We selectively monitored the Cr/PCr exchange rate during visual stimulation using filter exchange spectroscopy (FEXSY), a pulse sequence sensitized to exchange between two pools with different diffusion coefficients. A Significant difference was observed between the filtered and non-filtered D(tCr) during rest, and vanished during stimulation, indicating an increase in the rate of creation of a fast-diffusing species in the tCr complex during stimulation. This suggests an increased production of Cr from PCr resulting from increased metabolic demand, or an increase in “free” vs. enzyme-bound tCr. The non-filtered ADC of NAA significantly increased during activation, suggesting microstructural change during activation.
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