Two eyes, one vision, except when not.
We took part in another fruitful collaborative study led by vision neuroscience specialist Prof Janine Mendola, which was just published in the European Journal of Neuroscience in open access.
In this new study, Janine advances our understanding of the intriguing mechanism of binocular rivalry. Constantly, our two eyes capture two slightly different images of our environment that compete when they reach the brain, ultimately merging into one unified percept. In the lab, scientists have pushed this ability to the extreme by presenting two radically different images to each eye. In this situation of visual rivalry, we experience the stunning perception of seing only one image after the other, with dominance of one image and suppression of the other, alternating spontaneously over time.
The present study clarifies which cortical circuits and what type of neural dynamics enable the phenomenon of binocular rivalry. We exploited the millisecond temporal resolution of MEG brain imaging to track the brain signals related to each visual item, by flickering them very rapidly at slightly different frequencies, unbeknownst to the participants. By doing so, we were able to “tag” the brain regions that processed each image, either constantly or in alternance with the perceptual experience. We found that different brain networks were entrained by the dominant vs. suppressed stimulus, in temporal coherence with the subjective experience: the dorsal (top) part of the parietal cortex and its ventral (bottom) portion, respectively.
From the paper — Binocular rivalry is an example of bistable visual perception extensively examined in neuroimaging. Magnetoencephalography can track brain responses to phasic visual stimulations of predetermined frequency and phase to advance our understanding of perceptual dominance and suppression in binocular rivalry. We used left and right eye stimuli that flickered at two tagging frequencies to track their respective oscillatory cortical evoked responses. We computed time-resolved measures of coherence to track brain responses phase locked with stimulus frequencies and with respect to the participants' indications of alternations of visual rivalry they experienced. We compared the brain maps obtained to those from a non-rivalrous control replay condition that used physically changing stimuli to mimic rivalry. We found stronger coherence within a posterior cortical network of visual areas during rivalry dominance compared with rivalry suppression and replay control. This network extended beyond the primary visual cortex to several retinotopic visual areas. Moreover, network coherence with dominant percepts in primary visual cortex peaked at least 50 ms prior to the suppressed percept nadir, consistent with the escape theory of alternations. Individual alternation rates were correlated with the rate of change in dominant evoked peaks, but not for the slope of response to suppressed percepts. Effective connectivity measures revealed that dominant (respectively, suppressed) percepts were expressed in dorsal (respectively ventral) streams. We thus demonstrate that binocular rivalry dominance and suppression engage distinct mechanisms and brain networks. These findings advance neural models of rivalry and may relate to more general aspects of selection and suppression in natural vision.
Access the full publication here.