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Chapter 4

Chapter 4- Sensation + Perception.docx

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PSYC 1010
Gerald Goldberg

Ch apter 4 Sensation- stimulation of sense organs - Involves absorption of energy (ex: light, sound waves) by sensory organs (ears, eyes) Perception- selection, organization, interpretation of sensory input - Involves organizing + translating sensory input into something meaningful (ex: faces, stimuli) Psychophysics- study of how physical stimuli are translated into psychological experience Stimulus- detectable input from environment Threshold- divdiving point between energy levels that do + don’t have a detectable effect Absolute threshold - Specific type of sensory input (minimal amount of stimulation that an organism can detect) - Defines boundaries of an organism’s sensory capabilities Just noticeable difference (JND)- smallest differences in amount of stimulation that a specific sense can detect - Cousins of absolute threshold Weber’s Law- states that size of JND is a constant proportion of the size of the initial stimulus - Applies to all senses - Constant proportion (weber fraction)- different fraction apply to different types of sensory input Fechner’s Law - Magnitude of 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 perceived magnitude of sensation - Sensory experience= all relative (never absolute) Signal detection theory - Detection of stimuli involves decision process as well as sensory processes, which are both influenced by a variety of factors besides stimulus intensity - Detectability is measured in terms of probability + depends on decision making processes as well as sensory processes - Ex: more noise in a system, the harder to detect a weak signal Subliminial perception- registration of sensory input w/o conscious awareness - Below threshold= perception w/o awareness can take place - Generally produces weak effects (only detectable in lab conditions w/ undivided attention) - Ex: money, sex, religion, rock music Sensory adaptation- gradual decline in sensitivity due to prolonged stimulation - Automatic, built-in process that keeps ppl tuned into the changes rather than the constants in their sensory input - No one to one correspondence between sensory input + sensory experience - Sensory adaption is probably a behavioral adaption Light- form of electromagnetic radiation that travels as wave at speed of light - Varies in amplitude (height)- perception of brightness - Varies in wavelength (distance between peaks)- perception of colour - Varies in purity (how varied the mix is)- perception of saturation, richness of colour - Incoming visual input must be converted into neural impulses that’re sent to the brain Purpose of eyes - Channels light to the neural tissue that receives it (retina) and they house that tissue - Eye- living optical instrument that creates an image of visual world on light sensitive retina lining its inside back surface - Light enters thru cornea (transparent window) - Crystalline Lens (behind cornea) and cornea forms upside-down image of objects on retina Lens- transparent eye structure that focuses the light rays falling on the retina - Made of soft tissue (capable of adjustments called accommodations) - Accommodations occurs when curvature of lens adjusts to alter visual focus - Closer objects- lens gets fatter (distant object- lens flatten) - Nearsightedness- focus of light from distant objects falls a little short of retina - Farsightedness- focus of light from close objects falls behind the retina Iris- coloured ring of muscle surrounding the pupil (black centre of eye) Pupil- opening in centre of the iris that helps regulate the amount of light passing into the rear chamber of eye Saccades- eye movements (tiny movements= good vision)- small reduction- vision degrades - One form of saccade may give away your covert gaze even when you’re looking somewhere else Retina- neural tissue lining the inside back surface of the eye - Absorbs lights, processes images, sends visual information to brain - Piece of CNS (brain’s envoy in eye) - Complex network of specialized cells arranged in layers - Contains millions of receptor cells that are sensitive to light (located in innermost layer of retina) - 10% of light arriving at retina reaches receptors - Cones- specialized visual receptors that play a key role in daylight vision + colour vision  5 million to 6.4 million  Provide better visual acuity (sharpness, precise detail) than rods  Concentrated in centre of retina + quickly fall off in density towards it periphery  Fovea- tiny spot in centre of retina that contains only cones (visual acuity greatest here) - Rods- specialized visual receptors that play a key role in night vision + peripheral vision  100 million to 125 million  More sensitive in dim light (bright light will dazzle rods)  Handles more peripheral vision b/c they outnumber them in retina  Density- greatest outside of fovea (decreases towards periphery of