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Dear Sun, On Wed, Jan 26, 2011 at 4:17 AM, SUBSCRIBE SPM Sun Delin <[log in to unmask]> wrote: > Dear SPMers, > > I am doing some ERP analysis (2*2 designed) and I would like to use the EEG source reconstruction (group inversion) module provided by SPM8 to get the source intensity of each condition per subject, and put these data into the 2nd 2*2 ANOVA analysis in SPM. However, I am confused by some questions: > > 1. If the source intensity is viewed as the estimation of neural activity, could I say that higher source intensity means stronger neural activity? > The short answer is yes. The long answer is that depending on what you want to say you might need to keep in mind how EEG signals are generate, e.g. that they do not reflect action potentials but rather synchronized synaptic inputs etc. > 2. I found that the waveforms of one condition (e.g. condition A) and its reversed pattern (i.e. -1 * A) share the same source intensity. I would like to know whether there is a way to check the information of waveform's polarity from results of source reconstruction. > Yes, this makes perfect sense because the images you generate are a summary of source power, so they will be the same if you change the sign of your sensor data. However, if you plot the source waveforms using the 'ms or mm' textbox and the 'MIP' button, those should flip their sign because they show currents rather than power. But the sign is rather arbitrary in the first place because it depends on the orientation of the corresponding dipole. > 3. There are two ways to compare the source intensities of condition A and B: (1) get the contrast waveform of (A-B), and then get the source intensity by reversing the contrast, then put the source intensity into one-sample t test; (2) get the source intensity of A and B, respectively; then paired t test. However, in (1) the source intensity of no matter (A-B) or (B-A) is positive and the one-sample t test will show a very very significant result (since all data are greater than 0); whereas in (2), paired t test of (source of A vs. source of B) will show positive in some regions but negative in other regions. Could you please tell me which way is right? > The second way is the only valid way. The first way is indeed not valid because your values will be non-negative so the null hypothesis of the power being zero never holds. If you get more power in condition A in some regions and more power in condition B in other regions I don't see anything wrong with that. In general, localization of a contrast (difference between conditions) is not recommended because SPM makes some assumptions about the SNR which do not hold for difference waveforms. > 4. Could I compare the source intensity of some region in different time window? For example, the source intensity of fusiform area in the time window of 170+-20 ms vs. 500+-20 ms after facial stimuli presentation? > Yes, you can do that, but you should use the latest SPM update (4010) and when you press the 'Contrast' button specify both windows together separated by a semicolon, like [150 190; 480 520]. Best, Vladimir