Técnica baseada na função de coerência para avaliar o grau de sincronismo no EEG durante estimulação sensorial / A coherence-based technique for evaluating the degree of synchronism in the EEG during sensory stimulation

RESUMO

The coherence between the stimulation signal and the EEG has been used in the detection of evoked responses to rhythmic stimulation. Such detection is based on the knowledge of the sampling distribution of coherence estimate under the null hypothesis of lack of evoked response (zero coherence). In this work, the technique is extended for quantifying the degree of activation due to sensory stimulation. With this aim the sampling distribution for the non-zero response is investigated for evaluating the performance of the detector as well as obtaining the confidence limits of coherence estimate. The latter are obtained using an approximation to the probability density function of the estimate, since the closed-form inverse for the cumulative density function, and hence the critical values for non-zero coherence, cannot easily be found. The theoretical results are verified in Monte Carlo simulations and the techniques is further applied to the EEG signals from 14 subjects during t]rhythmic photic stimulation. The results indicate inter-hemispheric symmetry at homoloque posterior regions in the stimulation frequency of 6 Hz and also its harmonics for most of the subjects. As expected, such symmetry is more pronounced at occipital regions. Further, in 28 per cent of the subjects, the strength of the responses in 12 Hz depends on either such frequency is the stimulation frequency or the second harmonic of it. This difference may be due the fact that the stimulation frequency is lower or higher the subjects' alpha rhythm and need to be further investigated.