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Chapter 4

Chapter 4 Summary

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Dax Urbszat

Sensation is the stimulation of sense organs Perception is the selection, organization and interpretation of sensory output Sensation involves the absorption of energy (light or sound waves by sensory organs -> ears and eyes) Perception involves organizing and translating sensory input into something meaningful (ex. Your best friend's face or other environmental stimuli) Psychophysics: Basic Concepts and Issues - the study of how physical stimuli are translated into psychological experience - important dude: Gustav Fechner 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 stimulation that an organism can detect - define the boundaries of an organism's sensory capabilities - as stimulus increases, subjects' probability of responding to stimuli gradually increases - the absolute threshold as the stimulus intensity detected 50 percent of the time 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 - JNDs are close cousins of absolute thresholds - an absolute threshold is simply the just noticeable difference from nothing (no stimulus input) - JNDs vary by sense, and the smallest detectable difference is a fairly stable proportion of the size of the original stimulus - Weber's law states that the size of a just noticeable difference is a constant proportion of the size of the initial stimulus - Weber fraction - Weber fraction for lifting weights -> 1/30 - just be able to detect the difference between a 300-gram weight and a 310-gram weight (the JND for 300 grams is 10 grams) Psychophysical Scaling - 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 - ex. Dark room with a single lamp that has 3 bulbs of the same wattage. Turn on a switch, one bulb lights. Difference is big. Turn on a second bulb.Amount of light is doubled but the room is NOT double bright. Barely notice it. Turn on the last one -> smaller differences in magnitude of sensation 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 - your responses will depend in part on the criterion you set for how sure you must feel before you react - involves higher mental processes rather than raw sensation and depends on your expectations and on the consequences of missing a signal or reporting a false alarm - the more noise in the system, the harder it will be for you to pick up a weak signal - detectability is measured in terms of probability and depends on decision-making processes as well as sensory processes Perception withoutAwareness - subliminal awareness – the registration of sensory input without conscious awareness -> limen is another term for threshold, so subliminal means below threshold - ex. “eat popcorn” subliminal message -> popcorn sales skyrocketed - subliminal stimulation generally produces weak effects SensoryAdaptation - a gradual decline in sensitivity due to prolonged stimulation - ex. Garbage smells in kitchen. If you stay in the kitchen for a long time, smell fades b/c you're used to it - the stimulus intensity of the odour is stable, but with continued exposure, your sensitivity to it decreases - sensory adaptation is an automatic, built-in process that keeps people tuned into the changes rather than the constants in their sensory input - allows people to ignore the obvious and focus on changes in the environment that may signal threats to safety - a behavioural adaptation that has been sculpted by natural selection - no one-to-one correspondence between sensory input and sensory experience Our Sense of Sight: The Visual System The Stimulus: 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 (height) and in wavelength (the distance between peaks) -Amplitude affects mainly the perception of brightness - Wavelength affects mainly the perception of colour - the lights that humans normally see are mixtures of several wavelengths - light can also vary in its purity (how varied the mix is) - purity influences perception of the saturation, or richness, of colours - the visible spectrum is only a slim portion of the total range of wavelengths - vision is a filter that permits people to sense but a fraction of the real world The Eye:ALiving Optical Instrument - the eyes serve 2 main purposes: - they channel light to the neural tissue that receives it -> retina - they house that tissue -> retina - 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 - 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 - accommodation occurs when the curvature of the lens adjusts to alter visual focus - when you focus on a close object, the lens of your eye gets fatter (rounder) to give you a clear image - when you focus on distant objects, the lens flattens out - nearsightedness -> objects are seen clearly but distant objects appear blurry - the focus of light from distant objects falls a little short of the retina - cornea of lens bends the light too much, or when the eyeball is too long - farsightedness -> distant objects are seen clearly but close objects appear blurry - the focus of light from close objects falls behind the retina - the eyeball is too short - the eye can make adjustments to alter the amount of light reaching the retina - the iris is the coloured ring of muscle surrounding the pupil, the black centre of the eye - 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 - when the pupil constricts, it lets less light into the eye but sharpens the image falling on the retina - when the pupil dilates (opens), it lets more light into the eye 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 sharpened sacrificed to allow more light to fall on the retina so that more remains visible - eye is always moving -> eye movements are referred to as saccades - essential to good vision if there is even a small reduction in these voluntary eye movements our vision degrades - we would just focus on things and other things “fade from view” 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 - paper-thin sheet of neural tissue - 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 the eye - you cannot see the part of an image that falls on the optic disc -> blind spot Visual Receptors: Rods and Cones - light must pass through several layers of cells before it gets to the receptors that actually detect it - rods and cones - Cones are specialized visual receptors that play a key role in daylight vision and colour vision - cones handle most of our daytime b/c bright lights dazzle the rods - major role in the perception of colour - better visual acuity – sharpness and precise details, than rods - concentrated most heavily in the centre of the retina and quickly fall off in density toward its periphery - fovea is a tiny spot in the centre of the retina that contains only cones; visual acuity is greatest at this spot - focus on an object -> 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 - handle night vision b/c they are more sensitive than cones to dim light - greatly outnumber cones in the periphery of the retina - density of rods is greatest just outside the fovea and gradually decreases toward periphery of the retina - best to look slightly above or below the place where the object should be -> when you want to see a faintly illuminated object in the dark Dark and Light Adaptation - dark adaptation – the process in which the eyes become more sensitive to light in low illumination - go into a dark movie theatre -> so dark -> but after you're ok :) - virtually complete in about 30 minutes, with considerable progress occurring in the first 10 minutes - cones adapt more rapidly than rods - light adaptation is the process whereby the eyes become less sensitive to light in high illumination - going out from dark theatre – AHHH BURNING LIGHT -> ok now :) - light adaptation improves your visual acuity under the prevailing circumstances Information Processing in the Retina - the retina transforms a pattern of light falling into a very different representation of the visual scene - light striking the retina's receptors (rods and cones) triggers neural signals that pass into the intricate network of cells in the retina, which in turn send impulses along the optic nerve – a collection of axons that connect the eye with the brain - axons, which depart from the eye through the optic disk, carry visual info, encoded as a stream of neural impulses to the brain - the collection of rod and cone receptors that funnel signals to a particular visual cell in the retina (or ultimately in the brain) make up that cell's receptive field - the receptive field of a visual cell is the retinal area that, when stimulated, affects the firing of that cell - receptive fields in the retina come in a variety of shapes and sizes - common -> circular fields with a centre-surround arrangement - light falling in the centre 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 centre of its receptive field and decreased by light in the surrounding area - retina cells send signals both toward the brain and laterally (sideways) toward nearby visual cells - Lateral antagonism (also known as lateral inhibition) is the most basic of these interactive effects - occurs when neural activity in a cell opposes activity in surrounding cells - responsible for the opposite effects that occur when light falls on the inner vs. Outer portions of the centre-surround receptive fields - allows the retina to compare the light falling in a specific area against general lighting - compute the relative amount of light at a point instead of reacting to absolute levels of light Sense of Hearing: TheAuditory System The Stimulus: Sound - sound waves are vibrations of molecules, which means that they must travel through some physical medium, such as air - move at a fraction of the speed of light - sound waves are usually generated by vibrating objects (ex. Guitar string, loudspeaker cone, vocal cords) - sound waves can also be generated by forcing air past a chamber (pipe organ) or releasing a burst of air (clapping) - sound waves are characterized by their amplitude, their wavelength and their purity - the physical properties of amplitude, wavelength and purity affect mainly the perceived (psychological) qualities of loudness, pitch, and timbre, respectively Human Hearing Capacities - wavelengths of sound are described in terms of their frequency, which is measured in cycles per second or hertz (Hz) - higher frequencies are perceived as having higher pitch - humans can hear sounds ranging from a low of 20 Hz up to a high of about 20,000 Hz. - low-frequency sounds under 10 Hz
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