Friday, November 12, 2010

Grandmother Cells


The term "grandmother cell" was coined by Jerry Lettvin in 1969 to describe a hypothetical neuron that can be shown to represent a specific psychological concept. This cell would become active every time a person thinks about a complex thing, such as his or her grandmother. The question was: Are there any cells, anywhere in the brain, that are dedicated specificlly to processing information about one's grandmother? It seems that this may be the case despite the fact that neuroscientists were convinced, for decades, that this was a gross oversimplification.

It is known that millions of cells work together to help us visualize even the most simplistic visual objects. The retinas of the eyes relay information about what we look at to early visual processing areas in the back of the head. This data passes through a series of neural areas before objects are recognized. These areas, which have been fine-tuned by life experience, act as filters that allow the visual data to matchup against the brain's best existing representation of what is being seen. In this sense, when we see, we are not really looking at what is out there, but instead piecing together a collage of things that we know to try to recreate the scene. When our brain does this, large numbers of neurons in the simple visual areas that correspond to the lines and contours of what we are seeing send their information to a smaller number of more complex neurons that deal with shape and form. These neurons, in turn, converge on even smaller populations of neurons that code for recognizable objects like people, cars and animals. It is thought that populations of cells, in these high-order processing regions, can be dedicated to processing very specific objects. Grandmother cells are the theoretical limit to this convergence where the activities of a large interconnected structure of networks meet together to activate a single neuron that in some senses, holds much of the information of the entire network (because much of the network must be activated for it to fire).

Some cells that come close to meeting the requirements of a grandmother cell have been found. One study by Rodrigo Quiroga and colleagues used patients undergoing treatment for epilepsy where 100 tiny electrodes were implanted in their brains. Each subject saw around 100 images of famous people, places and things. Overall, almost 1,000 neurons were sampled and 132 of these reacted to at least one of the images. The objects that elicited increased activity were used in another round of recognition except different images of these objects were shown. For example, if a head-on photo of a truck elicited a response, then a profile picture might be shown next. The researchers were able to find grandmother-like cells in some of the participants. One female participant had a neuron that only responded to the actress Jennifer Aniston. The neuron did not respond to other pictures, even of similar-looking female actresses, with one exception. This neuron responded to some pictures of Lisa Kudrow, Jennifer’s costar on the show Friends.

Another female participant had a neuron that responded only to Halle Berry. Pictures of the actress, a line drawing, a profile, even the words of her name, all made this neuron fire. These neurons were generally in convergence areas – such as the hippocampus- where a lot of processing, in areas such as vision and audition, meet up. This neuron must have been highly tuned, not only to the physical aspects of Halle, but to the abstract representation of her overall persona because even a picture of Halle’s catwoman character made the neuron fire despite the fact that she was masked.

Early critics of the idea pointed out that there are not enough neurons in the brain to account for every possible sensory object in one-to-one correspondence. Grandmother cells don’t necessarily work this way though. It seems that sensory objects are represented by networks of neurons many of which overlap and intersect. The individual neurons that make up these networks usually have the capacity to contribute toward the perception and recognition of several different objects of sensation. The further down the processing stream these neurons lie - the closer in the brain they are to the retinal inputs - the more fundamental they are in the process of recognition and the more objects they contribute to. The contribution of an individual neuron is actually very weak. One neuron usually does not have the capacity to send a global message that can be perceived consciously. In fact, many, many neurons would have to be removed to abolish the ability to recognize your grandmothers. Even more (tens of thousands or perhaps millions), would have to be removed to ensure that you could not remember anything about your grandmothers.

Experience fine-tunes neurons and the inputs that they are receptive to. The brain will even tune some neurons so narrowly that they become dedicated processing specialists for things that you recognize frequently, especially things that you see over and over again. They may not become active to every representation of a grandmother, they may become active to totally unrelated representations and there may be a number of them in each person’s brain, that vary in their responsivity to grandmothers. Even so, grandmother cells, at least loosely defined, do seem to exist. It is probably a safe bet that the cartoon drawing of a grandmother at the beginning of this entry activated some of the same cells in your brain that respond uniquely to images of your parent’s mothers.

Quiroga, R. Q., Reddy, L., Kreiman, G., Koch, C. & Fried, I. 2005. Invariant visual representation by single neurons in the human brain. Nature 435: 1102-1107.

Read the full article that I wrote on this topic here:

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