In 1932 Haldan Keffer Hartline pushed a fine electrode into a single ommatidium of a horseshoe crab's eye and found something strange: shine light on the neighbours and the cell he was recording fired less. Each receptor inhibits the ones beside it — lateral inhibition.
The wiring builds a center-surround receptive field: a cell reports its own light minus a blurred average of its neighbours (a difference of Gaussians). On a flat field the two nearly cancel, so the retina barely responds to absolute brightness. At an edge the balance breaks — the bright side overshoots and the dark side is pressed below baseline.
Those over- and under-shoots are the Mach bands Ernst Mach drew in 1865: a bright fringe and a dark fringe hugging every edge, present in your response but never in the light itself. Push the inhibition too far and the dark band dives below zero — a firing rate a real neuron cannot deliver. The retina isn't a camera. It is a contrast machine, and the contrast is partly its own invention.
The simulation stopped unexpectedly — the lesson continues without it. You can move on; nothing you did was wrong.