Visual alpha generators in a full-density spiking thalamocortical model

The alpha rhythm (~10 Hz) is one of the most prominent features in waking electroencephalograms of a variety of mammals. It is mainly observed in occipitoparietal regions during the eyes-closed resting state. Although alpha is strongly associated with reduced visual attention, it is also related to other functions such as regulation of timing and temporal resolution of perception, and transmission facilitation of predictions to visual cortex [1]. Understanding how and where this rhythm is generated can elucidate its functions. Even today there is no definitive answer to this question, though several hypotheses put forward thalamus and cortex as possible protagonists.In this work, we built a full-density spiking thalamocortical model, including the primary visual cortex (V1) and the lateral geniculate nucleus (LGN), to study two potential alpha rhythm generators: 1) rhythmic bursts produced by pyramidal neurons in L5 at around 10 Hz [2]; 2) a thalamocortical loop delay of approximately 100 ms, as suggested in mean-field models [3]. The cortical component of our model covers 1 mm2 of V1 surface and is divided into four layers (L2/3, L4, L5, and L6), each containing excitatory and inhibitory populations. The thalamic network comprises an excitatory and an inhibitory population. All neurons were simulated by the adaptive exponential integrate-and-fire model. Cortical neurons in L4 and L6 receive thalamocortical connections, and L6 neurons provide feedback projections to the thalamus. We performed all network simulations using the NEST simulator. The resulting spiking activity was recorded and compared with experimental data by means of power spectra and Granger Causality (GC) analysis.Our results show that both mechanisms are capable of generating and spreading alpha oscillations through the layers, but with different laminar patterns. In Hypothesis 1, the GC analysis suggests that the alpha rhythm mainly originates in L5 and L2/3, as reported in experimental studies with macaques where top-down feedback alpha was observed [4]. On the other hand, Hypothesis 2 points to L4 and L6 as the primary source layers, which may be interpreted as feedforward alpha propagation and matches laminar patterns observed in another macaque study [5]. Furthermore, combining both mechanisms resulted in a summation of effects, with GC in the alpha range emanating from all layers. Thus, our findings suggest that the two mechanisms may contribute differently to alpha rhythms, with distinct laminar patterns, and may be expressed either separately or in tandem under different conditions