Abstract
An essential part of visual perception is segmenting images into figures and background. Previous studies have shown that this segmenting process relies on two complementary processes: boundary detection and subsequent region-filling. However, the neuronal mechanisms for these processes and whether they depend on awareness remain unclear. Here we performed psychophysical and fMRI experiments to examine these issues. In our experiments, each figure-ground stimulus had a regular Manhattan grid of 33 × 45 bars, presented on a dark screen. All bars were identically oriented except for a square figure of bars with another orientation in either the upper left or the upper right quadrant. There were three possible figures: the first and second consisted of 2 × 2 (Small Figure, SF) and 10 × 10 (Large Figure, LF) bars, respectively, and the third was a combination of the two (Combined Figure, CF). Low- and high-contrast masks, which had the same grid as the figure-ground stimuli, rendered the whole stimulus visible (Experiment 1) and invisible (Experiment 2) to subjects, respectively. In the psychophysical experiment, the Posner cueing paradigm was adopted to measure the spatial cueing effect of each figure on an orientation discrimination task with Small or Large Gratings, which had the same size as the SF and LF, respectively. For the Small Grating, there was no significant difference between the LF and CF, and both were significantly lower than the SF in Experiment 1; there was no significant difference between the SF and CF, and both were significantly higher than the LF in Experiment 2. No significant difference was found for the Large Grating in either Experiments 1 or 2. A further fMRI experiment confirmed these findings and suggested that boundary detection is an early, automatic process that is independent of awareness, whereas subsequent region filling is strongly modulated by awareness.