Deep brain stimulation in Parkinson’s Disease patients modulates visual attention and explorative behaviour during free-viewing

Deep brain stimulation (DBS) is an effective treatment for reducing motor symptoms in Parkinson’s disease (PD) patients and it might also be useful to compensate associated cognitive deficits. Here we focused on possible visual attention problems that have been described in reports of subclinical left-sided neglect in PD. Our aim was to extend these previous observations by studying the patients’ visual behaviour in a non-restricted free-viewing task of natural stimuli. The paradigm was intentionally void of predefined task goals in order to enable investigations in spontaneous attentional bias and exploration strategies. Additionally, we want to evaluate whether unilateral stimulation of the basal ganglia output nuclei for eye movement, the substantia nigra pars reticulata, modifies viewing biases. We recorded eyetracking data of PD patients with left-sided symptom onset and age-matched control subjects. The patients participated in the following four conditions: 1) bilateral clinical stimulation in the subthalamic nucleus, 2) no stimulation, and two conditions targeting the substantia nigra, including 3) unilateral ventral left stimulation, and 4) unilateral ventral right stimulation. In contrast to the unbiased behaviour of controls, PD patients showed a significant rightward viewing bias when the stimulation was off. Remarkably, this bias was also present during clinical stimulation but absent after ventral right stimulation. Additionally, we found that saccade lengths and peak velocities were reduced in patients compared with controls. Opposite to the attentional bias, this impairment was significantly improved by clinical DBS. We also evaluated explorative behaviour by examining the angular and length differences of consecutive saccades. Compared with controls, patients showed an increased number of consecutive saccades that followed the same direction, suggesting perseverative behaviour. This increase was least pronounced during clinical stimulation. We conclude that clinical DBS is effective in treating oculomotor symptoms but not in counterbalancing attentional bias. Unilateral ventral stimulation, however, might provide a therapeutic alternative for counteracting symptoms of neglect. Acknowledgements: This study was supported by the EU (FP7-ICT- 270212).