Hubel and Wiesel documented the existence of several types of neurons. Some  neurons seem to function as "dot detectors", also called center-surround cells.  If light shines onto the central region, the cell increases its firing rate; this is why the center is marked "+". If light shines onto the peripheral region, the cell decreases its firing. If both regions are illuminated, the cell will not change its firing rate in either direction. Stimuli presented outside the periphery have no effect on the cell.

Other cells fire at their maximum only when a stimulus containing an edge of just the right orientation appears within their receptive fields: "edge detectors".

Other cells fire maximally only if an angle of a particular size appears in their receptive fields; other fire maximally in respons to corners and notches. Still other cells apear to be "movement detectors".