Supplementary materials
Pharmacological design and procedures
During an initial intake session, participants were screened by a medical doctor and a research nurse. This screening included a Mini-International Neuropsychiatric Interview(Sheehan et al. 1998) to exclude (a history of) psychiatric diseases. Furthermore, an anamnesis and physical examination (including measurements for weight, pulse rate, blood pressure and an electrocardiogram) were completed to exclude (a history of) medical illness, (a history of) substance abuse or a family history of psychiatric diseases. Subjects practised the attention switching paradigm in the MRI scanner, while we obtained a structural scan and a diffusion-weighted scan. Finally, subjects completed a neuropsychological battery, including
- the Daneman and Carpenter listening span to test working memory capacity(Daneman and Carpenter 1980),
- a Dutch version of the North American Adult Reading Test to assess IQ(Schmand et al. 1991),
- Cloninger’s Temperament and Character Inventory (TCI)(Cloninger et al. 1993), the NEO Five-Factor Inventory (NEO-FFI)(McCrae and Costa 2004), the Eysenck Personality Questionnaire(Sanderman 1995), and the Barratt Impulsiveness Scale(Patton et al. 1995) to assess personality characteristics, and
- the Spielberger Trait Anxiety Inventory(8,9) and the Beck Depression Inventory(Beck et al. 1961) for screening purposes.
In the Dutch version of the Daneman and Carpenter listening span task, subjects listen to sets of two to seven sentences while answering written questions about the content of each sentence. After hearing the whole series of sentences, subjects write down the last word of each sentence, thus keeping the words in working memory over a certain period. There are three trials at each level and the span represents the maximum number of last words that were remembered correctly on at least two out of three trials (for more details, see (Salthouse et al. 1991). In this case, recall of final words was scored irrespective of recall-order. There were no associations between drug effects and baseline working memory capacity. Average scores of the participants on all questionnaires and tests are presented in Table S1.
Table S1. Demographics, baseline self-report scores and neuropsychology
Measure / Sub-scale / Mean (SEM) / ReferenceAge / 21.3 (0.4)
Handednessa / 12.4 (0.2) / Max: 13
Verbal IQb / 104 (2.2)
STAI trait / 32.9 (1.2) / Range: 20-80
Listening Spanc / Span / 5.2 (0.2) / Max: 7
Questions correct / 83.5 (0.1) / Max: 84
Words correct / 68.0 (1.4) / Max: 84
NEO-FFId / Neuroticism / 4.0 (0.4) / Max: 9
Extraversion / 6.2 (0.4) / Max: 9
Openness / 6.5 (0.3) / Max: 9
Altruism / 4.8 (0.3) / Max: 9
Conscientiousness / 3.4 (0.4) / Max: 9
TCI / Novelty seeking / 23.1 (1.3) / Max: 40
Harm avoidance / 11.2 (1.3) / Max: 35
Reward dependence / 16.6 (0.7) / Max: 24
Persistent / 3.4 (0.4) / Max: 8
Self-directedness / 35.6 (0.8) / Max: 44
Cooperativeness / 34.4 (0.9) / Max: 42
Self-transcendence / 7.8 (1.1) / Max: 33
EPQ / Neuroticism / 2.5 (0.5) / Max: 12
Extraversion / 9.2 (0.6) / Max: 12
Psychoticism / 3.2 (0.5) / Max: 12
Social desirability / 5.2 (0.3) / Max: 12
Barratt / Total / 67.8 (2.1) / Range: 30 - 120
Attention / 17.9 (0.6) / Range: 8 - 32
Motor / 21.4 (0.9) / Range: 10 - 40
Non-planning / 28.6 (1.0) / Range: 12 - 48
Beck / Depression / 2.7 (0.6) / < 10 = no depression
Values represent means and standard error of the means. IQ = Intelligent quotient, STAI = state/trait anxiety inventory, NEO-FFI = NEO-Five Factor Inventory, TCI = Cloninger’s Temperament and Character Inventory, EPQ =Eysenck Personality Scales, Barratt = Barratt Impulsiveness Scale, Beck = Beck Depression Inventory. a The score on the handedness questionnaire represents the number of actions performed with the right hand. All participants were right handed with a score >10. b Verbal IQ is measured with the Dutch adult reading test(Schmand et al. 1991). c Baseline working memory capacity was assessed with a Dutch version of the listening span(Salthouse et al. 1991). d Scores are within the normal range of the NEO-FFI norm scores: 1 = very low, 2-3 = low, 4-6: normal, 7-8 = high, 9 = very high(McCrae and Costa 2004).
After inclusion in the study, subjects were invited back for two scanning sessions. Subjects received placebo or the dopamine receptor agonist bromocriptine (Parlodel ®, Novartis, 1.25mg) in a counterbalanced order (11 subjects received bromocriptine first, 11 subjects received placebo first). All doses were administered in opaque, gelatine capsules. Dose selection was based on previous and similar studies, which had revealed good tolerance(Cools et al. 2007; Gibbs and D'Esposito 2005). Mean time to maximal plasma concentration of bromocriptine is about 2.5 hours with a plasma half-life of about 7 hours(Deleu et al. 2002). Accordingly, time of testing (110 minutes after drug intake) coincided with the time-window of maximal drug effects represented by a combination of plasma kinetics and physiological effects.
