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KeywordsSensory Preconditioning Hierarchical Association Higher-order Association Long-delay Conditioning Fear Dogs
Gradual development of an association between events that are encountered together.
Events that have biological relevance to the organism can elicit reflexive responses (e.g., food can elicit salivation). Through a similar process, organisms may produce responses in order to bring forth or prevent the occurrence of a consequence (e.g., following a command to obtain a food reward). In either situation, a link or association between the two events develops, and responses consistent with this association are observed. These responses have the important function of preparing the organism for an impending biologically relevant event or better suiting the organism to maximize reward and minimize punishment in its environment. The process by which paired events become associated is known as acquisition.
What Relationships Are Acquired?
Learning about environmental events can be broadly classified as occurring through classical conditioning or instrumental conditioning. In classical conditioning, an environmental event (e.g., a tone) consistently signals the occurrence of a (usually) biologically significant event that naturally produces a response. Consequently, the environmental event also comes to elicit a response (Pavlov 1927). For example, food is an unconditioned stimulus (US) that produces an unconditioned response (salivation). Through repeated pairings, an association between the tone and food is acquired, and the tone begins to elicit a salivation response conditional on having been paired with the US. Thus, the tone becomes a conditioned stimulus (CS) that produces a conditioned response (CR) of salivation. Instrumental conditioning is a related process but, in this situation, the association that is acquired is between a response (R) produced by the subject and the consequence of that response (reinforcement or punishment; SR; cf. Skinner 1938). For example, a child who cries (R) at the grocery store and receives a lollipop to comfort her (SR) may acquire an association between crying and obtaining candy, which will reinforce (increase the likelihood of observing in the future) the response of crying.
Parameters That Affect Acquisition of an Association
Contingency. Contingency refers to the likelihood that two events will occur together as opposed to apart (Rescorla 1968). Some stimulus-outcome or response-outcome pairs are encountered together more often than apart, that is, they have a positive contingency. For example, it is very likely that a fall will result in pain, that is, falling and experiencing pain have a strong positive contingency. In contrast, it is very unlikely that a fall will result in receiving a static electricity shock; thus, falling and experiencing a shock have a very weak (or near zero) positive contingency. Not surprisingly, we are more likely to acquire an association between falls and pain than between falls and shocks. Responses and outcomes also share some level of contingency. For example, if a child is ignored (outcome) every time she throws a temper tantrum (response), the strong positive contingency between the two events will make it more likely that an association between them is acquired than if they shared a weak positive contingency (e.g., if the child was only occasionally ignored when throwing a tantrum). It is also possible that events occur more often apart than together. For example, it is very unlikely that a fall will result in muscles becoming relaxed. Thus, there is a strong negative contingency between falling and experiencing relaxation. In this case, an association is also acquired, but between the occurrence of a stimulus (or response) and the omission of the outcome. Although there may be some controversy as to whether associations can be acquired to omitted events, some theorists assume that unfulfilled expectations can act as outcomes (e.g., Rescorla and Wagner 1972).
Stimulus duration. In general, long antecedents (stimuli or responses) and/or long outcomes reduce the strength of the response that their association produces. This is not to say that long events do not result in the acquisition of associations, but it is possible that only the portions of those long events that are temporally contiguous to each other become associated. For example, in inhibition of delay (or long-delay conditioning; Fig. 1) a long stimulus predicts an outcome in its final stages. Usually, subjects come to respond to the final segment of the stimulus, suggesting that they acquire information about stimulus duration and outcome timing. This suggests that “stimuli” and “responses” can be viewed as a sequence of events rather than a single, discrete event (see Discrete vs. real-time acquisition below).
Stimulus salience and relevance. In general, stimuli that are more salient or intense acquire associations to their outcomes faster than stimuli that are less salient or intense. In turn, more salient outcomes result in stronger responses than less salient outcomes (see Fig. 2). Operant responses cannot be assessed in terms of salience, but the extent to which they “belong” with the outcome can determine the strength of the association developed between response and outcome (e.g., Breland and Breland 1961). For example, a pigeon will be much faster to learn to peck to obtain food (the response of pecking has high “belongingness” with the outcome of food) than it is to coo in order to obtain food (the cooing response has low “belongingness” with the outcome of food).
