Photosensitive epilepsy is the most common type of stimulus-induced (i.e., visually triggered) epilepsy; however, the dynamical nature of the defensive mechanism which prevents the cortical hyper-excitation for control subjects is largely unknown. We investigated the dynamical characteristics of evoked magnetoencephalographic (MEG) signals to isoluminant flickering (10 Hz) stimuli of different color combinations (red-blue, red-green, and blue-green) from a group of control subjects, and from a photosensitive patient (seizure free intervals only). Wavelet based time-frequency analysis showed that two distinct responses - fundamental (corresponding to stimulus frequency), and harmonic (corresponding to first harmonic) - were found for control subjects, whereas only fundamental but no harmonic response was found for the patient. Three nonlinear measures - approximate entropy (measuring complexity), smoothness index (measuring determinism), coupling index (measuring interdependency) - were applied in combination with the method of phase randomized amplitude adjusted surrogate data. Strong indications of nonlinear structures in the MEG signals for control subjects were found, whereas remarkable absence of nonlinearity was reported for patient. Although such nonlinear structures were observed for all three chosen stimuli, the degrees of the applied nonlinear measures were found to depend on the color combination, thus suggesting a possible correlation between cortical excitation and chromatic sensitivity. These findings put forward the hypothesis that the neuronal responses to photic stimulation in healthy human brain are significantly nonlinear which might reflect an inherent mechanism defending against hyper excitation to chromatic flickering stimulus, and such nonlinear mechanism is likely to be impaired for patient with photosensitive epilepsy.
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