Neuromodulation of distributed neural networks by tonic and phasic release of norepinephrine

I investigated whether and how the locus coeruleus-norepinephrine (LC-NE) neuromodulatory system modulates the processing of cross-modal integration of sensory information in the cortex. I used a simple hierarchical neural network model in which two unimodal (feature) networks and one bimodal (integration) network interact through bottom-up and top-down processes. I assumed a tonic/phasic activation profile for the LC, which provided combinatorial (tonic and phasic) release of NE into the networks. Ongoing bursts of pyramidal cells mediated through tonic NE-release enabled the dynamic cell assemblies that participate in encoding sensory features, and their integrative (cross-modal) information to be transiently linked together across these networks, whereby the cognitive performance (sensory contrast: selective-responsiveness to sensory input) of the networks was enhanced. Transient hyperpolarization of pyramidal cells through phasic NE-release broke the links to stimulus-irrelevant dynamic cell assemblies, whereby stimulus-relevant information that is distributed over the networks could rapidly and preferentially be bound together. I suggest that combinatorial neuromodulation by tonic and phasic NE-release into the cortex may have important roles in the processing of cross-modal integration of information that is distributed over multiple sensory areas.