Spatiotemporal Inseparability of Natural Images and Visual Sensitivities

The visual system is concerned with the perception of objects in a dynamic world. A significant fact about natural time-varying images is that they do not change randomly over space-time; instead image intensities at different times and/or spatial positions are highly correlated. We measured the spatiotemporal correlation function -- equivalently the power spectrum -- of natural images and we find that it is non-separable, i.e., coupled in space and time, and exhibits a very interesting scaling behaviour. This behaviour is shown to be related to the motion in the images and the power spectrum is naturally separable into a spatial term and a velocity term. The same kind of spatiotemporal coupling and scaling exists in visual sensitivity measured in physiological and psychophysical experiments. By assuming that the visual system is optimized to process information of natural images, a quantitative relationship can be derived between the power spectrum of natural images and the visual sen..