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Chapter 4

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PSYC 100
Samuel Reed

CHAPTER 4: SENSATION AND PERCEPTION - gravity provides cues for people’s judgment concerning the spatial orientation of their bodies Figure: Ian Howard - pioneer in sensation/perception research (binocular vision and human spatial orientation) - used room that tilts sideways and plane that travels in a parabolic manner to experience normal, hypergravity and microgravity environment - human relies on visual, gravity and body direction >astronaut is dependent on visual cues Sensation: Stimulation of the sense organs Perception: selection, organization and interpretation of sensory inputs (into something meaningful) A) Psychopyhsics: Basic Concepts and Issues - study of physical stimulation and how it is translated into psychological experience - Figure: Gustav Fechner Thresholds: Looking for Limits - the dividing point between the energy levels that have do and do not have a detectable of effect - Absolute threshold: Minimum amount of stimulation that an organism can detect > just noticeable from nothing > also defined as: stimulus intensity detected 50% of the time *Under ideal condition, human abilities to detect weak stimuli were greater than previously thought Weighing the Differences: The Just Noticeable Difference (JND) - JND: smallest difference in the amount of stimulation that a specific sense can detect (cousin of absolute threshold) Figure: Ernst Weber - the size of JND is a constant proportion of the size of the initial stimulus > constant proportion: Weber fraction (differs for each sensory input) > e.g: for lifting weight; 1/30.For 300grams object, should be able to detect the different with 310g >> as stimuli increases in magnitude, JND becomes larger Psychophysical Scaling Unit of measurement: JND - Fechner’s Law: 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 Signal-Detection Theory (this theory is describing chaos) - detection of stimuli involves decision processes as well as sensory processes which are both influenced by a variety of factors besides stimulus intensity > your performance depends on the level of noise (the noiser it is, the harder it gets) Significance: this theory replaces Fechner’s sharp threshold with the concept of detectability - measured in terms of probability and depends on decision-making processes as well as sensory processes Perception without Awareness - introduces the Subliminal Perception: registration of sensory inputs without conscious awareness > limen is another word for threshold. Subliminal means below threshold - area of research: unconscious semantic priming, subliminal affective conditioning, subliminal mere exposure effects, subliminal visual priming, subliminal psychodynamic activation << all of these conclude that perception without awareness can take place - subliminal inputs can produce measurable but small effects - subliminal stimuli turn out to be nearly as subliminal as the stimuli themselves - not much practical impotance Sensory Adaptation - gradual decline in sensitivity due to prolonged stimulation - sensory adaptation: built-in process that keeps people tuned in to the changes rather than the constants in their sensory input > ignore the obvious and focus on changes B) Our Sense of Sight: The Visual System B)i) The Stimulus: Light - Light: electromagnetic radiation that travels as a wave - Light varies in: amplitude (height) and wavelength (distance between peaks) - Amplitude: affects perception of brightness - Wavelength: Affects perception of colour > Lights that humans see are mixture of several wavelengths - Vision is a filter that enables human to sense but a small fraction of the world The Eye: A Living Optical Instrument - Purpose of eyes: channel information (light) to the retina and house the neural tissues Flow of movement: 1) Light enters through cornea (transparent window); upside-down image is formed 2) Lens: transparent eye structure that focuses light on retina > made of soft tissues that facilitates a process called accommodation - close object: lens become rounder - distant object: lens flatten - farsightedness: can only see far object, close object becomes blurry > image falls behind retina - nearsightedness: can only see near object, distant object becomes blurry - eyeball is too short - iris: colored ring that surrounds the iris - pupil: the opening at the centre of the eye, helps regulate amount of light that enter the eyes > pupil constricts: less light enter, image becomes sharper > pupil dilates: more light enter, images become blurry - saccades: constant eye movement that scans the environment and making brief fixation at various part if stimuli > even small reduction in this movement, our vision degrades > people can track YOU by knowing your saccades B)ii) The Retina: The Brain’s Envoy in the Eye - neural tissue lining the inside back surface of the eye; absorbs light, processes images, sends visual information to the brain Visual Receptors: Rods and Cones - Retina contains two types of receptors: Rods (elongate) and cones (stubbier) - Rods: big in number, night vision and peripheral vision > more sensitive to dim light > density is greatest outside fovea and decreases towards its periphery - Cones: small in number, daylight vision and color vision > do not respond well to dim light > provide better visual acuity: sharpness and precise detail > concentrated in the centre of retina and decreases towards the periphery - fovea contains only cone and visual acuity is the greatest here > to focus, image is adjusted to the fovea Dark and Light Adaptation Dark Adaptation: eyes become accustomed to the light in low illumination > require less light to see > cones adapt more rapidly than rods Light Adaptation: eyes become more accustomed to light in high illumination *Both of these result from chemical change in rods and cones *cones adapt more rapidly than rods (probably because it is smaller in number) Information Processing in the Retina - axons which depart from the eye through optic disk, carry visual information that is encoded into neural impulses to the brain - complex information processing goes on in the retina - receptive field of a visual cell: the retinal area when stimulated affects the firing of that cell > light in centre of its receptive field: increases the rate of firing of a visual cell > light outside the centre of receptive filed: decreases the rate of firing of a visual cell Effect: - when receptive fields are stimulated, retinal cells send signal towards the brain and laterally - Lateral Antagonism (Lateral inhibitory): neural activity in a cell opposes activity in surrounding cells > allows retina to compare the light falling in the specific area against general lighting > significance: visual system can compute the relative amount of light at a point instead of reacting to absolute levels of light B)iii) Vision and the Brain - visual input is meaningless until it is processed in the brain http://www.youtube.com/watch?v=AoUgq8yy0Mo Visual Pathways to the Brain - optic chiasm: point where optic nerve from inside half of each eye cross over and