I haven't tried this, but in theorey, there is a way you could make the spike balls go abck and forth, but it would only work with spike balls, and it would have another possibly undesirable effect. You'd have to set the balls to actually swing around in 3D like they're meant to, and have the ball's path cross floating conveyors that go in opposite directions. It would go one direction when the ball is high, and the opposite direction when the ball is low. However, it would also be moving in and out of the background as it does this.