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

Field potentials from different neocortical areas and intracellular recordi ngs from areas 5 and 7 in acutely prepared cats under ketamine-xylazine ane sthesia and during natural states of vigilance in chronic experiments, reve aled 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 oscillati on (0.5-1 Hz) and could be synchronized over similar to 10 mm. The dependen ce of ripples on neuronal depolarization was also shown by their increased amplitude in field potentials in parallel with progressively more depolariz ed values of the membrane potential of neurons. The origin of ripples was i ntracortical as they were also detected in small isolated slabs from the su prasylvian gyrus. Of all types of electrophysiologically identified neocort ical neurons, fast rhythmic-bursting and fast-spiking cells displayed the h ighest firing rates during ripples. Although linked with neuronal excitatio n, ripples also comprised an important inhibitory component, Indeed, when r egular-spiking neurons were recorded with chloride-filled pipettes, their f iring rates increased and their phase relation with ripples was modified. T hus besides excitatory connections, inhibitory processes probably play a ma jor 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 an d rapid-eye movement (REM) sleep. The mechanisms of generation and synchron ization, and the possible functions of neocortical ripples in plasticity pr ocesses are discussed.