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Chapter 4

Chapter 4: Sensation & Perception; Textbook & Lecture

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York University
PSYC 1010
Jill Bee Rich

Chapter 4: Sensation and Perception Psychophysics: Basic Concepts and Issues Sensation is the stimulation of sense organs Perception is the selection, organization, and interpretation Sensation involves the absorption of energy, such as light or sound waves, by sensory organs such as the eyes & ears where as perception involves organizing & translating sensory input into external stimuli. Psychophysics is the study of how physical stimuli are translated into psychological experience Thresholds: Looking for Limits Sensation begins with a stimulus, which is 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 Absolute thresholds are not really absolute A just noticeable difference (JND) is the smallest difference in the amount of stimulation that a specific sense can detect JNDs vary by sense & the smallest detectable difference is a fairly stable proportion of the size of the original stimulus. As stimuli increase in magnitude, the JND become larger. Webers law states that the size of a just noticeable difference is a constant proportion of the size of the initial stimulus The constant proportion is called the Weber fraction and applies to all senses. Psychophysical Scaling Fechners law asserts that larger and larger increases in stimulus intensity are required to produce just noticeable differences in the magnitude of sensation. Constant increments in stimulus intensity produce smaller and smaller increases in the perceived magnitude of sensation. Perceptions cant be measured on absolute scales, everything is relative. Signal-Detection Theory Signal Detection Theory: Proposes that detection of stimuli involves decision processes as well as sensory processes, which are both influenced by a variety of factors besides stimulus intensity Response to a stimulus of a specific nature is pre set by a criterion which involves higher mental processing and expectations rather then raw sensations Performance is also dependant on the amount of noise in the system as it becomes more difficult to pick up a weak system when there is more noise. Perception without Awareness Subliminal (below threshold) perception is the registration of sensory input without conscious awareness Persuasive because people supposedly are defenceless against appeals operating below their threshold of awareness. In recent years, it has become apparent that perception can occur without awareness Sensory adaptation is a gradual decline in sensitivity due to prolonged stimulation as a result of adapting to the stimulus. Automatic, built-in process that keeps people focused on changes opposed to constants in their sensory input. Allows people to ignore the obvious & focus on other changes in their environment. Likely a trait developed via natural selection as a behavioural adaptation. This is an example of how there is no one-one correspondence b/w sensory input & experience. Our Sense of Sight: The Visual System The Stimulus Light Light is a form of EM radiation travelling as a wave at the speed of light Varies in amplitude(height) & wavelength (distance b/w peaks) Amplitude affects perception of brightness while wavelength affects perception of colour. Light that we see is only small portion of EM spectrum of all wavelengths. The Eye: A Living Optical Instrument Light varies in terms of wavelength, amplitude, and purity Eye serves 2 purposes: Channels light to neural tissue that receives it called the retina House retina tissue The lens is the transparent eye structure that focuses the light rays falling on the retina, forms an upside down image of objects onto the retina. Made of soft tissue, facilitates a process called accomadation that occurs when the curvature of the lens adjusts to alter visual focus. In nearsightedness, close objects are seen clearly but distant objects appear blurry Caused b/c focus of the light from distant objects falls short to the retina when the cornea or lens bends the light too much or when the eyeball is too long In farsightedness, distant objects are seen clearly but close objects appear blurry Caused b/c focus of light from close objects falls behind the retina, typically occurring when the eyeball is too short 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 Constricting pulpils allows less light into the eye, but sharpens the image falling on the retina Dialating pupils allow more light but the image is less sharp. Tiny imperceptible eye movements are referred to as saccades These saccades are essential to good vision and w/o vision begins to decay Light enters the eye through the transparent cornea and pupil and is focused on the retina by the lens
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