Researchers using intracranial recordings in 17 epilepsy patients report that brief hippocampal sharp‑wave ripples rise before unpredictable events and alter cortical processing. The team, led by Darya Frank and Bryan A. Strange, published the results in Nature Neuroscience (25 June 2026, doi:10.1038/s41593-026-02345-6).
Participants performed a visuomotor selection task while the probability distribution of visual stimuli varied across blocks. Trial‑by‑trial information measures separated entropy (predictability of upcoming items) from surprise (how unexpected a specific stimulus was). The authors found a prestimulus increase in hippocampal ripple count that scaled with entropy in a −800 to −600 ms window before stimulus onset and not with stimulus surprise.
Ripple duration also tracked predictability: prestimulus ripples became longer as entropy rose, whereas poststimulus ripples shortened with higher entropy. Trials that contained a prestimulus ripple yielded faster reaction times on average, consistent with the idea that ripples may boost precision weighting and accelerate evidence accumulation.
The study tested directed interactions between hippocampus and visual cortex. For surprise after stimulus onset, the authors observed increased bottom‑up gamma‑band information flow from fusiform cortex to hippocampus (approximately 52–60 Hz) on high‑versus‑low surprise trials. The paper interprets these patterns as hippocampal signaling of predicted uncertainty (entropy) that modulates cortical prediction‑error responses, while poststimulus bottom‑up gamma reflects processing of actual prediction errors.
Recordings, analysis code and preprocessed files are available from the authors' GitHub repositories at https://github.com/frdarya/GenerativeRipples. The study is based on invasive recordings in a clinical population and reports associations rather than causal manipulations.
Photo credit: media.springernature.com
Tags: hippocampal ripples, intracranial EEG, predictive coding, entropy, reaction time
Topics: Neuroscience & neuroplasticity, Neuroprosthetics & neural implants, EEG & neuro-sensing headsets