Photoreceptors strongly attenuate high temporal frequencies. Hence when an image moves, high spatial frequency components are lost if their direction of modulation coincides with the direction of movement, but not if it is orthogonal. The power spectra of natural images are remarkably consistent in having a 1/f 2 falloff in power in all directions. For moving images, the spatial power spectra will be distorted by becoming steeper in the direction corresponding to modulation in the direction of motion, and the contours of equal power will tend to become elliptical. This study demonstrates that the mammalian visual system is specifically sensitive to such anisotropic changes of the local power spectrum, and it is suggested that these distortions are used to determine patterns of optic flow. Convergent evidence from work on Glass figures, motion streaks, and sensitivity to non-Cartesian gratings is called on in support of this interpretation, which has been foreshadowed in several recent publications.