Purely temporal figure-ground segregation

Visual figure-ground segregation is achieved by exploiting differences in f eatures such as luminance, colour, motion or presentation time between a fi gure and its surround. Here we determine the shortest delay times required for figure-ground segregation based on purely temporal features. Previous s tudies usually employed stimulus onset asynchronies between figure- and gro und-containing possible artefacts based on apparent motion cues or on lumin ance differences. Our stimuli systematically avoid these artefacts by const antly showing 20 x 20 'colons' that flip by 90 degrees around their midpoin ts at constant time intervals. Colons constituting the background flip in-p hase whereas those constituting the target flip with a phase delay. We test ed the impact of frequency modulation and phase reduction on target detecti on. Younger subjects performed well above chance even at temporal delays as short as 13 ms, whilst older subjects required up to three times longer de lays in some conditions. Figure-ground segregation can rely on purely tempo ral delays down to around 10 ms even in the absence of luminance and motion artefacts, indicating a temporal precision of cortical information process ing almost an order of magnitude lower than the one required for some model s of feature binding in the visual cortex [e.g. Singer, W. (1999), Curr. Op in. Neurobiol., 9, 189-194]. Hence, in our experiment, observers are unable to use temporal stimulus features with the precision required for these mo dels.