It’s hard to pin down exactly what makes us remember things. When you see an image, what makes you decide you’ve seen it before? A new study has tackled this question, identifying a group of neurons that participate in the process of identifying images as familiar.

While this may seem counterintuitive—it probably feels like you either recognize something automatically or you don’t—your brain makes that determination using different aspects of your memory. “Determining whether a stimulus is novel or familiar is a complex decision involving the comparison of sensory information with internal variables,” the authors explain in their paper.

Am I sure I’ve seen this before...?

When your brain makes a decision, it's often accompanied by an assessment of how accurate that decision is. Was I right to buy that car? My brain would consider a number of factors—the driving experience, the gas mileage, and so on—before concluding it’s pretty likely I’m making the right decision. (Just an example; alas, there’s no shiny new car for my brain to assess). These confidence values are an essential part of the decision-making process, at least for humans, as it helps us navigate our complex environment.

The decision of whether you recognize something is no exception. But exactly how your brain makes confidence judgments about familiarity is not well understood. One model holds that the ability might rely on evaluating the decision after it has been made, an ability that might be unique to humans.

Other models propose that confidence judgments are an essential part of the decision-making process itself—confidence in the decision is assessed by the same mechanism that makes the decision in the first place. Unlike the other model, it doesn’t require advanced cognitive abilities exclusive to humans, and thus we would expect to find this in other animals.

Confidence judgments in perceptual decisions can be studied in animals, and this work has recently provided evidence for the latter model. These studies show that animals seem to have the ability to make confidence judgments about decisions they’re making, but this has not been tested when it comes to memory-based decisions.

Finding the right neurons

The brain's medial temporal lobe (MTL) has previously been implicated in memory-based decisions, and researchers have even identified specific populations of neurons within the MTL that might be involved in the process. Some of these neurons, researchers have suggested, may be marking certain stimuli the person encounters as familiar or as new.

This led to a prediction: the activity of those neurons should correlate with both memory strength and with confidence. That is, as a person looks at a stimulus she’s encountered before, these neurons should activate more when she’s most confident she’s seen the stimulus before, and when she has the clearest memory of it.

To test this prediction, the researchers needed a way to see what was going on in the brain as people made their decisions. The ideal candidates turned out to be individuals who’d had electrodes implanted in their brain to evaluate them for possible surgical treatment of epilepsy. The electrodes allowed the researchers to follow the activity of individual neurons; 28 individuals volunteered and were included in the study.

The researchers presented these participants with a series of images, all selected from easily recognizable categories: cars, animals, people, and so on. Later, in another session, the participants were presented with a second series of images from the same categories, with half being images the participants had seen during the first session. The task was simple: when shown an image for one second, report whether it’s familiar to you or whether you’re seeing it for the first time—as well as how confident you were in that evaluation.

Our brains do a pretty good job. The participants who reported higher confidence in their answer were consistently more likely to answer correctly. Subjects tended to correctly identify around 69 percent of the images they’d seen before as familiar, but they also mistakenly said they’d seen between 11 and 45 percent of the new images before.

A neuron never forgets

Similar to what other studies had shown, a small percentage of neurons in the amygdala and hippocampus, about 8.5 percent, responded differently when the participants were shown familiar images than when they were shown new ones. The researchers labeled these "memory-selective" neurons. The researchers identified two kinds of memory-selective neurons: one that fired when participants saw new images (novelty selective), and one that fired at familiar images (familiar selective).

These neurons responded differently based on confidence: the more confident the participant was that the image was familiar, the stronger the signal among the familiar selective neurons, and vice versa for the novelty selective ones. Both kinds decreased their firing when the opposite stimuli were shown: familiar selective neurons fired less when the stimulus was new, and vice versa. This last effect, however, did not correlate with the participant’s confidence.

The memory-selective neurons also reacted at a rate better than chance when the participant was shown images they’d previously seen but claim to have forgotten. By contrast, that reaction did not occur when the participant was looking at a truly new image. Apparently, stimuli that we think we’ve forgotten may not be gone from memory.

With their results, the researchers created a mathematical model that was able to consistently predict the familiarity decision a person will make when given the signals from these neurons. The researchers’ model makes specific predictions about new kinds of neurons that may be found through future work, neurons that would be involved in evaluating the evidence for familiarity and unfamiliarity and deciding which is stronger.

Taken together, the researchers’ results favor the second of the two ideas mentioned earlier: making a confidence judgment in one’s own decision doesn’t require meta-cognition (thinking about one’s own thoughts); instead, it’s an inherent property of the way brains make decisions. This implies that animals are probably capable of making confidence judgments about visual memories as well, something that should be possible to test.

Nature Neuroscience, 2015. DOI: 10.1038/nn.4041 (About DOIs)