Participants
Twenty-two early stage, non-demented PD patients who were not using dopamine replacement therapy and 40 healthy controls participated in this study. Prior to the analyses a number of participants was excluded, due to (1) presence of a comorbid psychiatric disorder (one patient), (2) scanner failure (one patient, one control), more than 3 mm/degrees of movement while performing the task (two controls), (3) extremely low scores on task performance (more than two standard deviations from the median) when compared within the own group (two patients; two controls). This resulted in a total of 53 subjects; 18 PD patients (mean age 59.7 ± 10 years) and 35 healthy controls (mean age 56.7 ± 10 years). All patients were recruited from the movement disorders outpatient clinic of the VU University medical centre (VUmc) in Amsterdam and were diagnosed using the UK Parkinson’s Disease Society Brain Bank criteria for idiopathic Parkinson’s disease [34]. The healthy controls were matched at the group level with the PD patients on age, gender, education and handedness. Education level was measured in 7 levels ranging from 1 (no finished education) to 7 (university training). Exclusion criteria to take part in this study for both groups were current psychiatric or neurological disorders other than PD, a Beck Depression Inventory (BDI) score >15 and a Mini Mental State Examination (MMSE) score <24. Written informed consent was obtained according to the declaration of Helsinki from all participants after reading the protocol, which was reviewed and approved by the medical ethical committee of VUmc (reference number: 2008/145).
Demographic and clinical characteristics and behavioural performance
As described in our previous study, the groups did not significantly differ in age (p = .24), gender (p = .78), or handedness (p = .56) and there was no difference in MMSE scores (p = .23). Patients and controls were similarly highly educated: the median education level for patients was 6 (range 2–7) and for the controls 6 (range 3–7), p = .81. The Beck Anxiety Inventory [BAI, median (range) PD group: 4 (0–16), controls: 0 (0–11)] and the Beck Depression Inventory [BDI median (range) PD group: 4.5 (0–11), controls 0 (0–10)] scores were significantly higher (p < .001 and p = .01, respectively), but clinically irrelevant, in the PD group compared with the control group. For the PD patients the mean UPDRS was 22 and the median Hoehn and Yahr stage 2. PD patients made more errors during repeat trials (HC 0.72 %; PD 2.2 %, p = .004) but not set-shift trials (HC 0.36 %; PD 0.5 %, p = .36), and had longer reaction times on both the shift (HC 902 ± 212 ms; PD 1083 ± 336 ms, p = .02) and repeat trials (HC 822 ± 200; PD 1019 ± 283, p = .01). For further detail see [15].
Set-shifting task
An arrow was presented on a screen outside the MRI scanner that was visible to the participants via a mirror attached to the head coil. The arrow appeared either on the right or the left side of a fixation cross, and was pointing up or down. Depending on the feature of the stimulus that was relevant at the moment of presentation, participants had to either indicate its location (right or left of the fixation cross) or direction (pointing up or down) using an MRI compatible response box (Cambridge Research Systems Ltd., UK) with four buttons (left, right, top and bottom) which were arranged in a diamond shape. The stimulus was presented for a maximum of 4000 ms, but was terminated upon a button press. When no response was given within this time window, a red screen appeared, indicating a time-out. Each button press was followed by a feedback screen with a fixed duration of 2000 ms, indicating whether the response had been correct (green screen), or incorrect (red screen). Based on the behavioural response made by the participant each trial was classified into one of five categories (see [15]). For this study, we divided the trials into three categories according to the given response, namely (1) “correct repeat” if no set-shift was indicated and the stimulus was correctly categorized according to the current rule, (2) “successful shift” if the preceding feedback signaled a set-shift, and the subsequent stimulus was correctly categorized according to the new rule, and (3) “error trials” were all trials that were not “correct repeats” or “successful shits”. After 4–7 correct repeat trials, a red screen followed a correct response, indicating a set-shift to the other classification rule. The session ended when 20 percent of all trials were correct set-shift trials, and took approximately 20 min to complete.
