Obsessed with reruns of the TV sitcom Friends? Well then you probably have at least one 鈥淛ennifer Aniston cell鈥� in your brain, suggests research on the activity patterns of single neurons in memory-linked areas of the brain. The results point to a decades-old and dismissed theory tying single neurons to individual concepts and could help neuroscientists understand the elusive human memory.
鈥淔or things that you see over and over again, your family, your boyfriend, or celebrities, your brain wires up and fires very specifically to them. These neurons are very, very specific, much more than people think,鈥� says Christof Koch at the California Institute of Technology in Pasadena, US, one of the researchers.
In the 1960s, neuroscientist Jerry Lettvin suggested that people have neurons that respond to a single concept such as, for example, their grandmother. The notion of these hyper-specific neurons, coined 鈥済randmother cells鈥� was quickly rejected by psychologists as laughably simplistic.
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But Rodrigo Quiroga, at the University of Leicester, UK, who led the new study, and his colleagues have found some very grandmother-like cells. Previous unpublished findings from the team showed tantalising results: a neuron that fired only in response to pictures of former US president Bill Clinton, or another to images of the Beatles. But for such 鈥済randmother cells鈥� to exist, they must invariably respond to the 鈥渃oncept鈥� of Bill Clinton, not just similar pictures.
Wired up, fired up
To investigate further, the team turned to eight patients currently undergoing treatment for epilepsy. In an attempt to locate the brain areas responsible for their seizures, each patient had around 100 tiny electrodes implanted in their brain. Many of the wires were placed in the hippocampus 鈥� an area of the brain vital to long-term memory formation.
They first gave each subject a screening test, showing them between 71 and 114 images of famous people, places, and even food items. For each subject, the researchers measured the electrical activity or 鈥渇iring鈥� of the neurons connected to the electrodes. Of the 993 neurons sampled, 132 fired to at least one image.
The team then went back for a testing phase, this time showing participants three to seven different pictures of the initial 132 photo subjects that hit. For example, one woman saw seven different photos of the Jennifer Aniston alongside 80 other photos of animals, buildings or additional famous people such as Julia Roberts. The neuron almost ignored all other photos, but fired steadily each time Aniston appeared on screen.
Conceptual connections
The team found similar results with another woman who had a neuron for pictures of Halle Berry, including a drawing of her face and an image of just the words of her name. 鈥淭his neuron is responding to the concept, the abstract entity, of Halle Berry,鈥� says Quiroga. 鈥淚f you show a line drawing or a profile, it鈥檚 the same response. We also showed pictures of her as Catwoman, and you can hardly see her because of the mask. But if you know it is Halle Berry then the neurons still fire.鈥�
Given more time and an exhaustive list of images, the team may well have landed upon other images that spiked the activity of the 鈥淗alle Berry鈥� neuron. In one participant, the 鈥淛en鈥� neuron also fired in response to a picture of her former Friends cast-mate, Lisa Kudrow. The pattern suggests that the actresses are tied together in the memory associations of this particular woman, says Charles Connor, a neuroscientist at Johns Hopkins University in Baltimore, US.
These object-specific neurons may be at the core of how we make memories, say Connor. 鈥淚 think that鈥檚 the excitement to these results,鈥� he says. 鈥淵ou are looking at the far end of the transformation from metric, visual shapes to conceptual memory-related information. It is that transformation that underlies our ability to understand the world. It鈥檚 not enough to see something familiar and match it. It鈥檚 the fact that you plug visual information into the rich tapestry of memory that brings it to life.鈥�
Journal reference: Nature (vol 435 p 1102)
