1. A method, comprising: applying a program signal having a first polarity and a first current compliance to a memory cell; switching the memory cell to a first program state from a non-program state in response to the first program signal by forming a filament in a resistive switching layer of the memory cell; applying an additional program signal having a second polarity to the memory cell; and switching the memory cell to an additional program state different from the first program state in response to the additional program signal by modifying at least one parameter of the filament in the resistive switching layer of the memory cell, wherein: the memory cell inherently resists switching back from the additional program state to the first program state, the second polarity is opposite to the first polarity.