Systems that have not fully relaxed to equilibrium are known to be capable of retaining memories of “training” to which they have been subjected. Understanding the basic operations of memory—imprinting, reading, and erasure—is a formidable challenge of immediate relevance to gaining control over learning and forgetting.
Towards this end, Nathan Keim and Sidney Nagel of the University of Chicago, US, report significant progress in their paper in Physical Review Letters. They generalize a seemingly simple process of cyclic shear by an external force applied to disk-shaped particles confined to a flat corral. The effect of the force is detected by following the fraction of misplaced particles, and a well-defined memory of that action is recorded and quantified. By applying cyclic strain of different amplitudes, more than one memory can be imprinted. All memories are present throughout the transient period, yet eventually all but the one with the largest effect are lost.
Most strikingly, adding noise to the system can stabilize multiple memories indefinitely. This is because noise disrupts self-organization, thus ensuring that the system never reaches its final state. That this remarkable form of memory appears to be generic opens the door to a better understanding of biological systems where cyclic behavior is the modus operandi, while noise is inevitable and, indeed, ubiquitous. – Yonko Millev