A point spread function, chosen to link contrast sensitivity and stimulus dimensions, can be obtained from measured thresholds by assuming small-signal-linearity and peak detection for the visual system. To that end a special case of summation of subthreshold signals (perturbation) is used, taking specific measures against the effect of sensitivity drift. The basic assumptions are tested simultaneously and confirmed. Other provisional assumptions like radial symmetry and homogeneity were evaluated along a horizontal and a vertical meridian through the fovea. In the fovea no deviation from radial symmetry was found. The effect of inhomogeneity within the central fovea. seems to be too small to cause a significant change in the point spread function. The validity for predicting thresholds of stimuli exposing larger areas is tested. Annuli with varying radii show no significant aberration if probability summation is taken into account. Predicted disk thresholds, however, show a large discrepancy with experiment for radii larger than 2 min arc. A possible extension of the model with multiple-detection units having tuned sizes is evaluated.