Useful resolution for picture perception as a function of eccentricity.

Observers inspected for different lengths of time pictures which contained high-resolution information within an eye-contingent viewing window and low-resolution information outside the area of that window. A recognition test followed in which the pictures inspected were presented together with other, distractor, pictures. The time required to reach 75% picture recognition (the criterion study time) was determined as a function of window size and degree of completeness of video sampling of information outside the window. For each level of information sampling density, criterion study time decreased as window size increased up to a critical size, and then remained approximately constant beyond this size. From these critical-window sizes, a function which describes the resolution actually used by the visual system at each eccentricity (the useful-resolution function) was obtained and this decreased monotonically with eccentricity. The useful resolution at each eccentricity was coarser than the resolution available at that eccentricity as determined by visual acuity, suggesting that useful resolution is not limited by visual acuity. The relationship between useful resolution and saccade length was also analyzed.