Altered functional connectivity relates to motor performance deficits in bipolar but not unipolar depression

Underpinnings of psychomotor deficits in bipolar and unipolar depression remain underexplored. Here, we hypothesize that motor performance deficits in patients may be partially explained by altered functional connectivities between hand primary motor cortex and posterior cingulate cortex with supplementary motor area. 95 participants between 18-65 years of age, including bipolar depressed, unipolar depressed, and sex-, age-, and education-matched healthy controls, participated in this observational study with two separate visits about five weeks apart, during which the patients received psychopharmacological treatment. Motor performance was measured with a finger-tapping-task and related to functional connectivity from individual seeds in hand primary motor cortex and posterior cingulate cortex as well as to the default mode and sensory motor networks from resting state functional MRI data. 79 participants (45.6% females, 21 bipolar depressed, 27 unipolar depressed and 31 healthy controls) were included in the analysis. Using a finger-tapping-task, the groups differed in motor performance (ANOVA factor "group" F(2,76) = 4.122; p = 0.020) and bipolar depressed but not unipolar depressed showed performance deficits compared to controls (post-hoc-test p = 0.023 and p = 0.158 respectively). Behavioral performance correlated with functional coupling of posterior cingulate cortex - supplementary motor area, but not with coupling of primary motor cortex - supplementary motor area at cluster-wise correction level p FWEc bipolar depressed, unipolar depressed>bipolar depressed) at second visit and in primary motor cortex - supplementary motor area (bipolar depressed>unipolar depressed) at both visits at cluster-wise correction level p FWEc<0.05. Motor performance did not relate to functional coupling of sensory motor network - anterior (visit 1 p = 0.375, visit 2 p = 0.700) or - posterior (visit 1 p = 0.903, visit 2 p = 0.772) default mode network. Motor performance deficits were seen exclusively in bipolar depressed and related to reduced posterior cingulate cortex - supplementary motor area functional connectivity at rest. Our results shed new light on a possible disruption in the anticorrelation between these regions, which seems fundamental for the preservation of motor skills. Given that nuisance factors were controlled for in the study, it is unlikely that the main results are biased by lefthanders, medication load, seed volumes, or differences in movements during MRI scanning. If these findings are confirmed, new targeted non-invasive interventions, such as repetitive transcranial magnetic stimulation, may be more effective against psychomotor deficits in bipolar depression, when aimed at modulating the supplementary motor area.