In the study published in Neuron, the researchers tested people who had a rare form of intractable epilepsy that could be treated only by surgically removing regions of the brain responsible for the seizures. As part of the diagnostic tests to pinpoint the origin of the seizures, the patients underwent placement of deep-brain electrodes. The patients permitted Schuman and her colleagues to thread bundles of microwires inside the diagnostic electrodes. These microwires were designed to detect activation of single neurons in the brain. Patients in the study agreed to undergo behavioral tests which allowed the researchers to make recordings of the brain as each patient was asked to consider a series of novel and familiar images.
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In the experiments — as the researchers recorded from the patients' brains — they asked the patients to look at a series of images of subjects such as planes, cars, bottles, animals, mountains or computers. They also asked the patients to remember the quadrant of the computer display in which the images appeared.
The researchers subsequently asked the patients to look at another series of images, both novel and familiar. To test the patients' recognition of the images, they were asked to identify which images were new and which were old. To test the patients' recollection of the images - a process distinct from recognition - the patients were asked to recall in which quadrant of the display the images had appeared.
“This experimental design gave us the ability to distinguish clearly between recognition and recollection,” said Rutishauser. “There has been considerable debate in the field about whether the hippocampus is involved in recognition, and this design enabled us to test it.
“We found two classes of neurons, which we called novelty detectors and familiarity detectors,” he said. “The novelty detectors only increased firing to new stimuli whereas the familiarity detectors increased firing exclusively to old stimuli,” he said.
This finding is important, said Schuman, because it showed that the researchers were seeing general novelty detectors, and not just those that are active during one phase of the experiment or the other. Novelty detection is critical for animals to create associations between behaviorally relevant stimuli. “The other important finding is that these experiments enabled us to identify for the first time, neurons in humans that show changes in responsiveness as a result of a single behavioral trial,” she said. “This is a landmark in that respect.”
Schuman said one surprising observation was that the patients' successful recognition of a familiar image — as measured by the neuronal response — didn't necessarily mean they correctly recollected where they have seen it. Also, a failure to recognize an image didn't necessarily mean the appropriate neurons didn't fire.
“When the subjects would make behavioral errors, and we looked at the corresponding neuronal firing pattern, we found that the neuronal firing actually indicated the correct response,” she said. “Thus, there is some sort of disconnect or failure at the connection between the information the brain has and how the subject decided to respond.
“There are examples of this in real life,” said Schuman. “For example, I might not remember meeting somebody before, even though they might insist that we had met. But later, I might remember that I had, indeed, met them before. That indicates there existed a certain neural substrate for the information on that person, but at the time I saw them, I perceived them as a new stimulus, not a familiar stimulus.”
