Introduction
Habituation and sensitization are the two most fundamental and widespread forms of learning in the animal kingdom. According to ethologists, learning is any modification in behavior that results from previous experience, in some way involves the nervous system, and is not caused by development, fatigue, or injury. More advanced forms of learning include association, perceptual or programmed learning, and insight; the two simplest (nonassociative) forms are habituation and sensitization. These two processes can be characterized as behavioral modifications that result from repeated presentation of simple environmental stimuli.
Habituation is a decrease in response to repeated presentation of a stimulus
—an environmental cue that can potentially modify an animal’s behavior via its nervous system. One of the most widely cited examples of this kind of learning involves the startle response exhibited by nestling birds in response to potential predators such as hawks. A young duck, for example, will exhibit an innate startle response whenever a hawk-shaped model or silhouette is passed overhead. With repeated presentation of the model, however, the intensity of the bird’s response will decline as the animal becomes habituated, or learns that the stimulus bears no immediate significance.
Common throughout the animal kingdom and even among some groups of protozoans, habituation is important for preventing repeated responses to irrelevant environmental stimuli that could otherwise overwhelm an organism’s senses and interfere with other critical tasks. In the case of a nestling bird, there is a clear advantage to an alarm response in the presence of a potential predator; however, a continued fixed response would result in an unnecessary expenditure of energy and distraction from other important activities, such as feeding.
In identifying a habituation response, it is necessary to distinguish between true habituation and sensory adaptation and fatigue. These latter two phenomena involve a waning in responsiveness that is caused by temporary insensitivity of sense organs or by muscle fatigue and thus are not considered forms of learning. In contrast, habituation results in a drop in responsiveness even though the nervous system is fully capable of detecting a signal and eliciting a muscle response.
In contrast to habituation, sensitization is the heightened sensitivity (or hypersensitivity) that results from initial or repeated exposure to a strong stimulus. Examples of sensitization include the increased sensitivity of humans to soft sounds following exposure to a loud, startling noise such as a gunshot and the increased responsiveness and sensitivity of a laboratory animal to mild (usually irrelevant) tactile stimulation after an electric shock. Sensitization increases an organism’s awareness and responsiveness to a variety of environmental stimuli, thereby preparing it for potentially dangerous situations.
Comparison of Responses
At first glance, habituation and sensitization seem to be opposite behavioral responses—one a decrease in responsiveness and the other an increase—but in fact, they are physiologically different processes, each with its own set of unique characteristics.
At the physiological level, the two responses are determined by contrasting neurological processes that take place in different parts of the nervous system. Habituation is thought to take place primarily in the reflex arc (or SR) system, which consists of short neuronal circuits between sense organs and muscles. In contrast, sensitization is assumed to occur in the state system, or that part of the nervous system that regulates an organism’s state of responsiveness. The SR system controls specific responses, while the state system determines an organism’s general level of readiness to respond. The interaction between habituation and sensitization and these systems determines the exact outcome of a response. At the cellular level, habituated sensory neurons produce fewer neurotransmitters on the postsynaptic membrane, while sensitized neurons are stimulated by other neurons to increase neurotransmitter production and hence responsiveness of the nerves. Thus, while their ultimate neurological effects seem linked, the mechanisms by which such effects are achieved are quite different.
Other important differences between habituation and sensitization include contrasting recovery times, opposite patterns of stimulus specificity, and differences in responsiveness to stimulus intensity. Sensitization is generally characterized by a short-term or spontaneous recovery, as are some cases of habituation. In certain situations, however, recovery from habituation may take several days, and even then it may result in incomplete or less intense responses.
Habituation is usually elicited by very specific sign stimuli, such as certain colors, shapes, or sounds. Thus, even after complete habituation to one stimulus, the organism will still respond fully to a second stimulus. Sensitization, on the other hand, can be characterized as a more generalized response, one in which a single stimulus will result in complete sensitization to a variety of stimuli. Such fundamental differences between these two learning processes reflect differences in their function and survival value. It is a clear advantage to an organism to increase its general awareness of a variety of stimuli, as occurs in sensitization, once it is alarmed. A similar generalized pattern of habituation, however, would shut down the organism’s sensitivity to many important stimuli and possibly put the organism in danger.
A final important difference between habituation and sensitization is the manner in which the two processes are affected by stimulus strength. Habituation is more likely to occur if the repeated stimulus is weak, while sensitization will occur when the stimulus is strong.
These various characteristics have important survival implications, especially for species that rely on stereotypic responses to avoid predation and other life-threatening situations. They ensure that the response is elicited in a timely fashion, that the animal is returned to a normal state in a relatively short period of time, and that the animal is not overwhelmed with sensory input.
Aplysia Research
Habituation and sensitization have been studied in a variety of contexts and in a number of organisms, from simple protozoans, such as the genus Stentor, to human subjects. Such studies have focused on the adaptive significance of these simple learning processes, their neurological control, and the range of behavioral responses that result from interaction between these two forms of learning.
One particular organism in which the neurological basis of habituation and sensitization has been extensively studied is the Aplysia genus of marine slugs. Eric Kandel
and his associates at Columbia University showed that when the mantle of this organism is prodded, the slug quickly withdraws its gills into a central cavity; but after repeated prodding, it learns to ignore the stimulus—that is, it becomes habituated. Conversely, when the slug is stimulated with an electric shock, its sensitivity to prodding increases greatly, and it withdraws its gills in response to even the slightest tactile stimulation—that is, it becomes sensitized.
