Traditional classification of the frequency bands, that are associated to different functions/states of the brain and consist of
delta,
theta,
alpha,
beta and
gamma bands. Due to the limited capabilities of the early experimental/medical setup to record fast frequencies, for historical reason, all oscillations above 30 Hz were considered as high frequency and were difficult to investigate.[1] Recent advance in manufacturing electrophysiological setups enables to record
electric potential with high temporal and space resolution, and to "catch" dynamics of single cell
action potential. In neuroscience nomenclature, there is still a reaming gap between ~100 Hz and
multi unit activity (>500 Hz), so these oscillations are often called high gamma or HFO.
Neurophysiological features
HFO are generated by different cellular mechanisms and can be detected in many brain areas.[8][9] In
hippocampus, this fast neuronal activity is effect of the population synchronous
spiking of
pyramidal cells in the
CA3 region and
dendritic layer of the
CA1, which give rise to a characteristic oscillation pattern (see more in
sharp waves and ripples).[10] The HFO occurrence during memory task (encoding and recalling images) was also reported in human patients from intracranial recordings in
primary visual,
limbic and higher order
cortical areas.[11] Another example of physiological HFO of around 300 Hz, was found in
subthalamic nucleus,[12] the brain region which is the main target for high-frequency (130 Hz)
deep brain stimulation treatment for patients with
Parkinson's disease.
Somatosensory evoked high-frequency oscillations
ECoG recordings from human
somatosensory cortex, has shown HFO (reaching even 600 Hz) presence during
sensory evoked potentials and somatosensory evoked magnetic field after
median nerve stimulation.[13] These bursts of activity are generated by thalamocortical loop and driven by highly synchronized
spiking of the thalamocortical fibres, and are thought to play a role in information processing.[14] Somatosensory evoked HFO amplitude changes may be potentially used as biomarker for neurologic disorders, which can help in diagnosis in certain clinical contexts. Some oncology patients with
brain tumors showed higher HFOs amplitude on the same side, where the tumor was. Authors of this study also suggest contribution from the thalamocortical pathways to the fast oscillations.[15] Interestingly, higher HFO amplitudes (between 400 and 800 Hz) after nerve stimulation were also reported in the EEG signal of healthy
football and
racquet sports players.[16]
Pathological HFO
There are many studies, that reports pathophysiological types of HFO in human patients and animal models of disease, which are related to different psychiatric or neurological disorders:
HFOs are visible in different brain regions just after
cardiac-arrest and are linked to near-death states.[21]
High amplitude HFOs (80–200 Hz) bursts correlates with psychotic-like state evoked with
PCP or subanesthetic dose of
ketamine (and other
NMDA receptor blockers).[6][22][23]
NMDA receptor hypofunction HFO
There are increasing number of studies indicating that HFO rhythms (130–180 Hz) may arise due to the local NMDA receptor blockage,[25][26][27][28] which is also a pharmacological model of schizophrenia.[26] These NMDA receptor dependent fast oscillations were detected in different brain areas including
hippocampus,[29]nucleus accumbens[6] and
prefrontal cortex regions.[30] Despite the fact that this type of HFO was not yet confirmed in human patients, second generation
antipsychotic drugs, widely used to treat schizophrenia and schizoaffective disorders (i.e.
Clozapine,
Risperidone), were shown to reduce HFO frequency.[6] Recent studies, reports on the new source of HFO in the olfactory bulb structures, which is surprisingly stronger than any other previously seen in the mammalian brain.[31][32] HFO in the bulb is generated by local excitatory-inhibitory circuits modulated by breathing rhythm and may be also recorded under ketamine-xylazine anesthesia.[33] This findings may aid understanding early symptoms of schizophrenia patients and their relatives, that can suffer from olfactory system impairments.[34]