Perceived Timing of Different Features at Surface Formation

Different features of a stimulus, such as color, motion, or orientation, are to some extent processed independently in the brain. It is not clear, however, whether a change within a feature is perceived with different delays for different features because different methods —temporal order, synchrony, and pairing judgments—show conflicting results. Previous methods provide estimates of perceived timing that often are quite variable across trials and observers suggesting the involvement of high-level strategies. Here, we report a new method to measure the perceived timing of feature changes that shows low variability. When four pac-men are presented properly aligned above a background defined by some feature, the illusory surface that pac-men produce appears to be filled in with the feature. We asked observers to report the feature value inside a briefly presented illusory surface when the feature in the background changed over time (Motoyoshi, 2007). We found that observers' estimates are very precise for both continuous and discrete feature changes. For example, when a background changes motion repetitively between two discrete values (60 ms leftwards and 60 ms rightwards), observers can precisely report the motion direction filling in an illusory surface presented for 60 ms. We propose that our method measures the perceived timing for different features at the level of surface formation.