Photic drive response in people with epilepsy: Exploring the interaction with background alpha rhythm

Abstract

Photic drive responses (PDRs) are used to explore cortical hyperexcitability. We quantified PDRs and interactions with the alpha rhythm in people with epilepsy (PwE). Fifteen PwE (mean age ± SD 47.3 ± 4.6 years; 8 males), and 15 control subjects (mean age 52.7 ± 4.6 years; 9 males) underwent EEG with modified intermittent photic stimulation (IPS). The modification allowed so-called alpha-band gain to be measured. None of the PwE had demonstrated photosensitivity. The modified IPS method alternated eyes-open and eyes–closed conditions with and without IPS. The alpha-band gain appeared as N-fold changes in PDR when IPS (or its harmonics) and the alpha-bands overlapped. An epileptic attack within 1 month of testing significantly increased alpha-band gain by 1.36×. Generalised epilepsy (but not focal epilepsy) significantly decreased alpha-band gain y 0.79×. Each decade of age beyond the mean age significantly increased alpha-band gain by 1.09×. Similar significant interactions were seen between alpha and the second harmonic of IPS driving frequencies that matched alpha frequencies, i.e. for recent attack and, generalized epilepsy. The interactions thus appeared to be occurring between cortical IPS outputs and the alpha generator. These changes were most evident at electrodes O1 and O2. Investigating alpha-band gain using modified IPS offers a way to quantify cortical hyperexcitability in epilepsy and other diseases. It also provides new information about alpha and so too predictive coding, which appears to be at least partly governed by alpha.