7 Sensation and Perception
STEP 4 Review the Knowledge You Need to Score High
IN THIS CHAPTER
Summary: If you had to give up one of your senses, which one would it be? Most people choose the sense of smell or taste; no one ever chooses sight or hearing. What we see and hear are more essential for our survival than what we smell and taste. Vision is the most studied sense and has the largest area of our cerebral cortex devoted to it of all our senses.
All species have developed special sensory mechanisms for gathering information essential for survival. Sensation is the process by which you detect physical energy from your environment and encode it as neural signals. Perception is the process that organizes sensory input and makes it meaningful. What you perceive is influenced by your memory, motivation, emotion, and even culture. The study of sensation and perception is rooted in physics. Psychophysics is the study of the relationship between physical energy and psychological experiences. Psychophysics asks questions about our sensitivity to stimuli.
This chapter focuses on the conversion of sensations to perceptions that give you your view of your world.
Chemical senses—taste (gustation) and smell (olfaction)
Gestalt organizing principles
Perceptual adaptation and perceptual set
Sensory sensitivity can be measured by the absolute threshold, the weakest level of a stimulus that can be correctly detected at least half the time. Measured in studies by Galanter about 50 years ago, our absolute threshold for sight/vision is a candle flame seen at 30 miles on a dark, clear night; for hearing/audition, the tick of a watch under quiet conditions at 20 feet; for taste/gustation, 1 teaspoon of sugar in 2 gallons of water; for smell/olfaction, 1 drop of perfume diffused in a three-room apartment; for touch, the wing of a bee falling on your cheek from a distance of 1 centimeter. Have you noticed that dental or medical procedures feel more painful when you feel tired? It’s not your imagination! According to signal detection theory, there is no actual absolute threshold because the threshold changes with a variety of factors, including fatigue, attention, expectations, motivation, and emotional distress. It also varies from one person to another. In a signal-detection experiment, a person needs to decide whether a signal is present or not. If the signal is present and the person thinks it is present, it is a hit. If the signal is present and the person thinks it is absent, it is a miss. If the signal is absent and the person thinks it is present, it is a false alarm. If the signal is absent and the person thinks it is absent, it is a correct rejection. Clearly, you want an air traffic controller in front of a radar screen with the ability to score both hits and correct rejections.
Subliminal stimulation is the receipt of messages that are below one’s absolute threshold for conscious awareness. Subliminal messages can have a momentary, subtle effect on thinking. Such stimuli can evoke a feeling, though not a conscious awareness of the stimulus. When you are just barely aware of a change in stimulus, such as an increase in volume of a CD or brightness on your computer screen, the difference threshold—the minimum difference between any two stimuli that a person can detect 50 percent of the time—has been reached. In order to survive, organisms must have difference thresholds low enough to detect minute changes in important stimuli. You experience the difference threshold as what Ernst Weber called the just noticeable difference (jnd). If you add 1 BB to a container with 10 BBs in it, you’re more likely to notice a difference than if you add one BB to a container holding 100 BBs. According to Weber’s law, which was quantified by Gustav Fechner, difference thresholds increase in proportion to the size of the stimulus. When stimulation is unchanging, you become less sensitive to the stimulus. This sensory adaptation permits you to focus your attention on informative changes in your environment without being distracted by irrelevant data such as odors or background noises.
Transmission of Sensory Information
Sensory information of stimuli comes from millions of sensory receptors in your eyes, ears, nose, tongue, skin, muscles, joints, and tendons. Different receptors detect different types of physical energy, such as light waves, mechanical energy, chemical energy, and heat energy. Receptors transduce energy from one form into another. In sensation, transduction refers to the transformation of stimulus energy to the electrochemical energy of neural impulses. Except for impulses for olfaction/smell transmitted directly to the olfactory bulbs on the underside of the cortex, impulses from sense organs are transmitted to the thalamus before the cortex. The cerebral cortex puts all the sensory information together and acts on it. Different areas of the cortex translate neural impulses into different psychological experiences, such as odor or touch. Visual information is first processed in the occipital lobes in the back of the cortex, hearing in the temporal lobes, body senses in the parietal lobes, taste at the junction of temporal and parietal lobes, and smell in the lower portion of the frontal lobes. These primary sensory centers then project the results of their activity to other areas in the cortex, the association areas, where more abstract information processing takes place and where you connect new information with old information stored in your memory. Perception is the process of selecting, organizing, and interpreting sensations, enabling you to recognize meaningful objects and events.