Familiar faces always bring instant recognition. True, but what is it that tells the brain within a split second of seeing a person that we know him or her?

Over the years, scientists have been searching for that one type of cells in the brain that would explain that flash of recognition when one happens to see a familiar face, like that of one’s grandmother. The efforts did not bear fruit as that single cell which was at the crossroads of sensory perception and memory or what was referred to as the “grandmother neuron”, could not be found.

This was true till recently when in the brain’s temporal pole region, a class group of neurons were found that link face perception to long-term memory. Although not a grandmother neuron, it is a group of cells – and not a single one – that together help us remember grandmother’s face.

According to an article in sciencedaily.com the findings of the study that reveal how the brain ingrains the faces of those people who are dear to us were published in Science.

Talking about this research, Winrich Freiwald, Professor of Neurosciences and Behaviour at Rockefeller University said: "When I was coming up in neuroscience, if you wanted to ridicule someone's argument you would dismiss it as 'just another grandmother neuron' — a hypothetical that could not exist. Now, in an obscure and understudied corner of the brain, we have found the closest thing to a grandmother neuron: cells capable of linking face perception to memory."

It was in the 1960s the concept of a brain cell that was capable of coding for a specific and complex concept came up. The idea was that there was one neuron for grandmother’s memory while there was another for the mother, so on and so forth. The aim was to demystify as to how our brain combined what was seen with memory which was long-term.

Even though subsequently sensory neurons that help in processing information concerning faces and cells which stored memory of personal experiences were found, the grandmother neuron never surfaced. Summing it adequately Freiwald said: "The expectation is that we would have had this down by now. Far from it! We had no clear knowledge of where and how the brain processes familiar faces."

Not long ago, Freiwald and his colleagues located in the brain a small portion in the temporal pole region that could be concerned with recognition of faces. Using functional magnetic resonance imaging to target the TP regions of two rhesus monkeys, the team documented the electrical signals of the neurons as the monkeys watched familiar and unfamiliar images they had seen on the screen.

Neurons in the TP region were found to be selective in their response to familiar faces which was stronger as compared to those which were not. They were able to differentiate between what was known to them with what was not.

What was truly amazing was that the response to the familiar was threefold more than to the unfamiliar though the latter had been seen many times by the monkeys on the screen. The first author of the paper and a neuroscientist Sofia Landi observed: "This may point to the importance of knowing someone in person. Given the tendency nowadays to go virtual, it is important to note that faces that we have seen on a screen may not evoke the same neuronal activity as faces that we meet in-person."

This discovery provides the very first evidence of something like the grandmother neuron which is hybrid in nature. Why hybrid? Because TP region cells act like sensory responding with speed and reliability to stimuli which are visual in nature while also playing the part of memory cells by reacting only to that stimuli that has been seen before by the brain. In this case it was the familiar individuals, thus showing a change in the brain because of earlier experiences.

Freiwald averred: "They're these very visual, very sensory cells — but like memory cells. We have discovered a connection between the sensory and memory domains."

Yet, these cells are not the grandmother neurons since they are not one cell but a collective that work in unison.

This finding will lead the scientists to study as to how familiar faces are encoded by the cells. "We can now ask how this region is connected to the other parts of the brain and what happens when a new face appears. And of course, we can begin exploring how it works in the human brain,” remarked Freiwald.

The study is important from another perspective too as it can help those who suffer from face blindness or prosopagnosia (from Greek prósōpon, meaning face and agnōsía, meaning non-knowledge).

Affecting one per cent of people, prosopagnosia leads to social isolation. "Face-blind people often suffer from depression. It can be debilitating, because in the worst cases they cannot even recognize close relatives. This discovery could one day help us devise strategies to help them,” said Freiwald.