Unsupervised learning of dependencies between local luminance and contrast in natural images

Separate processing of local luminance and contrast in biological visual systems has been argued to be due to the independence of these two properties in natural image data. In this paper we examine spatial, retinotopic channels formed by these two quantities and use Independent Component Analysis to study the possible dependencies between the channels. As a result, oriented, localized bandpass filter pairs are learned, where one filter processes the luminance channel and the other the contrast channel. We study the relationship of the learned filters and their pairings, and show that these are due to dependencies existing between local luminance and contrast. Subsequently, our results suggest that the separate processing of local luminance and contrast can not be attributed to their independence in natural images.