Reduced locus coeruleus integrity linked to response inhibition deficits in parkinsonian disorders

Parkinsons disease and progressive supranuclear palsy (PSP) both impair response inhibition, exacerbating impulsivity. Inhibitory control deficits vary across individuals, and have been linked with worse prognosis and lack of improvement on dopaminergic therapy. Motor and cognitive control are associated with noradrenergic innervation of the cortex, arising from the locus coeruleus noradrenergic system. Here we test the hypothesis that loss of structural integrity of the locus coeruleus explains response inhibition deficits in progressive supranuclear palsy and Parkinsons disease. This cross-sectional observational study recruited 24 people with idiopathic Parkinsons disease, 14 with PSP-Richardsons syndrome, and 24 age- and sex-matched controls. All participants undertook a stop-signal task and ultrahigh field 7T-magnetic transfer weighted imaging of the locus coeruleus. Hierarchical Bayesian estimation of the parameters of race models of go-versus stop-decisions was used to quantify the cognitive processes of response inhibition. We tested the multivariate relationship between locus coeruleus integrity and model parameters using partial least squares. Both disorders impaired response inhibition at the group level. Progressive supranuclear palsy caused a distinct pattern of abnormalities in inhibitory control, relative to Parkinsons disease and healthy controls, with a paradoxically reduced threshold for go responses, but longer non-decision times, and more lapses of attention. The variation in response inhibition correlated with variation in the integrity of the locus coeruleus, across participants in both clinical groups. Structural imaging of the locus coeruleus, coupled with behavioural modelling in parkinsonian disorders, confirms that locus coeruleus integrity is associated with response inhibition and its degeneration contributes to neurobehavioural changes. The noradrenergic system is therefore a promising target to treat impulsivity in these conditions. The optimisation of noradrenergic treatment is likely to benefit from stratification according to locus coeruleus integrity.