and linear perspective. Perceptual constancies allow us to perceive objects as stable despite changes in the sensory patterns they produce. Perceptual illusions occur when sensory cues are misleading or when we misinterpret cues What is your absolute threshold? • a. the point at which you detect a stimulus about half of the time • b. the point at which you detect any portion of a stimulus • c. the point at which you detect any stimulus set point • d. the point at which you detect a stimulus that registers on sensory memory Christina was skiing down an intermediate run when the run broke into two separate trails. One trail turned off at a 90-degree angle; the second trail appeared to continue in the same general direction she had been headed. If Christina takes the second trail her actions would be consistent with which Gestalt principle? • a. continuity • b. closure • c. proximity • d. common region Hearing  Audition: our sense of hearing  Three physical characteristics of sound waves that alter psychological experience of sound 1. Loudness: intensity/amplitude of pressure wave (dB) 2. Pitch: frequency of pressure waves (Hz); height or depth of tone 3. Timbre: complexity of the pressure wave; distinguishing quality of sound And Ear on the World  Cochlea: snail-shaped, fluid-filled organ in the inner ear, containing the structure where the receptors for hearing are located  Organ of Corti: structure in the cochlea containing the hair cells (cilia) that are the auditory receptors  Basilar membrane: rubbery membrane that stretches across the interior of the cochlea in which the hair cells are embedded Structures of the Ear  Eardrum: over shaped membrane  Hammer, anvil and stirrup: three bones that intensify the force of the vibration Constructing the Auditory World  Patterns of sound also organized to consult meaningful patterns  Gestalt principles can also relate to sound perception o E.g. Canadian researcher Bregman & “auditory senses”  Sound localization: relies on loudness and intensity of stimuli to tell us where a sound is coming from  Hearing (audition) is affected by the intensity, frequency, and complexity of pressure waves in the air or other transmitting substance, corresponding to the experience of loudness, pitch, and timbre of the sound. The receptors for hearing are hair cells (cilia) embedded in the basilar membrane, in the interior of the cochlea. These receptors pass signals along the auditory nerve. The sounds we hear are determined by patterns of
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