The relationship between the subjective and objective aspects of visual filling-in

We explored the relationship between filling-in processes and the known increase in detection sensitivity observed for targets presented between collinear flankers. Filling-in was probed using a Yes/No detection task by measuring the false-positive reports (false-alarm, FA) and hit rate (Hit) for a low-contrast Gabor target with different target-flankers distances. Observers increased the number of reports on the presence of a target (FA and Hit) when the flankers’ distance was within the known range of facilitatory lateral interactions. This bias in reporting was reduced with blocked stimulation, when the target-flanker distance was kept fixed across trials. When different distances were mixed by trials the bias followed the pattern of lateral interactions across distance. The effect was maximal when flankers and targets were aligned. These false perceptions are most likely the result of a filling-in process by lateral excitation that produces illusory contours.