Subconscious Signals Can Trigger Drug Craving
A team of researchers at the University of Pennsylvania, led by Dr. Anna Rose Childress and Dr. Charles O’Brien, showed cocaine patients photos of drug-related cues like crack pipes and chunks of cocaine. The images flashed by in just 33 milliseconds — so quickly that the patients were not consciously aware of seeing them. Nonetheless, the unseen images stimulated activity in the limbic system, a brain network involved in emotion and reward, which has been implicated in drug-seeking and craving.
“This is the first evidence that cues outside one’s awareness can trigger rapid activation of the circuits driving drug-seeking behavior,” said NIDA director Dr. Nora Volkow. “Patients often can’t pinpoint when or why they start craving drugs. Understanding how the brain initiates that overwhelming desire for drugs is essential to treating addiction.”
To verify that the patterns of brain activity triggered by the subconscious cues reflected the patients’ feelings about drugs, Childress and her colleagues gave the patients a different test two days later, allowing them to look longer at the drug images. The patients who demonstrated the strongest brain response to unseen cues in the fMRI experiment also felt the strongest positive association with visible drug cues. Childress notes, “It’s striking that the way people feel about these drug-related images is accurately predicted by how strongly their brains respond within just 33 milliseconds.”
Childress and her colleagues also found that the regions of the brain activated by drug images overlapped substantially with those activated by sexual images. This finding supports the scientific consensus that addictive drugs usurp brain regions that recognize natural rewards needed for survival, like food and sex.
According to Childress, these results could improve drug treatment strategies. “We have a brain hard-wired to appreciate rewards, and cocaine and other drugs of abuse latch onto this system. We are looking at the potential for new medications that reduce the brain’s sensitivity to these conditioned drug cues and would give patients a fighting chance to manage their urges.”
PLoS ONE 3(1): e1506. doi:10.1371/journal.pone.0001506
The human brain responds to recognizable signals for sex and for rewarding drugs of abuse by activation of limbic reward circuitry. Does the brain respond in similar way to such reward signals even when they are “unseen”, i.e., presented in a way that prevents their conscious recognition? Can the brain response to “unseen” reward cues predict the future affective response to recognizable versions of such cues, revealing a link between affective/motivational processes inside and outside awareness?
We exploited the fast temporal resolution of event-related functional magnetic resonance imaging (fMRI) to test the brain response to “unseen” (backward-masked) cocaine, sexual, aversive and neutral cues of 33 milliseconds duration in male cocaine patients (n = 22). Two days after scanning, the affective valence for visible versions of each cue type was determined using an affective bias (priming) task. We demonstrate, for the first time, limbic brain activation by “unseen” drug and sexual cues of only 33 msec duration. Importantly, increased activity in an large interconnected ventral pallidum/amygdala cluster to the “unseen” cocaine cues strongly predicted future positive affect to visible versions of the same cues in subsequent off-magnet testing, pointing both to the functional significance of the rapid brain response, and to shared brain substrates for appetitive motivation within and outside awareness.
These findings represent the first evidence that brain reward circuitry responds to drug and sexual cues presented outside awareness. The results underscore the sensitivity of the brain to “unseen” reward signals and may represent the brain’s primordial signature for desire. The limbic brain response to reward cues outside awareness may represent a potential vulnerability in disorders (e.g., the addictions) for whom poorly-controlled appetitive motivation is a central feature.