Reprinted from Borges and Memory: Encounters with the Human Brain, by Rodrigo Quian Quiroga. Copyright © 2013, by Massachusetts Institute of Technology. Used with permission of the publisher The MIT Press. See a previous excerpt on the famous Borges story, Funes the Memorious.

Technological innovations developed at UCLA (mostly having to do with the design of the electrodes) have resulted in recordings that allow us to see the activity of individual neurons in the human brain. The unique chance to perform such studies was one of the main things that enticed me to California ten years ago as a Sloan fellow. Though I certainly did not perform these studies on my own-some of the scientists with whom I collaborated include my mentor Christof Koch at Caltech and Itzhak Fried, one of the neurosurgeons who established this line of research at UCLAI-I will switch to the first person and describe without too many technical details my research on the neuronal activity in the human hippocampus.

Based on the connection between the hippocampus and the higher visual areas (and following earlier results obtained by Gabriel Kreiman, a friend and colleague from Argentina, now a professor at Harvard), I started by looking for responses to visual stimuli, in particular those stimuli that were familiar or relevant to the patients. The reason is simple: in principle one expects that more neurons will respond to an image of the patient's mother than to a picture of someone they don't know. This initial intuition turned out to be correct. For example, in a patient who happened to be a soccer fan I found a neuron that responded to Argentine player Diego Armando Maradona; with a patient obsessed with the Rocky films I tried different characters from the series until I found a neuron that fired when we showed an image of Mr. T; in yet a third patient, one who loved to watch documentaries from the Discovery Channel, I found a response to pictures of animals.

These examples, and several others showing similar results, make it clear that neurons in the medial temporal lobe respond to visual stimuli. Now the question is whether the neuron responded to Maradona himself or to some detail in that particular photo-for example, the neuron might have responded to the green turf, a reasonable (though disappointing) possibility given the existence of neurons in monkeys' visual areas that respond to colors. The neuron might also have fired to the colors of the Argentine national team, the motion implied by Maradona's posture, or simply the ball, How can one prove that the neuron is responding to the person and not to some detail? Very simple: we show many pictures of Maradona (in different environments, with different background colors, in different pos· tures, with different attire, etc.) and see if the neuron responds equally to all the photos (in other words, to the concept of Maradona) or only to that particular picture. We found that it indeed responded to the concept.

The first (and by far the most famous) of these neurons was one that responded to seven completely different images of actress Jennifer Aniston and to no other stimulus (including other people, animals, or places). The neuron fired when presented with different pictures of Aniston, but not when shown other celebrities like Kobe Bryant, Julia Roberts, Oprah Winfrey, or Pamela Anderson, places like the Golden Gate Bridge or the Eiffel Tower, or different animals. Another neuron from the same patient responded to different images of the Sydney Opera House, and yet another to the leaning tower of Pisa. The patient knew all of those people and landmarks quite well.

At this point it seems that the neurons in the hippocampus encode concepts such as a person or a particular place. To give even more conclusive evidence, we can take advantage of the fact that these experiments were performed in human subjects and see how the neurons respond to the names of the persons or objects eliciting responses. [One experiment showed] a neuron in the hippocampus of a patient that responded to different pictures of actress Halle Berry and to her written name (but to no other names). At the time the experiment was performed, Halle Berry was promoting one of her films, Catwoman, and the neuron also fired when presented with pictures of her in costume, even though her face was not visible. In total, the neuron responded to four different pictures of Halle Berry, to three of her in costume, to a caricature of her, and to her name spelled out on the screen, but failed to respond to five caricatures of other people, seven other names, and 78 pictures of people (including Cameron Diaz, Arnold Schwarzenegger, and Adam Sandler), animals, and landmarks.