Fig. 9: Testing the predicted origins of bursts in sensory-evoked cortical output.

a Schematic illustration of the proposed strategy. In response to the same direct and indirect input from the thalamus, PTs should transition from 1 AP responses into bursts conditional on their additional inputs, e.g., from ongoing activity, and these transitions should be different for bursts of 2 and 3 APs. b Simulation examples show how in silico manipulations of ongoing activity affect responses with 1 AP. From top to bottom: no manipulation, we scaled ongoing firing rates beyond those that we had recorded in the barrel cortex in control conditions by a factor of 1.1 and 1.5 for excitatory types in all layers. c, d In silico predicted relationships between scaling of ongoing activity and TC-driven PT output for our experimental in vivo condition. Box plots represent medians and 25^th to 75^th percentiles. Whiskers extend to 1.5 times the interquartile range (3237, 2108, 1193 in silico responses with 1, 2, 3 APs). e Left: schematic of our optogenetic experiments in anesthetized rats. The schematic of the rat was adapted and modified from “Diamond ME, von Heimendahl M, Knutsen PM, Kleinfeld D, Ahissar E. ‘Where’ and ‘what’ in the whisker sensorimotor system. Nat Rev Neurosci 9, page 602, 2008, Springer Nature”⁶⁸. Reproduced with permission from Springer Nature. Right: example recording for one of the PTs whose TC input along the dendrites is shown in Fig. 1c (most right), and whose in vivo and in silico responses are shown in Fig. 2a, b and Figs. 3–6. Bottom: the traces show somatic APs recorded during three example trials in which we activated TC synapses by a 10 ms light pulse (green shading). We estimated ongoing activity as the LFP amplitude in the barrel cortex at the onset of each light stimulus (blue) compared to baseline before the first light stimulus (dashed line). f, g. In vivo relationships between ongoing activity and TC-driven PT output (24 PTs from 17 rats) analogous to the in silico predictions in panels (d, e). Box plots represent medians and 25^th to 75^th percentiles. Whiskers extend to 1.5 times the interquartile range (2025, 744, 197 in vivo responses with 1, 2, 3 APs). Source data for panels (c, d, f and g) are provided in the Source Data file.