Mapping Sustained Attention on the brain (EEG)
You should consider using subject-specific alpha channels and frequency ranges. There are differences between individuals in alpha peak frequency. Also the placement of the EEG cap as well as small differences in orientations of cortex can affect which channels pick up perceptual activity.

One way to do this would be to record data from the subject with their eyes open and then their eyes closed, say for 60 seconds each. Then you can find the channels where there is a significant difference in power around the alpha range between these two conditions. You can also use this data to determine the peak frequency of each subject's alpha.