DYNAMICS OF SOMATO-DENDRITIC ACTIVITY IN CORTICAL PYRAMIDAL NEURONS DURING VISUAL PROCESSING

The apical dendritic compartment of cortical pyramidal neurons bidirectionally communicates with the soma via back-propagating action potentials and active, local electrogenesis. In the visual cortex, these signals are thought to crucially influence somatic tuning and sensory integration. However, recording them in vivo with high temporal resolution has proven extremely challenging; thus, their role in sensory processing remains unclear. We developed a method to simultaneously record from the apical dendrite and soma of single pyramidal neurons in awake animals using Neuropixels 1.0 and Neuropixels Opto, which enabled compartment-specific stimulation leveraging dendrite-localised ChRmine and of blue-shifted, soma-localised ChR2. We applied this in the visual cortex of mice viewing drifting gratings presented as disks or annuli centered on the local receptive fields, which are thought to differently engage the soma and apical dendrite of L5 neurons. A novel spike-by-spike analysis pipeline identified the extracellular signature of dendrite-evoked, forward-propagating events (dSpikes), and back-propagating action potentials (bAPs). All visual stimuli significantly increased the probability of dSpikes, with a stronger effect of stimuli aligned with the somatic tuning. Visual stimuli in the receptive field surround were the most effective at boosting bAPs, with a stronger upregulation from stimuli orthogonal to somatic tuning. In absence of stimuli, bAP extent was mostly modulated by the length of the preceding ISI. In summary, we show that visual stimuli can differently engage the two modes of somato-dendritic communication, with surround-driven inputs enhancing backpropagation, and surround and center stimuli promoting forward-propagating dendritic electrogenesis aligned with somatic tuning.