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Chapter 5

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Psychology 1000
John Campbell

Psychology Chapter 5 Sensation and Perception Synesthesia: “mixing of the senses” people who experience sounds as colours or tastes as touch sensations that have different shapes. Women are more likely to be synaesthetes. The five stages that constitute the process of sensory processing and perception of information: Stimulus is received by sensory receptors (sensation) Receptors translate stimulus properties into nerve impulses (transduction) Feature detectors analyze stimulus features Stimulus features are reconstructed into neural representation and is compared with previously stored information in brain Matching process results in recognition and interpretation of stimuli (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, the active process of organizing this stimulus input and giving it meaning Transduction: is the process whereby the characteristics of a stimulus are converted into nerve impulses. • There are more than the 5 classical senses (touch, taste, sight, auditory, smell) such as sense of balance, detection within the immune system, etc. Psychophysics: studies relations between the physical characteristics of stimuli and sensory capabilities. Concerned with: • Absolute limits of sensitivity. What is the softest sound or the weakest salt solution that humans can detect? • Differences between stimuli. What is the smallest difference in brightness that we can detect? Absolute threshold: the lowest level of stimulation that a person can detect correctly 50 percent of the time. The lower the absolute threshold, the greater the sensitivity. Decision criterion: a standard of how certain they must be that a stimulus is present before they will say they detect it. Signal detection theory is concerned with the factors that influence sensory judgments. Signal detection research shows us that perception is, in part, a decision. Subliminal stimulus: stimulus so weak or brief that, although it is received by the senses, it cannot be perceived consciously—the stimulus is well below the absolute threshold. • Higher-order facial recognition is a complex process involving several brain regions, including the LOA(lateral occipital area) and the FFA(Fusiform FacialArea) in addition to the primary visual cortex. • Individuals with prosopagnosia are unable to recognize familiar faces. • We may not be consciously aware of stimuli, but aspects of the stimuli could be processed at a different level and available for us to use in subsequent decisions. Difference threshold: the smallest difference between two stimuli that people can perceive 50 percent of the time a.k.a. the “just noticeable difference” (jnd) Weber’s law: states that the difference threshold, or jnd, is directly proportional to the magnitude of the stimulus with which the comparison is being made, and can be expressed as a Weber fraction. For example, the jnd value for weights is a Weber fraction of approximately 1/50. This number means that if you lift a weight of 50 grams, a comparison weight must weigh at least 51 grams in order for you to be able to judge it as heavier. If the weight were 500 grams, a second weight would have to weigh at least 510 grams (i.e., 1/50 = 10 grams/500 grams) for you to discriminate between them. The smaller the fraction, the greater the sensitivity to differences. Sensory adaptation: the diminishing sensitivity to an unchanging stimulus. In Review: • Sensation refers to the activities by which our sense organs receive and transmit information, whereas perception involves the brain's processing and interpretation of the information. • Psychophysics is the scientific study of how the physical properties of stimuli are related to sensory experiences. Sensory sensitivity is concerned in part with the limits of stimulus detectability (absolute threshold) and the ability to discriminate between stimuli (difference threshold). The absolute threshold is the intensity at which a stimulus is detected 50 percent of the time. Signal detection theory is concerned with factors that influence decisions about whether or not a stimulus is present. • Research indicates that subliminal stimuli, which are not consciously perceived, can influence perceptions and behaviour in subtle ways, but not strongly enough to justify concerns about the subconscious control of behaviour through subliminal messages. • The difference threshold, or just noticeable difference (jnd), is the amount by which two stimuli must differ for them to be perceived as different 50 percent of the time. Studies of the jnd led to Weber's law, which states that the jnd is proportional to the intensity of the original stimulus and is constant within a given sense modality. • Sensory systems are particularly responsive to changes in stimulation, and adaptation occurs in response to unchanging stimuli. The Sensory Systems Vision: • Cornea: a transparent protective structure at the front of the eye (where light waves enter). • Pupil: an adjustable opening that can dilate or constrict to control the amount of light that enters the eye. Low levels of illumination cause the pupil to dilate, letting more light into the eye to improve optical clarity; bright light triggers constriction of the pupil. • Iris: regulates the size of the pupil. • Ciliary muscles: regulate the shape of the lens. • Lens: (behind pupil) an elastic structure that becomes thinner to focus on distant objects and thicker to focus on nearby objects • Retina: a multi-layered, light-sensitive tissue at the rear of the eyeball. Contains specialized sensory neurons