Intracellular correlates of fast (> 200 Hz) electrical oscillations in ratsomatosensory cortex

Oscillatory activity in excess of several hundred hertz has been observed i n somatosensory evoked potentials (SEP) recorded in both humans and animals and is attracting increasing interest regarding its role in brain function . Currently, however, little is known about the cellular events underlying these oscillations. The present study employed simultaneous in-vivo intrace llular and epipial field-potential recording to investigate the cellular co rrelates of fast oscillations in rat somatosensory cortex evoked by vibriss a stimulation. Two distinct types of fast oscillations were observed, here termed "fast oscillations" (FO) (200-400 Hz) and "very fast oscillations" ( VFO) (400-600 Hz). FO coincided with the earliest slow-wave components of t he SEP whereas VFO typically were later and of smaller amplitude. Regular s piking (RS) cells exhibited vibrissa-evoked responses associated with one o r both types of fast oscillations and consisted of combinations of spike an d/or subthreshold events that, when superimposed across trials, clustered a t latencies separated by successive cycles of FO or VFO activity, or a comb ination of both. Fast spiking (FS) cells responded to vibrissae stimulation with bursts of action potentials that closely approximated the periodicity of the surface VFO. No cells were encountered that produced action potenti al bursts related to FO activity in an analogous fashion. We propose that f ast oscillations define preferred latencies for action potential generation in cortical RS cells, with VFO generated by inhibitory interneurons and FO reflecting both sequential and recurrent activity of stations in the corti cal lamina.