 This equation can be thought of as the sum of the lengths of two vectors.MacLeod (1972) expanded upon Schrödinger's work by proposing the following formula for the signal strength of a neural signal formula_5 in terms of internal noise formula_6, luminance difference across the contour formula_7, and threshold luminance difference formula_8.It is still uncertain exactly how this process is performed by the brain and remains an active area of research.