We found a significant Stroop interference effect in all participants. Furthermore we demonstrated that all PD patients made more errors than normal controls when off medication. We found that in their “on state” patients had a shorter RT on switching behaviour between congruent and incongruent trials, in keeping with previous studies [8
]. Our findings are also consistent with previous studies showing improvement of Stroop performance in PD patients with and without deep brain stimulation [9
There was no difference in RT between PD patients and normal controls in keeping with previous studies [10
]. Further patients on medication showed a trend to be slower in RT in congruent trials followed by incongruent trials compared to incongruent trials followed by incongruent trials (). This might be explained by an increased awareness caused by the previous conflicting trial. We found no difference in error rates between the patient groups which implies that the inability to suppress automatic responses and the inability to suppress, for example, the urge to gamble depend on different processes and neural systems. Our findings are also in line with two other studies which have shown no impairment on the Frontal Assessment Battery scores in PD patients with pathological gambling compared to those without ICBs [11
]. Thus, ICBs seem to be unimpaired in tasks that are mediated by frontal cortex, as for example occurs also with response suppression tasks [13
]. Our results are also consistent with another study done in PD patients with pathological gambling, which showed impairment in a risk assessment task but not in other cognitive domains [14
]. Our study extends the current literature and demonstrates there is no difference in cognitive flexibility between PD controls and patients with impulse control disorders irrespective of the type.
Furthermore performance of the Stroop test might not trigger mesolimbic dopamine release and could fail to activate limbic and reward centres of the brain which are known to be abnormal in PD+ICB patients. In line with this notion is the finding that there was no correlation between amygdala activation and Stroop performance [15
]. Since cognitive performance may vary during the day [16
], we tested patients in their “off” condition on average at about 8.30 am and patients of the “on” group at a similar time point, on average at 10.30 am.
There are however some limitations in our study. Patients who were tested first’ on’ then’ off’ were tested on separate days, whereas patients tested first off and then on were tested on the same day. Thus, the test-retest interval differed between the two groups and conceivably might have influenced the results. However, our significant results are within-subject effects comparing off vs. on, and they did not depend on the order of testing.
Future work using an emotionally charged Stroop test, which is more likely to activate the limbic system, could potentially demonstrate differences between the two PD groups.