Ultrafast frequencies: a new challenge for electroencephalography, with remarks on ultraslow frequencies

EEG frequencies are not limited to the usual 0.5-70/sec (or 0.3-100/sec) range. In recent years, ultrafast activities between 100 and 1000/sec have been the topic of various studies with regard to physiological and paroxysmal conditions. Personal work on ultrafast frequencies in deep structures (elicited with pentylenetetrazol in rats) is mentioned in passing and will be the object of a special study. Other work focusing on the sensorimotor cortex and thalamocortical connections has proved to be seminal for ultrafast EEG research in conjunction with evoked responses (N20 response, SSEP) and experimental neurophysiological studies of afferent volleys, including those causing paroxysmal cortical responses. The well known decremental seizures with initially flat tracings require clarification with ultrafast recordings. In the physiological-neurocognitive domain, Pfurtscheller's event-related desynchronization might also benefit from the use of ultrafast recording. A plea for additional ultraslow recording (DC recording) is also being made, since paroxysmal flattening (electrodecrement) may be associated with an ultraslow negative baseline deflection. The combination of ultrafast (facilitated by digital technique) and ultraslow (technically difficult in patients, easier in experimental animals) would finally denote the frequency-wise complete EEG.