Ten years ago, surgeons were stupefied by single neuron’s strange behavior.
Itzhak Fried, a neuroscientist at the University of California, Los Angeles, and his colleagues were looking at the medial temporal lobe (MTL) in epileptic patients who’d had part of their skulls removed so scientists could investigate the inner workings of their brains. Whenever the patient was shown a picture of Jennifer Aniston, one particular neuron in the patient’s brain would start to fire. Other individual neurons in the same brain region (which included the hippocampus, the mind’s headquarters for memory), were similarly associated with different celebrities.
What’s more, these neurons demonstrated a property called “invariance,” which means they fired in reaction to many versions of a generalized stimulus (in the case of the Jennifer Aniston neuron, the stimulus was… Jennifer Aniston). But it didn’t matter how she looked—whether she was wearing a black dress, had her hair in an up-do, or was sporting jeans and a T-shirt—the neuron still recognized Aniston for Aniston.
Presumably, that’s because the patient’s brain had learned to identify Jennifer Aniston in various scenarios (though interestingly, the neuron did not respond to a photo of Aniston holding hands with Brad Pitt). An update to this study, originally published in 2005, now illustrates the relationship between these neurons and episodic memory—the mysterious ways in which our brains organize autobiographical events.
Fried presented 14 epileptic people in the midst of surgery with randomly sorted images, including pictures of loved ones, celebrities like Clint Eastwood, and landmarks like the Eiffel Tower.
Here’s Emily Underwood, writing for Science:
Of the roughly 600 neurons the team recorded in each patient, between 2 and 28 cells fired vigorously in response to at least one image. Next, the researchers presented participants with digitally altered photographs in which a neuron’s “preferred” image, such as a photo of Clint Eastwood, was superimposed on a background the neuron had ignored in a previous trial, such as the Leaning Tower of Pisa. In a series of memory tasks, the participants were asked to match pairs of separate images based on the doctored composites. If they’d seen the Eastwood composite, for example, their task might be to pair a photo of Eastwood with a separate photo of the tower.
Even after one exposure to the composites, neurons that had previously fired exclusively in response to one picture—like that of Eastwood—significantly increased their firing rate when exposed to the image with which it had been combined—in one case, by 230%, Fried and colleagues report today in the journal Neuron. The fact that an individual neuron can adapt its firing rate so quickly could help explain how large, dynamic neuronal networks form complicated memories of past events, Fried says.
The new study could eventually contribute to efforts underway to alleviate memory loss in people who suffer from traumatic brain injury or Alzheimer’s disease. Scientists have few options when it comes to reversing traumatic brain injury, in particular.
Though the ethics of memory enhancement are not yet clear, a deeper understanding of individual neurons and how they work is accelerating progress toward an era in which these questions will be mandatory.