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To survive, an organism must receive information about the environment
and then interpret that information. These processes of sensation and
perception are complicated by the number of different types of stimuli
in the environment. Special receptors are used to receive each one; and
we must find ways to interpret the stimuli. The first task is to
determine whether a stimulus is even present. The smallest amount of a
stimulus we can detect, called the absolute threshold, is remarkably
tiny in many cases. For example, humans are capable of seeing a candle
flame at a distance of 30 miles on a dark, clear night. A differential
threshold involves being able to determine when a stimulus has
changed—that is, has become either stronger or weaker.
Most people fear losing vision more than any other sense. In fact, more neurons are devoted to vision than to any other sense. Specialized receptor cells called rods and cones make it possible for us to receive the electromagnetic waves that make up the visible spectrum. This information is then sent on its way to brain centers where it is interpreted.
A number of theories explain how we are able to perceive color. Various problems with sensory receptors cause people to have color deficiencies, which are more common in males than in females.
Although the term wavelength is associated with vision, it is also associated with hearing or audition. Sound waves received by the ear are the stimuli for hearing. You may have heard of the term decibels, which is a measure of the amount of energy produced by the vibrations we perceive as sound. High decibel levels such as those that occur during rock concerts can damage our hearing.
Taste and smell are called chemical senses because they involve molecules dissolved in liquid or mixed in the air. Stick out your tongue while standing in front of a mirror and you can see the taste buds on little protrusions called papillae. Olfaction or smell has been somewhat difficult to study because of the location of the smell receptors. Odor and taste work together: recall how food tasted the last time you had a cold. Several other sensory systems provide information about balance, bodily movements, and pain.
Most research on perception has focused on vision; psychologists have determined that what we perceive depends on factors such as motivation. How we perceive information depends on a number of factors including Gestalt principles of perceptual organization such as figure-ground relationships. If you have ever been in a car stopped alongside a tractor-trailer and felt you were moving forward, you have experienced a figure- ground relationship. Psychologists have also studied a wide range of visual perceptual illusions that seem to fool us and are often fun to decipher.
One of the most controversial areas of psychology is the study of paranormal phenomena— occurrence of behaviors of experiences that cannot be explained by information received by the senses. Is it possible to move a pencil across a table using only the power of the mind? Can some people predict the sum of a series of numbers offered by several randomly selected individuals? Debate about the existence of such phenomena has been going on for decades, with each side equally certain of its stance.