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Chapter 4

Chapter 4 - Sensation and Perception.docx

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Dax Urbszat

Chapter 4 - Sensation and Perception Sensation and Perception  One of the first Canadian experiments carried out was research on the manufacturing mirrors for use in space -carried out by two Ottawa students in 1985 on space shuttle: Atlantis  Sensation and perception sometimes viewed only as topics in basic science  People differ in terms of how they perceive 3-D images  Sensation: the stimulation of the sense organs o involves: absorption of energy (light or sound waves) by sensory organs (ears/eyes)  Perception: is the selection, organization and interpretation of sensory input o involves: organizing and translating sensory input into something meaningful (such as best friend’s face of other environmental stimuli) Psychophysics: Basic Concepts and Issues  Psychophysics: study of how physical stimuli are translated into psychological experience  Gustav Fechner: published seminal work on subject in 1860 Thresholds: Looking for Limits  Sensation begins with stimulus (input from environment)  Fechner wanted to know: for any sense, what is the weakest detectable stimulus? -concept central so psychophysics: threshold  Threshold: dividing point between energy levels that do and do not have a detectable effect  Absolute threshold: for specific type of sensory input is minimum amount of stimulation that an organism can detect -define boundaries of organism’s sensory capabilities  When lights varying intensity are flashed at subject, there is no single stimulus intensity at which subject jumps from no detection to completely accurate detection -stimulus increases  Define absolute threshold as 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 -close cousins of absolute thresholds -vary by sense  Weber’s law: states that the size of a just noticeable difference is constant proportion of the size of the initial stimulus -constant proportion called: Weber fraction o applies to weight perception and all senses  Fractions apply to different types of sensory input Psychophysical Scaling Fechner’s law: states that magnitude of a sensory experience is proportional to number of JNDs that the  stimulus causing the experience is above the absolute threshold o larger and larger increases in stimulus intensity are required to produce just noticeable differences in magnitude of sensation Signal-Detection Theory  Perceptions can’t be measured on absolute scales applies not only to sensory scaling but to sensory thresholds as well  Signal-detection theory: the detection of stimuli involves decision processes as well as sensory processes which are both influenced by a variety of factors besides stimulus intensity Noise comes from all irrelevant stimuli in environment and neural activity they elicit   More noise in signal, harder for you to pick up weak signal  Key point: the single-detection theory replaces Fechner’s sharp threshold with concept of detectability  Detectability is measured in terms of probability and depends on decision-making processes as well as sensory processes Perception without Awareness  Can sensory stimuli that fall beneath the threshold of awareness still influence behaviour? -issue centered on subliminal perception  Subliminal perception: the registration of sensory input without conscious awareness  Limen- another term for threshold  So subliminal means below threshold  Subliminal concept tied up with controversies like money, sex, religion and rock music  Using diverse methodological and conceptual approaches: like 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 stimuli can have important effects  The effects of subliminal perception are relatively weak and of little/ no practical concern Sensory Adaptation  Another factor that influences registration of sensory input  Is a gradual decline in sensitivity due to prolonged stimulation  Is a pervasive aspect of everyday life Is an automatic, built-in process that keeps people turned into the changes rather than the constants in  their sensory input  Allows people to ignore the obvious and focus on changes in their environment that may signal threats to safety  Is a behavioural adaptation that has been sculpted by natural selection  Also shows that there is no one-to-one correspondence between sensory input and sensory experience  Prolonged stimulation may lead to sensory adaptation, which involves a reduction in sensitivity to contact stimulation Our Sense of Sight: The Visual System The Stimulus: Light  For people to see there must be light  Light: form of electromagnetic radiation that travels as a wave, moving at speed of light  Light waves vary in amplitude and wavelength o Amplitude affects mainly perception of colour o Lights humans normally see are mixtures of several wavelengths  Light vary in purity  Purity influences perception of saturation or richness of colours  Saturation is difficult to describe  Visible spectrum: is only slim portion of total range of