Figures 2 and
3 present the landing positions of the initial saccades on test trials.
Figure 2 presents the mean landing position averaged across participants for the five different target stimuli and for distractor versus no-distractor trials. This shows that saccades in the no-distractor, control condition were relatively accurate, undershooting the center of the target by a little less than 10% of its eccentricity (for a review, see Becker,
1989), irrespective of its size and shape (see also Kowler & Blaser,
1995). Saccade endpoints in distractor trials were less accurate, thus confirming the presence of a global effect, and quite importantly, it varied with the 2-D spatial extent but not with the shape of the target. This result thus suggests, in contrast with the boundary hypothesis, that where stimulus boundaries were located did not matter. The deviation of the eyes' landing position away from the center of the target became greater as the circle target became smaller in size, but it was of about the same extent for the medium circle and both horizontal and vertical ellipsoid targets.
Figure 3, which presents the distributions of the landing positions, illustrates the consistency of the effect of stimulus size across the different bins as well as a lack of clear modulations of landing positions with the location of stimulus boundaries.
The analysis of variance confirmed this pattern. It first indicated a significant effect of target eccentricity, F(1, 7) = 444.00, p < 0.01, as well as a main effect of the distractor condition, thus confirming that when a visual distractor was presented along with the target, the eyes were deviated away from the target, F(1, 7) = 100.70, p < 0.01. There was a trend for an interaction between distractor condition and target eccentricity, F(1, 7) = 4.23 p = 0.079. However, post hoc tests indicated that the undershoot classically observed when the target is presented in isolation tended to be more pronounced for 6° compared to 4° target eccentricities (0.44° ± 0.10° vs. 0.33° ± 0.05°, respectively; p = 0.062). They also showed that in distractor conditions the deviation away from the target was more pronounced for 6° compared to 4° targets (1.50° ± 0.09° vs. 1.36° ± 0.08°, respectively; p < 0.01).
The visual properties of the target stimulus also influenced the saccades' landing position as indicated by a significant effect of target type, F(4, 28) = 14.42, p < 0.01. However, the significant interaction between target type and distractor condition, F(4, 28) = 18.92, p < 0.01, suggested that the target properties influenced where the eyes landed only when a distractor was present. Post hoc tests confirmed that there were no significant differences between the different target stimuli when presented in isolation (ps ≥ 0.14).
Planned comparisons were conducted in the distractor condition in order to test the predictions of the stimulus-extent and boundary hypotheses. First, the medium-circle target was contrasted with the two medium, ellipsoid target stimuli. Then, all three medium-size targets were contrasted with both the small- and the large-circle target. These planned comparisons revealed that the deviation was not significantly different between the medium circle and the two ellipses, whatever their orientation, vertical: F(1, 7) = 3.21, p = 0.12; horizontal: F(1, 7) < 1. However, the deviation was greater for the small circle, F(1, 7) = 14.52, p < 0.01, and smaller for the large circle compared to all three medium-size targets, F(1, 7) = 35.95, p < 0.01. Post hoc comparisons further indicated that the deviation was more pronounced for the small circle than for the vertical ellipse (p < 0.01) and less pronounced for the large circle than for the horizontal ellipse (p < .01). The two-way interaction between target type and eccentricity as well as the three-way interaction between all three manipulated variables were not significant, F(4, 28) = 1.28, p = 0.30 and F(4, 28) < 1, respectively, suggesting that these effects of target properties were comparable for both eccentricities.