Neuroscientists at MIT discovered millions of “silent synapses” in the adult brain, which are dormant neural connections that can be activated to form new memories. This finding suggests that the brain can preserve older memories while accommodating new learning, providing insights into memory flexibility and potential implications for aging.

• MIT researchers found millions of "silent synapses" in the adult brain—dormant connections on filopodia that have NMDA but not AMPA receptors, making them inactive by default
• These silent synapses can be rapidly switched on with glutamate plus a brief electrical signal, but the same trigger doesn't affect already-active synapses
• This gives the brain a built-in way to encode new memories by activating fresh connections rather than overwriting existing ones, potentially explaining how old and new memories coexist
• Filopodia turned out to be far more common in the adult visual cortex than previously assumed, raising questions about how this reserve of silent synapses changes with aging