FUNCTIONAL SPECTROSCOPY OF BRAIN ACTIVATION FOLLOWING A SINGLE LIGHT-PULSE - EXAMINATIONS OF THE MECHANISM OF THE FAST INITIAL RESPONSE

Localized proton spectroscopy using the PRESS sequence was performed t o measure the free induction decay of the water signal in a 2 x 2 x 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 th e stimulus (TS). The time course showed three different phases: An ini tial response was observed 400 ms after the onset of the stimulus, whi ch was demonstrated to be mainly a change in the apparent spin density and/or T-1 with a minor contribution of T-2. No significant change i n T-2 could 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 response. (C) 1995 John Wiley & Sons, Inc.