COMBINING FLEXIBLE ELECTRODE RECORDINGS WITH TWO-PHOTON IMAGING TO INVESTIGATE BRAIN-WIDE NETWORK COUPLING

Adaptive behavior is enabled by the dynamic coordination of neural activity across spatial and temporal scales, which remains difficult to investigate. Understanding this distributed brain activity may benefit from combining complementary tools. Extracellular electrophysiology offers sub-millisecond temporal resolution to resolve single action potentials or precise synchronization, while two-photon calcium imaging enables large-scale monitoring of neuronal populations with cell-type specificity. However, combining these two recording modalities poses significant challenges, since traditional rigid electrophysiological probes constrain optical access and suffer from light-induced recording artifacts.

Here, we demonstrate the advantages of ultra-flexible tentacle electrodes (UFTEs) for simultaneous optical and electrophysiological chronic recordings. UFTEs are ultrathin, low-impedance electrodes that can be chronically implanted with arbitrary connector placement. We implanted 9 L2/3-GCaMP6f mice with 64-channel UFTEs in the hippocampus and thalamus and prepared a 4-mm cranial window over the posterior cortex (SSp-bdf, VISrl, and VISp) for two-photon calcium imaging in awake head-fixed mice.

We show that this method enables stable, long-term neural recordings across multiple cortical and subcortical brain areas over weeks. We recorded on average 53 units with the UFTEs, out of which we could track 78% over two weeks (n=5 mice with longitudinal spike-sorting). Simultaneous two-photon calcium imaging with a low working-distance objective produced negligible light artifacts (5 µV mean maximum amplitude). We demonstrate the benefits of our multi-modal recording preparation by analyzing the interaction between thalamic, hippocampal, and cortical neural activity during behavioral state changes and hippocampal ripples. Our approach promises to reveal previously inaccessible and under-appreciated aspects of coordinated brain dynamics.