AbstractLocalized proton spectroscopy using the PRESS sequence was performed to measure the free induction decay of the water signal in a 2 × 2 × 2 = ml voxel located in the visual cortex after a short light pulse. The experiment was run continuously with a repetition time of 400 ms; the onset of the light pulse was triggered by the spectrometer. The FIDs were measured as a function of the echo time (TE) and the length of the stimulus (TS). The time course showed three different phases: An initial response was observed 400 ms after the onset of the stimulus, which was demonstrated to be mainly a change in the apparent spin density and/or T1with a minor contribution of T2*. No significant change in T2could be measured. Changes in the ionic cell environment and/or the subsequent increase in cell size upon stimulation are discussed as a potential mechanism for the observed signal respons