Figure 3 plots individual observers' coherence thresholds from
Experiment 1 as a function of viewing condition.
Panels (a) and (b) show data from the high and low contrast levels, respectively. For the monocular and segregated conditions, data from situations of signal in left eye and signal in right eye were averaged.
Panel (a) shows that performance in the segregated condition was better as compared to the monocular and balanced conditions at high contrast, but not at low contrast. This benefit of segregation was most evident for the two larger displacements (speeds). Thresholds of slow dots (1.2°/s) are also generally higher in Panel (a) than in Panel (b), whereas thresholds of medium and fast dots (3.3°/s and 8.1°/s) are generally higher in Panel (b). These results agree with Seitz, Pilly, and Pack (
2008), who reported that increasing the luminance contrast caused an improvement in performance (direction judgments became more accurate) for large two-frame displacements (fast dots), but worsened performance for small displacements (slow dots).
To ascertain the reliability of this difference, we did a repeated measures ANOVA on threshold SNRs with Dot Displacement, Contrast, and Viewing Condition as factors. The first order effects were significant for Dot Displacement, F(2, 12) = 49.6, p < 0.0001, and Viewing Condition, F(4, 24) = 11.5, p < 0.0001, but not for Contrast, F(1, 6) = 1.06, p = 0.34. The interaction between Dot Displacement and Viewing Condition was not significant, F(8, 48) = 1.27, p = 0.28. However, the interaction between Contrast and each of the other two factors was significant, F(2, 12) = 12.6, p = 0.001 for Contrast and Dot Displacement; F(4, 24) = 10.2, p < 0.0001 for Contrast and Viewing Condition. The three-way interaction between Dot Displacement, Contrast, and Viewing Condition was also significant, F(8, 48) = 5.02, p = 0.0001.
We found that large individual differences are a particular characteristic of the low speed, high contrast condition. For example, in
Figure 3a some data have a decibel SNR of 0 for threshold: The observer was at the maximum SNR, with a value of 1. It was therefore reasonable to redo ANOVA excluding the lowest speed condition. After this adjustment, the first order effects were significant for Viewing Condition,
F(4, 24) = 9.40,
p = 0.0001, and Contrast,
F(1, 6) = 10.8,
p = 0.017, but not for Dot Displacement,
F(1, 6) = 4.83,
p = 0.07. The interaction between Dot Displacement and Viewing Condition was not significant,
F(4, 24) = 1.84,
p = 0.154. The interaction between Contrast and each of the other two factors was still significant,
F(1, 6) = 11.5,
p = 0.015 for Contrast and Dot Displacement;
F(4, 24) = 19.1,
p < 0.0001 for Contrast and Viewing Condition. The three-way interaction between Dot Displacement, Contrast, and Viewing Condition was no longer significant,
F(4, 24) = 0.28,
p = 0.888.
Figure 4 replots the data from
Figure 3 as a difference in threshold SNR between low and high contrasts, as a function of dot displacement.
In
Figure 4, panel (a), the effect of contrast is plotted as a function of spatial displacement for each observer separately, collapsing across all viewing conditions. A positive value for the ordinate indicates a benefit at high contrast, and a negative value indicates a benefit at low contrast. Four observers (S3, S4, S6, and S7) had worse performance at high contrast at the smallest displacement but had better performance at high contrast at large displacement, in agreement with Seitz et al. (
2008); two observers (S1 and S5) had better performance at high contrast for all displacements; and one observer (S2) had better performance at high contrast, but only at the largest displacement.
Figures 3 and
4 show that observers behaved more similarly to one another at high speed than at low speed, both in terms of absolute threshold (
Figure 3) and in terms of the effect of contrast (
Figure 4).
Figure 4, panel (b) plots the mean effect of contrast across observers as a function of spatial displacement, with a separate series for each viewing condition. Within every viewing condition the contrast effect went from negative to positive as the spatial displacement was increased from 1.2°/s to 8.1°/s, showing that the cross-over interaction between contrast and spatial displacement is robust to the viewing condition. Yet the slope of the segregated-condition data appears to be larger than the slopes for the other two conditions. This was confirmed by a posthoc
t test: size of Contrast Benefit at Dot Displacement of 1.2 vs. 8.1,
t(6) = −3.7081,
p = 0.01.