The stimulus representation follows the one used by Poirson and Wandell (1993). The representation of the physical stimulus is based on the use of the Smith and Pokorny (1975) cone fundamentals. We use a version of the cone fundamentals that is normalized to a peak value of 1.0. The LMS coordinates of the uniform background are 5.322, 5.007 and 4.485. These three values are proportional to the rate of the photopigment absorptions created by a uniform field in the three cone classes for a standard observer