Biology's Best Kept Secret
    By Enrico Uva | January 12th 2011 04:49 AM | Print | E-mail | Track Comments
    About Enrico

    I majored in chemistry, worked briefly in the food industry and at Fisheries and Oceans. I then obtained a degree in education. Since then I have...

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    One of biology’s most beautiful hypotheses is 62 years old. Since first proposed in 1949, Donald Hebb’s concept of neuronal cell assemblies has been substantiated by a fair amount of laboratory evidence.

    In the 1980’s, it inspired John Hopfield to formulate mathematical neural network models. As described in a 2009 review by researcher Anders Lansner, Hebb’s cell assemblies have also inspired work on supercomputer simulations of associative memory. And yet this remarkable insight into how memory and learning operate has been kept a secret from most high school and freshmen biology students and teachers.

    When neurons are activated by the same stimulus, they connect to each other and form a cell assembly. Such a unit is seeded when one nerve cell repeatedly stimulates a neighboring one, leading to metabolic changes in at last one of the cells. As a result of these changes, the original cell becomes more efficient at communicating with its neighbor. If, for example, the stimulus was a child’s first experience with a dog, a cell assembly becomes the mental representation of that animal.

    Initially, independent assemblies form as the child sees different dogs, or dog-like objects and hears the associated word. When synaptic bridges between the related assemblies form, the child has formulated the concept of a dog and can generalize.

    As Hebb realized, the presence of the cell assemblies affects subsequent perceptions of similar stimuli, whether they are concrete or abstract pieces of external reality. But these basic units of our cortices don’t just curse us with subjectivity, they are also a basis for our individuality and creativity. In the same way that varying diction, sentence length and choice of imagery leads to a multitude of writing styles, there are almost an infinite number ways that these cell assemblies can link with each other.

    As we expose ourselves to the cell assemblies of different brains, we make our brain tapestry more exuberant and more similar to the external world. Take a forest for example. My son and I once met a photographer who was admiring the way bright November light was interacting with the bark of trees. Because the sun was at a lower angle at that time of the year, it created a visual effect that did not resemble what you would see at that time in the summer.

    In my brain, a new cell assembly has been added to my concept of a forest. When I was in college I integrated ecological interpretations of trees. Every time I walked through a forest I thought of what succession stage it was in. When I learned about evolution, I wondered about how the critters hiding in their bark would affect the genetic pool. My organic chemistry professor’s experiences with natural products allowed me to perceive every tree as a great natural repository of terpenes and unknown compounds. Then there is Monet who reminds us that as clouds drift and the earth turns, light is in constant flux so that our eyes never collect the same data from the forest. And then I occasionally revert to my childhood habit of looking for human faces in pruning scars of city trees, as I let my cell assemblies for face recognition have a little fun.