1. ¹Faculty of Life Sciences, The University of Manchester, Manchester, UK
2. ²Department of Biomedical NMR, AI Virtanen Institute, University of Kuopio, Kuopio, Finland
3. ³Vision Science Centre, Manchester Royal Eye Hospital, Manchester, UK

Correspondence: Dr RA Kauppinen, Faculty of Life Sciences, The University of Manchester, 1.124 Stopford Building, Oxford Road, Manchester M13 9PT, UK. E-mail: Risto.Kauppinen@manchester.ac.uk

Received 26 January 2005; Revised 26 May 2005; Accepted 17 June 2005; Published online 3 August 2005.

Abstract

Functional magnetic resonance imaging (fMRI) techniques were used to study haemodynamic and metabolic responses in human visual cortex during varying arterial blood oxygen saturation levels (Y_sat, determined by pulse-oximeter) and stimulation with contrast-reversing checkerboards. The visual-evoked potential amplitude remained constant at lowered Y_sat of 0.820.03. Similarly, fMRI cerebral blood flow (CBF) responses were unchanged during reduced Y_sat. In contrast, visual cortex volume displaying blood oxygen level-dependent (BOLD) fMRI response decreased as a function of Y_sat, but the BOLD signal change of 3.6%1.4% was constant. Oxygen extraction ratio (OER) during visual activation showed values of 0.260.03 for normal Y_sat. At lowered Y_sat, two OER patterns were observed. Firstly, a reduced OER of 0.140.03 in the visual cortex structures showing BOLD in hypoxia was observed. Secondly, signs of much higher OER in other parts of visual cortex were obtained. T₂^*-weighted magnetic resonance imaging revealed signal increases by 0.8%0.4% with visual activation during lowered Y_sat in the visual cortex structures, which showed BOLD of 3.6% in magnitude under normoxia. Because the CBF response in the visual cortex was quantitatively similar during stimulation in normoxia and hypoxia, attenuated T₂^*-weighted signal increase in parts of visual cortex indicated high OER during visual activation in hypoxia, which was close to that encountered in the resting brain. These spatially localised regions of tissue oxygen extraction and metabolism argue for dissociation between CBF and BOLD fMRI signals in mild hypoxia. The findings point to heterogeneity with regard to oxygen requirement and its coupling to the haemodynamic response in the brain.