The observations that Ipc neurons 1) are entrained by periodic input and 2) do not generate persistent oscillations when isolated from the OT suggest that either the Ipc is part of the gamma generating mechanism itself or that it receives input from a gamma generator in the i/dOT. To distinguish between these possibilities, we investigated whether the OT is capable of generating gamma oscillations on its own. In intact slices, retinal afferent stimulation evoked persistent oscillations not only in the sOT but also in the i/dOT (layers 10–15, Figures RAD001 research buy 6A, 6B, and 6C). The power spectrum of the oscillations was substantially broader in the i/dOT than in the sOT (Figures 6B, S5A, and S5B): oscillation power in the

i/dOT was distributed across both low (25–90 Hz) and high (90–140 Hz) gamma frequencies. This result mirrored observations made in vivo, where the power spectrum of gamma oscillations induced by a visual stimulus was broader in the i/dOT than in the sOT (Figures S5C and S5D). To test whether the OT alone can generate persistent gamma activity, we recorded multiunit spike and LFP activity in the i/dOT in transected slices. OT-Ipc transection, which simultaneously eliminated gamma activity in the sOT as described above (Figures S5E and S5F, top), did not eliminate gamma activity in the i/dOT (Figures S5E and S5F, bottom): retinal afferent stimulation continued to induce persistent, broadband

gamma oscillations in CAL-101 cost the i/dOT (median power in intact slices = 13.2 dB, transected = 11.4 dB, p > 0.2, U-test, n = 10, Figures 6A, 6B, and 6C). Furthermore, the oscillations were persistent, although a trend toward shortened durations was noted in transected slices (median: 172 ms, p > 0.1 compared to durations in intact slices, n = 10, Figure 6C). A rhythmic interplay of excitatory and inhibitory currents is a hallmark of gamma-generating networks (Bartos et al., 2007). We made whole-cell patch clamp recordings from layer 10 neurons in

the i/dOT to test whether postsynaptic currents (PSCs) were correlated with the LFP simultaneously recorded extracellularly < 100 μm away. Retinal afferent stimulation evoked persistent barrages of EPSCs and IPSCs that exhibited strong gamma coherence with the LFP, with peaks in the low-gamma TCL band (25–50 Hz; Figures 6D, S5G, and S5H, n = 17 neuron-field pairs). Analysis of the phase of the cross-spectrum between the PSCs and the LFP revealed that EPSCs led IPSCs by 53 ± 14 degrees of gamma cycle phase (Figure 6E), a lead of 4.0 ± 1.0 ms (mean ± SEM, n = 17, p < 0.01, Wilcoxon signed-rank test). In addition, we constructed a 25–50 Hz LFP trough-triggered average of the intracellularly recorded EPSCs and IPSCs. This analysis revealed low-gamma periodicity in the average PSC waveforms (Figure 6F). In addition, the EPSCs peaked at the trough of the LFP and the IPSCs peaked shortly thereafter (Figure 6F and S5I).