then project to the opposite half of the brain Rationality: ensure that both hemispheres receive signals The Flow: 1) Axons from the left half of each retina carry signals to the left side of the brain and axons from the right half of each retina carry signals to the right side of the brain 2) After reaching optic chiasm, optic fiber nerves diverge along the pathways which projects to the thalamus > synaption occurs in the lateral geniculate nucleus (LGN); also visual signals are processed here and then distributed to the occipital lobe 3) second visual pathway leaves optic chiasm proceeds to superior colliculus (an area in the midbrain) before travelling through the thalamus and proceeds to the occipital lobe > this 2 pathway is the coordination of visual input with other sensory input MAIN VISUAL PATHWAYS: - magnocellular: processes information regarding brightness - parvocellular: processes perception of color -involved in parallel processing (simultaneous extracting different kinds of information from the same input) Information Processing in the Visual Cortex Figures: David Hubel and Torsten Wiesel - experiment to see what shape turns us on - Result: primary visual cortex don’t respond to dots but more sensitive to lines, edges, more complex stimuli Hubel: - born in Windsor, Ontario - went to McGill Uni - interested in nervous system; influence by Wilder Penfield - discovery: simple cell respond to line with correct width, oriented at the correct angle, located in the correct position in its receptive field while complex cell respond to any position in their receptive fields - Conclusion: feature detectors; neuron that respond selectively to specific features of more complex stimuli - After visual input is processed in the primary visual cortex: 1) routed to other cortical areas for additional processing 2) the signals travel 2 streams: ventral stream (identify the object details) and dorsal stream (process the location of that object- motion and depth) 3) As signal move farther along the visual processing system, neurons become more specialized and fussy with what turns them on and stimuli that activate them become more complex Research Question: Why does the cortex have face detectors? - Probably: adaptive significance over the course of evolution - natural selection may have wired some brains of species to quickly respond to face Identify: face perception exists since from infancy - Visual agnosia: inability to identify object - Visual prosopagnosis: inability to recognize familiar faces (but can recognize voice better) - neurons in the ventral stream pathway that are involved in recognizing faces can learn from experience Multiple Methods in Vision Research (McCollough Effect) - after-image phenomenon that differs from other color after-image effects because it is contingent on both colour and pattern/form Figure: Peter Dodwell – use patient that suffers from visual agnosia Find out more: Mc Collough Effect B)iv) Viewing the World in Color The Stimulus for Color - perceived color is a function of the dominant wavelength - longest wavelength: red, shortest: violet *color is a psycho interpretation and NOT a physical property of light - wavelength: closely related to hue, amplitude to brightness and purity to saturation Two kinds of color mixture: - Subtracting: removing some wavelengths of the light leaving less light than original (object darker) - Additive: imposing some wavelength of the light adding more light to the original *pigment absorbs most wavelength Trichromatst Theory of Color Vindon Figure: 1 - Thomas Young, 2 – Herman von Helmholtz (modified) in 1852 - Theory states that: human eyes have 3 receptors with differing sensitivities to different light wavelengths (red, green and blue) > based on this model: people can perceive all colours of rainbow because the eyes does its own color mixing - Color-blindedness: defect in the ability to distinguish color > most people are dichromats: able to deal with 2 color channels Opponent Process Theory of Colour Vision - Complementary: pairs of colors that produce grey tone when mixed together > after staring at a strong colour and then stare at white background, you’ll see an afterimage > afterimage: a visual image that persists after a stimulus is removed > color of afterimage complements the color originally stared Research Question: If colors are reduced to 3 channels, why are four color names required to describe the full range of possible colors? Figure: Ewald Hering (Opponent Process Theorist) - states that : color perception depends on receptors that make antagonistic responses to three pairs of colors. - red vs green, yellow vs blue, black vs white - also explain some aspects of color-blindedness - grapheme-color synethesia: individuals perceive digit or letter but somehow associate it with colors - e.g: when you see number 7, you think of blue (unintentionally) Reconciling Theories of Color Vision - it takes both theories (trichromatic theory color of vision and opponent process theory color of vision) to explain color vision Figure: George Wald - demonstrates that eyes have 3 types of cones with each being sensitive to different type of band of wavelengths - 3 colors are predicted by trichromatic theory (red, green, blue) Research discovery: Cells in the retina, the LGN and visual cortex respond in the opposite ways to red vs green, blue vs yellow Effects of Color on Behavior Figure: Andrew Elliot - colors can have automatic unconscious effects on behavior (improve or impair) - hypothesis: exposure to the red color has a negative impact on performance B)v) Perceiving Forms, Patterns and Objects - reversible figure: drawings that are compatible with two interpretations that can shift back and forth > same visual input can result in radically different perceptions > people’s experience of the world is subjective - perceptual set: a readiness to accept stimulus in a particular way (perception is readily built) - form perception: depends on what people focus on - inattentional blindness: failure to see fully visible objects or events in a visual display > when asked to focus on something, the tendency to overlook details is high - inattentional blindness attributed to perceptual set whereby people tend to focus their attention on specific details/feature of a scene > the likelihood of inattentional blindness increases when people work on task that require a great deal of attention or that create a heavy perceptual load - same phenomenon: auditory parallel Feature Analysis: Assembling Forms Feature Analysis: process of detecting specific visual inputs and assemble them into a more complex form > e.g: line, edge, corners arranged into triangles, squares etc > assumes that form perception involves bottom-up processing (individual elements to the whole) - perceptions are usually created using top-down processing (from the whole to individual elements) > subject contour phenomenon: perception of contour where none actually exists - Conclusion: bottom-up and top-down perception have their own niches in form perception Looking at the Whole Picture: Gestalt Principles - Gestalt psycho
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