Change blindness, subset segmentation, and the perceptual underestimation of subset numerosity

How well can humans perceptually estimate subsets of collections of differently colored dots? People can simultaneously evaluate the numerosity of at least two color-defined subsets. But when equal (largish) numbers of light gray and white dots are presented on a medium-gray background, there appear to be fewer white dots than gray dots. The present paper presents 6 experiments designed to test the hypothesis that these behaviors are due to figure-ground segmentation based on color similarity, which can lead to incomplete segmentation, and thus perceptual underestimation of foregrounded dots. Ironically, subset matching is most accurate for sets of dots that are difficult to segment, such as light gray among white. This is demonstrated using a color-change detection task to show that (1) accurate subset estimation is only accomplished for sets that resist foreground selection, and (2) even stereoscopically backgrounded white dots fail to be segmented (i.e., are at chance for color change detection) when a frontal plane of gray dots is more successfully segmented. Although explicit attentional biasing is shown to shift performance between dots differing in chromatic color, it does not improve performance at selecting light gray dots among white. It is also shown that the perceptual underestimation of supersets of mixed colors may be consistent with combining an underestimated foreground with an accurately estimated background.