How global are olfactory bulb oscillations ? 1 2 Leslie

29 30 Previous studies in waking animals have shown that the frequency structure of olfactory bulb 31 (OB) local field potential oscillations is very similar across the OB, but large low-impedance 32 surface electrodes may have favored highly coherent events, averaging out local 33 inhomogeneities. We tested the hypothesis that OB oscillations represent spatially homogeneous 34 phenomena at all scales. We used pairs of concentric electrodes (200 μm outer shaft surrounding 35 an inner 2-3 μm recording site) beginning on the dorsal OB at anterior and medial locations in 36 urethane anesthetized rats and measured local field potential responses at successive 200 μm 37 depths prior to and during odor stimulation. Within locations (outer vs. inner lead on a single 38 probe), on the timescale of 0.5s, coherence in all frequency bands was significant, but on larger 39 timescales (10s) only respiratory (1-4 Hz) and beta (15-30 Hz) oscillations showed prominent 40 peaks. Across locations, coherence in all frequency bands was significantly lower for both sizes 41 of electrodes at all depths but the most superficial 600 μm. Near the pial surface coherence 42 across outer (larger) electrodes at different sites was equal to coherence across outer and inner 43 (small) electrodes within a single site and larger than coherence across inner electrodes at 44 different sites. Overall, the beta band showed the largest coherence across bulbar sites and 45 electrodes. Therefore, larger electrodes at the surface of the OB favor globally coherent events, 46 and at all depths coherence depends on the type of oscillation (beta or gamma) and duration of 47 the analysis window. 48 49