Top-down knowledge can affect perception when the input is ambiguous.

Does what we see depend on what we know? Many findings suggest that top-down factors such as emotions, desires, and categorical knowledge affect perception. However, these findings have been met with considerable criticism due to a variety of methodological flaws, replication failures, and extremely small effect sizes. Here, we focus on one specific case in which top-down knowledge has been claimed to affect perception: memory color. Specifically, we describe a novel variant of the memory color effect that was purposefully designed to avoid these common criticisms and serves as a clear example of a top-down factor affecting perception. Specifically, we theorized that under ambiguous viewing conditions, top-down knowledge is more likely to impact perception because that knowledge will be used to disambiguate underdetermined sensory input. To test this hypothesis, we showed observers' printouts of completely desaturated objects in an ambiguous viewing condition: extremely dim light. As predicted, we found a strong, subjectively appreciable memory color effect under dim, ambiguous light, but not under bright, unambiguous light. In addition, a series of control experiments demonstrated that these effects could not simply be attributed to experimental demand characteristics. These results demonstrate that in certain situations, top-down factors can directly affect perceptual experience and appreciably alter how items appear. (PsycInfo Database Record (c) 2026 APA, all rights reserved).