P300 at the three midline electrodes
Results of the 2 × 3 × 3 × 2 mixed ANOVA [Trial (“Go”, “No-go”), Money (45¢, 1¢, 0¢), Site (Fz, Cz, Pz) and Group (CUD, Control)] revealed the expected Site main effect [F(1.3,43)=9.5, p<0.01] and Site by Trial interaction [F(1.7,57.6)=20.0, p<0.0001], whereby P300 amplitudes were higher for Pz (and Cz) than Fz (Pz=Cz>Fz), especially during the ‘Go’ trials. This analysis also revealed a Group main effect [F(1,34)=4.6, p<0.05; CUD>control subjects], driven by the Fz [F(1,34)=4.3, p<0.05] and Cz [F(1,34)=5.7, p<0.05] electrodes but not by the Pz electrode [F(1,34)=0.8, p>0.4]. All other multivariate effects did not reach significance [F(2,33)< 2.0, p>0.2].
P300 at Pz
See for the means and standard deviations of all P300 amplitudes and latencies as a function of Trial, Money, and Group. The main 2 × 3 × 2 ANOVA for P300 amplitude revealed significant Trial [“Go”>”No-Go”; F(1,34)=14.4, p<0.01] and Money [45¢>0¢, F(2,33)=4.4, p<0.05] main effects. Although the Money by Group interaction was not significant [F(2,33)=0.4, p>0.7], a planned contrast revealed that the monetary effect was only significant in the control but not the CUD subjects, as best demonstrated during the “Go” trials [45¢>0¢; paired t(17)=−2.2, p<0.05; “No-Go”; paired t(17)=−1.8, p<0.09] (). There were no significant correlations between these Pz P300 “Go” amplitudes with depression, in the complete group or separately in both study subgroups (all r<|0.38|, p>0.1). Similarly, inspected with independent t-tests separately for each monetary condition and subject group (and for the complete sample), these amplitude measures did not differ by history of cigarette smoking (all t<|1.43|, p>0.2). For the current smokers, frequency of smoking and time since last cigarette were not associated with these amplitude measures. Thus, this differential P300 amplitude to money in the control group but not CUD subjects cannot be attributed to the differential effects of depression or cigarette smoking.
Table 2 The P300 Amplitude and Latency at Pz and Behavioral (reaction time, accuracy, and self-reported ratings) Dependent Variables for All Study Subjects as a Function of Group, Monetary Reward, and Trial Type (‘Go’ vs. ‘No-Go’). (more ...)
For the P300 latencies at Pz, significant Trial [“Go”<”No-Go”, F(1,34)=23.0, p<0.0001] and Group [CUD<control; F(1,34)=5.5, p<0.05] main effects demonstrated faster latencies for the “Go” trials and for the CUD subjects. Planned monetary contrasts did not reveal differences between the monetary conditions for any of the study groups or combined across all subjects [paired t<|2.0|, p>0.07]. Further, both main effects remained significant after entering depression as a covariate [F(1,33) >6.7, p<0.05]. After entering history of cigarette smoking, the Trial by Group interaction reached significance [F(1,33)=6.7, p<0.05], indicating faster latency in the CUD subjects for the “Go” trials only. For the current smokers, frequency of smoking and time since last cigarette were not associated with these latency measures.
See for the means and standard deviations of RT, accuracy, and the three rating scales as a function of Trial (where relevant), Money and Group. The main 2 × 3 × 2 mixed ANOVA on percent accuracy showed a Trial main effect [“No-Go”>“Go”; F(1,34)=38.8, p<0.0001], a Money main effect (0¢>1¢; F(2,33)=4.6, p<0.05), and a Money by Trial interaction [F(1.7,57)=14.6, p<0.0001]. Non-parametric comparisons showed that the monetary differences were driven by the “No-Go” trials, where the 1¢ condition was least accurate [45¢=0¢>1¢; Z>−5.5, p<0.0001]; the latter is an unexpected result that requires follow-up with clear hypotheses (e.g., could it reflect increased inhibitory control requirements under conditions of relative uncertainty/frustration?). Importantly, there were no differences between the study groups in any of these comparisons. Accuracy did not correlate with depression and was also not associated with history of cigarette smoking (including frequency of smoking and time since last cigarette).