A well-known side effect of bromocriptine is hypotension. To monitor our subjects and to assess drug effects, we measured heart rate and blood pressure, on both sessions, before drug intake, 45 minutes after drug intake, 165 minutes after drug intake and 240 minutes after drug intake. In one subject we did not measure heart rate on one of the time points. Drug effects on heart rate, diastolic and systolic blood pressure were tested with a repeated measures ANOVA with the factors drug (2) and time (4).
Secretion of the hormone prolactin is inhibited by dopamine D2 receptor stimulation in a dose-dependent manner. To measure the level of prolactin in blood plasma we drew blood twice during each of the sessions, once before drug intake, and once 165 minutes after bromocriptine intake. Plasma prolactin levels were determined by an electrochemiluminescence immunoassay on a Modular E170 Analyzer (Roche Diagnostics) by Professor Fred Sweep and Rob van den Berg at the Laboratory for Endocrinology of the Radboud University Nijmegen Medical Centre. From two subjects we failed to draw blood on one of the time points. These subjects are excluded from prolactin analyses. Drug effects on prolactin levels were tested with a repeated measures ANOVA with the factors drug (2) and time (2).
On each session, participants completed the following questionnaires: the State Anxiety Inventory(Spielberger 1970)(8,9), the Barratt Impulsiveness Scale, the Behavioural Inhibition/Behavioural Activation Scale (BIS/BAS)(Carver and White 1994) and the Positive and Negative Affect Scale. Background neuropsychological tests assessed at the end of each session day included the digit span test, a paper and pencil block completion and number cancellation test and letter fluency test. Paired t-tests were performed to assess drug effects on these neuropsychological tests and questionnaires.
Furthermore, to assess drug effects on subjective mood ratings, subjects completed the Bond and Lader visual analogue scales before drug intake, 165 minutes after drug intake and 240 minutes after drug intake, on each session. Effects of drug on the mood rating scales were measured with repeated measures ANOVA with the factors drug (2) and time (3) for each mood scale separately.
fMRI data acquisition
Whole-brain imaging was performed on a 3 Tesla MR scanner (Magnetom Trio Tim, Siemens Medical Systems, Erlangen, Germany). Functional data were obtained using a gradient-echo echo-planar scanning sequence with blood oxygenation level-dependent (BOLD) contrast (30 axial-oblique slices, repetition time = 1990 ms, echo time = 30 ms, voxel size = 3.5 x 3.5 x 3.0 mm, inter slice gap = 0.5 mm, field of view = 224 mm, flip angle = 80º). Visual stimuli were projected on a screen and were viewed through a mirror attached to the head coil. In addition, a high-resolution T1-weighted MP-RAGE anatomical scan was obtained from each subject (192 sagittal slices, repetition time = 2300 ms, echo time = 3.03 ms, voxel size = 1.0 x 1.0 x 1.0 mm, field of view = 256 mm). The diffusion tensor images were acquired using a twice-refocused spin-echo echo-planar imaging sequence (64 slices interleaved acquisition mode, repetition time = 8600 ms, echo time = 89 ms, voxel size = 2.2 x 2.2 x 2.2 mm, field of view = 220 mm). For each slice, seven images without diffusion weighting (b=0), and 61 images with diffusion weighting (b=1000 s/mm2) applied along non-colinear directions were assembled.
VBM analysis
White matter volume and grey matter volume are highly (negatively) correlated. To ensure that our findings reflect selectively anatomical connections, i.e. white matter, rather than grey matter volume, we applied a threshold mask in our FA correlation analyses excluding all voxels that showed an FA value lower than 0.2. This approach corresponds to applying a grey matter mask. Furthermore we conducted a voxel-based morphometry analysis. The VBM5.1 toolbox in SPM5 ( was used to segment and normalize the anatomical image of each subject(Ashburner and Friston 2000). Normalized modulated grey matter tissue probability maps were smoothed using a Gaussian kernel of 8mm full width at half maximum and tested in a second level one-sample t-test using the drug effect on switch-related BOLD signal extracted from our functional basal ganglia VOI as a covariate. We assessed positive correlations between grey matter volume and drug effects on switch-related BOLD signal.
Supplementary results
Additional VOI analyses
In the main article we report a significant drug effect on switch-related BOLD signal and a drug x FA correlation within our basal ganglia VOI. This VOI was defined as a sphere with a 6 mm radius around the peak voxel for the switch versus repeat contrast averaged across drug conditions within the left and right basal ganglia. Here we report the results of a basal ganglia VOI that was based on a cluster showing an increase for the switch versus repeat contrast at pFWE < 0.05 corrected for the whole brain. A large cluster encompassing the basal ganglia, thalamus and midbrain (see Table 1) was masked with the anatomical basal ganglia mask [defined as the caudate, putamen and pallidum, according to the Automated Anatomical Labelling (AAL) interface (Tzourio-Mazoyer et al. 2002)] to obtain the basal ganglia VOI.
