The effect of noise check size and shape on grating detectability

When noise checks are small, spatial noise mimics the effect of white noise in grating detection in the sense that contrast energy threshold is directly proportional to the spectral density of noise and the physical signal-to-noise ratio at threshold thus remains constant. We investigated how the size and shape of noise checks affect the masking properties of noise by using vertical and polar-circular gratings embedded in the spatial noise. The noise check shape and area was varied in both horizontal and vertical dimensions. For polar-circular gratings, noise mimicked the effect of white noise when both the noise check width and height were below a critical size. For vertical gratings there was a critical noise check width, but not critical noise check height. Thus, when the noise check side length was at or below the critical size across the grating bars, and at or below the stimulus size along the grating bars, contrast energy threshold was proportional to the spectral density of noise, calculated by multiplying the noise check area by the r.m.s contrast of noise squared both for one- and two-dimensional check noises.