Natural image statistics mediate brightness 'filling in'


Dakin SC, Bex PJ. Natural image statistics mediate brightness 'filling in'. Proceedings of The Royal Society of London [Internet]. 2003;270 :2341–2348.


Although the human visual system can accurately estimate the reflectance (or lightness) of surfaces under enormous variations in illumination, two equiluminant grey regions can be induced to appear quite different simply by placing a light- dark luminance transition between them. This illusion, the Craik-Cornsweet-{O'Brien} ({CCOB)} effect, has been taken as evidence for a low-level 'filling-in' mechanism subserving lightness perception. Here, we present evidence that the mechanism responsible for the {CCOB} effect operates not via propagation of a neural signal across space but by amplification of the low spatial frequency ({SF)} structure of the image. We develop a simple computational model that relies on the statistics of natural scenes actively to reconstruct the image that is most likely to have caused an observed series of responses across {SF} channels. This principle is tested psychophysically by deriving classification images ({CIs)} for subjects' discrimination of the contrast polarity of {CCOB} stimuli masked with noise. {CIs} resemble 'filled-in' stimuli; i.e. observers rely on portions of the stimuli that contain no information per se but that correspond closely to the reported perceptual completion. As predicted by the model, the filling- in process is contingent on the presence of appropriate low {SF} structure.