  Memories in the brain are separated in two categories: short-term and
long-term memories. Long-term memories remain for a lifetime, while short-term
ones exist from a few milliseconds to a few minutes. Within short-term memory
studies, there is debate about what neural structure could implement it.
Indeed, mechanisms responsible for long-term memories appear inadequate for the
task. Instead, it has been proposed that short-term memories could be sustained
by the persistent activity of a group of neurons. In this work, we explore what
topology could sustain short-term memories, not by designing a model from
specific hypotheses, but through Darwinian evolution in order to obtain new
insights into its implementation. We evolved 10 networks capable of retaining
information for a fixed duration between 2 and 11s. Our main finding has been
that the evolution naturally created two functional modules in the network: one
which sustains the information containing primarily excitatory neurons, while
the other, which is responsible for forgetting, was composed mainly of
inhibitory neurons. This demonstrates how the balance between inhibition and
excitation plays an important role in cognition.
