Phosphene threshold and 256 channels EEG in MCI patients as a measure of cognitive decline

Question: since phosphene has been considered as a measure of cortical excitability(Brigo et al., 2012; Abrahamyan et al. 2011) and EEG is a measure of the brain rhythms development in MCI (Mild Cognitive Impairment) patients(Babiloni at al.,2011), it has been described the correlation between motor and phosphene threshold compared with alpha posterior rhythms amplitude.The Aim of the study was to correlate the phosphene threshold with the alpha posterior rhythms in patients affected by MCI who were involved in a cognitive decline. Methods: a single pulse TMS stimulation (MX6000 Transcranial Magnetic Stimulator. ATES BIOMEDICA ITALY) was performed for each patient on the occipital right and left cortex. The frequency of stimulation was administered randomly. We marked stimulation sites on posterior regions as O1, O2 and OZ and P3,P4 positions. The Focal coil (figure of eight) was handled in a vertical position. Each patient was trained to say if he would have some bright sensation on his eyes i.e. phosphene. The basic stimulation was set at 30% of the simulator’s output increasing it at rates of 10% until 100% considered as maximum. A resting state 256 channels EEG was performed in order to evaluate the alpha brain activity of each patient. Results: only 20% (5 patients) saw phopshenes. Two of them at low intensity (46% stimulator's output), 3 of them at high intensity (90-100% stimulator's output). All our controls presented a phosphene threshold of 60-70% with motor threshold of 40-50% showing always a difference of 20% in the stimulator's output between the former and latter.Most of the patients presented a borderline pathological EEG dealing with decrease of alpha brain rhythms in occipital regions instead of a progressive spreading over the frontal areas, or in few cases an increase of theta rhythms over the temporal EEG derivations T3/T4;T5/T6. Conclusions: in MCI patients phosphene threshold can be considered as a measure of visual cortical excitability. Only a small percentage of patients were involved in phosphene (20%) and the major of them with a high intensity output stimulator's (60%), while the majority did not perceived any posphene. This finding is associated with a decrease in alpha rhythms and could be related to the degenerative involvement of cortical layers typical of such disease. These preliminary data suggest that changes in EEG rhythms associated with reduction of phosphene evoked by TMS could be a signature of cognitive decline