Experience, not childhood or genes, configures individual reward-circuit wiring
"Intriguingly, the pleasure centre and the behaviour it guides are sculpted mostly by life experiences rather than by our genes. This challenges previous assumptions that dopamine function could be straightforwardly inherited." —Paul Stokes, MD, PhD
Our reward center (or circuitry) is shaped primarily by experience. This remarkable finding places in doubt many long held assumptions: addiction is largely inherited, sexual tastes are set in stone, and experiences have little impact on this ancient circuit.
Almost weekly, research reveals the tremendous neuroplasticity of the human brain. However, most prior studies pointed to the adaptability of the larger, more superficial cerebral cortex. The deeper, primitive (limbic) brain was seen as an evolutionary holdover that simply responded to the impulses sent from the higher cortex.
Only addiction neuroscientists highlighted the multiple neuroplastic changes that occur in the reward circuit...of addicts. However, they presumed that a pathological process caused this phenomenon. Now, it appears that addiction is merely an extreme case of neuroplasticity. The same plastic mechanisms are deeply etched by day-to-day experience—especially during adolescence and early adulthood.
A closer look at the new research
Twin studies provide a powerful way of investigating the effects of genetic versus environmental factors. Recently, a UK team led by Paul Stokes, MD, PhD decided to look at the human dopamine system using a cleverly designed twin study.
The researchers divided the brain's dopamine systems into three overlapping regions based on function:
- memory and executive functions (implicated in ADHD),
- movement and coordination (the system damaged by Parkinson's disease), and
- reward (central to motivation, appetites and addiction).
We'll focus primarily on this last system, the reward circuitry, and the unexpected discovery that genes have a rather limited role in how individuals wire up the set of limbic structures that drive sexual, and other, tastes.
Next, the researchers used PET scans to measure dopamine activity in identical twins and fraternal twins (who served as controls) in order to see how similar or different the twins were in all three regions. This was the first twin study on the human dopamine system.
Result? Shared familial environmental factors had very little influence on any dopamine system. The greatest similarity (i.e., heritability) showed up in muscle control. In contrast, the reward function—the one that drives appetites and addiction—showed very little heritability.
In other words, the reward circuit of the brain is more plastic than other portions of the striatum (muscle control or memory). Post-childhood, individual life experience molds our appetites and addictions far more than genes and familial influences do.
As an evolutionary matter, this makes good sense. Our highly plastic reward function allows the brain to adapt with relative ease to environmental influences, especially during adolescence and very early adulthood, when the reward system of the brain is in overdrive. Primate adolescents of one gender or the other (depending upon species) change troops during adolescence. Similarly, humans traditionally exchanged young mates with other tribes. Greater plasticity enabled our ancestors to acclimate rapidly to new tribal sexual mores, local foods, and unfamiliar social hierarchies, as well as learn to pursue accomplishments valued by the new tribe.
Even so, the new finding surprised researchers, who had hypothesized that genetic inheritance would be a stronger influence than environment. The finding also brings into question the popular assumption that what one finds sexually rewarding is innate and cannot be consciously molded during adolescence or adulthood.
However, the new finding is consistent with the recent work of researcher James G. Pfaus, who suggests that rewarding experiences can condition human sexuality. As Pfaus points out, it would make good evolutionary sense for mammals to increase their reproductive success by adapting to new conditions relevant to mating.
In keeping with this point of view, the current researchers observe that the reward function of the brain in humans has already revealed an association with social hierarchy,
"In primates, striatal dopaminergic function can be altered by change in social hierarchy, and in humans striatal dopaminergic function is associated with social status and perceived social support."
In other words, if you are thrust into the role of alpha male, your brain does its best to get you fired up to make the most of your (their) opportunity!
The researchers also note that the new study has implications for understanding data related to neuropsychiatric conditions such as addiction and schizophrenia. Addiction is associated with unusually low dopamine signaling in the reward circuitry, schizophrenia with excessive dopamine signaling. While one may inherit susceptibility to such conditions, it's clear that not everyone who inherits them falls into pathology.
