1. The spatial properties of edge detectors were measured psychophysically with the technique of subthreshold addition. Subthreshold patterns used to add to an edge were lines, sine gratings, Gaussian edges, and ramps. 2. The sensitivity profile, determined from experiments on subthreshold addition of lines to an edge was an antisymmetric function, with peak sensitivity approximately ± 1·5′ from its midpoint. Its total extent was about ± 6′. 3. The spatial frequency response of edge detectors was measured in experiments on subthreshold addition of sine gratings to an edge. The spatial frequency response was peaked at about 3 c/deg, and was broadly tuned in frequency. It was approximately equal to the Fourier transform of the sensitivity profile, implying linearity of edge detectors. 4. The visibility of Gaussian edges and ramps could be explained largely in terms of the activation of edge detector neurones. 5. The role of edge detectors in perception, in creating apparent brightness, and as an explanation of contour illusions, is discussed.
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