Because of technical error, the behavioral data from six participants were lost; however, the ERP data for these subjects were included in the analyses. This left a total of 273 subjects that could be included in the behavioral analyses and 279 subjects included in the ERP and genetic analyses.
Performance measures in the overall sample, and as a function of DRD2 and DAT1 genotype, are presented in Table 1. Consistent with previous work, children were significantly faster on error trials than on correct go trials, t(1, 272) = 32.42, P < 0.001. Compared to the overall mean of correct trial reaction time, participants were slower to generate a correct response on trials that occurred after an error, t(1, 272) = 5.90, P < 0.001. However, there were no overall reaction time differences as a function of either DRD2 or DAT1 genotype, F1,269 = 0.02, P = 0.89, F1,269 = 2.90, P = 0.09, respectively; neither DRD2, F1,269 = 0.71, P = 0.40, nor DAT1, F1,269 = 0.46, P = 0.50, interacted with trial type to impact reaction time. However, there was a three-way interaction between trial type, DRD2, and DAT1, F1,269 = 4.56, P < 0.05.
Table 1. Means (SD) of behavioral measures (milliseconds), ERN, CRN and (ERN–CRN) amplitude (µV) at Cz for the entire sample and the genotypes
| ||All children (N = 279)||DRD2 A1 (N = 91)||DRD2 A2/A2 (N = 188)||DAT1 10/10 (N = 138)||DAT1 9 (N = 141)|
|Errors of commission||16.01 (7.62)||15.57 (6.51)||16.37 (8.10)||17.05 (8.13)||1520 (7.01)|
|Errors of omission||10.12 (11.05)||8.68 (9.58)||10.83 (11.66)||11.60 (12.06)||8.71 (9.83)|
|RT errors||509 (88)||511 (94)||507 (85.26)||514 (92)||503 (84)|
|RT correct||626 (72)||624 (77)||627 (70)||632 (71)||621 (72)|
|Post-error RT||655 (119)||647 (116)||660 (121)||655 (112)||656 (126)|
|Post-error slowing||28 (79)||22 (81)||32 (81)||23 (73)||35 (89)|
|ERN||0.09 (10.06)||−2.03 (7.37)||1.12 (10.99)||0.79 (11.34)||−0.59 (8.61)|
|CRN||9.18 (5.94)||9.32 (5.55)||9.11 (6.13)||8.56 (6.13)||9.78 (5.70)|
|ΔERN||−9.09 (10.26)||−11.35 (8.28)||−7.99 (10.95)||−7.78 (11.99)||−10.37 (8.07)|
As depicted in Fig. 3, post-hoc t-tests suggested that within the DRD2 A1 group, children who were homozygous for DAT1 10/10, were slower on correct trials than children with a DAT1 9 allele, t(1, 88) = −2.65, P < 0.01. Within the DRD2 A1 group, reaction time on error trials did not differ significantly between the two DAT1 genotypes, t(1, 88) = −1.01, P = 0.32. Within the DRD2 A2/A2 group, neither correct, t(1, 181) = 0.36, P = 0.72, nor error reaction times, t(1, 181) = −.59, P = 0.56, differed significantly between the DAT1 groups.
Additionally, post-error slowing did not differ by DRD2 genotype, F1,272 = 0.64, P = 0.42, or by DAT1 genotype, F1,272 = 0.004, P = 0.95, and there were no significant two- or three-way interactions involving genotypes and post-error slowing (all ps > 0.1).
Overall, participants committed an average of 16.10, SD = 7.62, errors of commission and an average of 10.12, SD = 11.05, errors of omission, out of a total of 240 trials. Children with at least one DAT1 9 allele made significantly fewer errors of commission and fewer errors of omission than children who were homozygous for the DAT1 10 allele, F1,272 = 4.08, P < 0.05 and F1,272 = 4.75, P < 0.05, respectively. All other effects and interactions did not reach significance (all ps > 0.1).
Means and standard deviations of ERN, CRN and ΔERN as a function of genotype are included in Table 2, response-locked waveforms at Cz for ERN and CRN for each genotype are included in Figs 1 and 2. The ERP response was more negative following errors than correct responses, F1,275 = 222.25, P < 0.001. There was no overall difference in brain activity as a function of the DRD2 or DAT1 genotypes, F1,275 = 2.98, P < 0.09, and F1,275 = 0.51, P = 0.48, respectively. However, the effect of trial type was qualified by a significant interaction with DRD2 genotype, F1,275 = 6.37, P < 0.01. Children with at least one DRD2 A1 allele had a larger (i.e. more negative) ΔERN than children who were homozygous for the DRD2 A2 allele, F1,277 = 6.67, P < .01. This effect was driven by the effect of DRD2 genotype on the ERN, F1,277 = 6.11, P < 0.01 such that children carrying at least one DRD2 A1 allele had a significantly larger (i.e. more negative) ERN than children carrying the DRD2 A2 allele (homozygous for A2). Children did not differ in CRN between the two DRD2 genotypes, F1,277 = 0.072, P = 0.79.
