In my June brain column for Discover, I wrote about the bizarre idea that there are single neurons in your head that can respond to individual people. The so-called “grandmother cell” started out 40 years ago as a thought experiment riffing on Philip Roth’s novel Portnoy’s Complaint. By the 1970s, most neuroscientists considered it more of a joke than a valid concept, but in the years since it hasn’t quite gone away.
In my column, I described the work of the work of Rodrigo Quian Quiroga of the University of Leicester:
For the past eight years, he and his colleagues have been studying epilepsy patients who have had electrodes implanted in a region of their brains called the medial temporal lobe, as part of a study to identify the source of their seizures. Quian Quiroga showed the subjects 100 pictures. The pictures included photos and drawings of celebrities as well as landmarks and various familiar objects. The patients had to press one button if a picture was of a human face and another if it was not.
In their first such study, Quian Quiroga and his team were able to observe the individual activity of 993 neurons. They found that 132 of them responded to at least one picture. And of those responding neurons, 51 fired in response to only a single person or thing. One neuron responded only to Halle Berry, for example.
Amazingly, the “Halle Berry” neuron responded to any picture of her, including one in which she was dressed as the masked Catwoman. Even the name Halle Berry triggered that neuron, which was silent at the sight of other actresses or their names.
Quian Quiroga does not, however, believe these neurons are grandmother cells, at least as they were initially conceived. He suspect that a very sparse network of neurons–perhaps hundreds out of the billions in our heads–can develop this kind of response to an individual. Quian Quiroga just happen to stick his electrodes near single neurons that belonged to these networks.
Which brings us to Quian Quiroga’s latest paper, published in Current Biology. He analyzed the signals from 750 electrodes implanted in seven patients as they looked at pictures of some celebrities like Oprah Winfrey. Quian Quiroga found neurons that responded strongly to the sight of these individuals–and they also responded strongly to their written names and even the sounds of their names.
Quiroga and his colleagues also ran the same test using themselves rather than the celebrities to probe for neurons. They discovered neurons that responded strongly only to individual researchers, too–and once more, the same neurons responded to the sight and sound of their name. Bear in mind–the patients had only met the scientists a day or two earlier. So these neurons had developed their grandmother-ish response in a very short time.
These results offer some clues to how these sparse networks are arranged. Some of the neurons probably get signals from other regions of the brain that recognize faces. Others tap into auditory networks, and others language centers. Yet, remarkably, the information from these far-flung parts of the brain get funneled into tiny sets of neurons that can then encode concepts of people.
They may not be the Grandmother Cells of legend, but in their own way, they’re very cool.
Reference: Quian Quiroga et al., Explicit Encoding of Multimodal Percepts by Single Neurons in the Human
Brain, Current Biology (2009), doi:10.1016/j.cub.2009.06.060