Biology subjects

Joo, H. R.

Publications and source records attributed to Joo, H. R..

4 recordsLinked to original sources

bioRxiv2020

Total recall: episodic memory retrieval,choice, and memory confidence in the rat

Episodic memory enables recollection of past experiences to guide future behavior. Humans know which memories to trust (high confidence) and which to doubt (low confidence). How memory retrieval, memory confidence, and memory-guided decisions are related, however, is not understood. Additionally, whether animals can assess confidence in episodic memories to guide behavior is unknown. We developed a spatial episodic memory task in which rats were incentivized to gamble their time: betting more following a correct choice yielded greater reward. Rat behavior reflected memory confidence, with higher temporal bets following correct choices. We applied modern machine learning to identify a memory decision variable, and built a generative model of memories evolving over time that accurately predicted both choices and confidence reports. Our results reveal in rats an ability thought to exist exclusively in primates, and introduce a unified model of memory dynamics, retrieval, choice, and confidence.

neuroscience↗

bioRxiv2019

A microfabricated, 3D-sharpened silicon shuttle for insertion of flexible electrode arrays through dura mater into brain

Electrode arrays for chronic implantation in the brain are a critical technology in both neuroscience and medicine. Recently, flexible, thin-film polymer electrode arrays have shown promise in facilitating stable, single-unit recordings spanning months in rats. While array flexibility enhances integration with neural tissue, it also requires removal of the dura mater, the tough membrane surrounding the brain, and temporary bracing to penetrate the brain parenchyma. Durotomy increases brain swelling, vascular damage, and surgical time. Insertion using a bracing shuttle results in additional vascular damage and brain compression, which increase with device diameter; while a higher-diameter shuttle will have a higher critical load and more likely penetrate dura, it will damage more brain parenchyma and vasculature. One way to penetrate the intact dura and limit tissue compression without increasing shuttle diameter is to reduce the force required for insertion by sharpening the shuttle tip. We describe a novel design and fabrication process to create silicon insertion shuttles that are sharp in three dimensions and can penetrate rat dura, for faster, easier, and less damaging implantation of polymer arrays. Sharpened profiles are obtained by reflowing patterned photoresist, then transferring its sloped profile to silicon with dry etches. We demonstrate that sharpened shuttles can reliably implant polymer probes through dura to yield high quality single unit and local field potential recordings for at least 95 days. On insertion directly through dura, tissue compression is minimal. This is the first demonstration of a rat dural-penetrating array for chronic recording. This device obviates the need for a durotomy, reducing surgical time and risk of damage to the blood-brain barrier. This is an improvement to state-of-the-art flexible polymer electrode arrays that facilitates their implantation, particularly in multi-site recording experiments. This sharpening process can also be integrated into silicon electrode array fabrication.

neuroscience↗

bioRxiv2019

Dorsal and ventral hippocampus engage opposing networks in the nucleus accumbens

Memories of positive experiences require the brain to link places, events, and reward outcomes. Neural processing underlying the association of spatial experiences with reward is thought to depend on interactions between the hippocampus and the nucleus accumbens (NAc)1-9. Hippocampal projections to the NAc arise from both the ventral hippocampus (vH) and the dorsal hippocampus (dH)6-12, and studies using optogenetic interventions have demonstrated that either vH5, 6 or dH7 input to the NAc can support behaviors dependent on spatial-reward associations. It remains unclear, however, whether dH, vH, or both coordinate memory processing of spatial-reward information in the hippocampal-NAc circuit under normal conditions. Times of memory reactivation within and outside the hippocampus are marked by hippocampal sharp-wave ripples (SWRs)13-19, discrete events which facilitate investigation of inter-regional information processing. It is unknown whether dH and vH SWRs act in concert or separately to engage NAc neuronal networks, and whether either dH or vH SWRs are preferentially linked to spatial-reward representations. Here we show that dH and vH SWRs occur asynchronously in the awake state and that NAc spatial-reward representations are selectively activated during dH SWRs. We performed simultaneous extracellular recordings in the dH, vH, and NAc of rats learning and performing an appetitive spatial task and during sleep. We found that individual NAc neurons activated during SWRs from one subdivision of the hippocampus were typically suppressed or unmodulated during SWRs from the other. NAc neurons activated during dH versus vH SWRs showed markedly different task-related firing patterns. Only dH SWR-activated neurons were tuned to similarities across spatial paths and past reward, indicating a specialization for the dH-NAc, but not vH-NAc, network in linking reward to discrete spatial paths. These temporally and anatomically separable hippocampal-NAc interactions suggest that dH and vH coordinate opposing channels of mnemonic processing in the NAc.

neuroscience↗

bioRxiv2018

A polymer probe-based system for high density, long-lasting electrophysiological recordings across distributed neuronal circuits

The brain is a massive neuronal network, organized into anatomically distributed sub-circuits, with functionally relevant activity occurring at timescales ranging from milliseconds to months. Current methods to monitor neural activity, however, lack the necessary conjunction of anatomical spatial coverage, temporal resolution, and long-term stability to measure this distributed activity. Here we introduce a large-scale, multi-site recording platform that integrates polymer electrodes with a modular stacking headstage design supporting up to 1024 recording channels in freely behaving rats. This system can support months-long recordings from hundreds of well-isolated units across multiple brain regions. Moreover, these recordings are stable enough to track 25% of single units for over a week. This platform enables large-scale electrophysiological interrogation of the fast dynamics and long-timescale evolution of anatomically distributed circuits, and thereby provides a new tool for understanding brain activity.

neuroscience↗