Abstract
[Purpose] A brightness difference between two identical gray stimuli on uniform light and dark surrounds becomes larger when the surrounds are replaced by the ones composed of many small patches having different luminances (‘articulated’ surrounds) while keeping the space-averaged luminance constant. To explore visual mechanisms underlying this articulation effect in view of global vs. local processing, the present study introduced the perception of transparency over the dark surround by manipulating global stimulus configuration alone, and investigated its effects on brightness perception on the surround.
[Methods] Light and dark surrounds were placed side-by-side which were either spatially uniform or articulated. By adding a contiguous region of lower luminance to the dark surround, the perception of transparency (i.e., impression of being covered with a larger dark filter or shadow) was produced under the transparency condition. Under the no-transparency conditions, the perceived transparency was eliminated by separating the dark from the light surround and also by introducing a gap at the border of the dark surround. Local stimulus configuration within the surround was kept constant under different conditions. The space-averaged luminances of the light and dark surrounds were 1.16 and 0.38 log cd/m2, respectively. Observers matched the brightness of the test stimulus (1.06 log cd/m2) on the dark surround by adjusting the luminance of the matching stimulus on the light surround.
[Results and Discussion] With the uniform surrounds, the test stimulus appeared brighter under the transparency condition than under the no-transparency conditions. In contrast, the brightness difference was not found with the articulated surrounds, although the manipulation of global configuration substantially changed the appearance of the stimulus on the dark articulated surround. The articulation effect was consistently found under all conditions. These findings suggest that brightness perception on the present articulated surround was determined almost exclusively depending upon local computation of brightness.