A wave fragment crossing the visual cortex, mapped back into the visual field.
The front is a circular arc in cortex whose ends contract as it goes; the apex angle falls to zero and the wave dies out near the periphery. Uneven contraction at the two ends lets the heading walk away from the horizontal.
Colour and pattern are pulled inward from the edge of the hole, losing detail the further they travel. Lower values leave the old scene visible for longer after you move.
Lines invert black to white and back, patch by patch and line by line. Set flicker to 0 Hz to hold them still.