Rapid functional magnetic resonance imaging sequence provides accurate information about timing of neural events
Development of non-invasive neuroimaging methods has been a prerequisite for emergence of and progress in the field of cognitive neuroscience. Indeed, the possibility to measure brain function indirectly in healthy volunteers without penetrating the skull is technologically outright amazing. One of the largest challenges that have remained has been due to the assumed sluggishness of the hemodynamic response that is measured with functional magnetic resonance imaging, which has limited attempts to measure temporal interplay between brain regions during task performance. However, there are some studies that have suggested presence of more accurate response-timing information in the hemodynamics than what has been predominantly believed, and at the same time (even an order of magnitude) faster MR acquisition sequences, such as dynamic magnetic resonance inverse imaging (InI), have been developed that allow whole-head functional volume acquisition as rapidly as 100 msec.
In their recent study, Dr. Fa-Hsuan Lin et al. (2013) combined InI and magnetoencephalography (MEG) to answer the question of whether the faster functional MRI sequences can be utilized to determine differences between response latencies between cortical regions. In the main experiment and two control experiments, altogether 41 subjects performed a simple visuomotor reaction time task during fMRI and MEG. The authors observed that with the faster acquisition rate fMRI could resolve even relatively small temporal delays in responses between cortical areas and the pattern of delays corresponded closely with those estimated with MEG. In one of the control experiments, the order of the visual and motor events was reversed to examine whether latency differences observed between cortical areas were caused by the hemodynamics genuinely measuring the latency of neural responses or, alternatively, whether the latency differences were due to inherent differences in hemodynamic responses properties of the underlying areas. The first hypothesis turned out to be the correct as indeed reversing the order of the visual and motor events reversed the order of hemodynamic responses across the respective areas.
These findings provide a highly exciting and novel methodological demonstration that significantly expands the usability of functional magnetic resonance imaging in cognitive neuroscience research in the future. The results provide evidence in support of the hypothesis that brain hemodynamics do contain (in the eyes of many even surprisingly) accurate information about the latencies of underlying neural events. These findings also stress the importance of methodological advances provided by the development of temporally more accurate fMRI sequences, an area of work that has been at times even belittled due to assumptions that hemodynamics would inherently not hold information about timing of neural events.
Reference: Lin F-H, Witzel T, Raij T, Ahveninen J, Tsai KWN, Chu Y-H, Chang W-T, Nummenmaa A, Polinemi JR, Kuo W-J, Hsieh J-C, Rosen BR, Belliveau JW. FMRI hemodynamics accurately reflects neuronal timing in the human brain measured by MEG. Neuroimage (2013) 78: 372–384. http://dx.doi.org/10.1016/j.neuroimage.2013.04.017