Byron,
current will flow if the potential is positive or negative. Thus, the current has two directions also for an ordinary 120VAC circuit.
Imagine a DC system with +120V and -120V to ground. A light bulb will work both between the +120V and the grounded conductor and between the -120V and the grounded conductor. Clearly, there is a current flow in both cases. This type of system was in fact in fairly extensive use in the first half of the 20th century.