Within this basal ganglia VOI, we found a significant effect of bromocriptine at the voxel-level, corrected for multiple comparisons across the VOI (cluster 1: MNI coordinates [16 0 -2], t = 3.91, psvc = 0.006; cluster 2: MNI coordinates [16 2 2], t = 3.82, psvc = 0.008). Moreover, drug effects on BOLD signal extracted from this VOI also correlated with FA values in the same basal ganglia VOI (MNI coordinates [16 6 0], t = 4.33, psvc = 0.01). Thus, using a basal ganglia VOI based on clusters showing a main effect of task (switch versus repeat) gives similar results as a basal ganglia based on peak voxels showing a main effect of task as reported in the main article. Note that these VOIs are not independent.
Whole brain results
Here we report the whole brain results for the drug and drug x FA analyses at an uncorrected threshold (p < 0.001). Table S2 shows all clusters within the main task network (switch versus repeat) that show an increase in BOLD signal after bromocriptine compared with placebo. In addition to the basal ganglia cluster that shows a significant drug effect corrected for multiple comparisons across our volume of interest (see main article), we observe 8 more clusters at the uncorrected level.
In Table S3 we report the whole brain results for the drug x FA analyses. The table reports FA clusters that show a correlation with the drug effect on switch-related basal ganglia BOLD signal at an uncorrected threshold.
Table S2– Whole brain drug effects at p < 0.001 uncorrected, within the attention switching task network (masked at p < 0.001 uncorrected).
local maximumRegion / cluster size / x / y / z / T-value
Hippocampus / 29 / 22 / -34 / -2 / 4.27
Midbrain / 5 / -4 / -36 / -8 / 4.09
Supplementary motor area / 23 / 12 / 12 / 60 / 4.02
Basal ganglia / 15 / 16 / 0 / -2 / 3.91
Midbrain/Thalamus / 32 / 6 / -14 / -6 / 3.79
Hippocampus / 6 / 30 / -28 / -12 / 3.53
Hippocampus / 1 / -26 / -34 / -8 / 3.33
Thalamus / 3 / 24 / -28 / 2 / 3.27
Midbrain / 1 / 0 / -24 / -12 / 3.22
Table S3– Whole brain correlation between drug effect on switch-related basal ganglia BOLD signal and FA values at p < 0.001 uncorrected.
local maximumRegion / clustersize / x / y / z / T-value
Basal ganglia / 14 / 18 / 6 / 0 / 4.86
Superior frontal gyrus / 2 / -14 / 20 / 56 / 4.27
Anterior cingulate cortex / 1 / -12 / 48 / 0 / 3.69
Additional control analyses
A number of additional analyses were performed to exclude possible confounding factors. First, white matter volume is often anti-correlated with grey matter volume. This might be especially true for the basal ganglia, where the capsula interna is embedded between the caudate and putamen. Therefore, we aimed to exclude the possibility that our white matter correlations were driven by differences in grey matter. To this end, we performed a voxel-based morphometry analysis, assessing whether switch-related BOLD signal in the basal ganglia could also be predicted from individual differences in grey matter volume. This was not the case (no supra-threshold effects within the basal ganglia even at p < 0.05 uncorrected for multiple comparisons). In addition, no other brain regions showed such a correlation at the threshold of 0.001 uncorrected for multiple comparisons.
Second, to strengthen our conclusion that our effects are driven by directional (axonal) organization rather than overall cell density, we repeated our correlational analysis with mean diffusivity (MD) maps. While the FA values represent the orientation-dependence of water diffusion, which is directional in white matter fibres, the MD reflects overall diffusion and depends on cell density. No associations were found between the drug effect on basal ganglia BOLD signal during attention switching and MD within our basal ganglia VOI, indicating that our results cannot be explained in terms of differences in cell density.
Drug effects on physiological measurements
Repeated measures ANOVA showed a significant main effect of drug on the prolactin levels (F1,19 = 9.6, p = 0.006), systolic (F1,21 = 34.5, p < 0.0005), and diastolic blood pressure (F1,21 = 11.8, p = 0.002), indicating that bromocriptine bound to dopamine receptors(Fitzgerald and Dinan 2008; Johns et al. 1984). Note that the drug-induced change in blood pressure might be associated with changes in neurovascular coupling, but this cannot explain our finding of a correlation between drug effect on basal ganglia BOLD signal and local white matter strength.
Drug effects on neuropsychological tests and mood ratings
Drug effects on questionnaires and neuropsychological tests were assessed. The only significant effect was found on subscales of the Behavioural Activation Scale (BAS) (Carver and White 1994), such that bromocriptine increased the score on the BAS Drive subscale (T21 = -2.1, p = 0.045), but decreased score on the BAS Fun subscale (T21 = 2.6, p = 0.018). However, there were no significant correlations between the drug effects on these scales and our critical measures, 1) drug effect on basal ganglia BOLD signal and 2) FA values, indicating that drug effects on these scales cannot account for our findings.
Bromocriptine had no effect on mood ratings(Bond and Lader 1974).
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