The cerebral representation of peripheral sensory surfaces is strictly ordered. In the case of the visual system, the scale of the cortical representation of visual space is non-linear: the area devoted to the fovea is greatly enlarged. Attempts have been made to quantify this retino-cortical relationship in animals and man: the result is the cortical magnification factor (M) which indicates the linear distance in mm along the striate cortex concerned with each degree of the visual field. The cortical magnification factor has indicated an alternative approach both to the theoretical interpretation of visual processing and to the clinical aspects of visual field examination. By M-scaling vision test stimuli, visual sensitivity, while quantitatively different at foveal and peripheral projections, remains qualitatively similar across the entire field.