Groups were generally well matched demographically. However, we found a significant effect of age between the 5 groups (F4,74
=3.5, p=0.01; controls, PD on, PD off, PD+ICB on and PD+ICB off). Post hoc analysis revealed that the PD on group was older than the PD+ICB on (p=0.03) but not to the PD+ICB off group (p=0.12). There was no difference between the control and the PD on group (p=0.13), no difference between the PD off and the PD+ICB on group (p=0.2) and all other patients groups (p>0.57). There was also a significant effect of age of onset (F3,49
=3.4, p=0.03). Post hoc analysis showed that the PD+ICB on group had an earlier disease onset (p=0.03) than the PD on group, consistent with previous studies (Voon V et al., 2007
; Weintraub D and MN Potenza, 2006
). There was no difference in age of disease onset between the PD+ICB on group and the PD off group (p=0.08) nor between the other groups (p>0.92). There was also no difference in the LEU dose (F3,48
=0.05, p=0.98) or the daily L dopa dose (F3,48
Analysis of punishment behaviour
We carried out an ANOVA with dependent variable the amount of punishment (), with group entered as five levels (PD on, PD off, ICB on, ICB off, controls). We found significant main effects of group (F4,73=11.17, p<0.001) and valence (F1,73=265.83, p<0.01), where valence was fair vs. unfair outcome. There was also a significant interaction between group and valence (F4,73=4.54, p=0.002). Given the interaction with valence, we ran separate ANOVAs on the fair and unfair rounds. In the fair rounds there was no effect of group (F4,73=1.95, p=0.111). In the unfair rounds there was a main effect of group (F4,73=9.24, p<0.001).
Average punishment score of participants in fair and unfair rounds. Error bars are +/− 1 s.e.m.
Next we compared the PD and ICB groups to directly examine a diagnosis of ICB as well as the effects of medication. Thus, group was split by ICB diagnosis (+ICB/−ICB) and medication (on/off dopamine replacement). The main effect of group just missed significance (F1,48=3.71, p=0.060). There was, however, a significant main effect of medication (F1, 48=5.76, p=0.020) and a significant interaction between group and medication (F1, 48=7.68, p=0.008). There was also a valence by group interaction (F1,336=4.97, p=0.026) and a significant valence by group by medication interaction (F1, 336=9.71, p=0.002). As there was a difference in age between groups, we also carried out an analysis where we covaried out the effect of age. Adding age as a covariate, however, did not affect significance of any parameters. Given the interactions with valence, we next split this ANOVA by valence and ran separate ANOVAs. In the fair rounds there was no effect of group (F1, 48=0.04, p=0.852) or medication (F1, 48=1.2, p=0.279). In the unfair rounds, however, there was a main effect of group (F1, 48=4.05, p=0.050), an interaction between group and medication such that PD on and off punished strongly, whereas PD+ICB on also punished strongly, but PD+ICB off punished less (F1, 48=8.24, p=0.006). The main effect of medication just missed significance (F1, 48=3.96, p=0.052).
Following this we carried out pair-wise post-hoc comparisons between all five groups in just the unfair rounds (Bonferroni corrected). This analysis showed that PD on, PD off and PD+ICB on punished significantly more than controls (p<0.01) whereas the PD+ICB off group punished similarly to controls (p=1.000). Furthermore PD on and PD+ICB on punished significantly more than the PD+ICB off group (p<0.05), but PD off only reached trend level vs. the PD+ICB off group (p=0.067).
As dopamine loss in PD progresses over the course of the disease we were interested in whether there would be any correlations between disease duration and the amount of punishment. Therefore, we carried out correlations between disease duration and the amount of punishment in the unfair condition, but found no significant effects (p>0.345). There was also no correlation between UPDRS scores and punishment (p>0.405).