We meet in a pub, we have a few drinks, some dinner and then you lean in for a kiss. You predict, based on our previous interactions, that the kiss will be reciprocated – rather than landing you with a slap in the face.
All our social interactions require us to anticipate another person's undecided intentions and actions. Now, researchers have discovered specific brain cells that allow monkeys to do this. It is likely that the cells do the same job in humans.
Keren Haroush and Ziv Williams at Harvard Medical School trained monkeys to play a version of the prisoner's dilemma, a game used to study cooperation. The monkeys sat next to each other and decided whether or not they wanted to cooperate with their companion, by moving a joystick to pick either option. Moving the joystick towards an orange circle meant cooperate, a blue triangle meant "not this time".
Neither monkey could see the other's face, or receive any clues about their planned action.
If the monkeys cooperated, both received four drops of juice. If one cooperated and the other decided not to, the one who cooperated received one drop, and the other received six drops of juice. If both declined to work together they both received two drops of juice.
The big reveal
Once both had made their selections, they could see what the other monkey had chosen and hear the amount of juice their companion was enjoying.
The monkeys played the game several thousand times. Like humans, they were more likely to cooperate if their opponent had recently cooperated in a previous trial.
During the experiment, the team recorded brain activity from single cells in the monkeys' anterior cingulate – an area thought to play a role in decision-making. They discovered that the activity of a specific set of neurons was associated with the monkey's own decision in each game.
But anticipating your opponent's action, on the basis of previous behaviour, is the key to receiving the greatest reward in the prisoner's dilemma. Haroush and Williams found another set of neurons within the same area that were responsible for predicting what the monkeys' opponent was intending on doing – before they did it.
In fact, when the team looked back at the results, in 79 per cent of trials, the activity of the neurons in one monkey predicted the subsequent move of the other monkey.
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