The idea that changes in synaptic strength (synaptic plasticity) is the foundation of memory and learning has a long history. For this reason long term potentiation (LTP) and long term depression (LTD), the electrophysiological manifestations of synaptic plasticity, have been intensely studied. However, the proof of causality, that by changing synaptic strength one can remove and reinstate a memory, had been missing.
In a recent paper, The Nabavi Lab provided the most direct evidence showing a cause-effect relation between synaptic plasticity and memory formation (Nabavi et al., 2014, Nature). With the use of optogenetics fortified by in vitro/in vivo recording and behavioral assays they generated an associative memory and showed that they could repeatedly turn off and on this associative memory simply by inducing LTD and LTP (see the figure).