Mechanisms of Memory Enhancement by Deep Brain Stimulation in Humans

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Project Summary/Abstract Loss of the ability to form new memories and retrieve old ones is one of the most dreaded afflictions of the human condition. It is present in various neurological disorders, including temporal lobe epilepsy, traumatic brain injury and is one of the first features of Alzheimer’s Disease (AD) affecting millions of people in the US and many more worldwide. Decades of research have established that declarative memory, the ability to remember recently experienced facts and events, depends on the hippocampus and associated structures in the medial temporal lobe (MTL), including the entorhinal cortex. Our laboratory has been a leader in single neuron physiology of the human MTL for last two decades and was the first group to publish findings using deep brain stimulation (DBS) of the entorhinal-hippocampal circuitry in humans to modulate human memory. Our approach is based on the unique opportunity to record activity of single neurons, neuronal assemblies and local field potentials (LFPs), as well as to apply deep brain stimulation of neural circuits in neurosurgical patients. These are patients with intractable epilepsy who have intracranial depth electrodes implanted in order to identify their seizure focus for possible surgical cure. Our initial findings showed dramatic spatial memory enhancement when DBS was applied to the entorhinal area during learning [1]. In the initial funding period of this project, we built on this success by testing DBS across a wide variety of hippocampal-dependent memory tasks and demonstrating that the critical predictor of whether stimulation would improve memory was the precise spatial targeting of the stimulating electrode to the white matter of the entorhinal area (angular bundle). In the renewed grant, we will further refine our modulation of the entorhinal–hippocampal circuitry by using microstimulation to more precisely identify the spatial and temporal features of applied DBS that lead to enhanced memory. Through simultaneous microstimulation and recording, the project will elucidate the complex relationship between single neuronal responses, LFP oscillations, and DBS that underlies memory enhancement. A primary objective will be to expand the investigation of DBS from encoding to the critical memory phases of consolidation and retrieval, across three memory tasks. Importantly, we will probe the effects of DBS on consolidation during sleep which provides an intriguing and feasible time window for potential clinical intervention. A critical component of our modulation will involve the use of novel closed-loop technology to provide stimulation coordinated in time with endogenous oscillations that have been shown to be critically important for encoding, retrieval, and for consolidation during sleep. The project aims at developing critical insights into the mechanisms of human memory and its enhancement through closed-loop DBS in humans, and thus may contribute significantly to the development of novel therapeutic approaches to human memory disorders.