Eluding oblivion with smart stochastic selection of synaptic updates

The variables involved in the equations that describe realistic synaptic dynamics always vary in a limited range. Their boundedness makes the synapses forgetful, not for the mere passage of time, but because new experiences overwrite old memories. The forgetting rate depends on how many synapses are modified by each new experience: many changes means fast learning and fast forgetting, whereas few changes means slow learning and long memory retention. Reducing the average number of modified synapses can extend the memory span at the price of a reduced amount of information stored when a new experience is memorized. Every trick which allows to slow down the learning process in a smart way can improve the memory performance. We review some of the tricks that allow to elude fast forgetting (oblivion). They are based on the stochastic selection of the synapses whose modifications are actually consolidated following each new experience. In practice only a randomly selected, small fraction of the synapses eligible for an update are actually modified. This allows to acquire the amount of information necessary to retrieve the memory without compromising the retention of old experiences. The fraction of modified synapses can be further reduced in a smart way by changing synapses only when it is really necessary, i.e. when the post-synaptic neuron does not respond as desired. Finally we show that such a stochastic selection emerges naturally from spike driven synaptic dynamics which read noisy pre and post-synaptic neural activities. These activities can actually be generated by a chaotic system.

Le Grand Retour ou les Grands Adieux ? Le roman de l’exil chez Kundera

Milan Kundera, an intellectual born in Moravia who emigrated to France in 1975, in L’Ignorance leans on the myth of Ulysses to question contemporary realities of exile and return, nostalgia and oblivion. Does the hope of returning to the place of origin really haunt the modern émigré? To what extent does the notion of homeland still have meaning for him? And what happens when the émigré, unlike Ulysses the great nostalgic, prefers to stay with Calypso his lover rather than return to his native land and faithful wife Penelope? With some cynicism, Kundera in L’Ignorance offers scenarios of exile which desecrate and destabilize historically and culturally available standards while allowing us to reflect on new paradigmatic figures of contemporary exile.