Optical implants to control and monitor neuronal activity in vivo have become foundational tools of neuroscience. Standard two-dimensional histology of the implant location, however, often suffers from distortion and loss during tissue processing. To address that, we developed a three-dimensional post hoc histology method called ''light-guided sectioning'' (LiGS), which preserves the tissue with its optical implant in place and allows staining and clearing of a volume up to 500 mm in depth. We demonstrate the use of LiGS to determine the precise location of an optical fiber relative to a deep brain target and to investigate the implant-tissue interface. We show accurate cell registration of ex vivo histology with single-cell, two-photon calcium imaging, obtained through gradient refractive index (GRIN) lenses, and identify subpopulations based on immunohistochemistry. LiGS provides spatial information in experimental paradigms that use optical fibers and GRIN lenses and could help increase reproducibility through identification of fiber-to-target localization and molecular profiling.

Dorsal Excitor motor neuron DE-3 in the medicinal leech plays three very different dynamical roles in three different behaviors. Without rewiring its anatomical connectivity, how can a motor neuron dynamically switch roles to play appropriate roles in various behaviors? We previously used voltage-sensitive dye imaging to record from DE-3 and most other neurons in the leech segmental ganglion during (fictive) swimming, crawling, and local-bend escape (Tomina and Wagenaar, 2017). Here, we repeated that experiment, then re-imaged the same ganglion using serial blockface electron microscopy and traced DE-3's processes. Further, we traced back the processes of DE-3's presynaptic partners to their respective somata. This allowed us to analyze the relationship between circuit anatomy and the activity patterns it sustains. We found that input synapses important for all of the behaviors were widely distributed over DE-3's branches, yet that functional clusters were different during (fictive) swimming vs. crawling.

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Successful navigation through the world requires the integration of sensory input with prior information about the environment. Although it has been shown that stimuli which match prior expectations can be detected faster and more accurately, little is known about the integration of prior information with incoming tactile stimulation in human somatosensory areas. It is also unknown if prior information can induce somatotopic activity in the primary somatosensory cortex (S1) in the absence of tactile stimuli. Based on a vibrotactile detection paradigm we assess how prior information impacts the behavioral performance of participants and how it concurrently modulates BOLD activity and multivariate representations of tactile stimuli in somatosensory areas within a human neuroimaging study. The supra-voxel somatotopic organization of S1 allows us to dissociate representations of tactile stimuli and the modulation thereof by prior information with the resolution permitted by fMRI. We find that vibrotactile stimuli that match the expectations of participants enhance stimulus perception, and that this behavioral enhancement is associated with higher decoding accuracies of stimulus representations in the S1 and a concurrent decrease in BOLD levels in the area. Additionally, we show that prior cues are capable of inducing somatotopic BOLD activity even prior to the onset of tactile stimulation, that tactile stimuli can be decoded from this preparatory activity and that the precision of the decoding is related to the upcoming behavioral performance of participants.

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