Watching brain oscillations drive perception.

Are the electrical oscillations observed in EEG recordings as we perceive images simply correlations, reflecting brain processes driving our visual experience, or are the oscillations themselves causal, driving the visual experience? Helfrich et al. address this basic question in a clever experiment in which they force brain oscillations of the left and right visual hemispheres into synchrony using transcranial alternating current stimulation. This causes human subjects to more often perceive an ambiguous figure in one of its perceptual instantiations, showing that the oscillations are driving the visual experience, not vice versa. Their summary:

Brain activity is profoundly rhythmic and exhibits seemingly random fluctuations across a very broad frequency range (less than 0.1 Hz to greater than 600 Hz). Recently, it has become evident that these brain rhythms are not just a generic sign of the brain-at-work, but actually reflect a highly flexible mechanism for information encoding and transfer. In particular, it has been suggested that oscillatory synchronization between different areas of the cortex underlies the establishment of task-relevant networks. Here, we investigated whether gamma-band synchronization (~40 Hz) is causally involved in the integration between the two brain hemispheres of alternating visual tokens into a coherent motion percept. We utilized transcranial alternating current stimulation (tACS), a novel non-invasive brain stimulation technique, which allows frequency-specific entrainment of cortical areas. In a combined tACS-electroencephalography study, we selectively up- and down-regulated interhemispheric coherence, resulting in a directed bias in apparent motion perception: Increased interhemispheric connectivity sustained the horizontal motion percept, while decreased connectivity reinforced the vertical percept. Thus, our data suggest that the level of interhemispheric gamma-band coherence directly influenced the instantaneous motion percept. From these results, we conclude that synchronized neuronal activity is essential for conscious perception and cognition.