observers, the JND at a50. 2 was greater than 0. 2; i. e., the exponent a2Dabecomes negative, and the slope of the 1/ fa2Daspectra for the comparison image is slightlypositive. This can occur only for the Below discrimina-tions and is the most likely explanation of the asymmetrybetween Above and Below thresholds. Fig. 6. Spatial JND's collapsed across temporal exponents. Here the spatial discriminations are plotted as a function of spatial expo-nent. Each subﬁgure is for an observer named by his initials. Fig. 7. Spatial JND's collapsed across spatial exponents. Here the spatial discriminations are plotted as a function of temporal expo-nent. The temporal exponent labeled ''0'' is a place holder for the static condition. Observers are identiﬁed by initials. 2410 J. Opt. Soc. Am. A/Vol. 18, No. 10/October 2001 Billock et al.