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Homeostatic control of synaptic rewiring in recurrent networks induces the formation of stable memory engrams

Fig 3

Evoked activity depended on the strength of a memory.

(A) Starting from a random network grown under the influence of unstructured stimulation (black dot), we repeatedly stimulated the same ensemble of excitatory neurons E₁ to eventually form an engram. Multiple stimulation cycles increased the recurrent connectivity within the engram. (B) Population activity of all excitatory neurons upon stimulation of E₁, for different levels of engram connectivity . Crosses depict the population rate observed in a simulation. Colors indicate engram connectivity , matching the colors used in panels (A) and (D). The grey line outlines the expectation from a simple mean-field theory. (C) Firing rate of E₁ engram neurons upon stimulation of 50% its neurons. Shown is the mean firing rate of the stimulated engram neurons (E₁S, orange), the mean firing rate of the non-stimulated engram neurons (E₁N, grey), and the mean firing rate of excitatory neurons not belonging to the engram (E₂, blue). Solid line and shading depict mean and standard deviation for 10 independent simulation runs, respectively. (D) Time-averaged overlap , for different fractions of E₁ being stimulated. The recurrent nature of memory engrams enabled them to perform pattern completion. The degree of pattern completion depended monotonically on engram strength. (E) Two engrams (orange and green) were encoded in a network. Both engrams had a different strength with regard to their within-engram connectivity (green stronger than orange). A simple readout neuron received input from a random sample comprising 9% of all excitatory and 9% of all inhibitory neurons in the network. (F) Raster plot for the activity of 10 different readout neurons during the stimulation of learned engrams and random ensembles, respectively. Readout neurons were active when an encoded engram was stimulated (orange and green), and they generally responded with higher firing rates for stronger engrams (green). The activity of a readout neuron was low in absence of a stimulus (white), or upon stimulation of a random ensemble of neurons (purple and blue).

doi: https://doi.org/10.1371/journal.pcbi.1009836.g003