Vernier and displacement thresholds in equiluminance

Two types of hyperacuity, vernier acuity and displacement threshold, were investigated in luminance(L) and equiluminance(EqL) conditions. The vernier task is to detect the spatial offset between two visual objects presented simultaneously and the displacement task is to detect the spatial offset between two sequentially presented objects. A one dimensional Gabor pattern, a Gaussian truncated sinusoidal grating was used as a stimulus. Three cycles of the grating were visible and spatial frequency was varied from 0.11 to 6.85 cpd. The stimulus was produced by superimposing a red grating on a green grating of 15 cd/m ² with 20% modulation. The mean luminance and contrast of the R grating was matched to those of the G grating by the minimum motion technique for each spatial frequency and for each subject. Superimposing in phase these R and G gratings produced an L stimulus while counter-phase superimposing produced an EqL stimulus. In the vernier task two identical 1D Gabor patterns placed vertically with horizontal offset were presented for 150 ms. In the displacement task two Gabor patterns were presented for 250 ms each without an interstimulus interval. A simple staircase method was used to measure the threshold. In the L condition, both kinds of thresholds reached about 10 second of arc, which is hyperacuity, but only in high spatial frequencies