Context is quick, knowledge is slow: rapid time-course of contextual modulations in the horizontal-vertical illusion

One of the oldest visual illusions consists of observers' tendency to estimate vertical lines as longer than equal horizontal lines. This Horizontal-Vertical Illusion has been studied widely; however, most studies may not generalize to natural contexts because they have not addressed the low-level processes that occur rapidly and in response to context-free lines vs the possibility for high-level processes that arise over time in response to context. These issues were investigated via a novel design: rapid time-sequencing embedded within the context of natural images. Participants saw different levels of context and the Horizontal-Vertical Illusion was measured. In traditional Horizontal-Vertical Illusion displays without natural context, there was a robust invariance of the illusion to stimulus duration. Within context, however, there was strong modulation: the magnitude of the illusion decreased by over 50% or increased over 30%, depending upon placement within context. Knowledge of object properties such as size or line length did not account for this effect. It appears that previous domain-general investigations of the Horizontal-Vertical Illusion did not address the specific higher-order processes that produce the illusion in natural contexts.