Now, in a study on mice, scientists have unravelled how the brain decides what to believe and how to distinguish fact from fiction itself.
The research by neuroscientists at Cold Spring Harbor Laboratory (CSHL) suggests that the estimation of confidence that underlies decisions may result because of a very basic kind of information processing in the brain, and is shared widely across species and not strictly confined to humans who are self-aware.
Having a sense of what we know -- and don't know -- is a universal human experience, and has often been assumed to be the hallmark of self-consciousness.
In a collaborative study, CSHL Assistant Professor Adam Kepecs, Ph.D., performed experiments on laboratory rats.
The researchers recorded signals from individual neurons in the rodents' brains while they were put to the test of distinguishing smells.
It was found that neurons in a part of the brain known as the orbitofrontal cortex (an area of the brain found in both rats and humans) signal the uncertainty of the decisions, "firing" much more vigorously in difficult tests compared with easier tests.
"These neurons seem to have been registering, after the rat made its decision, how uncertain the animal was that it was about to receive its desired reward. We tested several alternative explanations but the best explanation for the neural activity we observed was that these neurons were signalling the confidence of the animal about its decisions," Nature quoted a co-author of the study, as saying.
"Although previous work had suggested that confidence estimates might be a complex process restricted to humans and perhaps other primates, our results, to the contrary, show that for the brain to estimate confidence in a choice is no more complex than calculating the choice itself," said Kepecs.
The researchers then ran a series of olfaction trials that featured an important modification of the original task design. As in the first set of trials, rats made decisions involving the discrimination of two smells; they were rewarded, after a defined period of delay, if they decided correctly, and received nothing if their decision was incorrect.
In the modified task, the reward delay was increased substantially. However, while waiting for the reward, the rats were given the option to abort the trial - short of learning whether their decision was right or wrong - and return to the beginning to start a new trial.
"This new option to abort and restart constitutes a decision that should be made based on the level of confidence about getting a reward," said Kepecs.
The researchers concluded that if the rats were not confident about their original decision about the smell, they would be more likely to abort the trial.
"Ultimately, confidence about getting a reward is a direct function of the animals' confidence about the decision they have just made. In this way we sought to measure a variable internal to the animals - how confident they were about whether they made the right choice or not - by observing how it influenced their behaviour," said Kepecs.
The researchers found that rats preferentially aborted uncertain trials. This showed that they could not only calculate their level of confidence in a given decision, but also use that calculation in subsequent decisions to guide behaviour.
Kepecs said that taken together, these experiments reveal "that confidence estimation is not a complex function specific to humans but a core component of the process of decision-making probably found throughout the animal kingdom."
The study is reported this week in Nature.