Stimulus-induced gamma is stronger and co-exists with stimulus-free gamma in long-term open-eyed meditators

The electrophysiological signal recorded from the brain shows rhythmic activity (oscillations) at different frequencies. One such rhythm, called gamma (~30-80 Hz), is modulated by high-level cognitive processes such as attention, memory, and meditation and, therefore, could provide clues about mechanisms underlying high-level cognition. Interestingly, gamma oscillations can be “induced” in the brain by presenting certain stimuli such as bars, gratings, and color patches (especially red hues). Moreover, these stimulus-induced gamma oscillations weaken with age and the onset of Alzheimer’s Disease. While many meditation studies have observed changes in endogenous (stimulus-free) gamma with meditation, no study to date has tested if stimulus-induced gamma oscillations are also modulated by meditation. We recorded electroencephalography (EEG) signals from long-term Brahmakumaris meditators (N=35) who are trained to meditate with open eyes, as well as their age- and gender-matched controls (N=36). We presented achromatic stimuli that induce gamma before, during and after meditation. We found that stimulus-induced gamma, like stimulus-free gamma, is stronger in meditators. Interestingly, both gamma signatures co-exist during meditation but are unrelated and prominent in fronto-temporal and occipital regions, respectively. Further, the power spectral density (PSD) slope, which becomes shallower with ageing, was steeper for meditators in the fronto-central regions. The PSD slope reflects the excitation-inhibition balance, with steeper slopes reflecting more inhibition. Therefore, long-term meditation could boost inhibitory mechanisms, leading to stronger gamma and steeper PSDs, potentially protecting against aging and neurodegeneration.