  The idea that memory behavior relies on a gradually-changing internal state
has a long history in mathematical psychology. This chapter traces this line of
thought from statistical learning theory in the 1950s, through distributed
memory models in the latter part of the 20th century and early part of the 21st
century through to modern models based on a scale-invariant temporal history.
We discuss the neural phenomena consistent with this form of representation and
sketch the kinds of cognitive models that can be constructed using it and
connections with formal models of various memory tasks.
