In the biological system, a complex cell integrates information from multiple simple cells. Once different orientation lines are extracted by the orientation-detection circuit, these outputs can be used as inputs to the complex angle extraction circuit. Separate complex-cell cortical areas represent specific angles formed by combining a horizontal line with another oriented line. For instance, a 0° line together with a 45° line defines a 45° angle. Each of these complex-cell areas has a firing threshold set to the summed postsynaptic potentials (PSPs) arriving from the relevant source area, so firing occurs only when that specific combination is present, indicating the angle. The complex-cell regions corresponding to other angles operate using the same principle.