Figure 2.

Different rhythms allow period concatenation. (A) Cross-correlogram showing stable phase relationship between layer 2/3 and layer 5 field potentials in association cortex (S2). When excitation is high, layer 2/3 generates a gamma rhythm (c. 40 Hz) and layer 5 a beta2 rhythm (c. 25 Hz). Reducing excitation then concatenates these two rhythms to generate a beta1 (c. 15 Hz) frequency oscillation in both layers. (B) The asymmetric phase relationship seen for field potentials in (A) is also seen when comparing the timing of layer 2 units with layer 5 units. (C) Intracellular recordings, relative to an on-going beta1 frequency field potential, show the sequence of outputs from different neurons. Each beta1 period begins with a brief burst from layer 5 intrinsically bursting (IB) neurons. This triggers single spikes in superficial fast spiking interneurons (FS). Superficial regular spiking pyramids (RS) spike on the rebound of the resulting IPSP, triggering superficial low-threshold spiking interneurons (LTS) to fire. (D) A cartoon representation of the important cell types in the beta1 rhythm. To the PING model in Figure 1, we add a population of low-threshold spiking (LTS) interneurons and intrinsically bursting (IB) pyramidal cells. We note that the LTS cells inhibit the IB cells, and that the IB cells form excitatory synapses onto all inhibitory cells. A cartoon rastergram is on the right. The beta1 rhythm results from a process of period concatenation. Briefly, the beta1 rhythm propagates through the different cell types as follows. First, the IB cells spike (1) and cause the basket cells to spike (2). The basket cells inhibit the superficial pyramidal cells (3), which recover and spike after one gamma cycle. The superficial pyramidal cell spikes cause the basket and LTS cells to spike (4) which inhibit the IB cells. The IB cells then recover and spike after one beta2 cycle (interval indicated with beta2 label at bottom of this panel) and the rhythm repeats. (A–C) Adapted from Roopun et al. (2008).