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Sensory and Perceptual Processes

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Western University
Psychology 1000
Terry Biggs

Tuesday October 25th Sensory and Perceptual Processes Sensation: the process by which our sense organs respond to and translate stimuli into nerve impulses sent to the brain Perception: organizing the stimulus input and giving it meaning Stages of Sensation and Perception: • Stimuli activate sensory receptors • Sensory receptors translate information into nerve impulses • Specialized neurons analyze stimuli features • Stimulus pieces are reconstructed and compared to stimuli in memory • Perception is then consciously experienced Psychophysics: Sensitivity to Stimuli • Absolute limits of sensitivity - Dimmest light in which we can see objects - Softest sound we can hear • Recognizing differences between stimuli - Smallest difference in brightness detectable - Recognizing differences between tones Stimulus Detection • The Absolute Threshold - The lowest intensity at which at stimulus can be detected 50% of the time - However, the environment contains a background level of stimulation for each sense and this level (the ADAPTION level) must be overcome if a stimuli is to be detected The Absolute Threshold Stimulus Detection • The amount of energy required to overcome the Adaption level is known as the DIFFERENTIAL THRESHOLD and it is subject to variation with changes in circumstances • Locke’s experiment - Materials: 3 buckets/large bowls - Method: fill 1 with very hot water, fill with very cold water, and fill 1 with tepid water - Procedure: subject closes eyes- place one hand in hot and the other in cold for approx. 3 minutes. Then place both hands in tepid water. Ask subject to report sensation for each hand separately Instruct subject to open eyes. Although both hands are in the same water the sensations differ due to the prior ADAPTION. Signal Detection Theory • Decision criterion: A personal standard of certainty before a person will say that they detect a stimulus • Affected by: conservativeness or boldness, increasing rewards for hits or costs for misses • Bold subjects show high Hits and High False alarms • Conservative subjects show high misses and Correct rejections • However, Criterions can be manipulated by changing the payoff for each cell of the response matrix • This shows perception is, to some extent, a decision Subliminal Perception • A subliminal stimulus cannot be perceived consciously but do register in the nervous system - “Subliminal” advertising during a movie (Vicary’s Claim) Subliminal Perception: Research Results • Stimuli above threshold influence behaviour much more than subliminal stimuli • Subliminal stimuli have stronger effects on attitudes • Effects may be due to placebo effects The Difference Threshold • The difference threshold (just noticeable difference or JND) is the smallest difference between two stimuli that people can perceive 50% of the time - Weber’s Law: the JND is directly proportional to the magnitude of the stimulus with which the comparison is made (ex. 1/50 for weight) Weber’s Law Sensory Adaptation (Habituation) • Sensory neurons respond to a constant stimulus by decreasing their activity The Sensory Systems: Vision 1. Cornea: transparent protective covering 2. Retina: multilayered tissue at back of the eyeball • Optic Disk: small blind spot with no receptors where the optic nerve exits the eyeball • Fovea: small, central area of the retina where acuity is greatest 3. Pupil: adjustable opening controlling the amount of light entering the eye 4. Lens: becomes thinner to focus on distant objects and thicker to focus on closer ones 5. Iris: regulates the size of the pupil, colour component of eye Vision: Photoreceptors • Rods: black and white receptors, sensitive to dim light • Cones: located in fovea, colour receptors Vision: Transduction • Absorption of light by photo pigments produces a chemical reaction changing the rate of neurotransmitter release at the receptors synapse • The greater the change in release, the stronger the signal passed into the optic nerve Vision: Visual Pathways • At the level of the Retina once the rod and Cone receptors have transduced the photonic energy into electrical impulses the information is then passed to BIPOLAR cells and from there to GANGLION cells • This process condenses signals from 130 million receptors to a few million bipolar cells to less than a million ganglion cells • A major source of this condensing is due to the fact that as many as six Rods may be connected to one bipolar cell and several bipolar cells may converge on a single ganglion cell. This process is called LATERAL SUMMATION • This means that signals, which originate with Rods, have poorer resolution than signals from Cones, which are connected 1 to 1 or 2 to 1 with bipolar cells Factors which Influence Vision • Although we have seen the system reduced input from approx.. 130 million to just under 1 million this is still far too much information to be processed by interneurons so further reduction is required • One way information may be further reduced can be seen from experiments on STEADY STATE STIMULATION Steady State Stimulation • When a patch of receptors are in the light and another patch are in the dark rather than transmitting information about every receptor the system sends only information about the boundaries • This process is known as: LATERAL INHIBITION Lateral Inhibition • Basically this occurs because the photoreceptors are linked to higher level cells and those higher level cells are cross linked to one another • Through these cross links inhibitory signals cancel out the excitatory signals from all receptors except those at the
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