Excitability Modulations of Somatosensory Perception Do Not Depend on Feedforward Neuronal Population Spikes

Neural states shape perception at earliest cortical processing levels. Previous work in humans showed a relationship between initial cortical excitation, as indicated by the N20 component of the somatosensory evoked potential (SEP), prestimulus alpha oscillations, and the perceived intensity in a somatosensory discrimination paradigm. Here we address the follow-up question whether these excitability dynamics reflect changes in feedforward or feedback signals. To distinguish feedforward neural signals from feedback signals, we leveraged high-frequency oscillations (HFO) which have previously been shown to correspond to neuronal population spiking activity of the first excitatory feedforward volley in the somatosensory cortex. We examined these HFO in electroencephalography (EEG) data of 32 male human participants, performing a somatosensory intensity discrimination task. Spatial filtering and time-frequency analyses allowed to clearly distinguish HFO from the lower-frequency, conventional SEP. 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 prestimulus alpha and N20 effects of the conventional SEP. Given that the N20 component presumably reflects backpropagating membrane potentials toward the apical dendrites (distal dendritic sites), we argue that top-down feedback processes (e.g., related to alpha oscillations) may thus rely on activity modulations at those distal dendrites of involved pyramidal cells rather than on synchronous output firing changes at their basal compartments.