Detection Efficiency for Gratings with Various Frequency Gradients

We investigated the effects of spatial frequency gradients on human grating-detection efficiency. Both polar-circular and vertical cosine gratings of different central spatial frequencies (1 – 4 cycles deg⁻¹) and frequency gradients were used. The local spatial frequency either increased or decreased with the radial distance from image centre. Contrast energy thresholds were determined by a 2AFC method. The gratings were embedded in strong spatial noise, which allowed us to investigate detection performance in terms of efficiency. The experimental results were described as a function of a scale-invariant measure of image complexity (Z) calculated as Z=Af², where A is the grating area and f is the highest local spatial frequency of the gradient grating.

The results showed that efficiency behaves similarly for vastly different gratings in visual noise. Efficiency for all spatial frequency gradients obeyed a single function of complexity independently of the central spatial frequency or the steepness of the frequency gradient. At small values of complexity, efficiency was constant, but decreased at higher values of complexity. Thus, the proportion of contrast information used decreased with increasing image complexity. The results can be understood by assuming that the highest local spatial frequencies of the image determine the spatial extent over which the human visual system integrates contrast information of grating images. With increasing highest spatial frequency the extent of the integration area becomes smaller.