Portraits of Neuronal Communication
The brain is organized as a network of highly specialized networks of spiking neurons. To exploit such a modular architecture for computation, it is essential for the brain to be able to regulate the flow of spiking activity between these networks. Such communication critically depends on the pattern of anatomical links between neuronal networks and the physiology of the neurons involved. While the convergent-divergent connections generally found between neuronal networks facilitate communication, signal transmission is hampered by sparse connectivity, weak synaptic strength, noise in pre and postsynaptic neuronal activity and strong levels of inhibition. Over the last decade, both theoretical models and experimental data have considerably improved our understanding of inter-network communication. In particular, three prominent mechanisms (synfire communication, communication through coherence and communication through resonance) have been proposed, each one employing different biophysical mechanisms to ensure the flow of spiking activity, despite the presence of noise weak/sparse connectivity and inhibition. Here, we will link these mechanisms within a coherent framework that is rooted in the theory of dynamical systems. Communication between networks can thus be understood as trajectories in a two-dimensional state space, spanned by the properties of the input. Such 'portraits of neural communication' reveal whether communication will be successful or not. Moreover, we identify different regions within this state space that define the conditions under which each of these mechanisms contributes to a stable flow of spiking activity. Importantly, this synthesis sheds light on the complementary role of fast oscillations in the gamma range and non-oscillatory spike signals in neuronal network communication. The framework also considers nested oscillations as an important control mechanism for flexibly routing spiking signals to both allow and prevent communication between specific networks. In such nested oscillations, a low-frequency rhythm (e.g. alpha/theta-band oscillations) modulates the power of high-frequency oscillations (gamma band). We show that under some conditions, slow oscillations can modulate the time required to set off resonance in faster oscillating networks and, thereby, regulate communication between specific networks. Moreover, we discuss the potential role of competition in creating a contrast between opened and closed communication channels, and aberrant communication within psychiatric disorders.