Hippocampal ripples correlate with memory performance in humans

[Display omitted] •The increase of hippocampal ripple rate during memory retrieval is positively correlated to memory quotients.•Hippocampal IEDs and epileptic ripples are negatively correlated to memory quotients during memory-related task.•Correlations between the three signatures and memory quotients were not present during quiet wakefulness. 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.