Bispectral analysis of visual interactions in humans

Previous electrophysiological studies have demonstrated interactions between dichoptic visual stimuli presented to the same location in visual space. In this study, we used non-linear spectral analysis, in particular the bispectrum, to study interactions between the electrocerebral activity resulting from stimulation of the left and right visual fields. The stimulus consisted of two squares, one in each visual field, flickering at different frequencies. Bispectra, bicoherence and biphase were calculated for 8 subjects monocularly observing a visual stimulus. Both phase vs. frequency and biphase vs. frequency plots were made to determine weighted time delays from stimulus application to signal appearance in the EEG electrodes. Bispectral analysis reveals non-linear interactions between visual fields occurring with weighted delay times of 410 +/- 58 msec while non-interactive components propagated with weighted time delays of 202 +/- 39 msec. Evaluating these results in light of the predictions of various models, we were able conclude that this interaction does not occur in the retina. These results illustrate how bispectral analysis can be a powerful tool in analyzing the connectivity of neural networks in complex systems. It allows different neuronal systems to be labeled with stimuli at specific frequencies, whose connections can be traced using frequency analysis of the scalp EEG.