Learned behavior

Learned behavior is another class of behavior exhibited by invertebrates. The reasons for studying learning in invertebrates are varied and include gaining further knowledge of how biochemistry and physiology affect the process of learn ing, searching for similarities and differences within and between phyla, and using learning paradigms to explore applied and basic research questions (e.g., how pesticides influence the foraging behavior of the honey bee).

The term learning, like the term behavior, has several definitions. When reviewing studies of learning, the reader should be aware that definitions may vary from researcher to researcher. For example, a researcher may consider behavior controlled by its consequences (i.e., behavior that is rewarded or punished) as an example of operant behavior, while others believe that it depends upon the type of behavior being modified. Moreover, some believe that any association between stimuli represents an example of Pavlovian conditioning, while others believe that the "conditioned stimulus" must never elicit a trained response prior to the process of association.

We will define learning as a relatively permanent change in behavior potential that comes as a result of experience. This definition contains several important principles. First, learning is inferred from behavior. Second, learning is the result of experience. Third, temporary fluctuations are not considered learning; rather, the change in behavior identified as learned must persist as such behavior is appropriate. This definition excludes changes in behavior produced as the result of physical development, aging, fatigue, adaptation, or circa-dian rhythms. To better understand the process of learning in invertebrates, many behavioral scientists have divided the categories of learning into non-associative and associative.

Non-associative learning

This form of behavior modification involves an association developing from one event, as when the repeated presentation of a stimulus leads to an alteration of the frequency or speed of a response. Non-associative learning is considered to be the most basic of the learning processes and forms the building blocks of higher types of learning in metazoans. The organism does not learn to do anything new or better; rather the innate response to a situation or to a particular stimulus is modified. Many basic demonstrations of non-associative learning are available in scientific literature, but there is little sustained work on the many parameters that influence such learning (e.g., time between stimulus presentations, intensity of stimulation, number of repeated trials). There are two types of non-associative learning: habituation and sensitization.

HABITUATION

Habituation refers to a reduction in the response elicited by a stimulus as it is repeated. For a decline in responsiveness to be considered an instance non-associative learning, it must be determined that any decline related to sensory and motor fatigue do not exert an influence.

Studies of habituation show that it has several characteristics, including the following:

1. The more rapid the rate of stimulation is, the faster habituation occurs.

2. The weaker the stimulus is, the faster habituation occurs.

3. Habituation to one stimulus will produce habituation to similar stimuli.

4. Withholding the stimulus for a long period of time will lead to the recovery of the response.

SENSITIZATION

Sensitization refers to the augmentation of a response to a stimulus. In essence, it is the opposite of habituation and refers to an increase in the frequency or probability of a response. Studies of sensitization show that this process has several defining characteristics, including the following:

1. The stronger the stimulus is, the greater the probability that sensitization will be produced.

2. Sensitization to one stimulus will produce sensiti-zation to similar stimuli.

3. Repeated presentations of the sensitizing stimulus tend to diminish its effect.

Associative learning

A form of behavior modification involving the association of two or more events, such as between two stimuli, or between a stimulus and a response is referred to as associative learning. This form of learning allows a participant to aqcuire the ability to perform a new task, or improve on their ability to perform a task. Associative learning differs from non-associative learning by the number and kind of events that are learned and how the events are learned. Another difference between the two forms of learning is that non-associative learning is considered to be a more fundamental mechanism for behavior modification than those mechanisms present in associative learning; examples of these differences can easily be found in the animal kingdom. Habituation and sensitization are present in all invertebrates, but classical and instrumental conditioning seems to occur first in flatworms (phylum Platyhelminthes). In addition, the available evidence suggests that the behavioral and cellular mechanisms uncovered for non-associative learning may serve as building blocks for the type of complex behavior characteristic of associative learning. The term associative learning is reserved for a wide variety of classical, instrumental, and operant procedures in which responses are associated with stimuli, consequences, and other responses.

CLASSICAL CONDITIONING

Classical conditioning refers to the modification of behavior in which an originally neutral stimulus—known as a conditioned stimulus (CS)—is paired with a second stimulus that elicits a particular response—known as the unconditioned stimulus (US). The response which the US elicits is known as the unconditioned response (UR). A participant exposed to repeated pairings of the CS and the US will often respond to the originally neutral stimulus as it did to the US. Studies of classical conditioning show that it has several characteristics, including the following:

1. The more intense the CS is, the greater the effectiveness of the training.

2. The more intense the US is, the greater the effectiveness of the training.

3. The shorter the interval is between the CS and the US, the greater the effectiveness of the training.

4. The more pairings there are of the CS and the US, the greater the effectiveness of the training.

5. When the US no longer follows the CS, the conditioned response gradually becomes weaker over time and eventually stops occurring.

6. When a conditioned response has been established to a particular CS, stimuli similar to the CS may elicit the response.

INSTRUMENTAL AND OPERANT CONDITIONING

Instrumental and operant conditioning refer to the modification of behavior involving an organism's responses and the consequences of those responses. In order to gain further understanding of this concept it may be helpful to conceptualize an operant and instrumental conditioning experiment as a classical conditioning experiment in which the sequence of stimuli and reward is controlled by the behavior of the participant. Studies of instrumental and operant conditioning show that they have several characteristics, including the following:

1. The greater the amount and quality of the reward, the faster the acquisition is.

2. The greater the interval of time between response and reward, the slower the acquisition.

3. The greater the motivation, the more vigorous the response.

4. When reward no longer follows the response, the response gradually becomes weaker over time and eventually stops occurring.

Non-associative and/or associative learning has been demonstrated in all the invertebrates in which it has been investigated, including planarians and many protostomes (poly-chaetes, earthworms, leeches, water fleas, acorn barnacles, crabs, crayfish, lobsters, cockroaches, fruit flies, ants, honey bees, pond snails, freshwater snails, land snails, slug, sea hare, and octopus). While there is no general agreement, most behavioral scientists familiar with the literature would suggest that the most sophisticated examples of learning occur in several of the protostome taxa (crustaceans, social insects, gastropod mollusks, and cephalopods). Many of the organisms in these groups can solve complex and simple discrimination tasks, learn to use an existing reflex in a new context, and learn to control their behavior by the consequences of their actions.

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