  Biological synapses store multiple memories on top of each other in a
palimpsest fashion and at different timescales. Palimpsest consolidation is
facilitated by the interaction of hidden biochemical processes that govern
synaptic efficacy during varying lifetimes. This arrangement allows idle
memories to be temporarily overwritten without being forgotten, in favour of
new memories utilised in the short-term. While embedded artificial intelligence
can greatly benefit from such functionality, a practical demonstration in
hardware is still missing. Here, we show how the intrinsic properties of
metal-oxide volatile memristors emulate the hidden processes that support
biological palimpsest consolidation. Our memristive synapses exhibit an
expanded doubled capacity which can protect a consolidated long-term memory
while up to hundreds of uncorrelated short-term memories temporarily overwrite
it. The synapses can also implement familiarity detection of previously
forgotten memories. Crucially, palimpsest operation is achieved automatically
and without the need for specialised instructions. We further demonstrate a
practical adaptation of this technology in the context of image vision. This
showcases the use of emerging memory technologies to efficiently expand the
capacity of artificial intelligence hardware towards more generalised learning
memories.
