PURPOSE. The frequency-doubling illusion is an apparent doubling of spatial frequency when a sinusoidal grating is modulated rapidly in temporal counterphase. Recently, in the context of clinical testing, it has been proposed that the illusion arises from a spatially nonlinear ganglion cell class (Maddess, Goldberg, Dobinson et al. 1999). The current study reexamines this possibility and investigates other mechanisms which may underlie the illusion. METHODS. Human psychophysical thresholds were measured for a variety of grating phase discrimination and matching tasks. Responses of macaque magnocellular (MC-) retinal ganglion cells were also recorded to counterphase-modulated sinusoidal gratings of various spatial frequencies, and the spatial nonlinearity was assessed. RESULTS. No physiological evidence was found of a separate nonlinear MC-cell class. Also, cells' nonlinearity of spatial summation gives a doubled response in time but not in space; no spatially modulated signal can be expected from the nonlinear response of a ganglion cell. Observers were unable to distinguish the temporal phase of counterphase-modulated gratings above ~15 Hz, but could obtain spatial information provided by the grating zero-crossings. These and other measurements suggest that the illusion occurs because central mechanisms lose temporal phase information. CONCLUSIONS. These results argue against the hypothesis that spatially nonlinear retinal ganglion cells are the physiological substrate of the frequency-doubling illusion. A cortical loss of temporal phase discrimination may be the principle cause of the illusion, whereas spatial phase information (i.e., grating position) is retained.

Maddess, T., Goldberg, I., Dobinson, J., Wine, S., Welsh, A.H. & James, A.C. (1999). Testing for glaucoma with the spatial frequency doubling illusion. Vision Research, 39, 4258-4273.g