Hippocampal ripples correlate with memory performance in humans

Memory performance evaluation has generally been based on behavioral tests in the past decades. However, its neural correlates remain largely unknown, particularly in humans. Here we addressed this question using intracranial electroencephalography in patients with refractory epilepsy, performing an episodic memory test. We used the presurgical Wechsler Memory Scale (WMS) test to assess the memory performance of each patient. We found that hippocampal ripples significantly exhibited a transient increase during visual stimulation or before verbal recall. This increase in hippocampal ripples positively correlated with memory performance. By contrast, memory performance is negatively correlated with hippocampal interictal epileptic discharges (IEDs) or epileptic ripples in the memory task. However, these correlations were not present during quiet wakefulness. Thus, our findings uncover the neural correlates of memory performance in addition to traditional behavioral tests.

Automated summary

Memory performance evaluation has traditionally relied on behavioral tests, but the neural correlates, particularly in humans, have been unclear. In this study, we used intracranial electroencephalography in patients with refractory epilepsy to investigate the neural correlates of memory performance. We found that hippocampal ripples increased during visual stimulation or before verbal recall, and this increase was positively correlated with memory performance. On the other hand, memory performance was negatively correlated with hippocampal interictal epileptic discharges or epileptic ripples during the memory task. However, these correlations were not present during quiet wakefulness. Thus, our findings reveal the neural correlates of memory performance beyond traditional behavioral tests.