MRI data acquisition
Functional MRI data were acquired using a 3.0 T General Electric Signa MR750 MRI scanner at the VUMC in Amsterdam. The scanning included a sagittal three-dimensional T1-weighted scan for anatomical localization (256 × 256 matrix; voxel size = 1 × 0.977 × 0.977 mm; 172 sections). Functional images were obtained using a gradient echo-planar imaging (EPI) sequence (TR = 2100 ms; TE = 30 ms; field of view = 24 cm; 64 × 64 matrix; flip angle = 80°) with 40 ascending slices per volume (3.75 × 3.75 mm in-plane resolution; slice thickness = 2.8 mm; inter-slice gap = 0.2 mm).
Data analyses
Preprocessing and contrasts
As preprocessing, the EPI scans were slice-time corrected, realigned and unwarped, normalized, and smoothed with an 8 mm Gaussian kernel using SPM8 software (http://www.fil.ion.ucl.ac.uk/spm/software/spm8/). We included all trials during the presentation of feedback (with a fixed duration of 2000 ms) in a first level general linear model (GLM) and added the movement parameters as nuisance variables. Our contrast of interest was “successful shift > successful repeat”. We used this contrast to investigate which brain areas became more active when a feedback screen indicated a set-shift instead of a repeat, thereby thus capturing the neural process of the actual set-shift. Because no motor response was required while processing the feedback, this contrast was not contaminated with motor activity.
PPI analysis
We assessed the task-related functional connectivity of the bilateral DLPFC, bilateral PPC, and bilateral SFG using a generalized form of context-dependent psychophysiological interaction (gPPI) [35] (https://www.nitrc.org/projects/gppi/). A PPI analysis statistically tests in a whole-brain voxel-wise manner whether areas outside the seed region are functionally connected to the seed region during the task [25, 36]. We chose gPPI, instead of the traditional PPI, as it allowed us to model all psychological task conditions into one first-level design, thus improving the model fit [35]. We distinguished positive coupling (i.e. regions in which activity correlated positively with that of the seed region during the task) and negative coupling (i.e. areas in which activity correlated negatively with the seed region during the task). We employed the main effect of positive coupling as an inclusive mask to search for between-group differences in positive coupling, and the main effect of negative coupling to search for between group differences in negative coupling.
The coordinates of the designated seed areas were determined using the peak-voxels of the whole-group activations at second level (DLPFC; right: x = 39, y = 35, z = 31; left: x = −42, y = 26, z = 31; SFG; right: x = 27, y = −7, z = 58; left: x = −36, y = −7, z = 64. PPC; right: x = 45, y = −52, z = 49; left: x = −33, y = −52, z = 40). These coordinates were subsequently used as an initial starting point to find the individual peak-voxel at the first level-contrast “successful shift > successful repeat” within a radius of 5 mm around these previously mentioned coordinates to account for individual variability. The coordinates where manually verified to assure location in the designated area. Next, we constructed six spheres with a 6 (SFG and DLPFC) or 10 mm (PPC) radius around the individually determined peak-voxels, and again used the “successful shift > successful repeat” contrast in the MarsBar toolbox [37] (see Fig. 4).
The first-level models we used thus consisted of the three task conditions (successful shift trials, successful repeat trials, error trials), the time course of the seed of interest, three PPI terms (i.e. the three task conditions convoluted with the time course of the seed region), and six movement parameters. For each seed region, we constructed a separate first level GLM. We only used the PPI terms and our contrast of interest was “successful shift > successful repeat”.
For each of the six seed-regions, a second-level analysis was performed to assess between group differences on the “successful shift > successful repeat” PPI contrast, while employing an independent samples t test to compare the controls and PD patients. Because in our previous study [15] the PD patients had an increased RT on the successful shift trials, we included these in the second level analyses as a covariate. Because this is the first study to explore task-related functional connectivity in a group of unmedicated PD patients, we report all results at a voxel-level threshold of p = .001, with an extent threshold of 10 voxels.