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3. BIOBEHAVIOURAL PROCESSES UNDERLYING DEPENDENCE

serve as positive reinforcers (Johanson & Uhlenhuth, 1978). Additionally, the
property of eliciting pleasurable feelings also indicates that drugs are indeed
rewarding. Rewarding properties of drugs do not necessarily consist of sheer
sensations of pleasure like the “high” or the “rush” typical of amphetamine
and heroin or of inhaled crack (cocaine base) but can take milder forms of
hedonia, such as relief of tension, reduction of fatigue, increased arousal, or
improvement of performance. These positive sensations can explain why
drugs are used, but not necessarily why they can produce the behavioural
repertoire characteristic of dependence. In particular, drug reward alone
cannot account for drug dependence, a condition characterized by
compulsive, relapsing drug use and focusing of motivated behaviour on drugs
to the exclusion of alternative goals and in the face of familiar, social and
medical problems.
Clearly, the rewarding properties of drugs, at least as we understand them
from their comparison with conventional rewards, do not fully explain the
behavioural abnormalities associated with their use.
In the context of dependence, it is important to remember that over a
lifespan many people experiment with a variety of potentially dependence-
producing drugs, but most do not become dependent. Therefore, the question
specifically becomes:
—what is the process by which drug-taking behaviour, in certain
individuals, evolves into compulsive patterns of drug-seeking and
drug-taking behaviour that take place at the expense of most other
activities?

— what accounts for the inability of some compulsive drug users to stop
using drugs?
A complex interplay of psychological, neurobiological and individual
factors appears to be responsible. This section will cover some of the general
principles concerning effects of psychoactive substances on learning and
motivational processes that may come into effect during the development of
dependence. Clearly, an individual’s genetic and environmental background
will influence the ultimate behavioural expression of these influences. These
factors will be considered separately in other sections of this report. The
following discussion is intended to provide information on how substance
use interacts with motivational systems in the brain to contribute to the
development of dependence.

Drug dependence as a response to incentive-motivation

While not sufficient, the rewarding properties of drugs are nonetheless
necessary for their dependence-producing effects for at least two reasons.
First, drug reward, by promoting drug self-administration, is necessary for
repeated drug exposure. Secondly, the rewarding properties of drugs are
necessary for attributing – by an associative learning mechanism – positive

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