Acquisition of Higher-Order and Hierarchical Associations
Acquisition Versus Performance
It may be apparent from the previous section that acquisition of an association is not equivalent to behavioral performance based on that association. The examples described above presented a direct association between acquisition and performance: A tone paired with food came to produce a salivation response or a dog performed a sitting response to obtain a treat. However, there are also many situations in which there appears to be little relationship between acquisition and performance. For example, an individual on his way to work may often encounter a woman walking her dog. There may be no overt behavior to either woman or dog, but an association may be acquired between the woman and her dog. This association may become evident if, later on, the man develops a phobia of dogs and has a strong emotional reaction when encountering the woman by herself (this is known as sensory preconditioning because subjects can acquire associations between sensory events prior to conditioning; Brogden 1939). Thus, despite the fact that there was no overt behavior to demonstrate it, there was an association acquired between two sensory stimuli that were experienced together (the woman and dog). Furthermore, there are also situations in which there appears to be an inverse relationship between acquisition and performance. Situations characterized by a negative contingency between the stimulus or response and outcome may prevent a response (this is known as conditioned inhibition; Savastano et al. 1999). Thus, even if acquisition has taken place, behavioral performance based on that acquisition may not reflect the nature or strength of the association that was acquired (Miller and Escobar 2001).
Discrete Versus Real-Time Acquisition
At the theoretical level, there are many different views of acquisition. Some theories suggest that acquisition can be modeled as associations that acquire set amounts of strength in each trial. For example, Rescorla and Wagner (1972) suggested that acquisition is a function of the extent to which the outcome is still surprising (i.e., “how much” of the outcome is not yet predicted by the stimulus). Changes in predictability of the outcome (or associative strength) are presumed to reflect acquisition of the association. This approach is simple and predicts many phenomena, but it assumes that acquisition and performance have a direct relationship, which does not allow the model to explain situations in which an association may exist but it may not be observed in behavior (e.g., sensory preconditioning). Furthermore, a discrete-trials approach ignores the fact that time is a continuum and acquisition could be taking place each instant an event is experienced. Indeed, real-time approaches assume that all events can be divided into microelements, each of which can acquire different associations to other microelements within and across events (e.g., Wagner and Brandon 2001). Although these approaches are clearly more realistic, they are also extremely complex and have their own difficulties, such as which time units should be used to determine the duration of the microelements.
How Does Acquisition Take Place in the Brain?
Acquisition is the basic process that underlies learning and memory. When two events (e.g., a cue and outcome) are paired, their neural signature is activated together and this activity leads to the formation or new synapses and/or strengthening of existing synapses. Classic studies conducted by Eric Kandel in the 1960s with the sea slug Aplysia californica confirmed that experience not only results in changes in behavior but also results in cellular changes that can be correlated to the acquisition of new behavior. Experience with paired events can also potentiate synaptic transmission, making it more effective. Thus, subsequent experience of the events can activate the synapses more effectively (a process known as long-term potentiation; e.g., Rogan et al. 1997). This increased synaptic effectiveness parallels acquisition of the response, suggesting that the two processes (response acquisition and synaptic strengthening) are closely related.
Acquisition is a fundamental process that allows organisms to incorporate information about the environment and use it at a later time to adapt their behavior to the current situation. Many properties of the events that enter into associations determine the rate and extent of acquisition of the association. Associations will be stronger among events that occur in high temporal and spatial proximity (high contiguity), reliably occur together (high contingency), occur together many times (high number of pairings), and which naturally “belong” together (high relevance). Events that are absent can also enter into associations if one of their associates is present in a learning situation, creating higher-order associative links. How acquired associations translate into actual behavior is still a matter of debate because what was learned (acquisition) may not map directly onto behavior (performance). Modeling acquisition is a complicated endeavor because, although acquisition is often represented as resulting in discrete changes in behavior, organisms perceive and use information in real time. Acquisition results in changes in effectiveness of the synapses involved in detecting the relationship between events, suggesting that acquisition is a psychological process with clear physiological implications.
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