The role of early visual mechanisms in visual search

Abstract

We attempted to determine the extent to which visual search time is governed by the parallel and automatic front end of the visual system. The target patches and background fields were narrow-band-noise textures that differed in spatial frequency and/or orientation. In the first part of the experiment, accuracy for detecting the target in the background was measured as a function of eccentricity for many target-background pairs. In measuring these accuracy windows, the observer always knew the exact positions of the target. Because this 2AFC procedure minimizes the demand for high-level processing, the accuracy window should represent (approximately) the relative loss of information in the parallel front end. In the second part of the experiment, average search time was measured for the same targets and backgrounds that were used in the first part, except the targets were randomly located within the background region. We found that search time decreased monotonically as a function of the area under the accuracy window. These results support the hypothesis that the early visual mechanisms determine (for fixed sizes of the target and background regions) the rank-order search time across different combinations of target and background texture.