The table indicates that letter identity task responses were again slower in the incompatible condition than in the compatible and neutral conditions. As in the previous experiments, size task responses were faster for No-go than Go trials. There also was a tendency towards a compatibility effect in size task RTs, but only for Go trials. In No-go trials, letter task instructions did not influence size task performance—at least not in the usual direction. These observations were tested in separate ANOVAs for each task.
Size task
Reaction times were submitted to a 2 (trial type) × 3 (compatibility) within-subjects ANOVA. This analysis revealed a significant main effect of trial type, F(1,31) = 23.27, P < 0.01, MSE = 560.15, indicating that size task responses were faster for letter identity task No-go trials than Go trials. The main effect of compatibility was not significant, F(2,62) < 1, MSE = 181.10, but the interaction between trial type and compatibility was, F(2,62) = 6.66, P < 0.01, MSE = 149.16. Planned comparisons testing the compatibility effects (i.e., RTs in incompatible vs. compatible trials) separately for Go and No-go trials showed that the 6 ms difference in Go trials approached significance, F(1,31) = 3.96, P < 0.06, MSE = 291.97, whereas the effect in No-go trials did not, F(1,31) < 1, MSE = 438.31. Additional comparisons of the compatible and the neutral conditions indicated that RTs for neutral responses did not differ from compatible responses, neither for Go trials, F(1,31) = 1.4, P > 0.24, MSE = 271.79, nor for No-go trials, F(1,31) < 1, MSE = 299.77.
Error rates in the size task significantly differed between trial types, F(1,31) = 9.53, P < 0.01, MSE = 2.92, indicating that participants made more errors on No-go trials than on Go trials. Neither the main effect of compatibility nor the interaction between trial type and compatibility were significant (both Fs < 1, P > 0.6).
Additional analyses As in Exp. 1, we additionally conducted a 2 (trial type) × 3 (compatibility) MANOVA with RT and errors as dependent variables in order to back up the RT results. The MANOVA results were generally consistent with the RT ANOVA: the main effect of trial type was significant,
F(2,30) = 17.22,
P < 0.01, reflecting the fact that the canonical values were lower for No-go than Go trials (see Fig.
2a). The only other significant effect was the interaction between trial type and compatibility,
F(4,122) = 3.17,
P < 0.05, again suggesting that there was a difference between compatibility conditions in Go but not in No-go trials (see Fig.
2c). The main effect of compatibility was not significant,
F(4,122) < 1.
Finally, we directly compared Exps. 1 and 3, conducting a 2 (experiment) × 2 (trial type) × 2 (compatible, incompatible) mixed factors ANOVA. This ANOVA yielded significant main effects of trial type, F(1,54) = 50.75, P < 0.01, MSE = 1705.59, and compatibility, F(1,54) = 9.81, P < 0.01, MSE = 170.48. The RT difference between Go and No-go trials was larger in Exp. 1 than in Exp. 3, as indicated by a significant interaction between experiment and trial type, F(1,54) = 12.29, P < 0.01. Furthermore, the compatibility main effect was qualified by the two-way interaction between experiment and compatibility, F(1,54) = 5.43, P < 0.05, indicating that the compatibility effect was larger in Exp. 1 than in Exp. 3. The three-way interaction between experiment, trial type, and compatibility missed significance, F(1,54) = 2.82, P = 0.09, MSE = 168.48. However, post hoc comparisons suggest that the compatibility effects in Exps. 1 and 3 primarily differed in No-go trials: They revealed a highly significant compatibility effect across experiments in Go trials, F(1,54) = 9.12, P < 0.01, that did not interact with experiment, F(1,54) < 1. In comparison, the contrast testing the main effect of compatibility in No-go trials was not significant, F(1,54) = 2.23, P > 0.14, whereas the comparison testing the interaction between compatibility and experiment was significant, F(1,54) = 7.4, P < 0.01.
Letter identity task
The one-way ANOVA of RTs for letter identity Go trials with compatibility (compatible, neutral, incompatible) as a factor showed that compatibility conditions differed significantly from each other, F(2,62) = 10.01, P < 0.01, MSE = 185.30. Planned comparisons revealed that responses in compatible trials differed significantly from incompatible trials, F(1,31) = 9.56, P < 0.01, MSE = 386.32, whereas the difference between compatible and neutral trials was not significant, F(1,31) = 1.94, P > 0.17, MSE = 260.70.
The corresponding one-way ANOVA of key errors for Go trials mirrored the RT results: The effect of compatibility was significant, F(2,62) = 18.91, P < 0.01, MSE = 5.17, indicating that participants committed more errors after incompatible size task displays than neutral (non-overlapping) or compatible displays.
Unexpectedly, the percentage of commission errors for No-go trials also differed amongst compatibility conditions, F(2,62) = 6.74, P < 0.01, MSE = 2.65, indicating that participants made fewer commission errors following non-overlapping (neutral) size task stimuli than following overlapping (compatible and incompatible) displays.