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1 Department of Biosciences, University of Helsinki, Helsinki, Finland
2 Department of Biosciences, University of Helsinki, Helsinki, Finland; Department of Child Neurology, University Hospital of Helsinki, Hospital for Children and Adolescents, Helsinki, Finland
* To whom correspondence should be addressed. E-mail: juha.voipio{at}helsinki.fi.
Slow shifts in the human scalp-recorded EEG, including those related to changes in brain CO2 levels, have been generally assumed to result from changes in the level of tonic excitation of apical dendrites of cortical pyramidal neurons. We readdressed this issue using DC-EEG shifts elicited in healthy adult subjects by hypo- or hypercapnia. A 3-minute period of hyperventilation resulted in a prompt negative shift with a rate of up to 10 µV/s at the vertex (Cz) and an extremely steep dependence (up to 100 µV/mmHg) on the end-tidal Pco2. This shift had a maximum of up to -2 mV at Cz versus the temporal derivations (T3/T4). Hyperventilation-like breathing of 5% CO2 plus 95% air, which does not lead to a significant hypocapnia, resulted in a near-complete block of the negative DC shift at Cz. Hypoventilation, or breathing 5% CO2 in air at normal respiratory rate, induced a positive shift. The high amplitude of the voltage gradients on the scalp induced by hyperventilation is not consistent with a neuronal origin. Instead, the present data suggest that they are generated by extracortical volume currents driven by a Pco2-dependent potential difference across epithelia separating the cerebrospinal fluid and blood. Since changes in respiratory patterns and hence, in the level of brain Pco2, are likely to occur under a number of experimental conditions where slow EEG responses have been reported (e.g., attention shifts, preparatory states; epileptic seizures; hypoxic episodes) the present results call for a thorough re-examination of the mechanisms underlying scalp-recorded DC-EEG responses.
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