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Chapter 4

Ch. 4 - Sensation and Perception.docx

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PSYC 1010
Jennifer Steeves

Sensation and Perception - Ian Howard known internationally as pioneer in sensation/perception research, particularly binocular vision and human spatial orientation - People rely on three types of cues to determine which way is up: visual, gravity, body direction - Sensation is the stimulation of sense organs. Perception is the selection, organization, and interpretation of sensory input. o Sensation involves absorption of energy; light or sound waves, by sensory organs, such as ears and eyes o Perception involves organizing and translating sensory input into something meaningful, such as your best friend’s face or other environmental stimuli Psychophysics: Basic Concepts and Issues - Area of sensation and perception; psychophysics – the study of how physical stimuli are translated into psychological experience - Important contributor to psychophysics; Gustav Fechner, German scientist Thresholds: Looking for Limits - Sensation begins with a stimulus – any detectable input from environment - A threshold is a dividing point between energy levels that do and do not have a detectable effect o Eg. Hardware stores sell a gadget with photocell that automatically turns a lamp on when a room gets dark. The level of light intensity at which the gadget clicks on is its threshold - An absolute threshold for a specific type of sensory input is the minimum amount of stimulation that an organism can detect o Define boundaries of sensory capabilities Weighing the Differences: The JND - A just noticeable difference is the smallest difference in the amount of stimulation that a specific sense can detect - Ernst Weber: Weber’s law states that the size of a just noticeable difference is a constant proportion of the size of the initial stimulus o Constant proportion of the size is called Weber Fraction o Fechner’s law, which 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 o Three equal increases in stimulus intensity produce progressively smaller differences in the magnitude of sensation Signal-Detection Theory - 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 o Four possible outcomes; Hit (detecting signals when present), misses (failing to detect signals when present), false alarms (detecting signals when not present) and correct rejections (not detecting signals when absent) Perception without Awareness - Subliminal Perception – The Registration of Sensory Input without conscious awareness o Limen another term for threshold, subliminal means below threshold o Subliminal Perceptions have become tied up in highly charged controversies relating to money, sex, religion, and rock music - Subliminal messaging in advertisements make consumers buy more / appeal to message transmitted Sensory Adaptation - Sensory Adaptation is a gradual decline in sensitivity due to prolonged stimulation Review of Key Points - Psychophysicists use a variety of methods to relate sensory inputs to subjective perception. They have found that absolute thresholds are not really absolute - Weber’s law states that the size of a just noticeable difference is a constant proportion of the size of the initial stimulus. Fechner’s law asserts that larger and larger increases in stimulus intensity are required to produce just noticeable differences in the magnitude of sensation - According to signal-detection theory, the detection of sensory inputs is influenced by noise in the system and by decision-making strategies. Signal-detection theory replaces Fechner’s sharp threshold with the concept of detectability and emphasizes that factors besides stimulus intensity influence detectability. - In recent years, a host of researchers, using very different conceptual approaches, have demonstrated that perception can occur without awareness. However, research indicates that the effects of subliminal perception are relatively weak and of little or no practical concern - Prolonged stimulation may lead to sensory adaptation, which involves a reduction in sensitivity to constant stimulation Our Sense of Sight: The Visual System The Stimulus: Light - Electromagnetic radiation that travels as a wave - Light waves vary in amplitude (height) and wavelength (distance between peaks) and purity (how varied the mix of wavelengths are) - Purity influences perception of saturation – richness of colours - The Visible Spectrum is only a slim portion of the total range of wavelengths The Eye: A Living Optical Instrument - Two main purposes: Channel light to the neural tissue retina - Light enters eye through cornea – crystalline lens behind it form upside-down image of objects on retina - The lens is the transparent eye structure that focuses the light rays falling on the retina o Made up of soft tissue o Accommodations occur when the curvature of the lens adjusts to focus o Focusing on a closer object makes the lense fatter to give you a clear image, when you focus on distant objects, the lens flattens out to give you a better image of the objects - In nearsightedness, close objects are seen clearly but distant objects appear blurry – light from distant objects falls short on retina - In farsightedness, distant objects are seen clearly but close objects appear blurry – focus of light on close objects falls behind retina - The Iris is the coloured ring of muscle surrounding the pupil: an