Firing of human hippocampal units in relation to voluntary movements

During certain movements (termed "type I," "instrumental," or "voluntary"), the rodent hippocampal EEG is dominated by regular 7-10 Hz waves. This "theta rhythm" is accompanied by increased firing of hippocampal interneurons and dentate gyrus granule cells. No obvious theta rhythm is present in comparable situations in humans or other primates. However, a widespread scalp negativity (the "readiness potential") starts approximately 1 second before spontaneous key presses at long intervals. The readiness potential has been recorded in the monkey hippocampus. In this study, action potentials were recorded in the human hippocampus in relation to various movements. During a broadly ranging interview including various movements and memory tests, hippocampal units were found that fired during movements of the tongue and/or hands. Only movements that required a high degree of effort were effective. Other hippocampal units appeared to be correlated with either the transitions between tasks or the interruptions within tasks. In a second experiment, hippocampal units were found to change their firing in the seconds preceding spontaneous key presses. These data indicate that, like the rodent hippocampus, human hippocampal neuronal activity is strongly influenced by movement.