Noise Induced Perceptual Completion . 1 , 2

When an observer is faced with an ambiguous, occluded or degraded image, the image will frequently be perceptually completed. In perceptual completion observers report seeing the presence of features or objects which are actually absent that are the complement or “completion” of features or objects in the image. We report a novel completion effect resulting from adding half of a bilaterally symmetric image to visual noise, which results in an illusory appearance of the missing half-image in the noise. In this paper we investigate the role of prior knowledge and symmetry in this perceptual completion stimulus using a signal detection task. The task allows us to obtain separate measures of the magnitude of the completion effect (via false alarm rate) and the processing involved in the completion effect (via sensitivity, d’). With these separate measures, we can begin to distinguish between low-level and high-level theories of perceptual completion. We contrast low-level theories that produce differences in sensitivity between completion and no completion conditions with high-level theories that predict no change in detection sensitivity. Our results are most consistent with completion in our stimuli occuring as a result of a high-level perceptual inference or judgement rather than low-level, bottom-up processing. In addition, we show that additional symmetry information can augment the completion effect without changing sensitivity.