Abstract
It has been argued that salient stimuli capture attention automatically and rapidly. Also, it has been claimed that attention changes the appearance of objects. We investigated whether the appearance of salient stimuli differs from non-salient stimuli. Observers saw a circular array of eight bars. All but one bar were vertical. The orientation singleton differed by either 6° or 45° from vertical. Observers compared the luminance contrast of the orientation singleton to a non-salient element. The luminance contrast of the non-salient element had to be increased in order to match the singleton, showing that salient objects are perceived to have a higher luminance contrast. The increase in perceived luminance contrast was larger with highly salient (45° singleton) than with mildly salient (6° singleton) stimuli. Further, we investigated whether the perceived color saturation of an orientation singleton changed. We created equiluminant red stimuli and asked observers to judge their saturation. Unexpectedly, the mildly salient singleton appeared less saturated than the non-salient element. Consistent with our hypothesis, however, the highly salient singleton appeared more saturated. Further, we investigated whether stimuli that are salient because of their motion direction look different. One of eight drifting Gabors moved opposite to the others. Observers compared the Michelson contrast of the singleton Gabor to a non-salient Gabor. The salient Gabor appeared to have a higher contrast. Our results show that judgments of contrast and saturation are influenced by saliency. There has been a debate about the stage at which changes in appearance occur. Some have claimed that the effects reflect true perceptual changes while others maintain that the effects reflect response biases. We do not think that we can ultimately rule out response biases with behavioral measures alone, but we present some evidence against response biases for this series of experiments.
Swiss National Foundation PDFM1-129459 / 1.