Because Aplysia possesses only a few large neurons, it is an excellent organism in which to study the physiological basis of learning. Capitalizing on its unique system, Kandel and his colleagues have been able to establish the neurological changes that accompany simple forms of learning. In the case of habituation, they have shown that repeated stimulation interferes with calcium-ion channels in the nerve that, under normal circumstances, causes synaptic vesicles to release neurotransmitters, which in turn relay a nervous impulse between two neurons. Thus, habituation results in a blocking of the chemical signals between nerves and thereby prevents gill withdrawal. When Aplysia is stimulated (or sensitized) by an electric shock, on the other hand, an interneuron (a closed nerve circuit contained within one part of the nervous system) stimulates the sensory neuron by opening calcium-ion channels, increasing neurotransmitter production, and promoting gill withdrawal. Thus, the proximate neurological changes that take place during sensitization and habituation are nearly opposite, but they are achieved by very different neurological circuits.
Studies of the Sucking Reflex
A second area in which habituation and sensitization responses have been the subject of extensive investigation is the sucking reflex exhibited by human infants. When the cheeks or lips of a young child are touched with a nipple or finger, the infant will automatically begin sucking. In a study designed to explore how various stimuli affect this reflex, it was shown that babies respond much more vigorously to a bottle nipple than to the end of a piece of rubber tubing. In addition, repeated presentation of a bottle nipple causes an increase in sucking response, whereas repeated stimulation with rubber tubing causes a decrease in sucking. The sensitized or elevated response to a rubber nipple is a result of activation of the state system, which increases the baby’s awareness and readiness to respond. Sensitization, however, does not occur when the baby is stimulated with rubber tubing, and instead the child habituates to this stimulus.
Role in Emotional Reactions
In addition to influencing simple innate behaviors such as sucking reflexes and withdrawal responses, habituation is believed to be responsible for a number of more complex emotional reactions in humans. Explanations for the effects of habituation on emotions are derived primarily from the opponent process theory of motivation.
The opponent process theory holds that each emotional stimulation, or primary process, initiated by an environmental stimulus is opposed by an internal process in the organism. The emotional changes that actually occur in the organism are predicted to result from the net effect of these two processes. The opponent process detracts from the primary process, and summation of the two yields a particular emotional response. It is hypothesized that when the organism is repeatedly stimulated, the primary process is unaffected but the opponent process is strengthened, which results in a net reduction in the overall emotional response. In other words, repeated presentation of an emotion-arousing stimulus results in habituation in the emotional response, primarily as a result of the elevated opponent response.
An increase in drug tolerance, which results from repeated usage of a drug, is best explained by this kind of habituation. Habitual users of alcohol, caffeine, nicotine, and various opiate derivatives must consume greater quantities of such drugs each time they are ingested to achieve the same emotional stimulation. Thus, with repeated usage, there is a decline in the overall emotional response. This decline in the euphoric effects of a drug is primarily the result of an increase in the opponent process, which can be characterized as the negative effects of the drug. This is presumably why habitual users experience more severe physiological problems, such as headaches or delirium tremens, on termination of a drug.
Similar patterns of habituation have also been suggested to explain the human emotional responses associated with love and attachment and the extreme feelings of euphoria derived from various thrill-seeking activities such as skydiving. Thus, while habituation and sensitization are simple forms of learning, they may be involved in a variety of more complex behaviors and emotions as well.
Interaction of Learning and Instinct
Studies of habituation and sensitization have been especially helpful in clarifying the physiological and genetic mechanisms that control various forms of learning. Such investigations have also shown that habituation and sensitization are widespread phenomena with tremendous adaptive significance throughout the animal kingdom.
Ethologists, in marked contrast to psychologists (especially behaviorist psychologists), historically have emphasized the importance of underlying physiological mechanisms in the regulation of various behavioral phenomena. Traditionally, they argued that many forms of behavior are not only genetically determined, or innate, but also further constrained by the physiological hardware of the organism. They held that psychologists completely ignored these factors by focusing only on the input and output of experiments. Psychologists, on the other hand, have maintained that nearly all forms of behavior are influenced in some way by learning. These contrasting views, which developed largely as a result of different experimental approaches, eventually gave way to a more modern and unified picture of behavior.
One area of research that greatly facilitated this unification was the study of habituation and sensitization. By discovering the chemical and neurological changes that take place during these simple forms of learning, neurobiologists succeeded in demonstrating how the physiological environment is modified during the learning process and that such modifications are remarkably similar throughout the animal kingdom. Thus, it became quite clear that an understanding of proximate physiological mechanisms is central to the study of behavior and learning.
Other studies on sensitization and habituation also helped establish the generality of these processes among various groups of animals. They showed that simple forms of learning can occur in nearly all major animal phyla and that these learning processes often result in modification of simple innate behaviors as well as a variety of more complex responses. From these and other studies, it was soon evident that learning and instinct are not mutually exclusive events but rather two processes that work together to provide animals with maximum adaptability to their environment. The kind of learning that occurs during habituation and sensitization allows animals to modify simple, fixed behaviors in response to repeated exposure to environmental stimuli. Habituation allows an organism to filter irrelevant background stimuli, preventing sensory overload and interference of normal activities critical to its survival, while sensitization helps increase an organism’s awareness of stimuli in the face of potentially dangerous situations.
These two forms of learning represent important behavioral adaptations with tremendous generality in the animal kingdom. Even in humans, a variety of seemingly complex behaviors can be attributed to interactions between sensitization and habituation and the simple neurological changes that accompany them.
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