Homeostatic control of synaptic rewiring in recurrent networks induces the formation of stable memory engrams
Fig 1
Formation of memory engrams in a neuronal network with homeostatic structural plasticity.
(A) In a classical conditioning scenario, an unconditioned stimulus (US) was represented by a group of neurons that were connected to a readout neuron (yellow) via static synapses. The readout neuron spiked whenever neuronal activity representing the US was above a certain threshold (top). During a conditioning protocol, two other groups of neurons (CS1 and CS2) were chosen to represent two different conditioned stimuli (bottom). (B) During stimulation, the external input to a specific group of neurons was increased. The color marks indicate when each specific group was being stimulated. During the “encoding” phase, CS1 was always stimulated together with US, and CS2 was always stimulated alone. The time axis matches that of panel C (top). (C) Firing rate (top) and spike train (bottom) of the readout neuron. Before the paired stimulation (“baseline”), the readout neuron responded strongly only upon direct stimulation of the neuronal ensemble corresponding to the US. After the paired stimulation (“retrieval”), however, a presentation of C1 alone triggered a strong response of the readout neuron. This was not the case for a presentation of C2 alone. (D) Coarse grained connectivity matrix. Neurons are divided into 10 groups of 100 neurons each, shown is the average connectivity within each group. After encoding, the connectivity matrix indicates that engrams were formed, and we found enhanced connectivity within all three ensembles as a consequence of repeated stimulation. Bidirectional inter-connectivity across different engrams, however, is only observed for the pair C1-US that experienced paired stimulation. (E) Average connectivity as a function of time. The connectivity dynamics shows that engram identity was strengthened with each stimulus presentation, and that engrams decayed during unspecific external stimulation.