In previous work we showed that one-dimensional (1D) speed gradients are sufficient to produce a compelling impression of surface slant. Summing a 1D vertical shearing gradient or, less intuitively, a 1D horizontal shearing gradient with a random field of horizontally translating dots produces perceived slant about a horizontal axis. Similarly, a 1D vertical or horizontal compression gradient produces perceived slant about a vertical axis. Appropriately combining orthogonal 1D shears or compressions produces a purely deforming flow pattern. Here we asked whether both the vertical and horizontal components in such a stimulus contribute to perceived slant. Using a matching technique we found that for surfaces inclined about a vertical axis, this was indeed the case and that horizontal and vertical compression gradients contributed roughly equally to perceived slant. Similarly, for surfaces inclined about a horizontal axis, both vertical and horizontal shearing gradients contributed to the perceived slant, though here the horizontal gradient was given less weight than the vertical. We conclude that under appropriate conditions, the human visual system combines orthogonal speed gradients prior to the computation of slant from retinal flow.
Meese, T. S., & Harris, M. G. (1997). Computation of surface slant from optic flow: Orthogonal components of speed gradient can be combined. Vision Research, 37(17), 2369–2379. https://doi.org/10.1016/S0042-6989(97)00049-7