opening in the centre of the iris that helps regulate the amount of light passing into the rear chamber of the eye o When the pupil constricts, lets less light in but sharpens image on retina o When pupil dilates, lets more light but image less sharp - The eye movements are referred to as saccades o Saccades are tiny movements essential to good vision  Research on Saccades have measured gaze direction, attention and strength of visual distracters 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 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 Visual Receptors: Rods and Cones - Two types of receptors in retina; Rods and Cones o Rods are elongated and cones are stubbier - Cones are specialized visual receptors that play a key role in daylight vision and colour vision. o Handle most of daytime vision o Do not respond well to dim light, but provide better visual acuity (better sharpness and precise detail) - The fovea is a tiny spot in the centre of the retina that contains only cones; visual acuity is greatest at this spot - Rods are specialized visual receptors that play a key role in night and peripheral vision o More sensitive to dim light o Density greatest outside fovea, gradually decreasing toward the periphery of the retina Dark and Light Adaptation - Dark Adaptation; the process in which the eyes become more sensitive to light in low illumination - Light Adaptation is the process whereby the eyes become less sensitive to light in high illumination Information Processing in the Retina - The collection of rod and cone receptors that funnel signals to a particular visual cell in the retina (brain) make up the cell’s receptive field; the receptive field of a visual cell is the retinal area that, when stimulated, affects the firing of that cell - Laterial antagonism occurs when neural activity in a cell opposes activity in surrounding cells o Allows retina to compare light falling in specific area against general lighting Vision and the Brain - Light falls on the eye, but you see with your brain Visual Pathways to the Brain - Axons leaving the back of each eye form the optic nerves, which travel to the 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 o After reaching optic chiasm, optic nerve fibres diverge along two pathways; main pathway projects into thalamus; 90% of axons from retinas synapse in the Lateral Geniculate Nucleus o Visual signals processed in LGN, distributed to areas in occipital lobe that make up the primary visual cortex o The second visual pathway leaving optic chiasm branches off to an area in the midbrain called the superior colliculus before travelling through the thalamus and on to the occipital lobe  Principal function of second pathway is coordination of visual input with other sensory input o Main visual pathway subdivided into two more specialized pathways called the magnocellular and parvocellular channels; these channels engage in parallel processing – involves simultaneously extracting different kinds of information from the same input  Parvocellular handles perception of colour  Magnocellular processes information regarding brightness Information Processing in the Visual Cortex - Most visual input arrives in the primary visual cortex - Cells in primary visual cortex more sensitive to lines, edges and other more complicated stimuli - Hubel and Wiesel identified specialized cells in primary visual cortex o Simple cells respond best to a line of the correct width, oriented at the correct angel and located in the correct position in its receptive field o Complex cells also care about width and orientation, but respond to any position o Cells in visual cortex highly specialized, characterized as feature detectors, neurons that respond selectively to very specific features of more complex stimuli o Signals travel through two streams: Ventral stream – processes details of what objects are out there, and the dorsal stream – processes where the objects are - Some people exhibit visual agnosia – inability to recognize objects - Some people exhibit prosopagnosia – inability to recognize familiar faces Multiple Methods in Vision Research - McCullough effect is a well-known afterimage phenomenon that differs from other colour afterimage effects because it is contingent on both colour and pattern/form Review of Key Points - Light varies in terms of wavelength, amplitude and purity. Light enters the eye through the cornea and pupil and is focused upside down on the retina by the lens. Distant objects appear blurry to nearsighted people and close objects appear blurry to farsighted people - The retina is the neural tissue in the eye that absorbs light, processes images, and sends visual signals to the brain. Cones, which are concentrated in the fovea, play a key role in daylight vision and colour perception. Rods, which have their greatest density just outside the fovea, are critical to night vision and peripheral vision. Dark adaptation and light adaptation both involve changes in the retina’s sensitivity to light, allowing the eye to adapt to changes in illumination - The Retina transforms light into neural impulses that are sent to the brain via the optic nerve. Receptive fields are areas in the retina that affect the firing of visual cells. They vary in shape and size, but centre-surround arrangements are common. The optic nerves from the inside half of each eye cross at the optic chiasm and then project to the opposite half of the brain - Two visual pathways engage