Multiple generators of 600 Hz wavelets in human SEP unmasked by varying stimulus rates.

Human scalp-derived somatosensory evoked potentials contain a high-frequency wavelet burst, presumably reflecting repetitive synchronized population spikes. Here, the burst refractory behavior was characterized using median nerve electrostimulation with 18 frequencies (0.5-25Hz) for comparison with cellular burst characteristics. Above 10 Hz only a brief high-frequency (700 Hz) burst component remained discernible, which gradually decreased; possible generators comprise cells capable of generating spike bursts of extraordinarily high frequency, such as pyramidal 'chattering cells', cortical fast spiking inhibitory interneurons and some thalamocortical relay cells. At stimulation frequencies <4 Hz an additional late burst component appeared with only 494 Hz intraburst frequency. Comparably long refractory periods and low intraburst frequencies have been described for bursting cells driven by low-threshold calcium currents.