ABSTRACT
High-frequency oscillations (300–900 Hz) from the human somatosensory cortex have been the subject of great interest for the last 10 years. This is because physiological characteristics of the oscillations are so much different from the previously known lower frequency somatosensory evoked responses, such as N20, P25, and P27 components, to median nerve stimulation. In this chapter I present a short review of previous studies on somatosensory evoked high-frequency oscillations. I then describe the presence of tangential and radial dipoles for the highfrequency oscillations superimposed on N20 and P25 components, respectively. I also described briefly data on the detection of highfrequency oscillations to posterior tibial and digital nerve stimulation. These findings demonstrate that high-frequency oscillations are generated not only from area 3b but also from area 1 of the primary somatosensory cortex. Furthermore, high-frequency oscillations are not specific to median nerve stimulation but represent activity common to stimulation of any nerve. Developmental changes of the highfrequency oscillations or changes by an aging process are also presented. Highfrequency oscillations are now used as a tool for functional diagnosis of neurological diseases such as Parkinson's disease, multiple system atrophy and myoclonus epilepsy Reciprocal modulation of high-frequency oscilla-tions and the underlying N20 primary response by a wake-sleep cycle and by interference stimulation is a robust phenomenon. On the basis of the reciprocity between the 2 activities in the somatosensory cortex, I hypothesize that the high-frequency oscillations represent activity of fast-spiking GABAergic inhibitory interneurons in the somatosensory cortex, because it has been established that N20 is generated by ensemble excitatory postsynaptic potentials (EPSPs). Further evidence for the distinct generators for the 2 components is provided by the difference in the orientation of the dipoles calculated at the same time slices.