wavelengths  Vision: is a filter that permits people to sense but a fraction of real world The Eye: A Living Optical Instrument  Two main purposes: 1) channel light to neural tissue that receives it called retina 2) they house it  Each eye: living optical instrument that creates an image of visual world on light-sensitive retina lining inside back surface  Light enters eye through transparent window called cornea  Cornea and crystalline lens form upside-down image but brain knows rule of relating positions on retina to corresponding positions in the world  Lens: the transparent eye structure that focuses the light rays falling on the retina o made of soft tissue called accommodation o Accommodation: occurs when curvature of lens adjusts to alter visual focus o When focus on close objects, lens gets fatter(rounder) to give clear image o When focus on distant objects lens flattens to give better image of objects  Nearsightedness: close objects seen clearly but distant objects seen blurry because focus of light from distant objects fall short of retina (when eyeball is long)  Farsightedness: distant objects are seen clearly but close object appear blurry because the focus of light from close objects falls behind the retina (problem occurs when eyeball is short)  The iris: coloured ring of muscle that surrounds the pupil The pupil: the opening in the center of the iris that helps regulate the amount of light passing into the rear  chamber of the eye o When pupil open (dilates), more light in but image less sharp o Pupil in bright light take advantage of sharpened images o Dim light, pupil’s dilate; image sharpness sacrificed to allow more light to fall on retina so that more remains visible  Saccades: tiny movements of the eye are essential to good vision; even if there is small reduction in voluntary eye movements, our vision degrades o One form of saccade may give away your covert gaze even when looking elsewhere (McGill and James) The Retina: The Brain’s Envoy in the Eye  Retina: neural tissue lining the inside back surface of the eye; it absorbs light, processes images and sends visual information to the brain o contains complex network of specialized cells arranged in layers  Axons that run from retina to brain converge at optic disk  Optic disk: a hole in the retina where the optic nerve fibers exit the eye o can’t see part of image that falls on it, since it’s a hole  Have blind spot in each eye Visual Receptors: Rods and Cones  Retina contains millions of receptor cells that sensitive to light  Receptors located in innermost layer of retina  Light passes through several layers of calls before gets to receptors  Only 10% of light arriving at cornea reaches these receptors  Retina contains two types of receptors: rods and cones -names based on shapes  Cones: specialized visual receptors that play key role in daylight vision and colour vision -handles most of daytime vision because bright lights dazzle the rods -don’t respond well to dim light -that’s why don’t see well in low illumination -concentrated on center of retina and fall off in density towards its periphery  Fovea: a tiny spot in center of retina that contains only cones; visual acuity is greatest at this spot -if want something sharply, move eyes to center of object in fovea  Rods: specialized visual receptors that play key role in night vision and peripheral vision -handle night vision because more sensitive than cones to dim light -handle more of lion’s share of peripheral because outnumber cones in periphery of retina Dark and Light Adaptation  Dark adaptation: the process in which the eyes become more sensitive to light in the low illumination -virtually complete in about 30 minutes  The declining absolute thresholds over time indicate that you require less and less light to see Light adaptation: process of whereby the eyes become less sensitive to light in high illumination  -improves visual acuity under prevailing circumstances Information Processing in the Retina  Retina transforms a pattern of light falling onto different representation of the visual scene  Retina transforms light into neural impulses that are sent to the brain via the optic nerve  The information from over 100 million rods and cones converges to travel along only 1 million axons in the optic nerve  The receptive field of a visual cell: is the retinal area that when stimulated affected the firing of that wall -come in variety of shapes and sizes -light falling in center has opposite effect of light falling in surrounding of area  When receptive fields are stimulated retinal cells send signals both towards the brain and laterally towards nearby visual cells  Lateral antagonism: occurs when neural activity in a cell proposes activity in surrounding cells -responsible for opposite effects that occur when light falls on the inner vs. outer portions of center- surround receptive fields -allows retina to compare the light falling a specific area against general lighting -the visual systems can compute the relative amount of light at a point instead of reacting to