retina) Optic disk- hole in retina where the optic nerve fibres exits the eye (also called blind spot) Dark adaptation- process in which eyes become more sensitive to light in low illumination - Virtually completes in about 30 minutes (considerable progress occurring in first 10 min) Light adaptation- process whereby the eyes become less sensitive to light in high illumination - Both adaptation are due to chemical changes in rods + cones (also neural changes in receptors + retina) Receptive field of a visual cell (rod and cone receptors signals particular visual cell in retina) - Retinal area that, when stimulated, affects the firing of that cell - Comes in variety of shapes + sizes - In these receptive fields, light falling in the centre has opposite effect of light falling in surrounding area Lateral antagonism- neural activity in a cell opposes activity in surrounding cells - Responsible for opposite effects that occur when light falls on the inner vs. outer portions of centre surrounded receptive field - Allows retina to compare light falling in a specific area against general lighting - Compute relative amount of light (help recognize objects)- by contrast Optic Chiasm - Point at which optic nerves from the inside half of each eye cross over + then project to the opposite half of the brain - After reaching optic chiasm, optic nerve fibres diverge along 2 pathways - Main pathway- thalamus (brain’s major relay station)  90% of axons from retinas synapse in lateral geniculate nucleus (LGN)  Visual signals are processed in LGN + then distributed to areas in occipital lobe (primary visual cortex) - Other pathway  branches to midbrain (superior colliculus) before travelling thru thalamus + occipital lobe  function: coordination of visual input w/other sensory input - Visual input- subdivided into 2 more specialized pathways called Magnocellular + Parvocellular channel - Engages in Parallel processing  (simultaneously extracting different kinds of info from the same input)  Ex: Mag- brightness, Paryo- perception of colour - Individual cells in primary visual cortex (PVC) don’t respond to little spots (more sensitive to lines, edges, other complicated stimuli) - Hubel + Wiesel- identified various types of specialized cells in PVC that responded to different stimuli - Ex: simple cells- responds to line of correct width, angle, position in receptive field - Ex: complex cells- respond to any position in receptive field - Feature detector- neurons that respond selectively to very specific features of more complex stimuli Additional processing occurs after PVC (signals travels thru 2 streams) - Ventral stream- processes the details of what objects are out there - Dorsal stream- processes where the objects are - As signal move farther along in visual processing system, neurons become more specialized + stimuli that activates them become more and more complex - Visual agnosia- inability to recognize objects - due to damage somewhere along visual pathway that handles object recognition - Prosopagnosia- inability to recognize familiar faces - Compensates and develops heightened abilities to recognize voices - Neurons in ventral stream pathway that’re involved in perceiving face can learn from experience McCollough effect - After-image phenomenon that differs from other colour afterimage effects b/c it’s contingent (similar,common) on both colour and pattern/form - Effects mediated by primary visual cortex (V1) Subtractive colour mixing- removing some wavelengths of light, leaving less light than was originally there Additive colour mixing- superimposing lights, putting more light in mixture than exists in anyone light byitself - Human process of colour perception parallel additive colour mixing much more closely than subtractive Trichromatic theory of colour vision - Human eye has 3 types of receptors w/differing sensitivities to different light wavelengths - Red, green, blue - Light of any colour can be matched by additive mixture of three primary colour - Three colours are appropriately spaced out in visible spectrum can serve as primary colours Colour blindness- variety of deficiencies in the ability to distinguish among colours - Dichromats (only two colour channels) Complementary colours- pairs of colours that produce grey tones when mixed together Afterimage- visual image that persists after stimulus is removed - Complement of colour you originally stared at - Trichromatic can’t account for appearance of complementary afterimage Opponent process theory - Colour perception depends on receptors that make antagonistic responses to three pairs of colour - Explains complementary afterimage and needs four names (red, blue, green, yellow) to describe colours - Explains colour blindness - Go to pg 155 for more details Receptors that does the first stage of processing (cones) seem to follow principle outlined in trichromatic theory Later stages, some cells in retina, LGN, visual cortex- follows opponent process
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