Reverberating Brain Bits: Scrutinizing Information Routing by Synchronization

Abstract

At any given moment, the brain is subjected to a barrage of sensory information from the environment around us. Selective attention allows to focus on the behaviourally-relevant subset within this overwhelming information stream, depending on the task at hand, and ignore the rest. Attended signals are routed through to deeper parts of the brain, getting processed and reaching awareness, whereas the routing of non-attended signals is suppressed. Despite extensive evidence for attention-dependent changes in neural activity, the mechanism behind such flexible routing of information is not well understood.

One possibility is proposed by the routing by synchrony (RBS) mechanism -- wherein modulation of effective connectivity between neural populations is established by gamma-rhythmic synchronization and coherence of their neural activities. Neural populations receiving attended signals can align their gamma-rhythmic activity to that of the sending populations, such that incoming spikes would arrive when the receiving population is most excitable, enhancing signal transfer. Conversely, non-attended signals arrive unaligned to the receiver's rhythm, reducing their transfer. Experimental studies have already provided correlative evidence that gamma-rhythmic coherence between populations processing visual stimuli is modulated by attention. However, it remains to be shown whether the observed synchronization and coherence phenomena are responsible for flexible information routing causally. Here, we investigate RBS using two complementary approaches, aiming to establish a causal link between physiological phenomena and the neural mechanisms responsible for selective information processing.

In the first project, we make a prediction, based on previous experimental findings and the theory behind the RBS mechanism -- information encoded within neural activity should itself be conveyed through gamma-rhythmic packages. To investigate this prediction, we perform an intricate analysis on neural recordings collected from monkeys performing a visual selective attention task, which utilized visual stimuli allowing to assess their information content within neural activity. New methods are developed, allowing to compute how stimulus information content depends on the gamma phase and amplitude of neural activity. The results are in agreement with the prediction, showing that stimulus information is indeed conveyed via gamma-rhythmic packages, in support of the RBS mechanism.

In the second project, via a modeling approach, we investigate the consequences of direct microstimulation of neural circuits involved in selective attention. Using stimulation pulses in order to control gamma-rhythmic coherence between neural populations would allow to directly test the efficacy of the RBS mechanism and potentially control information routing itself. We propose a method that relies on the inherent oscillatory dynamics of the system -- applying minimal stimulation to nudge the natural activity of the network into a desired state. A closed-loop stimulation paradigm is developed, based on the phase-response characteristics of a biophysically realistic network performing selective routing of information. This allows us measure the contamination of stimulus information content by artificial pulses. Within the scope of our model, we demonstrate that precisely timed perturbations can be used to artificially induce the effect of attention by selectively routing visual signals to higher cortical areas and pinpoint caveats and limitations of utilizing this approach in vivo.