There was a significant Money linear contrast for RT (analyzed for the “Go” trials only) [F(1,34)=5.1, p<0.05], such that there was a trend for faster RT for the highest monetary condition. Planned comparisons revealed that the control subjects were somewhat faster than the CUD subjects, a difference that reached significance for the 1¢ condition [t(34)=2.1, p<0.05], with a trend for the 45¢ condition [t(34)=1.8, p<0.09]. Most importantly, the 45¢ vs. 0¢ differential was only significant for the control subjects [45¢<0¢; paired t(17)=2.7, p<0.05]. Entering depression as a covariate did not impact the monetary main effect and moved the Money by Group interaction closer to significance [Quadratic within-subjects contrast, F(1,33)=3.2, p<0.09]. History of cigarette smoking (including frequency of smoking and time since last cigarette) was not associated with RT.
Both interest and excitement rating scales showed a significant Money main effect [45¢>1¢≥0¢; F(1.4,46.9)>17.2, p<0.0001] and a significant Group main effect [CUD>control; F(1,34) >4.5, p<0.05] (). The Money by Group interaction was not significant. There were no significant results for ratings of frustration. When depression was entered as a covariate, results did not change for the interest ratings; the diagnosis main effect was no longer significant for the excitement ratings. Cigarette smoking (including frequency of smoking and time since last cigarette) was not associated with these rating scales.
Because the amplitude and latency P300 group differences were noted mostly during the “Go” trials as described above (P300 at Pz section), the following correlations with behavior were focused on the “Go” trials. There was a positive correlation between the P300 amplitude differential for the 45¢ minus 0¢ condition with the respective accuracy differential in the control subjects only; the higher the P300 amplitude differential, the better the accuracy for the high monetary condition as compared to the neutral cue (r=0.64, p<0.01; , left; this correlation remained significant after excluding the outlier on the upper right-hand corner of this figure: r=0.61, p<0.01). Similarly, a negative correlation with RT was only observed in the control subjects: the higher the P300 amplitude for the 1¢ minus 0¢ condition, the faster the respective change in RT (r=−0.6, p<0.01; , right). Further, only for the control subjects, there was a negative correlation between latency and accuracy (this reached significance for the 0¢ condition: r=−0.66, p<0.01): the faster the P300 latency, the higher the accuracy. Controlling for depression or cigarette smoking (with partial correlations), these correlations remained significant (r>|0.49|, p<0.05). The parallel correlations in the CUD were not significant (r<|0.29|, p>0.3). None of the correlations between the P300 measures (during “Go” trials) and the rating scales survived the nominal significance level. The parallel correlations for the “No-go” trials (except with RT) were not significant (r<|0.46|, p>0.06).
Figure 3 Correlations between the P300 and behavioral dependent variables. Left: positive correlation (R2=.32, p<1, regression line in bold; R2=.23, p<01 when one outlier is removed) between the P300 amplitude differential for the 45¢ minus (more ...)
Finally, we conducted analyses between these six variables (that showed P300-behavioral intercorrelations: P300 amplitude and accuracy differentials for 45¢ minus 0¢, P300 amplitude and RT differentials for 1¢ minus 0¢, and P300 latency and accuracy at 0¢ condition, all during the “Go” trials) and the selected 10 drug use variables in the CUD subjects (). One correlation reached nominal significance level: the higher the 45¢ minus 0¢ accuracy, the less frequent the cocaine use during the 12 months preceding this study (r=−0.80, p<0.0001; this correlation also reached significance for the 45¢ minus 1¢ accuracy differential, r=−0.72, p=0.001) (). These correlations remained significant after controlling for depression and history of cigarette smoking (including current frequency and time since last use; r>−0.66, p<0.01). Parallel correlations for the “No-go” trials did not reach statistical significance (r<|0.4|, p>0.1).
Figure 4 Correlations between accuracy differentials on the monetary incentive task and cocaine use. Left: negative correlation (R2=.53, p<01) between frequency of cocaine use in the last year and the differential accuracy for the 45¢ minus 0¢ (more ...)