Dynamics of lactate concentration and blood oxygen level-dependent effect in the human visual cortex during repeated identical stimuli.

In vivo (1)H NMR spectroscopy at 7 T was utilized to measure the changes in lactate concentration upon repeated identical visual stimuli, each lasting for 2 min. The average amplitude of these increases was found to be reduced over time (P < 0.01), from 0.13 +/- 0.02 micromol/g during the first half of the stimulation paradigm, to 0.06 +/- 0.02 micromol/g during the second half of the stimulation paradigm. In contrast, the blood oxygen level-dependent (BOLD) effect on the fMRI water signal and on the height of the total creatine signal at 3.03 ppm was persistent during the whole observation period. This finding may suggest a differential adaptation of cortical output that is not reflected at the level of the global excitation-inhibition activity of the cortical canonical circuits. Alternative possibilities that could account for an adaptation of [Lac] changes are also discussed.