Detecting the Unexpected in Photointerpretation

A methodology for measuring photointerpretation performance from a single glance at a novel scene is presented. Subjects attempted to detect the presence or absence of a target. object, specified in advance by the object's name, from a cued position in a 150-ms flash of a line drawing of a real-world scene. The cue, a dot, was presented immediately after the presentation of the scene, so that the subjects were uncertain as to the position of the cued object. Miss rates as a function of distance of the cued position from central fixation, target size, and degree of target camouflage were determined. The experiment also explored how these functions varied with objects that were in unexpected locations, such as a sofa floating in a street scene. Objects in these locations violated the usual constraints which characterize the organization of real-world scenes. Large, uncamouflaged targets in a normal relation to their context suffered only modestly from the effects of increasing distance from fixation. But targets that were small, camouflaged, or undergoing violations of the relational constraints suffered marked increases in miss rates when presented only 3 or 4 deg from central fixation. Since humans can assimilate visual information faster than the eye can move, the maximum rate at which a fixation can be made provides a limit–the saccadic barrier–of visual information processing. In addition to furnishing general guidelines for scene processing displays, the results of this experiment serve to restrict the breaking of the barrier-with the aid of high-speed, high-capacity display systems–to the extraction of the setting or gist of a scene. Individual targets, unless large, uncamouflaged, and in expected locations, require direct fixation.