Frontal subcortical executive dysfunction and minor hallucinations in Parkinson’s disease are linked to sensitivity to somatomotor conflicts

Background Minor hallucinations (MH) affect 30-60% of patients with Parkinson’s disease (PD), and are considered precursors to structured visual hallucinations and cognitive decline. While the link between structured visual hallucinations and dementia is well established, the neuropsychological correlates of MH in PD remain unclear; most studies finding no significant cognitive differences between patients with MH and those without any hallucinations. Objectives Presence hallucinations (PH) being among the most prevalent MH in PD, we used a robotic procedure delivering somatomotor conflicts inducing PH experimentally to investigate whether sensitivity to such robot-induced PH aids in detecting cognitive differences between patients with MH and without hallucinations. Methods 31 PD patients with MH (PD-MH) and 37 without hallucinations (PD-nH) underwent neuropsychological assessment and the robotic procedure inducing PH. The sensitivity to report robot-induced PH was analyzed in relation to cognitive performance in neuropsychological tests. Results PD-MH patients reported more robot-induced PH than PD-nH patients, supporting previous findings. While both groups showed comparable performance in neuropsychological testing, we found a significant association between increased sensitivity to the PH-induction and poorer performance in frontal subcortical functions (executive functions), in PD-MH patients, but not in PD-nH patients. Conclusions These findings demonstrate that sensitivity to robot-induced PH reveals a previously undetected link between MH and frontal subcortical cognitive deficits in PD, pointing to shared underlying mechanisms between executive dysfunction and somatomotor processes involved in MH. This approach offers a novel and clinically valuable means of identifying early cognitive vulnerability that assessments relying only on standard testing may overlook. Plain language summary Induced minor hallucinations are linked to executive function alteration in Parkinson’s disease This study investigated two groups of Parkinson’s disease (PD) patients: one group with minor hallucinations (MH) and another without any hallucinations. Using cognitive tests and a robotic task designed to temporarily induce a presence hallucination (hallucination of someone being there when no one is actually present), the study examined the relationship between cognitive impairment and sensitivity to robot-induced presence hallucinations (riPH). While both groups performed similarly on traditional cognitive tests, patients with MH were more sensitive to the riPH procedure. This increased riPH sensitivity was linked to difficulties with cognitive functions, especially executive functions, which are generally supported by the brain’s frontal and deeper regions (frontal subcortical network). These results from both tests (riPH, neuropsychology) suggest that elevated sensitivity to riPH is associated with mild signs of cognitive decline in PD patients that tradit