COMPETITION AND SHARING OF PROCESSING RESOURCES IN VISUAL-DISCRIMINATION

Discrimination thresholds for spatial frequency and contrast tested in dividually were compared with dual discrimination of contrast and spat ial frequency, and dual discrimination of 2 contrast or spatial freque ncy components. The components were presented overlapping, forming a c ompound grating or as side-by-side simple gratings. When observers had to judge contrast and spatial frequency simultaneously, discriminatio n thresholds increased by an amount predicted by a model of stimulus u ncertainty for orthogonal dimensions (1.7); when they had to judge 2 f requency or contrast components, discrimination thresholds increased b y a factor of 3-6 compared with the single-judgment task. The relative spatial location of the components did not interact with task complex ity. The results are consistent with a model assuming a set of paralle l special-purpose attentional mechanisms.