Four years ago, Rodrigo Quian Quiroga from Leicester University showed that single neurons in the brain react selectively to the faces of specific people, including celebrities like Halle Berry, Jennifer Aniston and Bill Clinton. Now, he’s back, describing single neurons that respond selectively to the concept of Saddam Hussein or Oprah Winfrey. This time, Quiroga has found that these neurons work across different senses, firing to images of Oprah or Saddam as well as their written and spoken names.
In one of his volunteers, Quiroga even found a neuron that selectively responded to photos of himself! Before the study began, he had never met the volunteers in the study, which shows that these representations form very quickly, at least within a day or so.
In his original experiments, Quiroga used electrodes to study the activity of individual neurons, in the brains of patients undergoing surgery for epilepsy. As the volunteers saw photos of celebrities, animals and other objects, some of their neurons seemed to be unusually selective. One responded to several different photos of Halle Berry (even when she was wearing a Catwoman mask), as well as a drawing of her, or her name in print. Other neurons responded in similarly specific ways to Jennifer Aniston or to landmarks like the Leaning Tower of Pisa.
The results were surprising, not least because they seemed to support the “grandmother cell theory“, a paradox proposed by biologist Jerry Lettvin. As Jake Young (now at Neurotopia) beautifully explains, Lettvin was trying to argue against oversimplifying the way the brain stores information. Lettvin illustrated the pitfalls of doing so with a hypothetical neuron – the grandmother cell – that represents your grandmother and is only active when you think or see her. He ridiculed that if such cells existed, the brain would not only run out of neurons, but losing individual cells would be catastrophic (at least for your poor forgotten grandmother).
The grandmother cell concept was espoused by headlines like “One face, one neuron” from Scientific American, but these read too much in Quiroga’s work. It certainly seemed like one particular neuron was responding to the concept of Halle Berry. But there was nothing in Quiroga’s research to show that this cell was the only one to respond to Halle Berry, nor that Halle Berry was the only thing that activated the cell. As Jake Young wrote, “The purpose of the neuron is not to encode Halle Berry.”
Instead, our brains encode objects through patterns of activity, distributed over a group of neurons, which allows our large but finite set of brain cells to cope with significantly more concepts. The solution to Lettvin’s paradox is that the job of encoding specific objects falls not to single neurons, but to groups of them.