absolute levels of light Vision and the Brain  Light falls on the eye, but you see with your brain  Visual input is meaningless until processed in brain Visual Pathways to the Brain  Axons leaving back of eye form optic nerve, which travel to optic chiasm  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  Signals from both eyes go to the brain, both hemispheres After reaching optic chiasm, optic nerve fibers diverge along two pathways   Main pathway projects to thalamus, the bran’s major relay station -90% axons from retinas synapse in lateral geniculate nucleus (LGN)  Visual signals processed in LGN and then distributed to areas in occipital lobe that make up primary visual cortex  2ndvisual pathway branches in midbrain called superior colliculus before travelling to thalamus on to occipital lobe nd -principal function of 2 pathway appear to be coordination of visual input with other sensory input  Main visual pathway subdivided into two more specialized pathways called the magnocellular and parvocellular channels  Channels engage in parallel processing  Parallel processing: involves simultaneously extracting different kinds of information from the same input (handles perception of colour) Information Processing in the Visual Cortex  Most visual input arrives in visual cortex, located in occipital lobe  Hubel and Wiesel (Noble prize winners) identified various types of cells in the primary visual cortex that respond to different stimuli  Simple cells: respond best to a line of the correct width, orientated at the correct angle and located in the correct position in its receptive field o Complex cells - respond to ANY position in their receptive fields Visual cortex seems to be highly specialized   Characterized as feature detectors  Feature detectors: neurons that respond selectively to very specific features of more complex stimuli  Visual input processed in primary visual cortex, and routed to other cortical areas for additional processing  Signals travel through two streams: the ventral stream and the dorsal stream  Ventral stream: details of what objects are out there (perception of form and colour) - vision for perception  Dorsal stream: where objects are (motion and depth) - vision for action  Signals move more further along in visual processing system, neurons become even more specialized and stimuli becomes more and more complex  Natural selection may have wired the brains of some species to quickly respond to faces  Discovery of what pathway and neurons inside it that respond to faces has shed new light on visual disorders  Visual agnosia: inability to recognize objects—even though eyes function just fine, probably damaged somewhere along visual pathway that handles object recognition  Prosopagnosia: inability to recognize familiar faces—including one’s own face  The neurons in ventral stream pathway are involved in perceiving faces can learn from experience Multiple Methods in Vision Research  The McCollough effect well-known after image phenomenon that differs from other colour afterimage effects because it is contingent on both colour, pattern or form  Vision researchers employ multiple, converging methods when trying to explain the role of the brain in visual experience Viewing the World in Colour The Stimulus for Colour  Perceived colour: primary function of dominant wavelength in these mixtures  In visible spectrum: lights with longest wavelengths appear red, whereas those with shortest appear violet  Colour is a psychological interpretation (not physical property of light itself)  Perception of colour depends on complex bends of all three properties of light  Wavelength: related to hue, amplitude to brightness and purity to saturation  Three dimensions of colour illustrated in colour solid  Two types of colour mixtures: subtractive colour mixing and Additive colour mixing o Subtractive colour mixing: works by removing some wavelengths of light, leaving less light than was originally there (mixing blue and yellow to get green) o Additive colour mixing: putting more light in the mixture than exists in any one light by itself (shining blue, red and green spot lights on white surface) Trichromatic Theory of Colour Vision  First stated by Thomas Young and later modified by Hermann von Helmholtz  Holds that the human eye has three types of receptors with differing sensitivities to different light wavelengths  Helmholtz: eye contains specialized receptors sensitive to the specific wavelengths associated with red, green and blue The impetus for trichromatic theory was that light of any colour can be matched by additive mixture of  three primary colours  Colour blindness: encompasses variety of deficiencies in the ability to distinguish among colours -more in males than females -most people who are colour blind are dichromats (make do with only two colour channels) Opponent Theory of Colour Vision  Complementary colours: pairs of colours that produce grey tones when mixed together  Afterimage: visual image that persists after a stimul
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