Inhibitory Motifs Quench Excessive Synchrony in Neuronal Networks

Biological neuronal networks are far from random; they exhibit specific, recurring patterns of connectivity known as di-synaptic motifs, such as reciprocal or convergent connections. While the over-representation of certain motifs among excitatory neurons is known to induce undesirable synchrony, the brain's cortical activity is typically asynchronous. This presents a key question: how is the synchrony spurred by these excitatory motifs naturally reduced to physiological levels?

To unravel this mystery, researchers systematically altered the prevalence of four key motifs—reciprocal, convergent, divergent, and chain—within an Excitatory-Inhibitory (EI) network. Their findings revealed that an overabundance of chain and convergent motifs specifically in the excitatory population led to increased firing rates and a greater synchrony. Crucially, as lead author Biswas notes in the paper, "However, this excess synchrony was quenched when we introduced the same type of motifs among inhibitory neurons."

The underlying mechanism for this quenching effect is fascinating. The widespread presence of motifs among inhibitory neurons led to some inhibitory cells receiving fewer recurrent inhibitory inputs, making them 'weakly coupled'. These less inhibited neurons were then primarily driven by uncorrelated external inputs, causing them to exert stronger inhibition on the excitatory neurons. This successfully reduced both the overall synchrony and firing rates in the network.

This study not only proposes a new mechanism by which synchrony can be finely controlled within excitatory-inhibitory networks but also offers a significant prediction: that the same types of di-synaptic motifs should be present in both the excitatory and inhibitory populations of neurons. This insight deepens our understanding of the delicate balance governing brain activity.