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The amount of data gathered with echo-planar blood oxygen level (BOLD)-weighted functional MRI can be staggering. With commercially available equipment, it is routine to gather 10 64 64 images per second almost continually for an hour (scanner, GE Signa 1.5 T, GE Medical Systems, Waukesha, WI; local head coil, Medical Advances, Milwaukee, WI). After reconstruction to 16 bit images, the data to be analyzed are acquired at about 250 megabytes per hour. Faster scanning and higher resolution are available with additional equipment, yielding data rates in excess of one gigabyte per scanning hour.

Only a small portion of such a vast accumulation of numbers relates to neural activity. Most of the content is due to the baseline MR signal; after that is removed, much of the variance is related to the cardiac and respiratory cycles, or to subject1 and scanner instabilities. 2 As far as neurological investigations are …