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NEUROSCIENCE OF PSYCHOACTIVE SUBSTANCE USE AND DEPENDENCE

conditioned. The second is an activational property that promotes a state of
motivational arousal. These two properties have their biological roots in the
mesolimbic dopamine system, and act together to direct behaviour towards
goals.

Motivation
Motivation is the allotment of attentional and behavioural resources to stimuli
in relation to their predicted consequences. Motivation therefore involves
learning of predictive relationships (contingencies) between neutral stimuli
and biologically meaningful ones, and between responses and their
outcomes. Learning of these contingencies enables the subject to act in ways
that lead to the most desirable outcomes.

Incentive-motivational responding

Incentive-motivational responding is responding based on the motivation
aroused by an external stimulus. Responding is a function of the perceived
value of the stimulus to the organism. The basis for this form of motivated
responding is hard-wired by evolution in the brain of organisms, including
humans. Thus, certain stimuli such as the taste of a sweet or the cry of a
predator, evoke responses that, depending on the stimulus, involve
approaching or avoiding the object or organism from which they originate.
Incentive-motivational responding is, however, subject to conditioning
principles, and therefore stimuli associated with the primary unconditioned
stimuli can take on incentive-motivational properties. Thus, individuals with
substance dependence may seek out people or environments previously
associated with drug use.
As an example of incentive-motivational responding, consider the earlier
example of the sight or smell of food. If a person is not hungry, this may have
little incentive-motivational value and hardly any attention will be paid to
the food, with no attempt to obtain it. If the person is hungry, the incentive
of food may cause him or her to orient towards the food, to begin to salivate,
and prepare to eat. If extremely hungry, the incentive-motivational value of
the food will be very high, and may cause the person to focus specifically on
the food to the exclusion of other stimuli, to become preoccupied with the
food, and possibly to engage in risky behaviour in order to obtain it. Similarly,
as described in the following sections, once drugs become conditioned
reinforcers, their incentive-motivational value can become higher than all
other competing motivations.

Drug reward alone does not explain drug dependence
The self-administration of drugs for non-therapeutic and non-medical use
is probably as old as human culture and civilization, and testifies that drugs

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