Nonlinear interactions in the visual system: a steady-state VEP study

To study the interaction between different receptive fields in human visual processing, the present study investigated the steady-state evoked potentials (ss-VEPs) by two light emitting diodes (LEDs) lit at the different temporal frequencies, which were presented separately to each hemivisual field of the subjects. Two LEDs set in front of the subject were energized at different temporal frequencies of combinations of 7, 9, 11, and 13 Hz. The subject was asked to look at a fixation point at the midpoint. Nineteen EEGs were recorded and the data were averaged 80-100 times triggered at the peak of a light intensity for each quartile range of phase lag. The phase dependency was studied by comparing steady-state visual evoked potentials (ss-VEPs) and also by using higher-order spectra (HOS). The interactions between background EEGs and ss-VEPs were also investigated. The power spectra of ss-VEPs exhibited peaks at the frequency other than those of stimuli. HOS analysis showed that nonlinear phase coupling between the peak frequencies of ss-VEPs were partially detected. Frequencies of the visual stimuli were suspected to reflect onto the bilateral visual area. The changes in the ss-VEPs triggered by the peak of stimuli were affected by the phase lag between bilateral stimuli. Our results are suggestive of some nonlinear interactions in the visual processing over the different areas.