Addiction comes in many forms: drugs, alcohol, cigarettes, and gambling have been the types that traditionally plagued society.
In recent years, the proliferation of technology has led to the rise of addiction to the internet and computer gaming. Even the promotion of a healthy lifestyle has led some to become hooked on exercise.
But do all addictions operate by the same biological mechanism? And is addiction an individual's choice or a disease of the brain?
Scientists have been studying addiction for years in order to improve treatments for harmful behaviour. They have found that powerful memories, often of highly pleasurable or intense experiences, underlie addiction. During such experiences the brain releases a chemical called dopamine that creates a reward circuit in the brain, by logging the intense experience as pleasurable and an important action to be repeated.
Dopamine release generally occurs in a region called the ventral tegmental area (VTA). In response to this, epigenetic changes happen in brain nerve cells to form reward memories. These chemical changes are a mix of DNA methylation and demethylation, which either turns genes off or on.
Such a system allows changes in how genes are expressed in cells without altering our genetic code, and forms a type of genetic memory.
Addiction to all four major classes of abused substances - psychostimulants, opiates, alcohol and nicotine - has been linked to the same parts of the brain associated with normal reward processing.
Because of this, scientists originally thought that drug addiction took over normal reward memory nerve pathways. However, a more nuanced picture is now emerging.
In normal reward processing, the VTA signals to another region of the brain called the nucleus accumbens (NAC).
Dr Alain Dagher, of McGill University, believes that abnormal interactions between different decision-making regions in the brain are the underlying cause of addiction.
A specific region in the brain called the dorsolateral prefrontal cortex regulates feelings of craving in response to cues. For example, a smoker craves a cigarette when they see or smell someone else smoking. By visualising how an addicted person perceives a substance, the imaging results can be used to predict consumption.
Applying a technique called transcranial magnetic stimulation, a method that uses magnets to induce weak electric currents in the brain without any form of surgery, the dorsolateral prefrontal cortex can be inactivated - and the craving response can be altered.
Dagher says: "Policy debates have often centred on whether addictive behaviour is a choice or a brain disease. This research allows us to view addiction as a pathology of choice. Dysfunction in brain regions that assign value to possible options may lead to choosing harmful behaviour."
Read the full article in the South China Morning Post.