A glaucoma screening device based on the visibility of the spatial frequency doubled (FD) illusion will be marketed by Welch Allyn Ltd in the next year (ANU Patients (Australia) 611,585, (USA) 5,065,767 and application PL 3130). An underlying assumption of the method is that retinal processes are being tested. To test this assumption we compared the visibility of the FD illusion over a range of conditions and in the same spatial locations as a multi-region pattern electroretinogram (PERG). Grating speed and contrast were good predictors of the psychometric functions and PERG amplitude and phase.