1 Lewis, Ni et al., Gamma-rhythmic input causes spike output
SummaryThe gamma rhythm has been implicated in neuronal communication, but causal evidence remains indirect. We measured spike output of local neuronal networks and emulated their synaptic input through optogenetics. Opsins provide currents through somato-dendritic membranes, similar to synapses, yet under experimental control with high temporal precision. We expressed Channelrhodopsin-2 in excitatory neurons of cat visual cortex and recorded neuronal responses to light with different temporal characteristics. Sine waves of different frequencies entrained neuronal responses with a reliability that peaked for input frequencies in the gamma band. Crucially, we also presented white-noise sequences, because their temporal unpredictability enables analysis of causality. Neuronal spike output was caused specifically by the input's gamma component. This gamma-specific transfer function is likely an emergent property of in-vivo networks with feedback inhibition. The method described here could reveal the transfer function between the input to any one and the output of any other neuronal group.
IntroductionAt the heart of the brain's processing abilities is the communication among neuronal groups. And at the heart of the brain's cognitive functions is the flexible modulation of this neuronal communication. Numerous studies suggest that neuronal communication is affected by neuronal synchronization. For example, computational models indicate that synaptic inputs to a neuron have a higher impact when they are synchronized (Salinas and Sejnowski, 2000); intracellular neuronal recordings demonstrate reduced spike thresholds for synchronized inputs (Azouz and Gray, 2003); and recordings from awake macaque monkey reveal that neurons transmitting attended stimulus information show enhanced gamma-band synchronization (Fries et al., 2008) that in turn speeds behavioral responses (Womelsdorf et al., 2006). The communication between two neuronal groups is also affected by the synchronization between them, a concept termed Communication-through-Coherence or CTC (Fries, 2005(Fries, , 2015. Computational models suggest that gamma-rhythmic inputs can entrain a postsynaptic neuronal target group and thereby enhance their impact, while at the same time reducing the impact of competing inputs (Börgers and Kopell, 2008); recordings from cats and macaques show that phase relations between neuronal groups affect their power-power covariation (Womelsdorf et al., 2007) and their transfer entropy (Besserve et al., 2015); and recordings from macaques performing selective attention tasks demonstrate that interareal gamma-band synchronization is enhanced along anatomical projections transmitting attended stimulus information (Bosman et al., 2012;Grothe et al., 2012), and that this synchronization actually improves behavioral performance (Rohenkohl et al., 2018).Thus, recordings of neuronal activity during visual stimulation and task performance have provided strong correlative evidence fo...