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What's in Brian's Brain?

Despite the progress of neuroscience, I still don’t know my own mind

Brian Hayes

Remembering When

Still another challenge awaits when trying to interpret brain architecture through the lens of computer technology. Digital computers rely on directly addressed data storage. A pattern of bits is written to a specific location; subsequent reading of the same location retrieves the data. Neuroscientists have searched everywhere in the nervous system for such an addressable read-write memory, without success.

The prevailing model of information storage in the brain is associative memory. Conventional computer memory works like a coat-check room. When you hand over your coat, you get a numbered ticket; later, when you present that ticket, you receive whatever coat is on the hanger with the matching number. Associative memory is a coat-check room that issues no tickets. To retrieve your coat, you list some of its attributes—blue wool jacket, missing top button—and the clerk brings out all the coats that match the description.

The brain’s implementation of associative memory is thought to be based on the adjustment of synaptic transmission in response to experience, as proposed by Hebb. This mechanism seems well suited to storing perceptual and motor patterns that we learn by repeated exposure or rehearsal: memories of places and faces, the habitual motions of the fingers when tying shoelaces or playing guitar chords. Each repetition is believed to strengthen the synaptic connections between neurons that fire simultaneously. The eventual result is a “cell assembly,” a set of neurons that tend to respond as a group whenever a sufficiently large subset of the assembly is stimulated.

Experiments are starting to reveal the biochemical nature of learning-induced changes in synapses. A harder question is how a shifting pattern of synaptic weights encodes an abstract concept or a narrative. Somewhere in my brain is an enduring record of the important fact that 7×9=63. There’s also a memory of the long, dreary struggle to implant that fact—the hours spent drilling with flash cards. I am intensely curious about how those two kinds of knowledge are represented in my head.

What about memories that are formed without the need for practice or repetition? People can narrate the plot of a movie (often in excruciating detail) after a single viewing. For that matter, honeybees can remember and report the location of flowers after a single foraging trip. Can these feats also be explained by some variant of Hebbian learning?

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