A Seoul National University (SNU) research team has discovered a correlation between hippocampus and memory function through a direct electrical stimulation on the brain.

There have been controversies over the relationship between the hippocampus and electrical stimulation.

The research team, led by Professor Chung Chun-kee and researcher Jun So-yeon, measured intracranial electroencephalogram (EEG) of 10 intractable epilepsy patients with implanted electrodes while performing two different verbal memory tasks under electrical stimulation to the hippocampus.

The study was divided into a word pair associative memory task and a single item memory task, and progressed to learning, resting, and recalling phases.

Participants performed the two different verbal memory tasks, both of which are known to recruit the medial temporal lobe, including the hippocampus during memory encoding. Thirty words and word pairs were in each session, and the participants performed a total of 120 words.

Each memory task comprised three successive stages: encoding, distractor, and retrieval.

To assess the efficacy of hippocampal stimulation on memory function, the patients had to press words on the screen to respond to memory recall while recalling two different tasks.

In the single word item memory task, patients had to respond to the word memory test with “old” or “new.” The study showed that the chance of getting the correct answer lowered from 86.1 percent of the patient group without electrical stimulation to 81.1 percent of the stimulated patient group.

However, in the associative test, the participants had to answer the questions with “intact,” “rearranged,” or “new.” The correct answer rate increased from 59.3 percent of the non-stimulated group to 67.3 of the stimulated group.

According to the results of the study, electrical stimulation on the hippocampus changed the behavioral results of memory functions differently depending on memory tasks.

The hippocampal theta activity engaged more in the associative memory task. It enhanced associative memory, but the item memory was lower than in the associative memory task due to less involved theta activity.

Besides, patients with lower cognitive function could benefit more from stimulation in improving memory function.

Brain activity was also different in the two memory tasks. The hippocampus elicited a strong theta power in encoding and recalling sections of the associative memory task rather than in the item memory task.

In particular, the research team found that the hippocampal theta power was stronger when the correct answer rate was high in the recalling section, revealing that strong theta power correlates with memory improvement.

“We found that different memory functions could be affected by hippocampal stimulation. This is the first time to see stimulation associated with increased combined memory function and memory task recall to be related with increased hippocampal theta power,” Jun said.

Professor Chung Chun-kee also said, “The study showed that hippocampal stimulation improved the combined memory function because it involved more theta activity in the hippocampus.”

The increase in the hippocampus theta activity could be a neurological mechanism of memory improvement, he added.

The results of the study were published in the latest issue of Brain Stimulation, an international journal in the field of brain science.

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