You are seeing motion blur caused by eye tracking on a sample-and-hold display. Eye movement causes the vertical lines to be blurred into thicker lines, filling the black gaps. This is motion blur from persistence (MPRT) instead of pixel transitions (GtG). See GtG versus MPRT. Low persistence displays (such as CRT or gaming monitors with ULMB) eliminate this display motion blur, so this demo looks different on those screens. For another amazing animation, see TestUFO Persistence of Vision.