Hijacking Addiction

Hijacking Addiction Neil Levy (bio) Keywords Addiction, dopamine system, love Neuroscientists and clinicians often speak of addictive drugs ‘hijacking’ the brain. Earp et al. (2017) want to do to the notion of addiction what drugs allegedly do to the brains of addicts; hijack it and put it to other purposes. There are, as they point out, clear commonalities (behaviorally and neutrally) between addiction and being in love. But there are also very important differences. These differences are significant enough to entail that it is at best highly misleading to describe love as an addiction. Hijacking addiction in this way is a move we should resist, both to better understand addiction and love, and to promote better responses to each. If anything justifies the hijacking metaphor with regard to drugs of addiction, it is their effect on the mesolimbic dopamine system. This system, it is widely held, has the role of tracking reward value. When it is working as it is designed (by evolution) to do, it signals unexpected reward. Evidence for this claim comes from work measuring phasic dopamine in the brains of monkeys when they were exposed to a cue followed by a reward (a piece of apple). For naïve monkeys, availability of the reward correlated with a spike in phasic dopamine, but once the association between reward delivery and the cue was learned, the spike in phasic dopamine occurred only in response to the cue, and dopamine returned to baseline on reward delivery (Schultz, Apicella, Scarnati, & Ljungberg, 1992). For the naïve the delivery of the reward is unexpected, but once it has learned the association between the cue and the reward, delivery is exactly what it expects, given the cue. Now it is the cue rather than the reward that indicates that the world is better than expected (if the cue is given but no reward follows, dopamine actually falls below baseline). This work has been replicated in monkey and related work indicates that human mesolimbic dopamine system works in the same way (Corlett et al., 2004; Schultz, Dayan, & Montague, 1997). By one causal route or another, drugs of addiction dysregulate this same system (Carter & Hall, 2012). They directly or indirectly drive up phasic dopamine, or they inhibit dopamine reuptake. It is this fact that justifies the hijacking metaphor: addictive drugs (and perhaps the unpredictable reinforcement patterns associated with gambling; Ross, Sharp, Vuchinich, & Spurrett, 2008) make it impossible for the prediction system to keep track of the reward value of drugs because they alter the very currency that the system uses to measure unexpected reward. Recall that the role of the system is to signal the availability of unexpected reward. For a drug user who (unexpectedly) encounters a cue that signals drug availability (someone they have used with in the past; a locale in which they have used— anything correlated with drug taking), this cue is a signal of unexpected reward. That signal has a host of effects, such as upregulating attention and motivating behavior. That is the system working as it was designed. But drugs have effects on the mesolimbic cortical system that are unique. When [End Page 97] other rewards are consumed—food, sex, and so on—there is no second spike in phasic dopamine and the system adjusts to the actual reward value. But with drugs, there is a second spike of dopamine, every single time the drug is consumed. In effect, the drug is always better than expected. The system tries to accommodate this response by responding more strongly the next time a cue signals drug availability, but because the spike always occurs on consumption, the system cannot adjust to a stable reward value; instead, reward value (as measured by this system) continues to ramp ever upward. This hijacking of the reward prediction system is at the very heart of addiction, and nothing like it is seen with natural rewards like love. The neural and behavioral similarities between love and addiction are relatively superficial. Both may involve strong appetites, strong enough to cause actions that the person may regret. Both centrally involve the ‘reward system’—the mesolimbic dopamine system, which I think is actually better thought of as a prediction...