We humans thrive on information. We love news, gossip, rumors, and, most importantly, information about our own future. What’s more, a number of studies by economists and psychologists have confirmed what we already know from experience: We want that information now, not later.
Do monkeys also share this desire to know what the future holds? And if so, does this information activate the same dopamine-using neurons in the ventral tegmental area (VTA) of the brain that are stimulated by intrinsically pleasurable stimuli like food and water? In other words, is information about the future pleasurable in and of itself?
These interrelated questions were addressed in a series of experiments performed by Ethan Bromberg-Martin and Okihide Hikosaka at the National Eye Institute in Bethesda, Maryland (Bromberg-Martin ES, Hikosaka O. Midbrain dopamine neurons signal preference for advance information about upcoming rewards. Neuron 63, 119–126, 2009). They trained two thirsty monkeys to perform a simple decision task: Two targets appeared on the left and right sides of a video screen, and the monkey would choose one by flicking its eyes to a given target. Then, following a delay of a few seconds, the monkey would receive either a large or a small water reward. It didn’t matter which target the monkey chose—the rewards were delivered randomly, and at the same overall frequency. The twist to this experiment was that choosing one of the visual targets produced an informative cue during the delay period—a symbol whose shape indicated the size of the upcoming reward—while choosing the other target produced a random cue in the delay period that had no meaning, no predictive value. So in this design, it doesn’t matter whether the monkey chooses to receive advance information or a meaningless symbol: It still has the same chance of getting the large water droplet and it will get it with the same delay.
Just like humans, when given the choice, the monkeys opted to receive information about the future. Within about ten trials, both monkeys were choosing the information-yielding target almost every time. Recordings made from individual dopamine neurons in the VTA of these monkeys revealed that these neurons briefly increased their firing rate when they saw the symbol that predicted a large amount of water, while the symbol that predicted a small amount of water briefly attenuated their ongoing firing rate. Crucially, after training, these same neurons were excited during probe trials in which the monkey only saw only the target that indicated upcoming information and were inhibited when they saw the target that indicated upcoming random, uninformative symbols. The same dopamine neurons that signal the expected amount of pleasure from water also signal the expectation of information, even when that information cannot be put to any use. The monkeys (and presumably humans as well) are getting a pleasure buzz from the information itself.
To my thinking this experiment is revolutionary. It suggests that something utterly useless and abstract—knowing merely for the sake of knowing—can engage the pleasure/reward circuitry. This is not pleasure obtained from essential things like the food or water or sex, which we need to propagate our genes. Nor is it the pleasure of monetary reward, which, while abstract, still represents some real-world benefit, as it can be exchanged for useful things. Nor is it even like the pleasure of charitable giving or the pleasure of receiving positive social feedback, which can also be evolutionarily beneficial for animals living in certain types of social group.
This experiment suggests that ideas are like addictive drugs. Certain psychoactive drugs co-opt the pleasure circuit to engage pleasurable feelings normally triggered by food, sex, and so on. In our recent evolutionary lineage (including primates and probably cetaceans), abstract mental constructs have become able to engage the pleasure circuitry as well, a phenomenon that has reached its fullest expression in our own species. The neuroscientist Read Montague, weaving together several strands of thought in cognitive neuroscience from a number of investigators, calls the human ability to take pleasure in abstract ideas a “superpower,” and I’m inclined to agree with him. From this perspective, human ideas can even directly oppose our most basic pleasure drives. For example, some people, acting on their religious principles, can forgo sexual activity in service to what they perceive as a more important goal. Likewise, the politically or spiritually motivated hunger-striker is activating her pleasure/reward center by furthering her own ideas, even when this requires acting in precise opposition to one of our most basic and ancient drives.