Recent behavioral data have shown that central gaze direction triggers reflexive shifts of attention to the cued location. Considering findings on the ecological adaptation of human vision, the authors expected that such a cueing effect would occur asymmetrically in horizontal and vertical planes. Participants detected a target letter appearing on a PC monitor after the brief presentation of a nonpredictive head-cue (Experiment 1), profile head-cue (Experiment 2), or arrow-cue (Experiment 3) directed up, down, left, or right of fixation. In all experiments, a cueing effect was observed only when the cue was directed horizontally, but no significant effect was observed in the vertical plane. The results also showed that RTs to a target appearing horizontally were shorter than RTs to a target appearing vertically, irrespective of the cue direction. These results might reflect humans' adaptation for extension of the visual field in the horizontal plane as a terrestrial primate species.

Key words: visual orienting, asymmetry, horizontal, vertical