Distribution, Amplitude, Incidence, Co-Occurrence, and Propagation of Human K-Complexes in Focal Transcortical Recordings

This fascinating paper suggests the existence of small, focal K-complexes in human sleep, adding to growing evidence that sleep may be a focal phenomenon (Krueger et al., 2008). I would just like to briefly respond to the authors’ criticisms of my previous paper examining the large intracranial fields associated with classical scalp EEG K-complexes (Wennberg, 2010).

Mak-McCully et al. (2015) argue that my presentation of intracranial EEG in a common average reference montage was ambiguous, and that the resultant waveforms could be interpreted to suggest K-complex generation in subcortical white matter. However, such an interpretation would be incompatible with analysis of the combined intracranial/extracranial field in its entirety (and entirely at odds with my own interpretation in the paper). The reasons behind the choice of using the average reference – for the specific purposes of that study – were explained in the paper’s Methods, and comparative examples of other referential derivations (linked ears, O2, Sp2, FCz) were provided in a Supplementary figure (Wennberg, 2010).

The bipolar transcortical derivations in Mak-McCully et al. (2015) are perfectly valid, and their Figure 3 showing a dorsal frontal K-complex depicts an intracranial electrical field identical to those presented in Wennberg (2010) and more recently in Voysey et al. (2015). Nevertheless, the bipolar derivations do not provide information not already present in the referential recordings; they simply augment amplitude by maximizing the voltage difference between the two depth electrode contacts making up the bipolar channel, one on the electronegative, superficial side of the cortical dipole layer (Gloor, 1985) and the other on the electropositive underside of the cortical mantle. The transcortical bipolar channel thus represents, in essence, a local active reference recording. That the same information is present in both bipolar and referential derivations is emphasized by the authors’ own methods: to identify contacts for reformatting into transcortical bipolar channels Mak-McCully et al. (2015) first “examined for successive contacts which recorded polarity-inverted spontaneous activity” along a given depth electrode – in referential recordings.

References

Gloor P (1985) Neuronal generators and the problem of localization in electroencephalography: application of volume conductor theory to electroencephalography. J Clin Neurophysiol 2:327-354.

Krueger JM, Rector DM, Roy S, Van Dongen HPA, Belenky G, Panksepp J (2008) Sleep as a fundamental property of neuronal assemblies. Nat Rev Neurosci 9:910-919.

Mak-McCully RA, Rosen BQ, Rolland M, Régis J, Bartolomei F, Rey M, Chauvel P, Cash SS, Halgren E (2015) eNeuro 2(4) e0028-15.2015 doi: 10.1523/ENEURO.0028-15.2015

Voysey Z, Martín-López D, Jiménez-Jiménez D, Selway RP, Alarcón G, Valentín A (2015) Electrical stimulation of the anterior cingulate gyrus induces responses similar to K-complexes in awake humans. Brain Stimul 8:881-890.

Wennberg R (2010) Intracranial cortical localization of the human K-complex. Clin Neurophysiol 121:1176-1186.