3.4: Non-associative Learning

Habituation

Habituation is not to be confused with sensory adaptation, which occurs when a sense organ becomes disabled, such as being temporarily blinded by a camera flash or having your hearing limited due to a loud sound like a gun being fired on a range. It should also not be confused with response fatigue or when a muscle is incapacitated due to repeated use such as weight training one’s legs and then not being able to run in an emergency. Habituation occurs within the nervous system while sensory adaptation and response fatigue occur outside it and in sense organs and muscles, respectively. It is a nearly universal occurrence in the animal kingdom and helps organisms save time and energy by not engaging in behaviors that are not functional.

Habituation generally ends if the stimulus is not present for a period of time and can either be short-term or long-term, indicating the length of time or durability of the habituation effect. When we think about learning, long-term habituation is what we are describing as it is a relatively permanent and stable change in behavior, as the definition of learning from Module 1 stated. The response slowly decreases with repeated stimulation and our ability to respond slowly recovers when the stimulus ends. In contrast, in short-term habituation, the response quickly decreases with repeated exposure to the stimulus and quickly recovers once gone.

Drug tolerance represents a form of habituation. According to the National Institute of Drug Abuse (NIDA), tolerance occurs when a drug is used repeatedly over time. The person no longer responds to the drug the way he or she initially did, meaning that higher doses of the drug are needed to obtain the same effect achieved early on. According to drugabuse.com, tolerance can take three forms. Acute, or short-term, tolerance results from repeated exposure to a drug over a short period of time while chronic, or long-term, tolerance develops when a person adapts to constant exposure to the drug over weeks or months. Finally, learned tolerance may result from frequent exposure to the drug. People who abuse alcohol for months or years may not appear intoxicated to others because they learn to compensate for the effects of alcohol on their coordination by practicing a task repeatedly while intoxicated. This tolerance disappears if the task is altered (Vogel-Sprott, 1997). Kesner and Cook (1983) found that 96 male Long-Evans rats developed tolerance to morphine in a nondistinctive environment due to repeated presentation of the drug, and that tolerance was quicker in massed rather than spaced presentations of the morphine.

Dishabituation occurs when an organism’s state of arousal is enhanced, leading to an increase in the response that previously was habituated. If you habituated to the sound of the clock ticking in your office, you might notice it again if an unrelated, but novel, stimulus presents itself such as a coworker talking to you. Dishabituation represents a separate process from sensitization.

Sensitization

As with habituation, sensitization ends when the stimulus is not present for a period of time. Unlike habituation, sensitization can generalize to similar stimuli. Consider that soldiers returning from war will jump at the sound of a car backfiring as it sounds like an explosion caused by an artillery shell or IED (Improvised Explosive Device).

Sensitization is also important in sexual behavior and responding to tactile stimuli such as kissing or hugging your significant other. When doing so, you become aroused, unlike when you engage in similar behaviors with your mother or a sibling. For instance, one study showed that vibrotactile sensitivity increased in a sample of 30 heterosexual, healthy males after viewing an erotic video, but did not occur in response to a non-erotic video (Jiao et al., 2007).

An Explanation for Habituation and Sensitization

The two processes of habituation and sensitization are controlled by different mechanisms that are described by the dual-process theory (Groves & Thompson, 1970). Habituation occurs due to the S-R system or through changes in neurons controlling our response. The S-R system is best represented by the reflex arc, and subsequent activation of this system causes the build-up of habituation. It is active with each occurrence of an eliciting stimulus.

On the other hand, sensitization occurs due to changes in an organism’s arousal, called the state system, which leads to excitability. The state system consists of those parts of the nervous system involved in our readiness to respond to an environmental event, and only activates due to arousing events.

The Malleability of Emotional States

Given what you have read so far, would it surprise you to learn that emotional states can also change due to the repeated exposure to environmental stimuli? The opponent process theory of emotion (Solomon & Corbit, 1974) states that our primary emotional reaction to an emotion-arousing stimulus, or a process, is followed by an opposite after-reaction, or b process, which counteracts this shift, and the primary reaction becomes weaker or habituates with repeated stimulation, while the after-reaction becomes stronger. This serves to maintain homeostasis or to keep our emotions fairly even or neutral. We seek to minimize highs and lows. For instance, in the first few stimulations opiate users will experience euphoria, a rush, and pleasure (a process) which will be followed by craving and aversive withdrawal signs for a short duration (b process). But after many such stimulations, they will experience a loss of euphoria, normal feelings, and no rush (a process), followed by an intense craving and agony lasting a long time, which is called abstinence agony (b process). Solomon and Corbit (1974) refer to this behavioral phenomenon of addiction as a “ghastly experience” (pg. 123).

Are Habituation and Sensitization Adaptive?

To understand the adaptive value of habituation and sensitization, we need to understand the role of stimulus intensity. Basically, low-intensity stimuli lead to habituation while high-intensity stimuli lead to sensitization. Why is that? Stimuli that are low intensity tend to be non-threatening, such as the fan in your computer running, while stimuli that are high intensity tend to be threatening, such as the sound of a gun firing. What is important is the threshold level of the organism when the first stimulus in a series occurs such that the greater the response the lower the organism’s sensory threshold prior to the stimulus. This initial response level will determine whether habituation or sensitization occurs (Eisenstein, Eisenstein, & Smith, 2001).

And habituation leads to a preservation of valuable energy that can be better spent by the organism. This represents the ability of an organism to change its behavior due to experience, which reflects learning.