Neurons in different locations across the cortex are connected through polysynaptic networks including both excitatory and inhibitory neurons. However, it is not known how the networks recruit synaptic excitation and inhibition. It is also not known whether their overall impact is increase or decrease in spike activity, and whether their impact is fixed or depends on other factors.

 To answer these questions, we triggered fixed amount of spikes optogenetically in a local region of mouse primary visual cortex (V1), and then investigate their causal effects on distal V1 regions. Extracellular measurements showed that this local activation had two effects on spike activity at distal locations1: activation when cortex was silent, and suppression when sensory-stimulated. The balance between the two depended on the strength of visual stimulation, and was correctly captured by divisive normalization, one of canonical cortical computations. Furthermore, in vivo whole-cell recordings revealed that activation was caused by increase in synaptic excitation and inhibition, whereas suppression was mediated not by increase in inhibition but by decrease in both excitation and inhibition2. These results indicate that distal network connectivity can flexibly recruit or disengage synaptic inputs, achieving adaptable impact on spike activity at distal cortical locations.

1) Sato TK*, Häusser M, Carandini M. (2014), Nature Neuroscience, 17(1):30-2

2) Sato TK*, Haider B, Häusser M, Carandini M*. (2016), Nature Neuroscience, 19(4):568-70