<title>Contrast sensitivity function (CSF) and image discrimination</title>

Abstract

In a previous study the simulation of image appearance from different distances was shown to be effective1 . The simulated observation distance accurately predicted the distance at which the simulated image could be discriminated from the original image. Due to the 1/f nature of natural images spatial spectra, the individual CSF used was actually tested only at one retinal spatial frequency. To test the CSF relevant for the discrimination task over a wide range of frequencies, the same simulations and testing procedure were applied to 5 contrast versions of the images. The lower contrast images probe the CSF at lower spatial frequencies, while higher contrast images test the CSF value at higher spatial frequencies. Images where individually processed for each of 4 observers using their individual CSF to represent the appearance of the images from 3 distances where they span 1, 2, and 4 deg of visual angle, respectively. Each of the 4 pictures at the 5 contrast levels and the 3 simulated distances was presented 10 times side-by-side with the corresponding original image. Images were observed from 9 different observation distances. Subject task was to determine which of the two was the original, unprocessed image. For each simulated distance the data was use to determine the discrimination distance threshold. Results of testing using simulations calculated with CSFs measured from a 2 m where veridical for the images in the 30 - 100% contrast range. The 10% image was discriminated at distances larger than the simulated distances. The 300% image was discriminated at a shorter distance. A second set CSFs were obtained from a range of observation distances (0.5 m to 8 m), to overcome limitations of the display. These data showed higher sensitivity for low frequencies and lower sensitivity at the higher frequencies as predicted by the simulation testing results. Replication of the simulation experiments with the combined CSFs resulted in a much better prediction of the discrimination distances. Thus, the CSF relevant for the image discrimination task was verified over a wide range of spatial frequencies while further validating the visual model and its use for simulations.