Task-dependent effects of intracranial hippocampal stimulation on human memory and hippocampal theta power

Abstract

Background: Despite its potential to revolutionize the treatment of memory dysfunction, the efficacy of direct electrical hippocampal stimulation for memory performance has not yet been well characterized. One of the main challenges to cross-study comparison in this area of research is the diversity of the cognitive tasks used to measure memory performance. Objective: We hypothesized that the tasks that differentially engage the hippocampus may be differentially influenced by hippocampal stimulation and the behavioral effects would be related to the underlying hippocampal activity. Methods: To investigate this issue, we recorded intracranial EEG from and directly applied stimulation to the hippocampus of 10 epilepsy patients while they performed two different verbal memory tasks e a word pair associative memory task and a single item memory task. Results: Hippocampal stimulation modulated memory performance in a task-dependent manner, improving associative memory performance, while impairing item memory performance. In addition, subjects with poorer baseline cognitive function improved much more with stimulation. iEEG recordings from the hippocampus during non-stimulation encoding blocks revealed that the associative memory task elicited stronger theta oscillations than did item memory and that stronger theta power was related to memory performance. Conclusions: We show here for the first time that stimulation-induced associative memory enhancement was linked to increased theta power during retrieval. These results suggest that hippocampal stimulation enhances associative memory but not item memory because it engages more hippocampal theta activity and that, in general, increasing hippocampal theta may provide a neural mechanism for successful memory enhancement.

The effects of direct brain stimulation in humans depend on frequency, amplitude, and white-matter proximity

“…A focus of many brain stimulation therapies is to recapitulate a target neuronal pattern [21,43,44]. Because we show the stimulation parameters that cause different types electrophysiological signals, our work offers guidelines for clinicians to select stimulation frequencies and amplitudes that recreate particular target patterns.…”

Theta-modulated oscillatory transcranial direct current stimulation over posterior parietal cortex improves associative memory

“…Furthermore, there is compelling evidence linking theta oscillations to hippocampal encoding and AM (e.g. [23][24][25][26]. Following this rationale, it was recently demonstrated that high definition theta band transcranial alternating current stimulation (tACS) improves AM performance 27 .…”

Frequency and Power Analysis of Eeg Theta Activity During Different Mathematical and Verbal Task in Males

“…The researchers observed an increase in theta activity during memory study and a decrease in the efficiency of the theta wave after memory tasks [9]. As a result of intracranial EEG recording from the hippocampus of 10 epilepsy patients, two different verbal memory tasks were performed, and it was reported to be proportional to theta strength that increased with memory performance by hippocampal stimulation [15]. İshii et al indicated that theta rhythms increased during focused attention on mental computation [10].…”