TITLE 1 Intrinsic heterogeneity in oscillatory dynamics limits correlation - induced neural synchronization 2 3 RUNNING HEAD 4 Heterogeneity limits stochastic synchronization

46 Synchronous neural oscillations are found throughout the brain and are thought to contribute to neural coding 47 and the propagation of activity. Several proposed mechanisms of synchronization have gained support through 48 combined theoretical and experimental investigation, including mechanisms based on coupling and correlated 49 input. Here, we ask how correlation-induced synchrony is affected by physiological heterogeneity across 50 neurons. To address this question, we examined cell-to-cell differences in phase-response curves (PRCs), which 51 characterize the response of periodically firing neurons to weak perturbations. Using acute slice 52 electrophysiology, we measured PRCs across a single class of principal neurons capable of sensory-evoked 53 oscillations in vivo: the olfactory bulb mitral cells (MCs). Periodically firing MCs displayed a broad range of 54 PRCs, each of which was well fit by a simple three-parameter model. MCs also displayed differences in firing 55 rate-current relationships and in preferred firing rate ranges. Both the observed PRC heterogeneity and 56 moderate firing rate differences (~10 Hz) separately reduced the maximum correlation-induced synchrony 57 between MCs by up to 25-30%. Simulations further demonstrated that these components of heterogeneity alone 58 were sufficient to account for the difference in synchronization among heterogeneous versus homogeneous 59 populations in vitro. Within this simulation framework, independent modulation of specific PRC features 60 additionally revealed which aspects of PRC heterogeneity most strongly impact correlation-induced 61 synchronization. Finally, we demonstrated good agreement of novel mathematical theory with our experimental 62 and simulation results, providing a theoretical basis for the influence of heterogeneity on correlation-induced 63 neural synchronization. 64 65