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Chapter 4

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Carolyn Ensley

PS101 Chapter 4 – Sensation and Perception Week 6 Psychophysics: Basic Concepts and Issues -Psychophysics is the study of how physical stimuli are translated into psychological experience Thresholds: Looking for Limits -Sensation begins with a stimulus, any detectable input from the environment -A threshold is a dividing point between energy levels that do and do not have a detectable effect -An absolute threshold for a specific type of sensory input is the minimum amount of simulation that an organism can detect -Absolute thresholds define the boundaries of an organism’s sensory capabilities Examples: Sense Absolute Threshold Vision A candle flame seen at 50 km on a dark clear night Hearing The tick of a watch under quiet conditions at 6m Taste 5mm of sugar in 7.5 L of water Smell One drop of perfume diffused into the entire volume of a six- room apartment Touch The wing of a fly falling on your cheek from a distance of 1 cm Weighing the Differences: The JND -A just noticeable difference(JND) is the smallest difference in the amount of stimulation that a specific sense can detect -Close cousins of absolute thresholds -Weber’s law states that the size of a just noticeable difference is a constant proportion of the size of the initial stimulus -As stimuli increase in magnitude, the JND becomes larger Psychophysical Scaling -If one light has twice the energy of another, do you necessarily perceive it as being twice as bright? When you are asked to make this kind of judgement, you are being asked to scale the magnitude of sensory experiences -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 -Constant increments in stimulus intensity produce smaller and smaller increases in the perceived magnitude of sensation -Perceptions can’t be measured on absolute scales Signal-Detection Theory -Signal-detection theory proposed 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 -Four possible outcomes: hits – detecting signals when they are present, misses – failing to detect signals when they are present, false alarms – detecting signals when they are not present, correct rejections – not detecting signals when they are absent -Signal detection attempts to account for the influence of decision-making processes on stimulus detection -According to signal-detection theory, your performance will also depend on the level of “noise” in the system -The more noise in your system, the harder it will be for you to pick up a weak signal PS101 Chapter 4 – Sensation and Perception Week 6 -Detectability is measured in terms of probability and depends on decision-making processes as well as sensory processes Perception without Awareness -Subliminal perception – the registration of sensory input without conscious awareness -Subliminal also means threshold -Subliminal perception has become tied up in highly charged controversies relating to money, sex, religion, and rock music -Includes ‘the messages were superimposed on only a few frames of the film, so that they flashes by quickly and imperceptibly -Subliminal self-help tapes intended to facilitate weight loss, sleep, memory, self-esteem, and the like have become a multimillion-dollar industry -Unconscious semantic priming, subliminal affective conditioning, subliminal mere exposure effects, subliminal visual priming, and subliminal psychodynamic activation have found evidence that perception without awareness can take place -Subliminal inputs can produce measurable, although small, effects in subjects who subsequently report that they did not consciously register the stimuli -Subliminal stimulation generally produces weak effects – these effects can be detected only by very precise measurement under carefully controlled laboratory conditions in which subjects are asked to focus their undivided attention on visual or auditory materials that contain the subliminal stimuli Sensory Adaptation -Sensory adaptation is a gradual decline in sensitivity due to prolonged stimulation -Ex. You find that the garbage in our kitchen has started to smell. If you stay in the kitchen without removing the garbage, the stench will soon start to fade – our sensitivity begins to fade -It is an automatic, built-in process that keeps people tuned in to the changes rather than the constants in their sensory input -Sensory adaptation probably is a behavioural adaptation that has been sculpted by natural selection -There is no one-to-one correspondence between sensory input and sensory experience Our Sense of Sight: The Visual System The Stimulus: Light -For people to see, there must be light -Light is a form of electromagnetic radiation that travels as a wave, moving, naturally enough, at the speed of light -Light waves vary in amplitude and in wavelength -Amplitude affects the perception of brightness and wavelength affects the perception of colour -Most objects do not emit light; they reflect it -Vision is a filter that permits people to sense but a fraction of the real world The Eye: A Living Optical Instrument -The eye serves two main purposes: they channel light to the neural tissue that receives it, called the retina, and they house that tissue -Each eye is a living optical instrument that creates an image of the visual world on the light-sensitive retina lining in its back surface PS101 Chapter 4 – Sensation and Perception Week 6 -The lens is the transparent eye structure that focuses the light rays falling on the retina -The lens is made up of relatively soft tissue, capable of adjustments that facilitate a process called accommodation -In nearsightedness, close objects are seen clearly but distant objects appear blurry -In farsightedness, distant objects are seen clearly but close objects appear blurry -The pupil is the opening in the centre of the iris that helps regulate the amount of light passing into the rear chamber of the eye -The eye itself is constantly in motion, moving in ways that are typically imperceptible to us -These eye movements are referred to as saccades The Retina: The Brain’s Envoy in the Eye -The retina is the neural tissue lining the inside back surface of the eye it absorbs light, processes images, and sends visual information to the brain -The retina processes images -The axons that run from the retina to the brain converge at the optic disk, a hole in the retina where the optic nerve fibres exit your eye Visual Receptors: Rods and Cones -The retina contains millions of receptor cells that are sensitive to light - it contains two types of receptors: rods and cones -Cones are specialized visual receptors that play a key role in daylight vision and colour vision -The fovea is a tiny spot in the centre of the retina that contains only cones; visual acuity is greatest at this spot -When you want to see something sharply, you usually move your eyes to centre the object in the fovea -Rods are specialized visual receptors that play a key role in night vision and peripheral vision -Rods are sensitive and that is why they handle night vision Dark and Light Adaptation -Ex. When you enter a dark theatre on a bright day, you stumble round almost blindly -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 -Light adaptation – the process whereby the eyes become less sensitive to light in high illumination -Both adaptations are due to chemical changes in the rods and cones Information Processing in the Retina -The receptive ield of a visual cell is the retinal area that, when stimulated affects the firing of that cell -When receptive fields are stimulated, retinal cells send signals both toward the brain and laterally toward nearby visual cells -Lateral antagonism occurs when neural activity in a cell opposed activity in surrounding cells -Lateral antagonism allows the retina to compare the light falling in a specific area against general lighting PS101 Chapter 4 – Sensation and Perception Week 6 Vision and the Brain Visual Pathways to the Brain -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 -After reaching the optic chiasm, the optic nerve fibres diverge along two pathways. The main pathway projects into the thalamus. The second goes to the midbrain -The main visual pathway is subdivided into two more specialized pathways called the magnocellular and parvocellular channels – these channels engage in parallel processing, which involves simultaneously extracting different kinds of information from the same input Information Processing in the Visual Cortex -Most visual input eventually arrives in the primary visual cortex, locate
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