Human memories—even the most precious—begin at a very granular scale. Your mother’s face began as a barrage of photons on your retina, which sent a signal to your visual cortex. You hear her voice, and your auditory cortex transforms the sound waves into electrical signals. Hormones layer the experience with with context—this person makes you feel good. These and a virtually infinite number of other inputs cascade across your brain. Kukushkin says your neurons, their attendant molecules, and resultant synapses encode all these related perturbations in terms of the relative time they occurred. More, they package the whole experience within a so-called time window.
But it would be a mistake to believe that those molecules, or even the synapses they control, are memories. “When you dig into molecules, and the states of ion channels, enzymes, transcription programs, cells, synapses, and whole networks of neurons, you come to realize that there is no one place in the brain where memories are stored,” says Kukushkin. This is because of a property called plasticity, the feature of neurons that memorize. The memory is the system itself.
Nick Stockton on the stuff memories are made of.