Besides this first-fixation effect, no other influences of the virtual
z-positions of search items on the search dynamics were found. For the benefit of conciseness, the corresponding analyses that were conducted will only be briefly described without detailed results. First, the effect of the
z-position of the target item of response time was examined. An effect would suggest some form of systematic scanning of the display in the
z-dimension. Second, RT was analyzed for different levels of the variance of search item positions along the
z-dimension. If shifting spatial attention in the
z-dimension requires additional time, then greater
z-variance among all search items should lead to longer RT. Third, fixation duration in the easy task was analyzed as a function of the variance in
z-position of the objects near fixation (in adjacent cells on the 8 × 8 grid) in the
x−
y-plane. If the visual span were limited in the
z-dimension, a smaller
z-variance would allow observers the processing of more objects during fixation, which should be reflected in longer fixation duration. Fixations followed by saccades shorter than two cells on the 8 × 8 grid were excluded from analysis to minimize potential interference of this measure with longer saccade planning processes due to greater
z-variance. Fourth, the likelihood of fixations landing on “singletons,” e.g., a red item surrounded by only green items, was compared between the situation when the surrounding items were on the same or on different
z-planes than the singleton. Bottom-up control of attention tends to direct a disproportionate amount of attention to singletons (“pop-out effect,” e.g., Wolfe,
1998), and therefore a greater effect for items on the same z-plane would indicate a perceptual z-distance limitation for pop-outs. Fifth, we compared the duration of fixations before saccades that switched between
z-planes and those that did not. Longer fixation duration before
z-plane switches would suggest a longer process of target selection and saccade programming. None of these five analyses yielded significant results or tendencies, all
ps > 0.2.