Achromatic brightness matches between two small patches were measured in a display containing ten larger regions of different luminances. The spatial organization of the ten regions was varied while keeping constant the immediate surround (and thus local contrast) of each patch as well as the average luminance of the entire stimulus. Various spatial arrangements were designed to alter the illumination inferred by the observer without changing the ensemble of luminances actually in view. Some spatial arrangements of the ten regions were consistent with five (simulated) surfaces under two distinct levels of illumination, with one luminance edge within the display (an 'apparent illumination edge') dividing the stimuli into an area of lower illumination and an area of higher illumination. In other spatial arrangements the ten regions were configured so that no luminance edge in the display could be interpreted as an ecologically valid illumination edge that provides a parsimonious interpretation of the ten regions; these conditions were designed to induce observers to infer ten surfaces under a single illuminant. When the ten regions were arranged with an apparent illumination edge, the patch within the area of lower perceived illumination was perceived as dimmer than when the same patch and immediate surround were presented with no apparent illumination edge. The results are interpreted by positing that the apparent illumination edge causes an observer to group together regions under the same perceived illuminant, with a consequent effect on brightness: lowering or raising the level of a perceived illuminant causes a patch of fixed contrast to be perceived as less bright or more bright, respectively, just as occurs when lowering or raising the level of real illumination. It is suggested that changes in brightness in a complex scene that result from a change in real illumination may be caused by a difference in inferred illumination at the perceptual level, not by simply a change in the amount of light absorbed by photoreceptors.