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 article 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 upon in support of this interpretation, which has been foreshadowed
in several recent publications.