how we’re wired

Sebastian Seung’s book explores the study of neuroscience and ‘how the brain’s wiring makes us who we are.’

Over the ages, philosophers came up with a list of principles by which associations can be learned. At the top of the list is coincidence, sometimes called contiguity in time or place. If you see photos of a pop singer with her baseball-player boyfriend, you will learn an association between them. A second factor is repetition. Seeing these celebrities together just once might not be enough to create the association in your mind, but if you see them ad nauseam day after day in every magazine and newspaper, you will not be able to avoid learning the association. Ordering in time also seems important for some associations. As a child you recited the letters of the alphabet repeatedly until you knew them by heart. You learned the association from each letter to the next, since the letters always followed one another in the same sequence. In contrast, the association between the pop singer and her boyfriend will be bidirectional, since they always appear simultaneously.

So philosophers propose that we learn to associate ideas when one repeatedly accompanies or succeeds another. This inspired connectionists to conjecture:

If two neurons are repeatedly activated simultaneously, then the connections between them are strengthened in both directions.

This rule of plasticity is appropriate for learning two ideas that repeatedly occur together, like the pop singer and her boyfriend. For learning associations between sequential ideas, connectionists proposed a similar rule:

If two neurons are repeated activated sequentially, the connections from the first to the second is strengthened.

In both rules, by the way, it’s assumed that the strengthening is permanent or at least long-lasting, so that the association can be retained in memory.

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