An extension of Helmholtz's explanation of Listing's law

Helmoholtz argued that the eye moves in accordance with Listing's law to ensure that objects have the same apparent direction despite changes in the point of fixation. Listing's law ensures that the same pattern of image displacements is associated with a given eye movement, independent of the initial point of fixation. His explanation has been extended to the problem of ensuring that objects remain at the same apparent depth, with both direct and indirect fixation. This problem is simplified if disparity is analysed in the region of the frontal plane.