and is actually an extension of the brain. The lens reverses the image from right to left and top to bottom when it is projected on the retina, but the brain reconstructs the visual input into the image that we perceive. The retina contains two types of light-sensitive receptor cells, called rods and cones because of their shapes • The ability to see clearly depends on the lens's ability to focus the image directly onto the retina • Myopia (nearsightedness). In nearsighted people, the lens focuses the visual image in front of the retina (too near the lens), resulting in a blurred image for faraway objects. This condition generally occurs because the eyeball is longer (front to back) than normal. • Hyperopia (farsightedness) occurs when 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 typically causes the eyeball to become shorter over time, contributing to the development of hyperopia and the need for many middle-aged people to use glasses. • Rods: black-and-white brightness receptors. More sensitive to light than the cones, function best in dim light. • Cones: colour receptors, function best in bright illumination. • In humans, 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, and the periphery of the retina contains mainly rods. • Fovea: a small area in the centre of the retina that contains only cones. • Bipolar cells have synaptic connections with the rods and cones. The bipolar cells, in turn, synapse with a layer of about one million ganglion cells, whose axons are collected into a bundle to form the optic nerve. Typically, many rods are connected to the same bipolar cell. They therefore can combine or “funnel” their individual electrical messages. • In the fovea, however, the densely packed cones each have their own “private line” to a single bipolar cell.As a result, our visual acuity, or ability to see fine detail, is greatest when the visual image projects directly onto the fovea. • The optic nerve formed by the axons of the ganglion cells exits through the back of the eye not far from the fovea, producing a blind spot. Our perceptual system “fills in” the missing part of the visual field. • Transduction: the process whereby the characteristics of a stimulus are converted into nerve impulses. Rods and cones translate light waves into nerve impulses through the action of protein molecules called photopigments. • Brightness Vision and Dark Adaptation • Rods have much greater brightness sensitivity than cones throughout the colour spectrum except at the red end. • Cones are most sensitive to low illumination in the greenish-yellow range of the spectrum • Dark adaptation: is the progressive improvement in brightness sensitivity that occurs over time under conditions of low illumination. If the eye has been exposed to conditions of high illumination, such as bright sunlight, a substantial amount of photopigment will be depleted. During the process of dark adaptation, the photopigment molecules are regenerated, and the receptor's sensitivity increases greatly. • The trichromatic (three-colour) theory: (1800s) three types of colour receptors in the retina. All cones can be stimulated by most wavelengths to varying degrees; individual cones are most sensitive to wavelengths that correspond to blue, green, or red. The visual system combines the signals to recreate the original hue. If all three cones are equally activated, a pure white colour is perceived. • Opponent-process theory: also assumed that there are three types of cones. 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. • The dual-process theory combines the trichromatic and opponent-process theories to account for the colour transduction process. o The cones contain one of three different protein photopigments that are most sensitive to wavelengths roughly corresponding to the colours blue, red, and green. Different ratios of activity in the red, blue, and green-sensitive cones can produce a pattern of neural activity that corresponds to any hue. o Certain ganglion cells respond in an opponent-process fashion by altering their rate of firing. If red light is shone on the retina, an opponent-process ganglion cell may respond with a high rate of firing, but a green light will cause the same cell to fire at a very low rate. • Trichromats: people with normal colour vision • Dichromat: person who is colour-blind in only one of the systems (red-green or yellow-blue) • Monochromat is sensitive only to the black-white system and is totally colour- blind. • Primary visual cortex in the occipital lobe at the rear of the brain. • Feature detectors: fire selectively in response to stimuli that have specific characteristics. Viewing the letter “A” comes from feature detectors seeing: –, /, \ • Parallel processing: separate but overlapping modules within the brain simultaneously analyze colours, shape, distance, and movement and constructing a unified image of its properties • Visual association cortex: complex features of the visual scene are combined and interpreted in light of our memories and knowledge Audition • Sound waves: form of mechanical energy. Successive waves of compression and expansion among the air molecules surrounding the source of the sound. • Frequency is the number of sound waves, or cycles, per second. The hertz (Hz) is the technical measure of cycles per second; 1 hertz equals one cycle per second. Frequency = pitch • Amplitude refers to the vertical size of the sound waves—that is, to the amount of compression and expansion of the molecules in the conducting medium. Decibels (db), a measure of the physical pressures that occur at the eardrum.Amplitude = loudness •
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