B. Methods 1. Participants and Apparatus These were the same as in Experiment 1. 2. Stimuli The stimuli were identical to those used in Experiment 1, with the sole exception that static fractals were also em-ployed. 3. Procedure The smallest detectable increase (Above JND) and de-crease (Below JND) in the spatial exponent band in the temporal exponent awere separately measured with an adaptive staircase procedure. For each of the four typesof discrimination, a reference and a comparison imagewere presented side by side (1. 1 mm apart) for 2. 133 s oruntil the observer responded, whichever came ﬁrst. Ineach case, the subject was given instructions that wereequivalent to identifying the reference stimulus. The lo-cation of the reference stimulus (left versus right) wasrandomized across trials. Immediately after the ob-server responded (using a computer keyboard), feedbackwas provided on the accuracy of this response. If the ob-server did not respond within the display time, the screenwas blanked and the observer was prompted for an an-swer. If the observer accurately identiﬁed the referencestimulus three times in a row (79% correct criteria), the difference between the exponents of the comparison andreference images was decreased by 10%. If the observerresponded incorrectly, the difference between the expo-nents was increased by 10%. Each staircase continuedfor eight reversals, and the mean of the last six reversalswas used as a measure of the threshold. When spatial JND's were measured, the reference and the comparison fractals were assigned the same temporalexponent and the spatial exponent of the comparisonstimulus was adaptively varied. The observers wereasked to identify the reference image. That is, they wereto ﬁnd the more ''ﬁne-grained'' texture for the Above dis-criminations and the less ﬁne-grained texture for the Be-low discriminations. When temporal JND's were measured, the spatial ex-ponent was held constant while the temporal exponentwas adaptively varied. The observers were again askedto identify the reference image. In other words, theywere to ﬁnd the faster and more jittery texture for the Above discriminations and the slower and less jittery tex-ture for Below discriminations. For all subjects, the 80 Above spatial discriminations were presented ﬁrst, followed by the 80 Below spatial dis-criminations, then the 70 Above temporal discrimina-tions, and ﬁnally the 70 Below temporal discriminations. Fig. 4. Mac Adam's ellipses. 33Discrimination thresholds (JNDs) for various directions in CIE 1931 color space. Ellipses are for Mac-Adam's observer PGN and are shown at ten times their actual size. Reproduced by permission of the Optical Society of America. 2408 J. Opt. Soc. Am. A/Vol. 18, No. 10/October 2001 Billock et al.