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3. BIOBEHAVIOURAL PROCESSES UNDERLYING DEPENDENCE

conditioning is important in substance use and dependence, since a person
performs an operant response when choosing to acquire and use a
psychoactive substances to experience its effects. Mesolimbic dopamine
systems are also thought to be important in instrumental learning about the
effects of psychoactive substances.
The following sections will examine aspects of learning theory as they relate
to dependence.

Reinforcer
A reinforcer is commonly defined as a stimulus that strengthens responses
upon which it is contingent (i.e. which it reliably follows). Thus, if one puts
money in a vending machine to obtain a bar of chocolate, the chocolate acts
as a reinforcer for the behaviour of putting money into the machine.

Reward
Reward is a term frequently used in the psychobiology of substance
dependence, to describe the pleasurable or enjoyable effects of a drug. In
general, rewards are stimuli that provide positive motivation for behaviour.
A fundamental feature of rewards is that of transferring their motivational
properties to stimuli that predict their occurrence, and of strengthening
responses upon which they are contingent. For this reason, rewards are
reinforcers. Although many drugs are taken for their pleasure-producing or
“rewarding” properties, this alone cannot account for the entire range of
behavioural processes involved in substance dependence (Robinson
&Berridge, 2000). Many stimuli can serve as rewards, but few take on the
profound, all-consuming value that psychoactive substances do, such that
they can lead to the symptoms and behaviours characteristic of dependence
(see Chapter 1).

Incentive
The term incentive was originally used to refer to the ability of certain stimuli
to elicit species-specific response patterns such as orienting, approaching
or exploring (Bindra, 1974). This term implies that responding is a
consequence of the stimuli (incentives). Accordingly, while reinforcers act as
consequences of responding, incentives act as premises. An example of an
incentive is a stimulus associated with food, such as smell, the sight of a
restaurant, or an advertisement for food. These stimuli may elicit certain
responses that direct attention and behaviour towards the acquisition of the
food, and activate the motivational circuits in the brain in order to acquire
the food. This example illustrates that incentives have two properties. One is
a directional property that promotes responses directed towards the
incentive, and towards the reward to which the incentive has been

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