Spatial frequency interference effects and interpolation in vernier acuity

Discrete spatial sampling of sinusoidal gratings has a detrimental effect upon vernier acuity for such stimuli if the sampling rate is less than 20 c/deg. We have examined the possibility that interpolation failure is due to masking by spatial frequency components near to the signal frequency. In an explicit masking paradigm, vernier acuity for a sine wave grating was found to be adversely affected by the presence of a masking grating falling within a critical band of frequencies near the test target. In the sampled stimulus, removal of sampling replicas similar in frequency to the test improved acuity, but the extent of the residual interference by high frequency components could not be predicted from the masking data. The high frequency interference effect depended on fixed phase relations between frequency components, creating narrow bright bars in the spatial luminance profile, and was greatly reduced by phase randomizing the sampling replicas. Various explanations of this nonlinear behaviour are discussed, including the Marr-Hildreth theory of edge detection.