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 of learning 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 be tween 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, or 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.

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