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Sensation and Perception.docx

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Joanne Lee

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Sensation and Perception • Sensation is the process through which the senses detect visual, auditory and other sensory stimuli and transmit them to the brain (stimulation of sense organs) - absorption of energy such as sound waves through the ears • Perception is the process by which sensory information is actively organized and interpreted by the brain (selection, organization and interpretation of sensory input) – translating sensory information into something meaningful • Psychophysics is the study of how physical stimuli are translated into psychological experience What can we sense? How much stimulation do we need? • Must reach a threshold before we pay attention • Sensation begins with a stimulus that is any detectable input from the environment • What counts as detectable depends on who you are • A threshold is a dividing point between energy levels that do and do not have a detectable effect Absolute Threshold • The difference between not being able to perceive a stimulus and being able to just barely perceive it -- the smallest amount of stimulation necessary for you to be able to detect • The minimum amount of sensory stimulation that can be detected 50% of the time Difference Threshold • The smallest increase or decrease in a physical stimulus that is required to produce the “just noticeable difference” (JND) in sensation that is detectable 50% of the time o When you notice something is there and the difference between two sensations Just Noticeable Difference • Weber’s Law states that the size of a just noticeable difference is a constant proportion of the size of the initial stimulus (Webers fraction) • Amount of increase needed to make a difference o If you are carrying 2kg, 1 additional gram will not make a difference o If you carry only 20 grams, 1 gram will make a difference • Real life example is in radiology • Focus on the physical stimulus • How strong or weak it is, • How much or how little there is • What about individual variations? • Individuals differ from each other • Individuals can differ depending on context o Fechner’s Law states that the magnitude of a sensory experience is proportional to the number of JND’s that the stimulus causing the experience is above the absolute threshold (perception cant be measured on absolute scales) Signal Detection Theory • Detection of sensory stimulus involves noticing a stimulus from background noise and a decision about whether the stimulus is present • Decision = probability of stimulus occurrence + potential gain or loss with deciding whether the stimulus was present or not • Attempts to account for the influence of decision making processes on stimulus detection • Depends on the criteria • If you have to pick up a cousin that you don’t know from an airport with an old picture Perception without Awareness • Subliminal Perception the registration of sensory input without conscious awareness Sensory Adaption • Process of becoming less sensitive to an unchanging sensory stimulus over time • Automatic process • Gradual decline in sensitivity due to prolonged stimulation • Allows us to shift attention Vision • Precursor – light (a form of electromagnetic radiation that travels as a wave moving naturally enough at the speed of light) • Amplitude affects perception of brightness and the wavelength affects the perception of the colour • Most objects do not emit light they reflect it • We can’t see any object unless light is reflected from it or given off by it Blind spot is where the optic nerve is • Sensation gives us the visual cues • Perception fills in the blanks and identifies what you are sensing • Vision: The Eye • Channel light to the neural tissue the receives it (retina) and they house that tissue • Living optic instrument that creates an image of the visual world on the light-sensitive retina lining its inside back surface - Cornea - light enters through the transparent film of the cornea  Lens (the transparent eye structure that focuses the light rays falling on the retina)  Made up of tissue that is capable of accommodation (when the curvature of the lens adjusts to alter visual focus)  Lens flattens when you focus on distant objects and bulges when on near  Crystalline lens and cornea form an upside down image on the retina o Nearsightedness is when you cant see distant objects because the lens bulges too much(falls short of the retina) o Farsightedness is when you cant see close objects because the eye is too short (falls behind the retina)  Retina is at the back of your eyeball (receives the light sensory information and converts it to neuro activity)  Projected upside down on your retina Pupil • the opening in the centre of the iris that helps to regulate the amount of light passing into the rear chamber of the eye • when it constricts less light gets in but the image is sharper (bright light) • dialate it is open but the image is less sharp (dark) • Saccades are constant eye movements that make brief fixations at various parts of the stimuli o Essential to good vision Retina (the neural tissue lining the inside back surface of the eye; it absorbs light, processes images and sends visual information to the brian) • It is a piece of the central nervous system • Paperthin sheet of neural tissue • Optic disk is a hole in theh retina where the optic nerve fibres exit the eye (your blind spot because you cannot see the images that fall on the whole) - Contains sensory receptors for vision • Rods – specialized visual receptors that play a key role in the night vision and peripheral vision o Black and white and grey o More sensitive to dim light o Dim light • Cones - specialized visual receptors that play a key role in daylight vision and colour vision o Colour o No dim light o Provide good visual acuity (sharpness and precise detail) o More heavily in the center of the retina and quickly fall off in density towards the periphery • Fovea o Fine detail • Optic Nerve Vision – Colour • Perceive small part of electromagnetic spectrum • Light waves measured in nanometers (billionths of meter) o From 700 nm to 400 nm Dark Adaptation is the process in which the eyes become more sensitive to light in low illumination (declining absolute thresholds – takes about 30 mins) Light Adaptation is the process whereby eyes become less sensitive to light in high illumination (coming out of a movie theatre and it is bright) Due to chemical changes in rods and cones Information Processing • Retina transforms light into a different representation of the scene • Rods and cones trigger neural signals that pass into the netwok of cells in the retina • Sends impulses along the optic nerve (made of axons that connect to the brian) • Depart through the optic disk and carry neural impulses to the brain • Receptive Field of a visual cell is the retinal area that, when stimulated, affects the firing of that cell o Circular field arrangement is most common (light falls in the center has the opposite effect of the light in the surrounding area) • Lateral Antagonism occurs when neural activity in a cell opposes activitiy in surrounding cells o Allows the retina to compare the light falling in a specific area against general lighting o Can compute the relative amount of light at a point instead of the absolute level of light • Visual information is meaningless until it is processed in the brain • Optic Chiasm is the point t which the optic nerves from the inside half of each eye cross over and then project to the opposite half of the brain o Ensures that signals from both eyes go to both hemispheres on the brain o Axons from the left carry singles from the left and vice versa o Main pathway into the brain is into the thalamus o 90% of the axons synapse in the lateral geniculate nucleus (where sigals are processed and then distributed to areas in the occipital lobe that make up the primary visual cotex o Two main pathways are divided and engage in parallel processing which involves simultaneously extracting different kinds of information from the same input • Feature detectors are neurons that respond selectively to very specific features of more complex stimuli Visual Agnosia is an inability to recognize familiar objects Three Dimensions of Colour Vision • Hue is the specific colour perceived, wavelengths • Brightness refers to the intensity of the light energy that is perceived, amplitude of wave • Saturation refers to the purity of the colour • Perceived colour is primarily a function of the dominant wavelength in the mixtures • A Colour Solid demonstrates that people can perceive many different colours o Subtractive colour mixing works by removing some wavelengths of light, leaving less light than was originally there o Additive colour mixing works by superimposing lights, putting more light in the mixture than in an one light by itself • Trichromatic theory (Young-Helmholtz) o Three types of color receptors in retina with different sensitivities to different light wavelengths o Cones most sensitive to blue, green, red wavelengths o Visual system combines activity from these cells o Colours are perceived by additive mixture of impulses o If all are equally activated -
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