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Tuesday April 22nd 2014


Aversive Stimuli

Aversive Stimuli

Aversive stimuli can be defined as any cues or events that produce negative emotional feelings or a negative outcome. Any stimulus can potentially be considered aversive, because it is the production of the undesirable feeling associated with the stimulus that determines whether it is aversive or not. In general terms, the aversive stimulus most often attenuates or eliminates behavior that is paired with that aversive stimulus. As such, aversive stimuli are importantly related to various principles of learning and have major implications for a number of basic and applied research and therapeutic settings.

Aversive stimuli are a key element to escape learning (i.e., the type of learning that is based on negative reinforcement). For instance, animals can learn to escape electric footshock by pressing a bar or moving to a different part of their environment. Aversive experiences are also very important for human learning. The human nervous system has evolved so that even newborns are prepared to find certain types of sensory experiences aversive, so that those aversive stimuli will be avoided thereafter. A wealth of research data indicate that any number of additional stimuli that precede the aversive stimulus can, themselves, become conditioned aversive stimuli. The presentation of the conditioned aversive stimulus can cause a number of behavioral responses that reflect avoidance of, or escape from, the conditioned stimulus. A response that prevents the occurrence of an aversive stimulus, because of the presence of a conditioned aversive stimulus, is called avoidance conditioning. Avoidance learning generally occurs after escape learning has occurred, such that the response is eventually made before the aversive stimulus is encountered, so that the aversive stimulus will be entirely avoided. The relationship between escape and avoidance learning is highlighted in the two-stage theory of learning: The first stage involves classical conditioning of fear, and the second stage involves operant conditioning and the reduction of fear by avoidance responding. The negative emotional reaction to aversive stimuli is caused by the activation of parts of the limbic system of the brain, including the amygdala, hypothalamus, and anterior cingulate cortex. In many instances, an aversive stimulus is the same as a punishing stimulus. Indeed, the use of an aversive stimulus is, in many instances, synonymous with punishment. Unlike positive and negative reinforcement, which act to increase the likelihood of a response, punishment acts to reduce the likelihood of a response, or behavior, occurring again. In other words, punishment produces the opposite outcome to reinforcement. In both experiments and applied settings, punishment may well suppress unwanted behavior—but usually only temporarily and only in situations or circumstances in which the punishment seems unavoidable. Punishment can also lead to additional behavioral issues, such as increased aggressiveness or other exaggerated emotional behavior.

Aversive stimuli potentiate certain types of learning in virtually all animals, as demonstrated by the unique ability of noxious (aversive) sensations to cause longlasting and often permanent avoidance of the related stimuli. The Garcia Effect is the avoidance of a novel food item that has been associated with illness after only a single exposure. This type of one-trial learning enables the organism to learn the consequences of consuming that food and thus avoid poisonous food sources in the future. In recent years researchers have probed the central nervous system mechanisms that produce the aversiveness of noxious stimuli. Use of the escape/ avoidance methodology uniquely facilitates an assessment of the aversive nature of noxious sensory events. Damage to limbic system structures, such as the anterior cingulate cortex, has thus been shown behaviorally to decrease the noxiousness of aversive stimuli. Aversive stimuli are used clinically in a type of behavioral therapy called aversion therapy. This approach is based on classical (or Pavlovian) conditioning, so that the undesirable behaviors are paired with stimuli that produce undesirable feelings or outcomes. The goal is to attenuate or eliminate undesirable behavior. For instance, in the treatment of alcoholism, the undesirable behavior of alcohol consumption can be paired with a drug called Antabuse, also known by the generic name disulfiram. Disulfiram in the presence of alcohol produces a number of aversive physical sensations, including flushing, sweating, soreness in the head and neck, nausea, and vomiting. Theoretically, the repeated pairing of alcohol consumption with these aversive sensations will decrease alcohol consumption. However, the use of this and other types of pharmacological aversive stimuli that cause uncomfortable consequences are typically associated with poor compliance. It should be noted that behavioral modification techniques utilizing aversive stimuli are often misunderstood. For instance, many times the same aversive stimulus is associated with either negative reinforcement or punishment. In negative reinforcement, the aversive stimulus precedes the undesirable behavior, whereas in punishment, the aversive stimulus follows the undesirable behavior. In combination with adequate cognitive-behavioral therapy, the use of aversive stimuli to treat substance abuse and other disorders may have utility, but there remains controversy as to the ultimate effectiveness of such an approach.

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