Conserving synaptic weight

Long-term potentiation (LTP), a phenomenon by which previously stimulated synapses become increasingly sensitive to stimulation such that the same level of presynaptic input induces a larger postsynaptic output, and long-term depression (LTD), which conversely reduces efficacy at such synapses, have been implicated in memory formation and storage. But computer models predict that without a balancing force of some kind, LTP and LTD will cause neural circuits to go haywire. Now, Sébastien Royer and Denis Paré have identified just such a force that can maintain balance in a network—and might imply that humans have a limited memory capacity.

“There have been lots of studies about what happens in LTP and LTD, but little attention has been paid to the synapses that are not stimulated when you induce LTP,” says Paré, despite the fact that each cell...