This dynamical regime can be represented by a simplified two-state model (described as the “monolithic brain” by Tononi 11) in which the cortex is either up (spiking) or down (not-spiking). Conversely, up states result in a widespread spiking of cortical neurons at the opposite phase of the delta cycle. During these periodic down states, cortical neurons switch to a more hyperpolarized resting membrane potential, increasing the difficulty of spiking and communicating with other neurons 9, 10. For this reason, our mini-review does not particularly emphasize the scalp location of delta (but see the column “Spatial topography” in Table 1).ĭepending on the location of the reference electrode, either the peak or trough of each oscillatory cycle in the delta band corresponds to a down state of cortical silence 8. Due to the inverse relationship between oscillatory frequency and spatial extent 7, delta activity is often extensive across the scalp. Given their mechanistic similarities and the lack of meaningful boundaries between frequency bands 7, we treat both delta and slow oscillations as belonging to the delta band for the purpose of this mini-review thus, our principal focus is on oscillations occurring at frequencies at or below 4 Hz. By doing so, we illuminate PPDs in which the oscillatory regime exhibited by the EEG may be safely and reversibly switched independently of consciousness in the laboratory, e.g., as a means of validating candidate biomarkers of consciousness. Our mini-review comprehensively covers literature in both non-human animals and human participants from over 70 years ago to the present day. Here, we continue reviewing paradoxical EEG patterns by describing pharmacological drug challenges that induce HADOs without loss of consciousness. However, the above review focused largely on neurological conditions and less so on pharmacological manipulations. Furthermore, understanding how such PPDs are possible is crucial for understanding how neural activity relates to consciousness.Ī recent review 5 described a range of conditions in which high amplitude delta oscillations (HADOs) appear in the awake and conscious state including, most notably, Angelman syndrome, a rare genetic condition characterized by diffuse HADOs during conscious wakefulness in children 6. Such paradoxical pharmacological dissociations (PPDs) are useful for falsifying putative spectral electroencephalogram (EEG) markers of conscious and unconscious states, e.g., if a substance can induce cortical slow waves during wakeful consciousness, then slow waves are not a universal indicator of loss of consciousness. Nonetheless, some pharmacological agents are known to enhance activity at these frequencies while sparing consciousness. Neural oscillations in the delta (1–4 Hz) and slow (<1 Hz) frequency bands are generally indicators of unconsciousness 1, 2, 3 or severely reduced consciousness 4, especially when activity is diffuse and of high amplitude.
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