The effect of hypercapnia (an increase in CO(2) concentration in the blood) on 
the functional magnetic resonance imaging (fMRI) blood oxygenation level 
dependent (BOLD) haemodynamic response has been well characterised and is 
commonly used for BOLD calibration. However, relatively little is known of the 
effect of hypercapnia on the electrical brain processes that underlie the BOLD 
response. Here, we investigate the effect of hypercapnia on resting and stimulus 
induced changes in neural oscillations using a feed-forward low gas flow system 
to deliver a reliable and repeatable level of hypercapnia. 
Magnetoencephalography (MEG) is used in conjunction with beamformer source 
localisation algorithms to non-invasively image changes in oscillatory 
amplitude. At rest, we find robust oscillatory power loss in the alpha 
(8Hz-13Hz), beta (13Hz-30Hz) and low gamma (30Hz-50Hz) frequency bands in 
response to hypercapnia. Further, we show that the spatial signature of this 
power loss differs across frequency bands, with the largest effect being 
observed for the beta band in sensorimotor cortices. We also measure changes in 
oscillatory activity induced by visual and motor events, and the effect of 
hypercapnia on these changes; whilst the percentage change in oscillatory 
activity on activation was largely unaffected by hypercapnia, the absolute 
change in oscillatory amplitude differed between normocapnia and hypercapnia. 
This work supports invasive recordings made in animals, and the results have 
potential implications for calibrated BOLD studies.