Simultaneous measurement of spatial-frequency summation and uncertainty effects.

The predictions for summation and uncertainty effects from several multiple-spatial-frequency-channels models were calculated. The models differed in their assumptions about the shape of the channels' underlying probability-density functions and in the decision rule used to combine the channels' outputs. Varying these assumptions resulted in quite different predictions about the magnitudes of these effects. Simultaneous summation and uncertainty experiments measured the detectability of gratings containing one (simple) or two (compound) spatial frequencies. Performance was assessed in two types of blocks of trials: either each stimulus was in a separate block or three stimuli (two simple gratings and their compound) were randomly intermixed in one block. Quantitative comparisons of the models with the data showed that the increasing-variance Gaussian models (in which the decision variable is the sum of the monitored channels' outputs) provided the best overall fit.