Attentional biases in Parkinson?s disease are task dependent and respond to ventral subthalamic deep brain stimulation

Parkinson’s disease is associated with oculomotor and attentional deficits, typically revealed in rather simplistic tasks. Here we evaluated these deficits during more complex viewing tasks, closer to daily life requirements. Additionally, the study of patients undergoing treatment with DBS of the subthalamic nucleus (STN) offers a unique opportunity to disentangle the roles of the STN and the ventrally adjacent substantia nigra pars reticulata (SNr) in attentional control.
We tested N patients under DBS and pharmacological treatment. Eye-movements were acquired in two tasks: free-viewing of natural scenes and serial visual search. Four conditions were evaluated: bilateral clinical DBS-ON, DBS-OFF and unilateral stimulation of the most ventral electrode pole, presumably located within the SNr.
As expected, clinical bilateral DBS-ON settings improved oculomotor features like saccade amplitude and velocity. Attentional deficits were expressed as a bias to explore the right hemifield in free-viewing, but not in visual search. Accordingly, the reaction times for detection of left- or right-side search items did not differ. Remarkably, the free-viewing bias was present in both the DBS-OFF and -ON conditions, but absent when unilateral ventral stimulation was applied.
We interpreted the observed divergence in viewing bias as differences in the motivational and arousal sets evoked by the two tasks. Alternatively, serial visual search might depend only on cortical-brainstem mechanisms. Given that only unilateral ventral but not clinical STN-DBS balanced exploration, we speculate that unilateral SNr-stimulation activated both crossed and uncrossed nigro-collicular projections in a non-selective fashion, eventually giving rise to balanced output of the superior colliculi.