Focal synchronization of ripples (80-200 Hz) in neocortex and their neuronal correlates

Field potentials from different neocortical areas and intracellular recordings from areas 5 and 7 in acutely prepared cats under ketamine-xylazine anesthesia and during natural states of vigilance in chronic experiments, revealed the presence of fast oscillations (80-200 Hz), termed ripples. During anesthesia and slow-wave sleep, these oscillations were selectively related to the depth-negative (depolarizing) component of the field slow oscillation (0.5-1 Hz) and could be synchronized over ~10 mm. The dependence of ripples on neuronal depolarization was also shown by their increased amplitude in field potentials in parallel with progressively more depolarized values of the membrane potential of neurons. The origin of ripples was intracortical as they were also detected in small isolated slabs from the suprasylvian gyrus. Of all types of electrophysiologically identified neocortical neurons, fast-rhythmic-bursting and fast-spiking cells displayed the highest firing rates during ripples. Although linked with neuronal excitation, ripples also comprised an important inhibitory component. Indeed, when regular-spiking neurons were recorded with chloride-filled pipettes, their firing rates increased and their phase relation with ripples was modified. Thus besides excitatory connections, inhibitory processes probably play a major role in the generation of ripples. During natural states of vigilance, ripples were generally more prominent during the depolarizing component of the slow oscillation in slow-wave sleep than during the states of waking and rapid-eye movement (REM) sleep. The mechanisms of generation and synchronization, and the possible functions of neocortical ripples in plasticity processes are discussed.