in parallel processing and send signals to different areas of the primary visual cortex. The main pathway is routed through the LGN in the thalamus. After processing in the primary visual cortex, visual information is shuttled along the what and where pathways to other cortical areas. - Nobel Prize-winning research by Hubel and Wiesel suggests that the visual cortex contains cells that function as feature detectors. The discovery of the what pathway and the neurons inside it that respond specifically to faces have shed new light on visual disorders that have perplexed scientists for decades. - 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 - Colour is a psychological interpretation, not a physical property of light - Perception of colour depends on complex blends of all three properties of light o Wavelength closely related to hue, amplitude to brightness, and purity to saturation - Two kinds of colour mixture: 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 exists in any one light by itself  If you shine red, green and blue spotlights on a white surface, will have an additive mixture Trichromatic Theory - The trichromatic theory of colour vision holds that the human eye has three types of receptors with differing sensitivities to different light wavelengths - People can see all colours of rainbow because eye does its own mixing - Colour blindness encompasses a variety of deficiencies in the ability to distinguish among colours o Misleading, colour-blind people are dichromate, only two colour channels instead of three Opponent Process Theory of Colour Vision - Complementary colours are pairs of colours that produce grey tones when mixed together - If you stare at a strong colour, then look at a white background, you’ll see an afterimage – a visual image that persists after a stimulus is removed - The opponent process theory of colour vision holds that colour perception depends on receptors that make antagonistic responses to three pairs of colours o Red vs. Green, yellow vs. blue, black vs. white - Grapheme-Colour Synesthesia o Rare condition, individuals perceive a letter or digit, concurrently and unintentionally experience the perception of an associated colour Reconciling Theories of Colour Vision - Eye has three types of cones, with each being most sensitive to a different band of wavelengths - Found cells in retina that respond opposite to red vs. green, blue vs. yellow Effects of Colour on Behaviour - First, people learn associations based on certain colours being paired repeatedly with certain experiences - Second, over the course of human evolution, certain colours may have had adaptive significance for survival or reproduction - Red is a sign of danger, negativity; subjects who took a test of an with a red cover scored significantly lower than with a white or green cover - However, red leads to significantly higher attractiveness, sexual desirability and dating interest ratings than blue (Red blouse vs. blue blouse Perceiving Forms, Patterns and Objects - Reversible figures; a drawing that is compatible with two interpretations that can shift back and forth - Perception involves interpretation of sensory input, and the process can be manipulating people’s expectations - Perceptual set – a readiness to perceive a stimulus in a particular way o Creates a certain slant in how someone interprets sensory input - Intentional blindness – failure to see fully visible objects or events in a visual display Feature Analysis: Assembling Forms - Feature Analysis is the process of detecting specific elements in visual input and assembling them into a more complex form o Assumes that form perception involves bottom-up processing from individual elements to the whole - Top-Down processing, a progression from the whole to the elements Looking at the Whole Picture: Gestalt Principles - Max Wertheimer – The phi phenomenon is the illusion of movement created by presenting visual stimuli in rapid succession - Gestalt’s Principles: o Figure and Ground: Figure is the thing being looked at, and the ground is the background against which it stands. Figures have more substance and shape, and appear closer, and stand out in front of the ground o Proximity: Elements that are close to one another tend to be grouped together o Closure: Viewers tend to supply missing elements to close or complete a familiar figure o Similarity: Elements that are similar tend to be grouped together o Simplicity: Viewers tend to organize elements in the simplest way possible o Continuity: Viewers tend to see elements in ways that produce smooth continuation - Two kinds of stimuli: Distal and Proximal o Distal stimuli are stimuli that lie in the distance (that is, in the world outside the body).  Distant – eyes to not touch them, formed by patterns of light falling on your retinas  These images are Proximal stimuli, the stimulus energies that impinge directly on sensory receptors - In visual stimulation, proximal stimuli distorted 2d versions of actual 3d counterparts. - A perceptual hypothesis is an inference about which distal stimuli could be responsible for the proximal stimuli sensed. o Eg. Eyes present trapezoidal image on retinas, but perceptual system guesses correctly that it’s a square Review of Key Points - Perceptions of colour (hue) are primarily a function of light wavelength, while amplitude affects brightness and purity affects saturation. There are two typ
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