Distinct functional properties of the vertical and horizontal saccadic network in Health and Parkinson's disease: An eye-tracking and fMRI study

Saccadic behaviour ranges from reflexive (e.g., prosaccade) to goal oriented voluntary movements (e.g., antisaccade). Behavioural asymmetries between vertical and horizontal saccades have been described both in normal individuals (greater delay of vertical prosaccades) and in disease states such as Parkinson's disease (PD) (prosaccades are short and antisaccades are delayed, especially in the vertical plane, possibly due to a frontostriatal deficit). Importantly, the cortical mechanisms for the generation of vertical saccades are largely unknown, both in health and disease, when compared with their horizontal counterpart. Moreover, studies exploring saccadic neural correlates and putative compensatory mechanisms at a functional level in PD are scarce. We investigated horizontal and vertical prosaccades and antisaccades in an eye tracking paradigm in 19 PD patients off medication and 22 healthy controls, followed by a block-design functional Magnetic Resonance Imaging (fMRI) study, consisting of two runs (prosaccade, antisaccade) of 6 blocks each (3 vertical, 3 horizontal). While saccade metrics were not significantly different between groups, PD showed left frontal underactivation during horizontal prosaccades and right parietal overactivation during horizontal and vertical prosaccades and horizontal antisaccades. Moreover, controls showed greater deactivation of the default-mode network (DMN) during antisaccades. Vertical prosaccades were associated with greater right frontal and cerebellar activity in controls, and cuneus hypoactivity in PD. Vertical antisaccades were associated with greater DMN deactivation in both groups and left frontal hypoactivity in PD. Putative functional compensatory changes in the right parietal cortex in PD patients may help to keep saccadic behaviour at the same level as the healthy controls. We provide first time evidence showing that functional cortical asymmetries between vertical and horizontal saccades occur distinctively in PD patients and healthy controls.