In general terms, any event that initiates behavior; or more specifically, any energy change that activates a sense organ.The idea of energy change needs clarification. When we put our hand on a hot radiator, the sudden change in energy level stimulates the nerve endings for warmth and pain embedded in our skin. However, if the temperature of the radiator were the same as the room, it would not stimulate these sense organs, although it might stimulate the touch endings. Stimulation, then, depends on a change in energy level; if the change ceases, the stimulation will also cease. As an example, when we first come into a room, we may notice the ticking of a clock due to the sudden rise in the sound level; but we soon become unaware of the ticking because this level remains so constant. This process of adaptation occurs in all sense modalities. Broadcasters sometimes try to overcome its effect by turning up the volume during commercials. This is an attempt to capture attention by increasing the stimulus level.Psychologists make a distinction between two major types of stimuli. The physical energy which excites the sense organs is termed the distal stimulus, and the activity of the sense organ itself is termed the proximal stimulus. The light energy reflected by the print you are now reading is a distal stimulus, but the chemical changes taking place in your retina constitute a proximal stimulus. This means that we do not actually experience the properties of the distal stimulus, but only its effects on the receptors. Yet, surprisingly, the two can appear the same even when they are different—for example, if a person stands five feet from you he produces a larger proximal stimulus than when he is 75 feet away, yet you do not see him as a full grown man in the first case and as a midget in the second. Instead, you see him as a grown man at different distances from you.We are responsive to only a limited number of stimuli because our receptors are sensitive only to a small range of energy changes. In hearing, this range is 20 to 20,000 cycles per second, although we can respond in other ways than hearing to frequencies that are both below and above that range—for example, earthquake tremors and ultrasonic waves. In vision, our eyes are sensitive to only a small range of electromagnetic waves known as the visible spectrum. We would be living in quite a different world if we could see ultraviolet and radar waves, or hear the sound made by insects, bats, and a gently swaying tree. And our world would be stiff different if we could taste as well as the lowly worm, respond to changes in water pressure as well as fish, and smell as well as a hunting dog.Our receptors are also limited to six different types of stimuli. Our eyes are sensitive to photic energy, our ears to acoustic energy, and receptors in our nostrils and tongue to chemical energy. Some receptors in our skin are sensitive to thermal changes (heat and cold), while others are responsive to mechanical energy (pressure or touch), or to destruction of tissue (pain). Electrical energy has the unique ability of exciting all sense channels, since the nerve impulse is electrical in nature. These six types of stimuli are not so fixed as they sound, since one type of energy may affect the operation of an organ that is primarily geared to another type —for instance, heating the skin makes it more sensitive to mechanical pressure, and a blow to the eye can make us “see stars.”Most stimuli come from the external world, but we are sensitive to internal energy changes as well. Much of our behavior is guided by stimuli which activate receptors embedded in our muscles, joints, and internal organs. If we did not receive them we would not be aware of our need for food or water, or know that our stomach is upset. We could not even tie our shoelaces or type a letter, since these activities consist of a behavioral chain in which each new response produces a feedback effect that becomes the stimulus for the next response.Stimuli play an indispensable part in all psychological experimentation. The standard laboratory technique is to vary a stimulus systematically and observe how this affects a response. If we want to find out whether a two- year-old child can distinguish between square and circular blocks, we might keep hiding a piece of candy under the square one, and interchange the position of the square and circle to determine whether he will respond to the shape alone. If our problem is to determine how internal stimuli affect behavior, we might see whether a rat will learn a maze more quickly when he is hungry than when he is satiated. We might also systematically vary the stimulus by depriving the rat of food for a longer or shorter time to see whether his learning ability changes proportionately.These experiments bring up a final point about stimuli. If the child discriminates correctly between a square and circle, and if the rat’s learning ability changes as his hunger changes, exactly what are they responding to? Is the child responding to the whole cube or only to a part of it—and in the rat experiment, what is the specific stimulus that is actually controlling his behavior? Attempts are often made to pinpoint these “effective stimuli,” as they are called, but in most cases, including the two just given, it is extremely hard to devise experiments that will provide a definitive answer.