Binding of visual hemifields investigated using nonlinear properties of SSVEP responses during passive viewing and binocular rivalry

Two views exist regarding the mechanism of binocular rivalry: one is that rivalry occurs between eyes, and the other holds that rivalry occurs between percepts. In the first two studies we try to distinguish physiological correlates of percept rivalry from eye rivalry. We conducted behavioral and EEG experiments using displays favorable to two types of rivalry: MO rivalry, in which perception alternates between two images presented one to each eye, and IO rivalry, in which perception alternates between percepts formed by interocular grouping of visual hemifields one from each eye. In our display, each visual hemifield was presented with gratings sine-flickered at different rates. SSVEP responses to each hemifield demonstrate a large-scale network that is enhanced by conscious perception during MO rivalry but not during IO rivalry.;We also observed SSVEP responses that were nonlinear combinations of our four fundamental frequencies, produced by flicker in complementary visual hemifields, but not from hemifields that occupy the same visual fields across both eyes. Combination responses were modulated by the type of rivalry but not by conscious perception, suggesting they are related to the binding of the potential percepts during rivalry. We further investigated the properties of combination responses in our third study. We delivered sine-flickered gratings to two visual hemifields, either in the same eye, or in different eyes. Stimulus properties (orientation and color) were systematically varied to study the combination response. We recorded first order SSVEP combination responses (f1 + f2) in all conditions, but found highest SNR at channels over occipital, parietal and medial frontal lobes during monocular stimulation with iso-oriented, solid-colored gratings. We propose that callosal pathways preferentially linking ocular dominance columns associated with the same eye generate the enhanced combination responses. Enhanced combination responses from viewing gratings in different eyes may also be generated by binocular neurons in later visual areas.;The paradigm we investigated extends the frequency-tagging method, where flicker elicits strong SSVEP responses to nonlinear interactions between flickers that generate combination responses. These responses unambiguously reflect the functional interaction between neurons responding to each flicker, and hence a potentially powerful tool to study the binding problem.