Next, we examined whether the magnitude of flanker tilts in the uni- and bidirectional flankers condition affected thresholds (see
Figure 3). Uni- and bidirectional flanker configurations had absolute average deviations from the vertical of 10, 15, and 20 degrees. Absolute tilts averaged to 10 and 20 degrees when symmetrical, and to 15 degrees when asymmetrical. For example, asymmetrical bidirectional flankers with one flanker tilted by 10 degrees to the left and the other by 20 degrees to the right have an average absolute tilt of 15 degrees. To test for any differences in threshold depending on tilt magnitude of the flankers, we ran a repeated measures ANOVA with flanker configuration (uni- and bidirectional tilts), absolute deviation from vertical (10 degrees, 15 degrees, and 20 degrees), and contrast polarity (uniform and alternating) as factors, and thresholds as dependent variable. A main effect of deviation from vertical (F(2,14) = 6.94,
p < 0.01, η² = 0.02) indicated better performance with 10 degree tilts compared to both other tilts (10 degrees versus 15 degrees:
p < 0.05; 10 degrees versus 20 degrees:
p < 0.05). Thresholds were lower for alternating compared to uniform triplets (F(1,7) = 17.70,
p < 0.01, η² = 0.04), and for uni- relative to bidirectional tilts (F(1,7) = 46.84,
p < 0.001, η² = 0.51). As shown in our first analysis, the effect of contrast polarity depended on flanker configuration (F(1,7) = 16.81,
p < 0.01, η² = 0.05). Furthermore, we found a three-way interaction between flanker configuration, average flanker tilt, and contrast polarity (F(2,14) = 5.07,
p < 0.05, η² = 0.02).