transduction or uncertainty, or a combination of the two. For contrast discrimination, the pedestal removes intrinsic uncertainty, and contrast gain control reduces the effective exponent; both processes result in a shallower slope of the psychometric function. Manipulating extrinsic uncertainty experimentally should affect both threshold and slope but, despite its theoretical importance, this test has not been performed previously at both detection threshold and above. Here we manipulated spatial uncertainty for detection and discrimination of a pair of horizontal 4 cycles degÿ1 Gabor patches placed equidistant from a central fixation point on the circumference of a virtual circle. In a temporal 2AFC paradigm, there were 1, 2, 4, or 8 possible locations for the target pairs, indicated by low contrast rings. The level of uncertainty was fixed within a block of trials, with target contrast levels determined by the method of constant stimuli. For contrast discrimination, the experiment was identical except that pedestals were presented in all locations on every trial. Thresholds and slopes increased with extrinsic uncertainty for both detection and discrimination. However, the threshold effect was greater for discrimination than for detection, confirming our prediction that intrinsic uncertainty is greater at threshold than above. We report estimated levels of intrinsic uncertainty for a range of transducer exponents (1: 3). A detailed understanding of the effects of intrinsic and extrinsic uncertainty are critical for examining effects such as collinear facilitation, for which uncertainty reduction is a common explanation.