Dear Robert and Tom,
I'm sorry for not attaching the images. I have tried many times to attach two .pnm image files to the message board, but I failed. I don't know why. Maybe because the message board don't support this kind of file type(pnm), or because the two image files are too large.
Anyway, thank you very much! I have tried your suggestions and I think you are right. The problem is due to trunction artifact. I converted the original BRIK file to the float datatype and then filtered it using the 3dFourier command. Now the filtered time series look very smoothing and the short horizontal lines in the time series disappeared.
Thanks again!
Meng