Abstract

Weber’s law is usually found for relative localization tasks, such as discriminating the separation of two squares or discriminating the vernier alignment of two lines. In other words, thresholds increase linearly with increases in the separation between the stimuli. This phenomenon may be due to various stimulus factors that are usually confounded with separation. In particular, widely separated stimuli must necessarily have one or both stimuli at larger eccentricities than can be achieved for narrowly separated stimuli: two points separated by 4° cannot both fit in the fovea. Consequently, stimulus eccentricity may contribute to the effect of separation. The current study investigates the role of eccentricity by holding it constant and manipulating separation alone. This is done by placing two squares along an imaginary circle centered around fixation. With such an arrangement, the separation can be increased while the eccentricity of each square is held constant. For 0.5° squares, separation discrimination is found to follow Weber’s law even when eccentricity is held constant at 5°. Further measurements at eccentricities of 2 and 18° also follow Weber’s law and for some cases even show similar Weber fractions. Thus eccentricity cannot in general mediate the effect of separation found in localization tasks.

© 1988 Optical Society of America