INFLUENCE OF PRE-STIMULATION RHYTHMS ON PHOSPHENE PERCEPTION IN THE CONTEXT OF VISUAL HALLUCINATIONS

Background
Complex visual hallucinations are a common symptom in Parkinson’s disease dementia (PDD). Cortical alpha rhythm is a measure of attention and visual perception in healthy controls, and is reported to dysfunction in PDD. Additionally, previous research has related the likelihood to experience visual hallucinations with levels of neuronal excitability in alpha band rhythms from the visual cortex.

Methods
A group of controls participants (n=17) and recently diagnosed PDD patients (n=26) where recruited for the study. Concurrent transcranial magnetic stimulation and electroencephalogram (TMS-EEG) was delivered to all participants and brain signals recorded. Using stereotactic navigation, the cortical region for optimal phosphene experience was determined within the visual cortex in an incremental power stimulation design, where TMS power was increased up to 75% of stimulator output. Only 11 PDD (mean age = 75.47 ± 5.4) and 9 control participants (mean age 73.82 ± 5.4) experienced phosphenes on 50% of trials (240 trials). Mixed effect analysis was implemented to investigate the effects of scalp region (anterior, posterior), pre-stimulus EEG power (within theta, low-alpha and high alpha bands), and participant group on phosphene experience.

Results
In the anterior region, there was a significant effect of phosphene or no-phosphene condition on the low-alpha relative power (F(1,18)=4.72, p=0.043, η2=0.208). Both groups showed decreased low-alpha power before phosphene experience and low-alpha power was lower in PDD participants when compared to controls. When separating the PDD hallucinators from the non-hallucinators, a significant effect of phosphene condition was found in high-alpha band (F(1,17)=7.79, p=0.013, η2=0.314). The posterior high-alpha band power was lower in the PDD hallucinator subgroup at both phosphene/no-phosphene conditions, but higher in the non-hallucinator subgroup when phosphenes were not experienced.

Conclusions
Decreased neural excitability at low-alpha band within the visual cortex is a driver of phosphene experience in both control and PDD patients. However, our data suggests that patients that normally experience complex visual hallucinations are in constant state of decreased alpha power that may elicit this symptom.