Increased Synchronization of Neuromagnetic Responses during Conscious Perception

During binocular rivalry, where each eye sees a different image and perception alternates between them, the brain activity corresponding to the consciously perceived image shows increased synchronization. Red vertical and blue horizontal gratings were presented to each eye, each flickering at a distinct frequency. Magnetoencephalography recorded stronger steady-state magnetic fields at the frequency of the perceived grating over occipital, temporal, and frontal cortices. Coherence analysis revealed increased interhemispheric and intrahemispheric synchronization at the stimulus frequency when that stimulus was perceived. The results demonstrate a direct correlation between conscious perception and synchronous activity of large populations of neocortical neurons.

Study at a glance

Abstract

In binocular rivalry, the observer views two incongruent images, one through each eye, but is conscious of only one image at a time. The image that is perceptually dominant alternates every few seconds. We used this phenomenon to investigate neural correlates of conscious perception. We presented a red vertical grating to one eye and a blue horizontal grating to the other eye, with each grating continuously flickering at a distinct frequency (the frequency tag for that stimulus). Steady-state magnetic fields were recorded with a 148 sensor whole-head magnetometer while the subjects reported which grating was perceived. The power of the steady-state magnetic field at the frequency associated with a grating typically increased at multiple sensors when the grating was perceived. Changes in power related to perceptual dominance, presumably reflecting local neural synchronization, reached statistical significance at several sensors, including some positioned over occipital, temporal, and frontal cortices. To identify changes in synchronization between distinct brain areas that were related to perceptual dominance, we analyzed coherence between pairs of widely separated sensors. The results showed that when the stimulus was perceived there was a marked increase in both interhemispheric and intrahemispheric coherence at the stimulus frequency. This study demonstrates a direct correlation between the conscious perception of a visual stimulus and the synchronous activity of large populations of neocortical neurons as reflected by stimulus-evoked steady-state neuromagnetic fields.