Introduction: Human attention requires switching between states that are optimized to process external and internal stimuli. The hippocampus is thought to be the interface between these states but the exact mechanisms remains unclear. The purpose of this study is to determine whether hippocampus theta-gamma phase-amplitude coupling subserves the distinction between internal and external attention states.
Methods: EEG data from 1 surgical epilepsy patient undergoing continuous monitoring for seizure localization was acquired and processed. Phase amplitude coupling (PAC) was computed using the Kullback-Leibler based Modulation Index. Raw PAC values were normalized by z-scoring to a surrogate distribution created by randomly combining the phase and amplitude signals from different trials 200 times. The patient performed a modified auditory Oddball task where blocks of tones were preceded by instructions to focus on the tones and indicate whether a high or low pitch tone was heard or focus on internal thoughts and ignore tones. A self-assessment was given at the end of each pair of blocks asking the patient to rate how successful he/she was in ignoring the tones on the “thoughts” block or vice versa on the “tones block”.
Results: The patient indicated somewhat being able to ignore tones on the “thoughts” blocks and being able to pay full attention to the tones with high consistency on the “tones” blocks. We found strong coupling between the phase of 6 – 10 Hz oscillations and the amplitude of 60 – 140 Hz oscillations in the Hippocampus during internal attention (z > 6). During external attention, this pattern held and was expanded to lower frequencies (3 – 5 Hz) entraining high frequency activity as well. Upon taking the element-wise difference between comodulograms, it appeared that external attention recruits greater theta/alpha-gamma coupling (difference: z > 3).
Conclusion : Phase-amplitude coupling in the hippocampus may underlie biasing attention states to favor internal vs external. Further research is required to explore whether distant regions participate in entrainment of hippocampus high frequency activity.
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