Often dismissed as an irrational or overinflated sense of concern, paranoia extends from a very rational ability to keep our wits about us in a chaotic environment.
Being able to adapt quickly can keep us alive when circumstances do change. At an extreme, however, delusional beliefs in the ill intentions of others can be socially isolating, robbing people of the ability to maintain healthy relationships and hold down jobs.
To better understand why some brains figuratively jump at shadows more than others, a team of researchers led by Yale University psychiatrists Praveen Suthaharan and Summer Thompson analyzed the results of a simple test on a group of male rhesus macaque monkeys and human volunteers.
Called a probabilistic reversal learning (PRL) task, the test revolved around the selection of a symbol for a chance of a reward – food for monkeys, and points for humans.
Different symbols granted varying chances of success, so given a choice of three on a screen, the subject was given an opportunity to learn which symbol was most likely to earn a reward.
Just when the subjects thought they had it all figured out after half of the test runs, the trial outcomes were flipped, with the luckiest symbol paying out rewards less frequently, and the unluckiest symbol now becoming the optimal choice.
"So participants have to figure out what's the best target, and when there's a perceived change in the environment, the participant then has to find the new best target," says Yale psychologist Steve Chang.
Six of the 20 macaques had undergone a neurological procedure previously in separate studies that affected either their dorsal thalamic nuclei – a region thought to play a role in planning, abstract thinking, and organization – or an area in their prefrontal cortex involved in decision-making.
Human volunteers, on the other hand, were required to complete a thought scale questionnaire to assess their level of paranoia, and a second survey to determine signs of any depression.
By analyzing the behaviors of the monkeys and humans before and after the switch had occurred and comparing the results with those of the surveys, the team were able to judge which of the impeded brain areas may affect the monkey's ability to coolly navigate the volatile gaming environment.
"Not only did we use data in which monkeys and humans performed the same task, we also applied the same computational analysis to both datasets," says Yale psychiatrist Philip Corlett.
The data indicated both the magnocellular mediodorsal thalamus (MDmc) within the dorsal thalamic nucleus and locations in the orbitofrontal cortex (known as Walker's areas 11, 13, and 14) all affected the monkey's behaviors following the test's switch, in subtly different ways.
Among those with impeded Walker's areas, the sudden loss of reward had little impact on their decisions to switch. The monkeys kept on tapping what they thought was the 'winning' ticket with reckless abandon.
Those whose MDmc had been damaged demonstrated the very opposite behavior, switching back and forth even after discovering the new 'high chance' symbol was paying out rewards, almost as if they suspected the system was rigged against them personally.
This was similar to the behavior observed in humans with survey responses that indicated higher levels of paranoia.
While delusions and acts of paranoia are undoubtedly complicated behaviors involving diverse thinking and various areas of the brain, tracing a line between one particular area and volatile decision-making could inform future studies that might lead to new therapies or help us better understand how some actions increase the risk of a psychosis.
"Maybe down the road we can use it to find new ways to reduce paranoia in humans," says Chang.
This research was published in Cell Reports.