Does this finding also point to windows of vulnerability? Does it mean that certain highly stimulating experiences may wire up the reward circuit in unexpected directions during these windows?
Researcher Paul Stokes explains that the changes to the reward circuit "are normally experiences that happen a little bit later on in life, in adolescence or early adulthood." (emphasis added) That is, the reward circuitry is more malleable, and more influential in shaping our lives, than experts thought. Again, earlier research made it clear that the cortex undergoes tremendous rewiring during adolescence. Stokes's twin research suggests that the reward circuit deep in the brain does too.
In all mammals, adolescence is a time of superlearning as the brain forms new connections and rapidly discards unused nerve connections (prunes itself). In an environment free of synthetic, supernormal stimulation, this would typically have resulted in an adolescent learning vital life skills handed down from tribemates.
In the modern world, however, this accelerated rewiring period is an unusually vulnerable time. Kids can easily choose to expend their superlearning-window on the compelling "lessons" of drugs, Internet porn, video games, and junk food, often with little adult guidance (or relevant adult experience). They tend to marry late, after their reward circuits have lost much flexibility.
This research suggests that heavy adolescent Internet use may be riskier than we realized—especially highspeed porn use because of its impact on mating behavior. During adolescence, the reward function of the brain is naturally in overdrive, and more vulnerable to addiction (erratic, unhelpful wiring). And now it appears that the adolescent reward circuit also lacks the compass that a more fixed genetic blueprint would provide.
This new finding (reward circuitry flexibility during adolescence) lines up neatly with our earlier posts, especially:
The current research may help explain why young guys with porn-related ED need far longer to recover than older guys. They've wired to pixels during a time when their brains were especially plastic. To recover, they often have to stop sexual activity completely for a time, that is, unhook their feelings of reward from previous sexual activity, and then rewire to real partners. At the moment, older guys recover faster because they wired to real partners prior to highspeed Internet. Those earlier brain circuits are still in place. When the intense stimulation of ever novel porn is no longer competing, the earlier attractions revive.
This research may also help to explain why young guys who claim not to be addicted still sometimes develop sexual dysfunctions (which abate after they stop using Internet porn).
"Born to be an addict" won't fly, because environmental influences on addiction now appear to be more powerful than genes. It's evident why highspeed porn use could have serious effects on adolescents while the same use would have less impact on older adults.
For years, experts have pointed out that there is extreme growth, rewiring and pruning in the adolescent cortex. But this new study is the first evidence of a corresponding rewiring deep in the ancient limbic structures that govern reward.
It's consistent with other evidence of hyperplasticity during adolescence. For example, Delta FosB turns on genes in the reward circuit in response to sustained overconsumption of rewarding stimuli, thus playing a big role in addiction-related brain changes. Delta FosB is naturally higher during adolescence, probably to assist rewiring and learning. Childhood learning has already taken place, so this special adolescent period has a different focus: improving the skills for reproductive success and adulthood.
We suspect the new finding also helps to explain the phenomenon of escalation to unexpected sexual tastes so often reported by today's avid highspeed porn aficionados.
Across history, cultures have exhibited a wide array of sexual practices. There are tribes in Africa that don't masturbate. There are tribes that encourage early marriage and tribes that reserve marriage for older men. There are cultures that practice polygamy and cultures where monogamy is the rule, and cultures that wink at infidelity while others punish it brutally.
Adolescent humans need a large degree of plasticity to wire reproductive strategies to the unique circumstances in which they find themselves. No wonder young brains are unexpectedly wiring to all sorts of never-before-seen things in today's profusion of porn.
Most have to pick their way through a host of synthetic, extremely tempting sexual stimuli that can wire their reward circuitry such that real mates won't fire it up when our heroes finally get up close and personal.
As Nietzsche once wrote,
All philosophers have the common failing of starting out from man as he is now and thinking they can reach their goal through an analysis of him. They involuntarily think of “man" as an aeterna veritas, as something that remains constant in the midst of all flux, as a sure measure of things.
Thanks to recent research, we now know that when it comes to the reward circuitry of the brain what remains constant is that it is malleable. We're hardwired to be adaptable—and especially so during adolescence. It's risky to imagine otherwise.