The dependence of visual scanning performance on search direction and difficulty

Phillips and Edelman [Phillips, M.H., & Edelman, J.A. (2008). The dependence of visual scanning performance on saccade, fixation, and perceptual metrics. Vision Research, 48(7), 926-936] presented evidence that performance variability in a visual scanning task depends on oculomotor variables related to saccade amplitude rather than fixation duration, and that saccade-related metrics reflects perceptual span. Here, we extend these results by showing that even for extremely difficult searches trial-to-trial performance variability still depends on saccade-related metrics and not fixation duration. We also show that scanning speed is faster for horizontal than for vertical searches, and that these differences derive again from differences in saccade-based metrics and not from differences in fixation duration. We find perceptual span to be larger for horizontal than vertical searches, and approximately symmetric about the line of gaze. (C) 2008 Elsevier Ltd. All rights reserved.