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I was reading about a few studies on estimating functional connectivity between brain areas using fMRI signals. However, as far as I know that fMRI has a very poor time resolution, roughly in the order of 1 second. Individual neurons communicate with delay less than 100 ms, and many brain functions happen within a second, in which case fMRI wouldn't have enough time resolution to determine if things happened simultaneously, or in a sequence at all. This problem would be even greater for causality analysis such as Granger causality framework.

So my question is, what is the theoretical, and practical limit of time resolution in fMRI on human?

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The quick and dirty answer is: fMRI doesn't perform well in the temporal domain, but excels in the spatial domain among non-invasive imaging methods. If you need an answer as to where the action is, use fMRI. If you need answers to temporal questions, I recommend using electrophysiology such as EEG, which excels in the temporal domain, or MEG for a combination of good temporal and spatial resolution.

As to what the limit exactly is, I would say a few seconds. fMRI measures blood oxygentaion responses, (See the answer on the question: "What does fMRI measure exactly"), which are inherently slow. Neural processes in local neural networks occur in milliseconds, in larger networks in the order of hundreds of milliseconds. Blood flow differences, in contrast, occur in the order of seconds.

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  • $\begingroup$ I don't think it excels in spatial domain either. Though there has been a recent report about correlations at a single neuron level. See Schulz, K., Sydekum, E., Krueppel, R., Engelbrecht, C. J., Schlegel, F., Schroter, A., Rudin, M., and Helmchen, F. (2012). Simultaneous BOLD fMRI and fiber-optic calcium recording in rat neocortex. Nat Meth, 9(6):597-602. dx.doi.org/10.1038/nmeth.2013 $\endgroup$
    – Memming
    Jan 21, 2015 at 21:13
  • $\begingroup$ I say excel because there is no non-invasive imaging method that matches that accuracy as far as I am aware $\endgroup$
    – AliceD ♦
    Jan 21, 2015 at 22:57

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