Colour discrimination deficit in REM sleep behavior disorder: an analysis of dopaminergic depletion, cognition, and brain morphology

Abstract

Isolated REM sleep behavior disorder (iRBD) is associated with impaired colour discrimination, cognitive deficits and morphological changes. This study evaluates whether colour discrimination deficits in iRBD are mediated by cognitive functions or related to dopaminergic denervation and brain morphology. A sample of 73 patients with iRBD and 77 controls underwent neuropsychological assessment, and colour discrimination assessment using the Farnsworth Munsell 100 Hue Test, DAT-SPECT, and MRI. The data were analyzed using multiple regression, mediation analysis, and voxel-based morphometry. Significant between-group differences were found in total colour discrimination as well as in the red-yellow spectrum. The association between iRBD and performance in the yellow-green spectrum was mediated by cognitive functions, as measured by the Montreal Cognitive Assessment. In controls, a positive correlation between the yellow-green spectrum and the left inferior frontal gyrus was observed compared to patients, however, this association was largely driven by a single data point. The performance in the green-blue spectrum was associated with the activity of dopamine transporters in the caudate nucleus. No interactions were found for total colour discrimination in any analysis. The present findings demonstrate a colour vision deficit in iRBD, which is not directly linked to any of the proposed potential explanatory mechanisms.

Keywords: Cognitive functions; Farnsworth-Munsell 100-Hue Test; REM sleep behavior disorder; Synucleinopathy; Vision.

Fig. 1

Competing causal assumptions Note. In all DAGs represented in this figure we assume that demographic variables age, sex and education level constitute common causes of iRBD, cognition, colour discrimination and sample selection and were thus appropriately adjusted for in all analyses. VisSyst = visual system; ColVis = colour vision; Cog = cognition; SNDop = substantia nigra dopamine; DiffMorf = diffuse morphological changes

Fig. 2

Inclusion flowchart

Fig. 3

Predicted marginal means of FM-100 by group. Note. The whiskers represent the 95% confidence interval. FM-100 = Farnsworth-Munsell 100 Hue Colour Vision Test; TES = total error score; RY = red-yellow

Fig. 4

Group × MoCA interaction in mediation analysis of group and FM-100 (YG) with MoCA mediator. Note. For better display, the points are depicted with additional variability of .3. FM-100 = Farnsworth-Munsell 100 Hue Colour Vision Test; YG = yellow-green; MoCA = Montreal Cognitive Assessment

Fig. 5

Full factorial analysis of FM-100 scores and group. Note. Significant cluster for steeper dependence of iRBD vs. CON with FM-100 (RY). Z-coordinates in the Montreal Neurological Institute space (in millimeters) are indicated next to each slice (top right)