For obvious and sensible reasons, nature has evolved neurochemicals with which to reward or discourage particular behaviours, which is why we find intrinsic pleasure in things such as sex and nice food. But the system is open to abuse: there are drugs that imitate or trick the brain into dispensing the neurochemicals of reward. Addiction to those drugs is a corrupted form of learning. If you come to associate the use of a particular drug with feeling good, you can come to depend upon it in order to feel good. That's basic Pavlovian conditioning.
And if only things were that simple, addiction would not be such a damaging social problem, and Marc Lewis's memoir would not be anywhere near so full or fascinating. Professor Lewis misspent his youth tripping on acid and acquiring the taste for heroin with a ménage à trois called "the Heap" in late-Sixties San Francisco; haunting the opium dens of Calcutta; and burgling homes and hospitals in Toronto to feed an addiction to pharmaceutical-grade opiates. He was arrested on several occasions, and almost died on several others. But he's now a respectable and respected neuroscientist, and as distinct from other drug-abuse memoirists, views these misadventures through the prism of scientific understanding.
There is a new cross-sectional diagram of the brain every chapter or so, and a lot of chemicals to remember and acronyms to keep straight. (In fact, a glossary would have been nice.) But his understanding of the biological underpinning lends a welcome precision to Lewis's descriptions of altered states of consciousness. (If anyone has come up with a better image of the inside of a stoner's head than "a mansion of ramifying corridors", I've yet to read it.) And the picture of his brain activity with which Lewis furnishes us is at just the right resolution for an interested lay reader. He patiently guides along the bidirectional neural pathways that explain how an addict gets caught in a feedback loop of anticipatory craving. "The biological laws of synaptic sculpting and neurochemical enhancement, each reinforcing the other, are what constrict the addict's mind, his behaviour, his hopes [and] his dreams," Lewis explains. "I wish this was just an exercise in biological reductionism, or neuroscientific chauvinism, but it's not. It's the way things really work."
So Lewis's brain wiring did not lead to his addictions any more directly than the fact that he was lonely at boarding school. But there was a complex self-reinforcing interplay between the neural substrate of his emotions, and his subsequent actions. And it's only after zooming into that extra level of explanatory detail that one can make sense of some of an addict's more irrational patterns of behaviour. In fact, without it, all other addiction memoirs – if not all human narratives – are liable to seem like they only tell half of the story.