Detection of coarse patterns with minimum contribution from rods

The visibility of coarse patterns was determined under a variety of experimental conditions. The contrast thresholds for patterns below 0.6 c/deg are nearly inversely proportional to spatial frequency. This relationship is relatively independent of the experimental variables (the number of cycles in a grating and modes of presentation). The inverse proportionality also holds over a large range of retinal illumination provided that cone outputs predominate in detection while the sensitivity of rods is greatly reduced by a pre-exposure to a bleaching field. It is proposed here that the inverse proportionality cannot be accounted for by a single detection mechanism with an asymmetrical sensitivity profile.