Excitability modulations of somatosensory perception do not depend on feedforward neuronal population spikes

Neural states shape perception at earliest cortical processing levels. Previously we showed a relationship between the N20 component of the somatosensory evoked potential (SEP), pre-stimulus alpha oscillations, and the perceived intensity in a somatosensory discrimination paradigm (Stephani et al., 2021, eLife). Here we address the follow-up question whether these excitability dynamics reflect changes in feedforward or feedback signals. Re-examining the previous EEG data, we leveraged high-frequency oscillations (HFO) as a metric for neuronal population spiking activity of the first excitatory feedforward volley in the cortex. Using Bayesian statistics, we found evidence against the involvement of HFO in moment-to-moment variability of perceived stimulus intensity, in contrast to previously observed pre-stimulus alpha and N20 effects. Given that the N20 component presumably reflects backpropagating membrane potentials towards the apical dendrites, we argue that top-down feedback processes (e.g., related to alpha oscillations) may thus rely on modulations at distal sites of involved pyramidal cells rather than on output firing changes at basal compartments.