Delay discounting and frontostriatal fiber tracts: a combined DTI and MTR study on impulsive choices in healthy young adults

Abstract

Delay discounting, a measure of impulsive choice, has been associated with decreased control of the prefrontal cortex over striatum responses. The anatomical connectivity between both brain regions in delaying gratification remains unknown. Here, we investigate whether the quality of frontostriatal (FS) white matter tracts can predict individual differences in delay-discounting behavior. We use tract-based diffusion tensor imaging and magnetization transfer imaging to measure the microstructural properties of FS fiber tracts in 40 healthy young adults (from 18 to 25 years). We additionally explored whether internal sex hormone levels affect the integrity of FS tracts, based on the hypothesis that sex hormones modulate axonal density within prefrontal dopaminergic circuits. We calculated fractional anisotropy (FA), mean diffusivity (MD), longitudinal diffusivity, radial diffusivity (RD), and magnetization transfer ratio (MTR), a putative measure of myelination, for the FS tract. Results showed that lower integrity within the FS tract (higher MD and RD and lower FA), predicts faster discounting in both sexes. MTR was unrelated to delay-discounting performance. In addition, testosterone levels in males were associated with a lower integrity (higher RD) within the FS tract. Our study provides support for the hypothesis that enhanced structural integrity of white matter fiber bundles between prefrontal and striatal brain areas is associated with better impulse control.

Keywords: DTI; delay discounting; frontostriatal tracts; impulsivity; testosterone.

Figure 1.

FS tracts of 1 individual (A) and binary group average VOI (B). The FS white matter tract within an individual subject, after applying inclusion and exclusion ROIs in model space (A). A VOI was created across the whole sample, where a voxel was flagged when that particular voxel had a FS fiber running through in at least 75% of the sample (B).

Figure 2.

Delay-discounting curves of males and females (N = 40). In the Y-axis, the subjective value of €10 (with SD) is plotted against various delays (no delay, 2 days, 30 days, 180 days, and 365 days) in the X-axis. The AUC is significantly smaller in males than in females (P< 0.04).

Figure 3.

The higher MD within FS fiber tracts is associated with faster discounting. A smaller AUC means steeper discounting, hence an increased preference for (smaller) immediate rewards. Sex and age effects on MD are regressed out, and unstandardized residuals are displayed (a smaller value relates to lower MD). The correlation between AUC and FS MD is −0.50; P= 0.001.

Figure 4.

The association between testosterone and RD in males. The correlation between FS RD (in mm²/s) and male testosterone: ρ = 0.62 (P= 0.01). Note that one outlier was removed from the male group (>3 SD; N = 19). Including this outlier in the analyses did not change the results.