Selectivity for multiple orientations in visual search

Observer strategies in visual search for targets containing multiple orientations were considered in the study. We used our efficient search framework (Tavassoli et al., 2007), a reverse-correlation based technique distinguishing between non-foveal and foveal processes, to examine how observers search for low-contrast targets created from sums (Experiment 1) and mosaics (Experiment 2) of Gabors. Eye movements were recorded while observers searched for one target randomly embedded in one tile of a grid of 1/f noise tiles. Fixated noise-only tiles (non-foveal false alarms) and a subset of these noise tiles selected by the observer as target candidate (foveal false alarm category) were analyzed off-line. We present several key findings. First, we show a strong presence of visual guidance in saccadic targeting in search for our composite targets, demonstrated by selectivity for spatial frequencies and (in some cases) orientations close to the characteristics of each target. Second, we show that, in most cases, the multiple orientations composing the targets were represented in both saccadic programming and target selection, but were not always equally weighted. Third, different mosaic configurations produced different tunings in orientation, but visibly idiosyncratic to each observer (Experiment 2). A local spectral analysis on sub-regions of noise tiles was performed to examine the use of phase. Fourth, a curious presence of close-to-vertical structures is observed in the results, although the search targets did not contain vertically-oriented structures (Experiment 2).