Hippocampal CA1 synaptic plasticity as a gamma transfer function

The capacity of activity-dependent synaptic modification is essential in processing and storing information, yet little is known about how synaptic plasticity alters the input-output conversion efficiency at the synapses. In the adult mouse hippocampus in vivo, we carefully compared the input-output relationship, in terms of presynaptic activity levels versus postsynaptic potentials, before and after the induction of synaptic plasticity and found that synaptic plasticity led synapses to respond more robustly to inputs, that is, synaptic gain was increased as a function of synaptic activity with an expansive, power-law nonlinearity, i.e. conforming to the so-called gamma curve. In extreme cases, long-term potentiation and depression coexist in the same synaptic pathway with long-term potentiation dominating over long-term depression at higher levels of presynaptic activity. These findings predict a novel function of synaptic plasticity, i.e. a contrast-enhancing filtering of neural information through a gamma correction-like process.