*** Appendix: Textbook definitions of learning, habituation, sensitization and dishabituation

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*** SUMMARY of Conclusions reached References

Charles Abramson. Picture courtesy of the Public Information Office, Oklahoma State University.

Learning is commonly defined by psychologists as "a relatively permanent change in behaviour potential as a result of experience" (Abramson, 1994, p. 2). This definition excludes changes in an organism's behaviour that are due to other factors such as physical development, aging, fatigue, sensory adaptation, or circadian rhythms (Abramson, 1994, p. 2). The simplest and most ubiquitous form of learning is non-associative learning, which the Encyclopedia Britannica (1989) defines as "those instances where an animal's behaviour toward a stimulus changes in the absence of any apparent associated stimulus or event (such as a reward or punishment)." Only one kind of event (the stimulus) is involved in this kind of learning. In the case of non-associative learning, "[t]he animal does not learn to do anything new or better; rather, the innate response to a situation or a particular stimulus is modified" (Abramson, 1994, p. 37).

The two main forms of non-associative learning are habituation and sensitization. Habituation can be defined as a "response decrement as a result of repeated stimulation" (Abramson, 1994, p. 106), which is the same as the definition used by Di Primio, Muller and Lengeler (2000, p. 7). Habituation has obvious survival value: it enables an organism to ignore stimuli that do not transmit any biologically significant information (e.g. about food or the presence of a predator), thereby minimizing energy that may be wasted on fleeing from "false alarms" and giving it more time to engage in behaviour that enhances reproduction. Habituation also makes it possible to engage in consummatory behaviour in unpalatable but otherwise suitable situations (Abramson, 1994, p. 107). Habituation has been described as the cornerstone of other forms of learning, since in order to focus on relevant stimuli, one must be capable of ignoring irrelevant information.

Sensitization (which will be discussed in a later section) is "the opposite of habituation and refers to an increase in frequency or probability of a response" to a stimulus (Abramson, 1994, p. 105). Non-associative sensitization occurs when an organism is exposed to an intense stimulus that elicits a strong innate response (e.g. electric shock). Sensitization allows an organism to remain responsive to significant or dangerous stimuli, and to rapidly learn adaptive behaviour patterns (Abramson, 1994, p. 107).

Dishabituation can be defined as a "facilitation of a decremented or habituated response" (Rose and Rankin, 2001, p. 63). It is often used to counteract the effects of fatigue in habituation experiments - e.g. by the administration of an electric shock. Dishabituation was formerly thought to be a special form of sensitization, but research on molluscs and leeches has now confirmed that it is a biologically and behaviourally distinct process (Rose and Rankin, 2001, p. 63).