Mesial-Temporal Epileptic Ripples Correlate With Verbal Memory Impairment

Area of Science:

• Neuroscience
• Epileptology
• Cognitive Science

Background:

• High frequency oscillations (HFOs), including ripples (80-200 Hz) and fast ripples (200-500 Hz), are potential biomarkers for epilepsy.
• Differentiating epileptic from physiological HFOs, particularly in temporal epilepsies, remains challenging.
• Physiological sleep-spindle-ripple formations are implicated in memory consolidation during slow-wave sleep.

Purpose of the Study:

• To investigate the correlation between mesial-temporal spindle-ripple rates and memory performance in epilepsy patients.
• To determine if surgical removal of spindle-ripple-generating tissue impacts memory.
• To test the hypothesis that higher rates of overall ripples or spike-associated ripples correlate with poorer memory.

Main Methods:

• Intracranial EEG and neuropsychological memory testing were performed on epilepsy patients undergoing surgery.
• Automatic detection of ripples, epileptic spikes, and sleep-spindles was conducted.
• Spearman correlation analysis assessed the relationship between mesial-temporal ripple rates and memory performance.

Main Results:

• Ripple rates were significantly higher in seizure onset zone channels (p < 0.001).
• Patients with pre-surgical verbal memory impairment exhibited higher overall ripple rates in left mesial-temporal channels (p = 0.039).
• Significant negative correlations were found between verbal memory performance and left mesial-temporal spike-associated ripples (r_s = -0.458; p = 0.007) and overall ripples (r_s = -0.475; p = 0.006).

Conclusions:

• This study did not establish a definitive link between spindle-ripples and memory performance.
• Spindle-ripples appear to constitute a minor component of physiological ripples involved in memory.
• The findings suggest that spindle-ripples do not diminish the predictive value of overall ripples for temporal epilepsy, as most ripples correlate with memory deficits.