text: fMRS was developed as an extension of MRS in the early 1990s. Its potential as a research technology became obvious when it was applied to an important research problem where PET studies had been inconclusive, namely the mismatch between oxygen and glucose consumption during sustained visual stimulation. The H fMRS studies highlighted the important role of lactate in this process and significantly contributed to the research in brain energy metabolism during brain activation. It confirmed the hypothesis that lactate increases during sustained visual stimulation  and allowed the generalization of findings based on visual stimulation to other types of stimulation, e.g., auditory stimulation, motor task  and cognitive tasks.
  H fMRS measurements were instrumental in achieving the current consensus among most researchers that lactate levels increase during the first minutes of intense brain activation. However, there are no consistent results about the magnitude of increase, and questions about the exact role of lactate in brain energy metabolism still remain unanswered and are the subject of continuing research.
  C MRS is a special type of fMRS particularly suited for measuring important neurophysiological fluxes in vivo and in real time to assess metabolic activity both in healthy and diseased brains (e.g., in human tumor tissue ). These fluxes include TCA cycle, glutamate–glutamine cycle, glucose and oxygen consumption. C MRS can provide detailed quantitative information about glucose dynamics that can not be obtained with H fMRS, because of the low concentration of glucose in the brain and the spread of its resonances in several multiplets in the H MRS spectrum.
  C MRSs have been crucial in recognizing that the awake nonstimulated (resting) human brain is highly active using 70%–80% of its energy for glucose oxidation to support signaling within cortical networks which is suggested to be necessary for consciousness. This finding has an important implication for the interpretation of BOLD fMRI data where this high baseline activity is generally ignored and response to the task is shown as independent of the baseline activity. C MRS studies indicate that this approach can misjudge and even completely miss the brain activity induced by the task.
  C MRS findings together with other results from PET and fMRI studies have been combined in a model to explain the function of resting state activity called default mode network.
  Another important benefit of C MRS is that it provides unique means for determining the time course of metabolite pools and measuring turnover rates of TCA and glutamate–glutamine cycles. As such, it has been proved to be important in aging research  by revealing that mitochondrial metabolism is reduced with aging which may explain the decline in cognitive and sensory processes.