  To explore what features of multi-dimensional training can be remembered in
granular materials, the response of a small, two-dimensional packing of
hydrogel spheres to two independent types of shear is measured. Packings are
trained via the application of several identical shear cycles, either of a
single shear type or combinations of the two types. The memory is then read out
using a standard protocol capable of revealing memories as a cusp at the point
where readout reaches the training strain. The ability to read out a memory is
sensitive not only to the type of deformation applied but also to the order in
which different types of training are performed. These results underscore the
importance of thinking of memories not as single remembered value (amplitude)
but as a learned path through phase space.
