Visually Evoked Gamma Responses in the Human Brain Are Enhanced during Voluntary Hyperventi.pdf

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Visually Evoked Gamma Responses in the Human Brain Are Enhanced during Voluntary Hyperventi.pdf

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Visually Evoked Gamma Responses in the Human Brain Are Enhanced during Voluntary Hyperventi

NeuroImage 15, 575–586 (2002) doi:10.1006/nimg.2001.1013, available online at onVisually Evoked Gamma Responses in the Human Brain Are Enhanced during Voluntary Hyperventilation Ole Jensen,* Riitta Hari,* , ? and Kai Kaila? *Brain Research Unit, Low Temperature Laboratory, Helsinki University of Technology, FIN-02015 HUT, Helsinki, Finland; ?Department of Clinical Neurophysiology, Helsinki University Central Hospital, Helsinki, Finland; and ?Department of Biosciences, University of Helsinki, Helsinki, Finland Received April 10, 2001Hypocapnia induced by hyperventilation (HV) has powerful effects on neuronal excitability and synaptic transmission. We have studied the effect of hyperven- tilation on the phase-locked oscillatory components of the evoked responses in the human brain. We recorded visually evoked magnetoencephalographic responses before, during, and after voluntary hyperventilation to pattern-reversal checkerboard stimuli. Gamma- band (30–45 Hz) responses phase-locked to the stimuli were generated in the occipital visual cortex. A wave- let-based time-frequency analysis revealed that the gamma responses increased during HV whereas their frequency did not change significantly. A recent in vitro study in the rat hippocampus demonstrated that the stability of spontaneous gamma activity increases during hypocapnia as a result of enhanced GABAergic transmission. To test if a similar mechanism could account for our findings, we performed simulations on a network of 100 Hodgkin–Huxley neurons connected by inhibitory synapses. We found that enhanced GABAA transmission, paired with enhanced excitabil- ity, can explain the increase in evoked gamma activity without changing the frequency. ? 2002 Elsevier Science (USA) INTRODUCTION Hyperventilation (HV) is known to lead to alter- ations in physiological and cognitive functions. For in- stance, cognitive performance was reduced in a Stroop- like task following a period of voluntary HV (Diest et al., 2000). HV can pro