Simultaneous enhancement of stimulus-induced and stimulus-free gamma in open-eye meditators

Visual stimuli-induced "narrowband" gamma oscillations (30-70 Hz) are generated due to excitation-inhibition interactions and are linked to attention/binding. These oscillations attenuate with aging and neurodegeneration, potentially indicating declining inhibitory function. In contrast, meditation enhances endogenous stimulus-free gamma (power in a broad frequency range above 25 Hz). However, it remains unknown how meditation affects stimulus-induced gamma or its interaction with endogenous broadband gamma, which could reveal whether meditation mitigates age-related decline in inhibitory function. We recorded electroencephalography (EEG) from long-term meditators and their healthy matched controls, performing open-eye meditation while gamma-inducing stimuli were presented before, during, and after meditation. We found that stimulus-induced gamma, like stimulus-free gamma, was stronger in meditators. Interestingly, both gamma signatures co-existed during meditation but were unrelated and prominent in occipital and fronto-temporal regions, respectively. This indicates that these two gamma signatures have different mechanisms and may be modulated differently. Reconstructed cortical sources showed meditation-induced broadband gamma localized in fronto-temporo-parietal regions, which was stronger in the parietal areas of meditators. Stimulus-induced gamma was localized to occipital areas and was stronger in meditators. Further, the power spectral density (PSD) slope, which becomes shallower with aging, was steeper for meditators. We also recorded personality traits and mystic experience using self-reported questionnaires and found that meditators were more mindful, less stressed, and had more mystical experience than the controls, but none of these scores were correlated with gamma power. Meditation could boost inhibitory mechanisms, leading to stronger gamma and steeper PSDs, potentially mitigating age-related neural changes.