1. A method of operating a crossbar memory array comprising a plurality of two-terminal memory devices and a plurality of selector devices, wherein each of the plurality of two-terminal memory devices is associated in series with one selector device from the plurality of selector devices, wherein each selector device is associated with a first electrical characteristic in response to an applied voltage less than a threshold voltage, and associated with a second electrical characteristic in response to an applied voltage greater or equal to the threshold voltage, the method comprising: applying a first voltage greater than the threshold voltage to a first memory structure comprising a first two-terminal memory device in series with a first selector device; applying a second voltage, concurrently with applying the first voltage, that is less than the threshold voltage to a second memory structure comprising a second two-terminal memory device in series with a second selector device; and determining a current in response to applying the first voltage concurrent with applying the second voltage, wherein: the current comprises a first current associated with the first selector device and a second current associated with the second selector device; and a current ratio of the first current to the second current is within a range of ratios selected from a group of ranges consisting of: about 1,000 to about 10,000, about 10,000 to about 100,000, about 100,000 to about 1,000,000, and about 1,000,000 to about 10,000,000.