The temporal dynamics of feature integration for color and form

When two visual stimuli are presented in rapid succession only one fused image is perceived. For example, a red disk followed by a green disk is perceived as a single yellow disk (e.g., Efron, Perc Psychophys, 1973). For two verniers that last 30ms each, it has been shown that the fusion process can be disrupted up to 420ms after stimulus onset by applying TMS over visual cortex (Scharnowski et al., J Vis, 2009). Here, we show that feature fusion can also be modulated with light masks. In Experiment 1, we used the same procedure as in the above TMS experiment: Two verniers were presented in rapid succession with opposite offset directions. Subjects had to indicate the offset direction of the fused vernier. In Experiment 2, a red and a green disk were presented in rapid succession and subjects had to indicate whether the fused disk appeared red or green. Verniers were presented for 30ms and disks for 10ms each. Light masks were presented from -100 to 240ms relative to the onset of the first stimulus. When masks were presented before the stimuli, the second stimulus dominated the percept and when masks were presented after the stimuli, the first stimulus dominated the percept. Feature fusion was modulated for a period of 240ms for both color and vernier stimuli. Our results indicate that for this duration individual memories exist for each vernier or color stimulus separately. Fusion is not completed beforehand. Taken together, our results show that light masks have similar effects on feature fusion as TMS and provide an adequate alternative to investigate feature integration. In addition, we showed that the process of unconscious feature fusion outlasts the physical duration of the stimuli by far.