Table 2. Overall and Incremental results from hierarchical regression analysis of genotype predicting ERPs at Cz
|Step 1: DRD2||0.022||6.11**||0.00||0.072||0.024||6.67**|
|Step 2: DAT1||0.025||3.64*||0.01||1.49||0.038||5.52**|
|Step 1: DRD2||0.02||6.11**||0.00||0.072||0.024||6.67**|
|Step 2: DAT1||0.004||1.17||0.01||2.91||0.015||4.28*|
In addition, the difference between ERN and CRN also varied as a function of DAT1 genotype, F1,275 = 3.88, P < 0.05. Although neither the ERN nor the CRN differed between the two genotypes alone, F1,277 = 1.32, P = 0.25 and F1,277 = 2.95, P = 0.09, respectively, children with the DAT1 10 allele (homozygous for DAT1 10) had a significantly smaller (i.e. less negative) ΔERN than children with the DAT1 9 allele (with at least one DAT1 9 allele), F1,277 = 4.53, P < 0.05.
Neither the DAT1 by DRD2 two-way interaction, F1,275 = 3.62, P = 0.06, nor the three-way interaction between trial type, DAT1 genotype, and DRD2 genotype reached significance, F1,275 = 0.01, P = 0.92, suggesting two independent effects on error-related brain response related to the DAT1 and DRD2 genes. To investigate the possibility that the genotypes related differently to frontal/posterior electrode sites, a repeated-measures anova was conducted that suggested that the effect of trial type at Pz was also qualified by a significant interaction with the DRD2, F(1, 275) = 6.053, P < 0.01, and the DAT1 genotype, F(1, 275) = 6.46, P < 0.01. An additional repeated-measures anova suggested that the effect of trial type at Fz was qualified by a significant interaction with the DRD2 genotype, F(1,275) = 5.74, P < .05, but not the DAT1 genotype, F(1,275) = 2.51, P = 0.12.).
Hierarchical multiple regression analyses
To test for unique contribution of each genotype on the ERN, CRN and ΔERN we conducted separate hierarchical multiple regression analyses in which each of the ERPs were the dependent variables and potential predictor variable were the DRD2 and DAT1 genotypes. Results are shown in Table 2. As can be seen from the table, the additional variance accounted for in the ERPs by adding DAT1 as a predictor was significant in the case of ΔERN, R2 = 0.015, P < 0.05, although not in any of the other ERP measures. The variance in the difference score ΔERN accounted for by DRD2 alone was 2.4% and after DAT1 was added, the variance accounted for increased to a total of 3.8%, a significant increment. Thus, DAT1 significantly predicts the difference score ΔERN even after controlling for DRD2. This suggests an additive effect of the two genotypes. Overall and incremental results from the hierarchical regression analyses are included in Table 2.
A follow-up repeated measure anova suggested that when overall accuracy, reaction time (on error and correct trials), and age are added as covariates, the interaction between trial type and DRD2 remained significant, F1,265 = 6.07, P < 0.01, however, the effect of trial type was no longer qualified by the interaction with DAT1, F1,265 = 1.94, P = 0.17. Statistical analyses were conducted to test the potential mediation of accuracy on the relationship between DAT1 and ΔERN. The original beta for the relationship between DAT1 and ERN was 2.60, t(278) = 2.13, P < 0.05 and the beta for the relationship between DAT1 and accuracy was −4.75, t(272) = −2.73, P < 0.01. In the second regression analysis, the beta for accuracy predicting ERN was -.12, t(272) = −2.51, P < 0.01 and the beta for DAT1 predicting ERN was reduced to 2.03, t(272) = 1.63, P = 0.12. This reduction was significant; Sobel's test Z = 1.85, P < 0.05 (Fig. 3).
Figure 3. Correct reaction time (milliseconds) for the combined genotype groups and significance for post hoc t-tests.
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A follow-up repeated measure anova suggested that when gender was added as a covariate, it did not result in a significant interaction with ERN, F1,274 = 0.16, P = 0.69, and the interaction of DRD2 and ERN, F1,274 = 6.35, P < 0.01, and DAT1 and ERN, F1,274 = 3.99, P < 0.05, remained significant. However, a previous study has suggested that males and females differ in DRD2 binding in the striatum (Pohjalainen et al. 1998b) so further post hoc analyses were completed by dividing the sample into males and females. In males, the effect of trial type was not significantly qualified by an interaction with DRD2 genotype, F1,155 = 0.88, P = 0.35. However, in females there was a significant interaction of trial type with DRD2 genotype, F1,123 = 7.20, P < 0.01, such that females with the DRD2 A1 allele had a significantly more negative ERN, M = −3.67, SD = 7.53, than females without the DRD2 A1 allele, M = 1.01, SD = 11.04.