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Chapter 5

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Psychology 1000

Psychology – Chapter 5: Sensation and Perception Sensation: the stimulus-detection process by which our sense organs respond to and translate environmental stimuli into nerve impulses that are sent to the brain Perception: making “sense” of what our senses tell us—is the active process of organizing this stimulus input and giving it meaning Sensory Processes: - Five classical senses: vision, audition (hearing), touch, gestation (taste), and olfaction (smell) - Human sensory systems are designed to extract from the environment the information that we need to function and survive Psychophysics: studies relations between the physical characteristics of stimuli and sensory capabilities - Concerned with two kinds of sensitivity: the absolute limits of sensitivity, and the differences between stimuli The Absolute Threshold for Stimulus Detection: How intense a stimulus must be before we can detect its presence The Absolute Threshold: the lowest intensity at which a stimulus can be detected correctly 50 percent of the time. The lower the absolute threshold, the greater the sensitivity. Signal Detection Theory: Concerned with the factors that influence sensory judgments - Decision Criteria: a standard of how certain a person much be that a stimulus is present before they will say they detect it - Can change depending on fatigue, expectation, and the potential significance of the stimulus. - Four possible outcomes in signal detection: o A hit: a tone is presented and the participant detects it o A miss: a tone is presented and the participant does not detect it o A false alarm: no tone is presented and the participant detects it o A correct rejection: no tone is presented and the participant does not detect it The Difference Threshold: the smallest difference between two stimuli that people can perceive 50 percent of the time; sometimes called the just noticeable difference - Weber’s law: the difference threshold is directly proportional to the magnitude of the stimulus with which the comparison id being made, and can be expressed as a Weber fraction - Example: the Weber fraction for weights id 1/50. If you lift a weight of 50 grams, another weight must be 51 g to detect a difference; 500g to 510g etc. Sensory Adaptation: the diminishing sensitivity to unchanging stimuli; due to adaptation or habituation The Sensory Systems: Vision: normal stimulus is electromagnetic energy, or light waves; measures in nanometers Our visual system is only sensitive to wavelengths from 700 nm (red) to 400 nm (blue-violet) The Human Eye: - Cornea: where light waves enter the eye, a transparent protective structure at the front - Pupil: behind the cornea, an adjustable opening that can dilate or constrict to control light entry - Iris: colored area around pupil, contains muscles that control the pupil’s size - Lens: behind the pupil, an elastic structure that becomes thinner or thicker to focus - Retina: multi-layered tissue at the rear of the eyeball, where the lens focuses the visual image o The ability to see clearly depends on the lens’s ability to focus the image onto the retina Myopia (nearsightedness): The lens focuses the visual image in front of the retina (too near the lens) resulting in a blurred image for faraway objects. This generally occurs because the eyeball is longer than normal Hyperopia (farsightedness): The lens does not thicken enough and the image is therefore focused on a point behind the retina (too far from the lens). The aging process causes the eyeball to become shorter over time, causing hyperopia Photoreceptors: - The retina contains two types of light-sensitive receptor cells, called rods and cones - There are about 120 million rods and only 6 million cones Rods: function best in dim light, black and white brightness receptors, 500 times more sensitive to light than cones, but do not give rise to colour sensation Cones: colour receptors, function best in bright illumination Rods are found throughout the retina except in the fovea (a small area in the centre of the retina that contains only cones) Cones decrease in concentration as one moves away from the centre of the retina Rods and cones synapse with a layer of bipolar cells, which then synapse with a layer of ganglion cells to communicate with the brain The axons of the ganglion cells are collected into a bundle to form the optic nerve Visual Transduction: the process whereby the characteristics of a stimulus are converted into nerve impulses - Photopigments: the protein molecules trough which rods and cones translate light waves into nerve impulses Brightness Vision and Dark Adaptation: - Rods are far more sensitive that cones under low illumination - Rods have much greater brightness sensitivity than cones throughout the colour spectrum except at the red end, where rods are relatively insensitive. - Cones are most sensitive to low illumination in the greenish-yellow range of the spectrum Dark Adaptation: the progressive improvement in brightness sensitivity that occurs over time under conditions of low illumination - Photoreceptors become depleted of their pigment molecules for a period of time after absorbing light - In dark adaptation, the photopigment molecules are regenerated and the receptor’s sensitivity increases - Cones regenerate faster than rods Colour Vision: two different theories of colour vision The Trichromatic Theory: There are three types of colour receptors in the retina - Although all cones can be stimulated by most wavelengths to some degree, individual cones are sensitive to wavelengths that correspond to blue, green, or red - Each of these receptor classes sends messages to the brain based on the extent to which they are activated - The visual system combines the signals to recreate the original hue Opponent-Process Theory: Each of the three cone types responds to two different wavelengths. One type responds to red or green, another to blue or yellow, and a third to black or white Dual-Process Theory: Combines the trichromatic and opponent-process theories to account for colour transduction Colour-Deficient Vision: - Trichromats: people with normal colour vision, sensitive to all 3 systems—red-green, yellow-blue, black-white - Dichromats: people who are colour-blind in only one of the systems (red-green or yellow-blue) - Monochromats: people who are sensitive only to the black-white system and are totally colour blind Analysis and Reconstruction of Visual Scenes: After light energy is transformed in to nerve impulses, the messages received from the photoreceptors are combined into the perception of a visual scene Feature Detectors: groups of neurons within the primary visual cortex (in the occipital lobe) that receive and integrate sensory nerve impulses originating in specific regions of the retina; fire selectively in response to stimuli that have specific characteristics Parallel Processing: simultaneous analyzing of something’s colour, shape, distance, and movement to construct a unified image of its properties Visual Association Processes: information analyzed and recombined by the primary visual cortex is routed to the visual association cortex where more complex features of the visual scene are combined and interpreted Audition: stimuli for our sense of hearing are sound waves, a form of mechanical energy Sound is actually pressure waves in the air, water, or some other conduction medium Frequency: the number of sound waves or cycles per second- - cycles per second are measured in hertz (Hz) - sound waves’ frequency is related to pitch—higher frequency = higher pitch Amplitude: the vertical size of the sound waves - the amount of compression and expansion of the molecules in the conducting medium - the primary determinant of the sound’s perceived loudness - expressed as decibels (db) Auditory Transduction: Transduction system made up of tiny bones, membranes, and liquid-filled tubes designed to translate pressure waves into nerve impulses - sound waves travel in an auditory canal - leads to the eardrum: a movable membrane that vibrates in response to the sound waves - middle ear: a cavity housing three tiny bones (the hammer, anvil, and stirrup) beyond the eardrum o these bones vibrate to amplify the sound waves more than 30 times o hammer – attatched to the eardrum, o stirrup- attatched to the oval window- a membrane that forms the boundary between the middle and inner ear - inner ear: contains the cochlea o cochlea: a coiled snail-shaped tube that is filled with fluid and contains the basilar membrane o basilar membrane: a sheet of tissue that runs the length of the cochlea o organ of Corti: rests on the basilar membrane, containing thousands of tiny hair cells—the actual sound receptors o the hair cells synapse with the neurons of the auditory nerve that sends im
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