A biological imperative, faced by all creatures, is to survive long enough to reproduce. Because of this, behavior related to survival and reproduction is often built into the organism. That is, organisms come into the world with a range of behavior that aids survival and reproduction. Creatures that fly to avoid predators are likely born with the ability to fly. Thus, flying does not need to be learned; it results from the organism’s species history. The complex array of motor movement and coordination involved in flying could be learned, but it is much more dependable when this behavior is based on genetic endowment. For most animals, survival at birth depends on being able to breathe, digest food, and move about. When a worm is dangled over a young robin’s head, this stimulus evokes opening the mouth and chirping. The behavior of the chick is the result of biological mechanisms and is released by the sight of the dangling worm. The relationship between the dangling worm (stimulus) and the open mouth (response) is a reflex. Presumably, in the evolutionary history of robins, chicks that presented a gaping mouth and chirped were fed, and those that did not may have been ignored. In humans, reflexive crying by an infant ensures more effective care from the child’s parents. Parents engage in a variety of caretaking behaviors in attempts to stop crying. Usually, parental responses such as changing a wet diaper, feeding, or burping the infant will stop the fussing.
The principles that describe the reflex (and its conditioning) are similar for many different kinds of reflexes. For example, the laws that govern pupil contraction when a light is shined in the eye or principles describing the relationship between a sudden loud noise and a startle response also hold for the salivation produced when you eat a meal. Early work by Sherrington (1906) focused on the reflex and the behavioral laws that he discovered, almost a century ago, still apply to a remarkable variety of stimulus–response relationships. When food is placed in a dog’s mouth, the salivary glands produce saliva. This relationship between food in the mouth and salivation is a reflex that is based on the genetic endowment of the organism and is not learned. Many reflexes serve defensive, protective, or survival functions. Frequently such reflexes are not learned because they have to function before adequate experience is provided.
All organisms are born with a built-in set of reflexes, but many are particular to a species. Thus, humans are born with an array of responses that are elicited by specific stimuli. As illustrated earlier, tactile stimulation of the human infant’s cheek evokes the rooting 3. Reflexive Behavior and Respondent Conditioning response—turning toward the stimulation with mouth open which then receives the nipple. Also, as we have noted, in young robins, the so-called “begging” reflex (open mouth and chirping) serves a similar function—getting fed. Because these relationships are relatively invariant and biologically based, we refer to the eliciting event as the unconditioned stimulus (US). The related behavior following the stimulus is called the unconditioned response (UR). The term unconditioned is used because the reflex does not depend on an organism’s experience or conditioning during its lifetime (i.e., learning). When an unconditioned stimulus elicits an unconditioned response (US → UR), the relationship is called a reflex. Reflexive behavior is automatic in the sense that a physically healthy organism will always produce the unconditioned response when presented with an unconditioned stimulus. You do not choose whether to salivate when you have food in your mouth; the US, which is; “food in the mouth” draws out or elicits the UR of salivation. This is the way the animal (you) is built. However, there are times and conditions described in the following where the US does not elicit the UR. When repeated presentations of the US lead to a reduction of the UR, we call that process habituation.
Laws of the Reflex
Aristotle about 350 b.c. developed principles of association that were re-discovered by psychologists, and in the 1900s by Pavlov a physiologist (Hothersall, 1990, p. 22). Sherrington (1906) studied many different types of reflexes and formulated the laws of reflex action. Because reflexive behavior occurs across most or all animal species from protozoa (Wawrzyncyck, 1937) to humans (Watson & Rayner, 1920) and because associative or respondent conditioning builds on reflexive behavior, it is important to describe the laws of the reflex. The laws are general in that they hold for all eliciting or unconditioned stimuli (e.g., food in the mouth, a touch of a hot surface, a sharp blow just below the knee, a light shining in the eye) and the corresponding unconditioned responses (salivation, quick finger withdrawal, an outward kick of the leg, pupil contraction).
The unconditioned stimuli that elicit unconditioned responses may vary in intensity. For example, light that is shining in the eye may be bright enough to hurt or so faint that it is difficult to detect. A tap below the knee, causing a kick, may vary from a modest to a heavy blow, etc. The intensity of the eliciting US has direct effects on the elicited reflex. These effects are described by the three primary laws of the reflex.
1. The law of the threshold is based on the observation that at very weak intensities a stimulus will not elicit a response, but as the intensity of the eliciting stimulus increases there is a point at which the response is evoked. That is, there is a point below which no response is elicited and above which a response always occurs. The uncertainty region, where roughly 50% of the stimuli that are presented produce a response, is called the threshold.
2. The law of intensity–magnitude describes the relationship between the intensity of the eliciting stimulus and the size or magnitude of the evoked response. As the intensity of the US increases so does the magnitude of the elicited UR. A light tap on the patella tendon (just below the kneecap) will evoke a slight jerk of the lower leg; a stronger tap will produce a more vigorous kick of the leg (the patella reflex). Of course, there are upper limits to the magnitude of the tap. If a hammer is used to smash into the knee, the result is a broken kneecap and no movement for a long time.
3. The law of the latency concerns the time between the onset of the eliciting stimulus and the appearance of the reflexive response. Latency is a measure of the amount Phylogenetic Behavior of time that passes between these two events. As the intensity of the US increases, the latency to the appearance of the evoked UR decreases. Thus, a strong puff of air will elicit a quick blink of the eye. A weaker puff will also elicit an eye blink, but the onset of the response will be delayed.
These three laws of the reflex are basic properties of all reflexes. They are called primary laws, because taken together they define the relationship between the intensity of the eliciting stimulus (US) and the unconditioned response (UR). Reflexes, however, have other characteristics, and one of these, habituation, has been shown in animals as simple as protozoa and as complex as humans.
One of the better documented secondary properties of the reflex is called habituation. Habituation is observed to occur when an unconditioned stimulus repeatedly elicits an unconditioned response and the response gradually declines in magnitude. When the UR is repeatedly evoked, it may eventually fail to occur at all. For example,Wawrzyncyck (1937) repeatedly dropped a 4-g weight onto a slide that the protozoa Spirostomum ambiguum was mounted on. The weight drop initially elicited a contraction, startle response, that steadily declined to near zero with repeated stimulation.
An interesting report of human habituation, in a dangerous setting, appeared in the July 1997 issue of National Geographic. The small island of Montserrat has been home to settlers since 1632. Unfortunately, the relatively silent volcano on the island reawakened in July 1995. Suddenly the quiet life that characterized living on Montserrat was rudely interrupted. Before the major eruption of the volcano, a large group of inhabitants refused to evacuate the island, and these people suffered through several small volcanic explosions.