Spatial aliasing constrains: The function of Y-cells and indicates their role in human vision

Purpose. The extraction of image motion from the visual environment is fundamental to visual perception. The Y retinal ganglion cells are commonly thought to be the inputs to motion computation and to have a different retinal magnification to other retinal cell types. Several studies indicate the possibility of "spatial under-sampling", in the Y-cell pathway. Methods. Several lines of evidence suggest that the spatial frequency doubled (FD) illusion is mediated by Y-cells. Motion reversal of periodic stimuli is a common feature of systems exhibiting under-sampling. We therefore tested for the presence of spatial frequency dependent m.otion reversal of FD stimuli and related phenomena. Results. Motion reversed patterns having the features of spatial aliases characteristic of undersampling were observed in 14 subjects. The critical spatial frequency for motion reversal at different retinal eccentricities shows the units mediating the FD illusion to have a density which is equal to estimates of the proportion of M-cells(15 to 24%) which are Y-like (i.e. M_y-cells). Conclusions. The results exclude Y-cells as the direct inputs to image motion computation and also show that, except for a scaling factor, their retinal magnification is little different to other cell types. This leaves to Y-cells the role of rapidly conveying information about dynamic retinal gain control to the brain, a role unaffected by under-sampling. There are also implications for testing Y-cells for the diagnosis of the eye disease glaucoma.