Effects of luminance oscillations on simulated lightness discriminations

The speed of processes underlying lightness constancy was studied by having observers discriminate small differences in simulated lightness under an o scillating illumination. The period of oscillation varied from 0.25 to 120 sec. The target was a lo square which appeared for 150 msec at random inter vals either directly against a uniform background or separated from the bac kground by a 1 degrees dark gap. When the target and background were adjace nt to each other, discrimination accuracy approached control levels (fixed illumination) at all but the shortest periods of oscillation. When the gap was introduced, accuracy increased as the period of oscillation increased, but never approached control levels. The results suggest that a fast local contrast mechanism is the primary mediator of lightness constancy for this task, but that there is also a slower mechanism that may be related to adap tation.